Review of Vadose Zone Measurement and
Monitoring Tools for Yucca Mountain
Performance Confirmation Plan
Dani Or
Department of Civil and Environmental Engineering
University of Connecticut
Storrs, CT 06269-2037
Markus Tuller
Soil and Land Resources Division
University of Idaho
Moscow, ID 83844-2339
Report prepared for:
Center for Nuclear Waste Regulatory Analysis
Southwest Research Institute
6220 Culebra Road
P.O. Drawer 28510
San Antonio, Texas, 78228-0510
December 29, 2005
TABLE OF CONTENTS
1.
Introduction ................................................................................................................................ 5
2.
Review of Key Vadose Zone Activities in DOE PC Plan .......................................................... 6
3.
A Review of VZ Sensing Technology and Measurement Characteristics ................................ 10
3.1
Measurement of Water Content .............................................................................. 10
3.1.1
Neutron Scattering .......................................................................................... 10
3.1.2
Dielectric and Electric Measurement Methods ............................................... 14
3.1.2.1 Time Domain Reflectometry (TDR) ........................................................... 14
3.1.2.2 Capacitance and Frequency Domain Methods ........................................... 19
3.1.2.3 Impedance Sensors (Amplitude Domain Reflectometry) ........................... 20
3.1.2.4 Phase Transmission Sensors ....................................................................... 21
3.1.2.5 Ground Penetrating Radar (GPR) ............................................................... 22
3.1.2.6 Electrical Resistivity Methods for Water Content Monitoring ................... 25
3.2
Water Potential - Definitions .................................................................................. 25
3.2.1
Matric Potential Measurement ........................................................................ 28
3.2.1.1 Tensiometer................................................................................................. 28
3.2.1.2 Heat Dissipation Sensors ............................................................................ 30
3.2.1.3 Psychrometers ............................................................................................. 33
3.3
Soil Pore Water Solution Extraction Methods ........................................................ 36
3.3.1
Suction Cups ................................................................................................... 36
3.3.2
Combined Solution Sampling – Tensiometer Probes ..................................... 38
3.3.3
Suction Lysimeters.......................................................................................... 39
3.3.4
Passive Capillary Samplers ............................................................................. 40
3.3.5
Capillary Absorbers ........................................................................................ 42
3.3.6
Solution Extraction from Soil & Rock Samples ............................................. 43
3.4
Indirect Methods for Monitoring Bulk EC ............................................................. 44
3.4.1
Electrical Resistivity Methods ........................................................................ 44
3.4.2
Electromagnetic Induction Methods ............................................................... 47
3.4.3
Fiber Optic Sensors ......................................................................................... 50
3.5
Temperature Measurement ..................................................................................... 52
3.5.1
Thermocouples ................................................................................................ 52
3.5.2
Resistance Temperature Detectors (RTD) ...................................................... 53
3.5.3
Thermistors ..................................................................................................... 54
3.5.4
Fiber Optic Thermometry ............................................................................... 55
3.6
In situ Measurement of Relative Humidity in Soils and Fractured Rock ............... 56
3.6.1
Psychrometers ................................................................................................. 56
3.6.2
Chilled Mirror Hygrometers (Dew-Point Technique) .................................... 57
3.6.3
Capacitive Humidity Sensors .......................................................................... 59
3.6.4
Resistive Humidity Sensors ............................................................................ 60
3.6.5
Thermal Conductivity Humidity Sensors ....................................................... 61
3.7
In situ Measurement of Water Flux ........................................................................ 61
3.7.1
Water Flux Meter ............................................................................................ 61
3.7.2
Heat Pulse Sensors – Water Content, Thermal Properties, and Water Flux ... 62
3.8
In situ Measurement of Gaseous Fluxes ................................................................. 63
3.9
Monitoring of Deep Percolation in Fractured Rock ............................................... 66
3.10
Sensor pairing for in-situ characterization and monitoring .................................... 69
4.
Potential Additions of VZ Activities to Repository Performance Confirmation Plan .............. 73
5.
Summary and Recomendations ................................................................................................ 73
6.
References ................................................................................................................................ 8x
7.
Appendix A: Manufacturers ..................................................................................................... 88
Executive Summary
The report provides preliminary evaluation of vadose zone (VZ) characterization and
monitoring activities proposed in the repository performance confirmation (PC) plan
developed by DOE. The PC plan is stipulated by the U.S. Nuclear Regulatory Commission
(NRC) and its primary purpose is to confirm that the actual subsurface conditions and
potential changes in these conditions during construction and waste emplacement operations
in Yucca Mountain are within the limits assumed in the licensing review.
The activities related to vadose zone processes proposed within the PC plan (TDR-PCS-SE000001 REV 05) reflect a combination of ongoing and new characterization and monitoring
activities. Cursory inspection of the PC plan reveals important omissions such as definitive
quantification of deep percolation flux, and confirmation and quantification of the existence
and extent of capillary diversion. Additionally, the PC plan often lumps long term monitoring
activities with short term characterization needs, a clear separation of these related efforts
would enhance the effectiveness of repository PC plan. To broaden the basis for discussion
of potential alternative VZ measurement and sensing technology, we devote a considerable
portion of the report to an overview of available sensors and measurement methods.
The disparity between VZ monitoring state-of-practice on the one hand, and the extent and
longevity of monitoring needs stipulated for the implementation of the PC plan on the other,
require a major paradigm shift with respect to long term VZ characterization and monitoring.
Evaluation of performance records reported in the literature and experiences within our group
and those of colleagues show that most existing technologies and sensors for VZ
measurements were not designed for multiyear, uninterrupted operation. Typical
hydrological monitoring networks and sensors are constructed for short term and relatively
high maintenance operation (with daily or weekly service schedule). Most sensors reviewed
in this work and proposed in DOE’s PC plan are not sufficiently robust for deployment at
depths and the natural environment surrounding the repository, nor designed for the duration
of uninterrupted operation required for PC plan and beyond. We conclude that a reliable and
robust VZ hydrological monitoring network for confirmation of waste isolation function of
the repository must rely on redesigned suite of sensors, following rigorous and thorough
testing protocols. The design must incorporate inherent redundancy and supplemented by
detailed maintenance, upgrading and replacement procedures.
For near-term VZ monitoring (while robust sensors and protocols are being developed), we
propose several additional key activities to improve quantification of deep percolation fluxes
by near surface monitoring of matric potential and even direct flux interception. Additionally,
we propose installation of banks of instruments above, at the plane, and below waste
emplacement drifts to measure the onset and extent of capillary diversion. This could be
accomplished by coupling matric potential (tensiometers and psychrometers) and water
content (neutron probe and TDR) measurement devices collocated. Finally, rather than
relying on natural tracers for dating water flux and potential pathways as proposed in the PC
plan, we propose enhancing these capabilities by active marking of water through the use of
well-defined and nonreactive tracers released at intervals of a decade or more. Such well
marked events would provide a traceable hydro-chemical signature marking rates and
providing quantifiable means for resolving actual flux rates and travel pathways the VZ.
1. Introduction
The U.S. Nuclear Regulatory Commission (NRC) stipulates development of a performance
confirmation (PC) program for the proposed repository in Yucca Mountain aimed at
confirming that the actual subsurface conditions and potential changes in these conditions
during construction and waste emplacement operations are within the limits assumed in the
licensing review. The Center for Nuclear Waste Regulatory Analyses (CNWRA) under
contract with NRC is responsible for assessing the adequacy of DOE proposed PC for the
repository. The primary objectives of the PC plan from regulatory point of view are to:





Confirm that subsurface conditions, geotechnical and design parameters are as
anticipated and that changes to these parameters are within limits assumed in the
License Application.
Confirm that the waste retrieval option is preserved.
Evaluate information used to assess whether natural and engineered barriers
function as intended.
Evaluate effectiveness of design features intended to perform a postclosure function
during repository operation and development.
Monitor waste package condition.
This report reviews measurement and monitoring technology for key hydro-physical
processes taking place in the vadose zone (VZ) and relevant to predicted repository
performance. Vadose zone conditions played an important role in the selection of repository
location motivated by the following VZ characteristics (DOE PC plan - TDR-PCS-SE000001 REV 05)





Semiarid climate with limited precipitation.
Thick rock and soil above the repository ranging from 215 to 365 meters.
Hydrogeologic and geochemical conditions limiting radionuclide movement.
Geologic and geomechanical setting supporting design and construction of an
effective Engineered Barrier System.
Depth to groundwater below repository emplacement drifts 250 to 400 meters.
These characteristics are key to the function of the natural and engineered barriers that isolate
waste by minimizing contact with naturally occurring water fluxes and retard radionuclide
transport (by sorption and drift shadow) in the event of a breach of waste package at the
repository level. In addition to reviewing measurement and monitoring technology, we
discuss several important hydrological processes for repository performance, and propose
additional measurement needs, reconfiguration of various proposed monitoring activities to
reduce potential informational gaps.
The report is organized as follows: Section 2 evaluation of DOE PC plan and identification
of potential gaps and proposed changes/additions. Section 3 reviews vadose zone monitoring
tools, adaptive plan to changes in technology and conditions, maintenance and periodic
reviews, thresholds for remedial action.
2. Review of Key Vadose Zone Activities in DOE PC Plan
Our evaluation of DOE PC plan requires better understanding of the objectives and scope of
the proposed plan. We begin with a brief review of DOE’s methodology and planned
activities based on the following general eight steps:
1. Select performance confirmation parameters and test methods
2. Predict performance and establish a baseline
3. Establish bounds and tolerances for key parameters
4. Establish test completion criteria and variance guidelines
5. Plan activities, and construct and install the performance confirmation program
6. Monitor, test, and collect data
7. Analyze and evaluate data
8. Recommend corrective action in the case of variance.
Based on thorough evaluation of various aspects and alternatives related to repository
performance and potential impact on total system performance, DOE narrowed the list of
potential activities to the following ongoing and future activities as the basis for their
proposed PC plan (direct vadose zone processes are highlighted).
Ongoing activities:
1. Precipitation monitoring (precipitation quantities and composition measured at the
Yucca Mountain site
2. Seepage monitoring (seepage monitoring and analysis in alcoves on the repository
intake side and in repository thermally accelerated drifts)
3. Subsurface water and rock testing (chloride mass balance and isotope chemistry
analysis of water samples collected at selected underground locations)
4. Unsaturated zone testing (field-testing of transport and sorptive properties of
unsaturated zone rock in an ambient seepage alcove or a drift with no waste
packages emplaced)
5. Saturated zone monitoring (measurements of water level, electrochemical potential,
hydrogen potential, and background radionuclide concentrations in saturated zone
wells at the repository site and in Nye County)
6. Saturated zone alluvium testing (tracer testing of alluvium transport properties in the
Alluvial Test Complex)
7. Subsurface mapping (mapping of fractures, faults, stratigraphic contacts and
lithophysal characteristics of rock in the underground openings)
8. Seismicity monitoring (monitoring of regional seismic activity and observation of
fault displacements following significant seismic events)
9. Construction effect monitoring (measurement of construction deformation of
underground openings/confirmation of related rock mechanical properties)
10. Corrosion testing (laboratory samples testing of waste package, waste package pallet,
and drip shield materials corrosion behavior in the range of expected repository
environments)
11. Waste form testing (laboratory testing of waste form dissolution and waste package
coupled effects including use of scale mockups of waste package).
New activities (post construction/ operation):
12. Saturated zone fault zone hydrology testing (hydraulic and tracer testing in fault
zones).
13. Drift inspection (periodic inspection of emplacement drifts and thermally accelerated
drifts using remote inspection and measurement techniques).
14. Thermally accelerated drift near-field monitoring (monitoring of rock mass and
water properties in the near-field of a thermally accelerated emplacement drift).
15. Dust buildup monitoring (monitoring and laboratory evaluations of quantity and
composition of dust on engineered barrier surfaces and samples).
16. Thermally accelerated drift environment monitoring (monitoring and laboratory
evaluations of environmental conditions in a thermally accelerated drift including
gas and water compositions, temperatures, film depositions, microbes, radiation
and radiolysis effects using remote techniques).
17. Thermally accelerated drift thermal-mechanical effects monitoring (monitoring of
drift and invert degradation in a thermally accelerated drift).
18. Seal testing (testing of effectiveness of borehole seals in the laboratory, shaft and
ramp seals in the field, and backfill emplacement techniques).
19. Waste package monitoring (monitoring of integrity of waste packages using visual
inspection and/or internal pressure measurement employing remote monitoring
techniques).
20. Corrosion testing of thermally accelerated drift samples (laboratory testing of waste
package, waste package pallet, and drip shield samples obtained from the thermally
accelerated drift).
Although many of the proposed activities are useful and reasonable and clearly contribute to
confirmation of processes essential to ensuring performance within prescribed parameters,
the plan appears incomplete, the outlined strategies for information gathering and monitoring
activities are sketchy, and a few critical processes were not adequately addressed in this plan.
We are cognizant of the intent to engage in more detailed planning for PC implementation as
discussed in length in section 5.2, nevertheless, the emphasis of these activities appears to
focus on QA and safety issues at the risk of overlooking critical scientific choices that form
the basis for the proposed activities to be “subsequently” designed in detail.
Limitations concerning proposed VZ-related activities in the PC plan affect several facets:
(1) Activity design, sensor selection and data quality associated with ongoing activities
that would be phased into the PC plan and upon which key parameters have been
estimated are incomplete and often lack the necessary definitiveness for PC plan
(seepage monitoring, example ii below). Transition from “ongoing” to PC plan should
be accompanied by detail review and adjustments in design and sensor selection.
Ongoing and planned VZ characterization and monitoring activities based on
suboptimal ad-hoc solutions reflecting historical constraints of time, budget, and
technology should be revised.
(2) Review of VZ sensing technology and DOE experiences should make it clear that
commercially available VZ sensors are not designed for long term operation
(multiyear) at the reliability required for monitoring the hydrologic performance of a
nuclear waste repository. We believe that special sensors must be designed,
constructed, and tested to ensure resilient monitoring backbone, one that incorporates
inherent redundancy and enable retrievability of sensors for calibration, replacement
or upgrades.
(3) The proposed PC plan overlooks several VZ processes and scenarios that could
potentially introduce unwanted vulnerabilities. For example, the plan proposes to
estimate deep percolation from precipitation monitoring on the one hand (example i,
below), or from convoluted and poorly defined series of water/rock tests (example iii,
below). We believe that such important variable should be measured more directly
through use of near surface banks of instruments (tensiometers and neutron probe),
flux interception, and prescribed released of markers as explained below. Another
overlooked important process is the existence and extent of capillary diversion – this
is a critical element of repository performance that must be verified in-situ.
(4) Finally, the plan represents a mix of characterization with monitoring activities.
These two types of VZ related activities involve different time horizons (days-months
for characterization vs. years-decades for monitoring), could rely on different sensors
and produce different streams of data. A clear separation and proper sequencing of
these activities would be helpful for both. For example, proper characterization could
enhance sensor placement and value of information obtained for monitoring purposes.
The following are examples highlighting some of the shortcomings of VZ-related activities
proposed in DOE’s PC plan – this is not a comprehensive critical review of the proposed
plan, but rather an attempt to substantiate comments above using a few illustrative examples:
i.
Linkage between precipitation monitoring (section 3.3.1.1) and seepage monitoring
(section 3.3.1.2) through independent measurement or confirmation of deep
percolation is needed. The proposed estimation of deep percolation from
atmospherically-based water balance closure is lacking – review of DOE reports
reveals the two key atmospheric parameters are precipitation and potential
evaportanspiration. Measurement of actual evaportanspiration using eddy covariance
or other techniques (lysimeters would provide more accurate estimates of this
parameter. Moreover, we propose direct monitoring of deep percolation through
banks of water content (TDR) and matric potential (tensiometers; heat dissipation)
below rooting zones at a few locations (measure gradients and changes in water
content as will be elaborated shortly).
ii.
Seepage monitoring (section 3.3.1.2) seems to rely primarily on qualitative
observations and measurements of relative humidity and temperatures at ventilation
check points. It is not clear whether RH and temperature measurements are aimed at
monitoring conditions for onset of seepage or designed to quantify seepage through
mass balance (a flawed concept). In short, the methodology is vague at best,
moreover, it is based to a large extent on untested model therefore represent a serious
gap in confirmation of this important process.
iii.
Subsurface water/rock tests (3.3.1.3.) - despite reliance on geochemical signatures in
pore water for assessing travel times and pathways – very little attention was given to
strategies for in-situ and nondestructive sample of rock pore water (and separation of
matrix and fracture collection zones) – some methods used in soil solution sampling
will be presented and discussed. It is not clear if the methodology is capable of
quantifying actual fluxes under conditions of steady state (again, the need for a direct
measure of deep percolation
iv.
Only limited efforts are devoted to confirmation of lateral diversion and the role of
capillary contrast (as opposed to say preferential flow in fractures bypassing the
drift). This important gap in PC plan will be elaborated in recommended monitoring
strategies in section 5 of this report.
v.
Repeated reference and heavy reliance on borehole psychrometeric measurements of
water potentials in the range of less than 1 bar is potentially misleading!
Psychrometers cannot reliably resolve potential differences at that range, hence
inversion of matric potential measurements for repository scale unsaturated transport
parameters may be unreliable and should be revisited with direct measurements or
samples.
vi.
Near-field and in-drift plan of activities for the thermally accelerated scenarios is
inconsistent and relies on geophysical methods such as GPR to assess dry out zones
etc under conditions that will not allow independent confirmation (unless other
techniques are implemented).
In the following we review VZ sensing technology in general, and subsequently propose
potential sensors and deployment scenarios to address some of the deficiencies discussed
above and others.
3. A Review of VZ Sensing Technology and Measurement Characteristics
3.1 Measurement of Water Content
The two most important characteristics of the liquid phase are: (i) the amount of water in the
porous medium, and (ii) the forces by which water is held in the pores (matric or capillary
potential). These attributes are related to each other through the soil water characteristic
(SWC) curve. The liquid phase characteristics affect the pore space gaseous phase and the
rates of exchange between these phases, as well as other transport properties such as the
hydraulic conductivity. Many geotechnical and hydrologic practices and studies require
knowledge of the amount of water contained in the soil or rock formations. In the following
section describe some of the methods used to determine water content focusing on
continuous and in-situ measurements relevant to long term monitoring and PC plan.
3.1.1 Neutron Scattering
This method is commonly used for field measurement of volumetric water content and in
some industrial and construction applications (www.berthold.com). It is based on the
propensity of hydrogen nuclei to slow (thermalize) high energy fast neutrons. A typical
neutron moisture meter consists of: (i) a probe containing a radioactive source that emits high
energy (2-4 MeV) fast (1600 km/s!) neutrons, as well as a detector of slow neutrons; (ii) a
scaler to monitor the flux of slow neutrons; and optionally (iii) a datalogger for storing and
retrieving data (Fig.1). The radioactive source commonly contains a mixture of Americium241 and Beryllium in 10 to 50 millicurie amounts. The alpha particles emitted by the decay
of the Americium-241 collide with the light Beryllium nuclei resulting in emission of fast
neutrons.
When the probe is lowered into an access tube, fast neutrons are emitted spherically into the
surrounding medium where they collide with various atomic nuclei. Collisions with most
nuclei are virtually elastic, i.e., resulting in only minor losses of kinetic energy by the fast
neutrons. However, collisions with light hydrogen nuclei, which have similar mass to
neutrons, cause significant loss of kinetic energy slowing down the fast neutrons (consider a
marble colliding with a bowling ball vs. another marble). When the speed of fast neutrons
diminishes to that of particles at ambient temperature (about 2.7 km/s) with corresponding
energies of about 0.03 eV, they are called thermalized or slow neutrons. Thermalized
neutrons rapidly form a "cloud" of nearly constant density near the probe, where the flux of
the slow neutrons is measured by the detector. The average loss of the neutrons' kinetic
energy (thus the relative number of slow neutrons) is therefore proportional to the amount of
hydrogen nuclei in the soil. The primary source of hydrogen in soil (and the most variable in
time) is water. Several other non-hydrogen substances which may be present in trace
amounts in some soils may also effectively thermalize fast neutrons; these may generally be
effectively compensated through soil-specific calibration.
Figure 1: An illustration of neutron probe lowered into an access tube for repetitive
and in-situ measurements of porous medium water content.
Calibration of the neutron probe to account for background hydrogen sources and other local
effects (e.g. local bulk density, trace neutron attenuators, etc.) is conveniently achieved by
simultaneous measurements of water content from samples acquired during installation of
access tube or nearby destructive sampling and actual neutron probe counts at the same
locations. The calibration curve (Fig.2) is typically linear and relates volumetric water
content to slow neutron counts or count ratio (CR):
 v  a  b (CR )
(1)
where CR is the ratio of slow neutron counts at a specific location in the soil to a standard
count obtained with the probe in its shield. For many soils the calibration relation is
approximately the same.
Figure 2: Calibration curve for CPN 502 neutron probe in Millville silt loam soil, Logan,
Utah (Or, 1990)
The sphere of influence about the radiation source varies between about 15 cm (wet soil) to
perhaps 70 cm (very dry soil), depending on how far fast neutrons must travel in order to
collide with a requisite number of hydrogen nuclei (see illustrated sphere of influence in
Fig.1). An approximation to estimate the radius of influence (r) in cm as a function of
ambient water content is given by:
r [cm]  15  v 1 3
(2)
Thus, the neutron scattering method is unsuitable for measurements near soil surface or rock
walls because a portion of the neutrons may escape. Typically, reliable measurements are
obtained at depths (or distances from rock wall) exceeding 15-20 cm. Limitations or
disadvantages of this method include the radiation hazard and attendant licensing
requirements, relatively poor and uncertain spatial resolution, unsuitability for near-surface
measurements, and soil-specific calibration requirement.
Figure 3: Alternative applications of neutron scattering for monitoring water content:
(a) in industrial applications using stand-alone probe (www.berthold.com);
and (b) automated scanning through horizontal access tube using manual or
motorized winch (Troxler Electronics Lab, and Sandia National Labs).
Measurement Range:
 Entire range of water contents
Accuracy:
 ± Volumetric water content with calibration
Limitations:
 Radiation hazards
 Requires site specific calibration
 Variable volume of measurement
 Not suitable for near-surface measurements
 Provides “snap shots”, difficult to automate
 Installation and measurements are labor intensive
Advantages:
 Repetitive and non destructive measurements at the same volumes
 Provide reliable and robust measurements (following calibration)
 Cost effective - one device can serve many access tubes
 Measurement of total water content for entire  range not sensitive to phase and
energy state of water (liquid, bound, and frozen water)
Applicability for PC Plan:
 May be used for monitoring near-field variations in water content
 In combination with deep (advanced) tensiometers may be used to independently
quantify deep percolation flux at top boundary.
3.1.2 Dielectric and Electric Measurement Methods
Dielectric-based techniques infer water content from measurement of the bulk dielectric
permittivity or dielectric constant (b) of porous medium (Hilhorst et al., 2001). The value of
this composite property in rock and soils is dominated primarily by the presence of liquid
water, due to its high dielectric constant (~81) relative to constituents such as 2–5 for soil and
rock minerals, ~3 for frozen or bound water, and 1 for air. Dielectric methods rely on
interactions between porous media and applied electromagnetic waves or fields to deduce the
(unknown) value of the dielectric permittivity of the medium under study.
3.1.2.1 Time Domain Reflectometry (TDR)
Time Domain Reflectometry (TDR) is a relatively new method for water content
measurement (Topp et al. 1980). The main advantages of the TDR method over other
methods for repetitive water content measurement such as the neutron moisture meter are: (i)
superior accuracy to within 1 to 2% of volumetric water content; (ii) calibration requirements
are minimal - in many cases soil-specific calibration is not needed; (iii) averts radiation
hazards associated with neutron probe or gamma-attenuation techniques; (iv) excellent
spatial and temporal resolution; and (v) measurements are simple to obtain, and the method is
capable of providing continuous soil water measurements through automation and
multiplexing.
The propagation velocity (v) of an electromagnetic wave along a transmission line (probe or
waveguide) of length L (Fig.4) embedded in a rock or soil matrix is determined from the time
response of the system to a pulse generated by the TDR cable tester. The propagation
velocity (v=2L/t) is a function of the soil/rock bulk dielectric constant (b) according to:
 c   ct 
 b      
 v   2L 
2
2
(3)
where c is the velocity of electromagnetic waves in vacuum (3x108 m/s), and t is the travel
time for the pulse to traverse the length of the embedded waveguide (down and back = 2L).
The definition of the dielectric constant is given in Eq.3; it simply states that the dielectric
constant of a medium is the ratio squared of propagation velocity in vacuum relative to that
in the medium. The bulk dielectric constant (b) is governed by the dielectric of liquid water
w  81, as the dielectric constants of other soil constituents are much smaller, e.g., soil
minerals s=3 to 5, frozen water (ice) i=4, and air a=1. This large disparity of the dielectric
constants makes the method relatively insensitive to soil composition and texture and thus a
good method for liquid water content measurement. The bulk dielectric permittivity is
determined from analyses of TDR waveforms (reflection coefficient vs. time or distance)
such as depicted in Fig.5.
Figure 4: TDR cable tester with 3-rod probe embedded vertically in surface soil layer.
Figure 5: A series of TDR waveforms demonstrating increasing travel time as the
permittivity of the medium (fluids) increases. Tangent lines are fitted to water
waveform, the intersection being the point from which the time is measured
(Robinson et al., 2003)
Two basic approaches have been used to establish the relationships between b and
volumetric soil water content (v). The first approach is empirical, whereby mathematical
expressions are simply fitted to observed data without using any particular physical model.
Such an approach was employed by Topp et al. (1980) who fitted a third-order polynomial to
the observed relationships between b and v for multiple soils (Fig.6a). The second approach
uses a model of the dielectric constants and the volume fractions of each of the soil
components to derive a relationship between the composite (bulk) dielectric constant and soil
water (i.e., a specific component). Such a physically-based approach, called a dielectric
mixing model, was adopted by Birchak (1974), Dobson et al. (1985), and Roth et al. (1990).
TDR calibration establishes the relationship between b and v. For this discussion we
assume that calibration is conducted in a fairly uniform soil without abrupt changes in soil
water content along the waveguide. The empirical relationship for mineral soils as proposed
by Topp et al. (1980):
v  5.3 102  2.92 102  b  5.5 104  b 2  4.3 106  b3
(4)
provides adequate description for the water content range <0.5, which covers most of the
range of interest in mineral soils, with an estimation error of about 0.013 for v. However,
Eq.4 fails to adequately describe the b-v relationship for water contents exceeding 0.5, and
for organic soils or mineral soils high in organic matter, mainly because Topp's calibration
was based on experimental results for mineral soils and concentrated in the range of v<0.5
(see Fig.6b). Birchak et al. (1974) and Roth et al. (1990) proposed a physically-based
calibration model which considers dielectric mixing of the constituents and their geometric
arrangement. According to this mixing model the bulk dielectric constant of a three-phase
system may be expressed as:



 b  v  w  (1  n) s  (n  v ) a

1
 
(5)
where n is the soil's porosity, -1<<1 summarizes the geometry of the medium in relation to
the axial direction of the wave guide (=1 for an electric field parallel to soil layering, =-1
for a perpendicular electrical field, and =0.5 for an isotropic two-phase mixed medium), 1n, v and n-v are the volume fractions and s, w and a are the dielectric constants of the
solid, aqueous and gaseous phases, respectively. Note that v = Vw /VT, (1-n) = Vs/VT, and
(n-)=Vair/VT, so these components sum to unity. Rearranging Eq.5 and solving for v yields:
 b   (1  n) s   n  a 
v 
 w   a 
(6)
which determines the relationship between b measured by TDR and v. Many have used
=0.5 which is shown by Roth et al. (1990) to produce a calibration curve very similar to the
third-order polynomial proposed by Topp for the water content range of 0<v<0.5. If we
introduce common values for the various constituents such as =0.5, w=81, s=4, and a=1
into Eq.6 we obtain the simplified form
v 
 b  (2  n)
8
(7)
Note that the soil's porosity must be known or estimated when using the mixing model
approach. A comparison between Topp's expression (Eq.4) and a calibration curve based on
Eq.7 with n=0.5 is depicted in Fig.6b Summarizing, Eq.4 establishes an empirical
relationship between bulk soil dielectric and volume water content, while Eq.5 is based on
physical and geometrical considerations. Eq.7 provides a simplified version of Eq.6.
Figure 6: Calibration approaches for establishing relationships between bulk dielectric
permittivity and v, (a) the empirical expression of Topp (1980) fitted to
experimental results; and (b) comparison between Topp’s empirical
expression and physically-based dielectric mixing model.
Limitations or disadvantages of the TDR method include relatively high equipment expense,
potential limited applicability under highly saline conditions due to signal attenuation, and
the fact that soil-specific calibration may be required for soils having large amounts of bound
water or high organic matter contents. Fig.7 depicts currently available TDR systems.
Figure 7: Overview of commonly used TDR systems. (a) Trace System (Soilmoisture
Equipment Corp.); (b) TDR100 (Campbell Scientific Inc.); and (c) Tektronix
1502C general purpose cable tester.
Measurement Range:
 Entire range of water contents
Accuracy:
 ± Volumetric water content
Advantages:
 Superior accuracy to within 1-2% of volumetric water content
 Minimal calibration requirements (usually no soil specific calibration necessary)
 No radiation hazard such as associated with neutron probe or gamma ray
attenuation techniques
 Excellent spatial and temporal resolution
 Continuous measurements through automation and multiplexing
Limitations:
 Expensive – typical system costs ~ $4000
 Limited performance in saline porous media
 Potential temperature effects
 Specialized – no “off the self” systems; requires training
Applicability for PC Plan:
 Monitoring near-field and in drift variations in water content (using large probes
for fracture integration)
 Paired with deep (advanced) tensiometers, a TDR bank may be used to
independently quantify deep percolation flux at top boundary.
 Confirmation of establishment of drift shadow below repository level.
3.1.2.2 Capacitance and Frequency Domain Methods
When two electrodes (parallel plates or rods) are inserted into a soil they form a capacitor
(with the soil as dielectric medium). Capacitance is strongly dependent on the dielectric
constant, dominated by the amount of water in the porous medium. The relationships
between dielectric constant and electrical capacitance between two parallel plates of area A
and spacing d is given as:
C=
A *  0
d
(8)
where ε* is the complex dielectric constant of the soil or rock. The complex dielectric
constant contains both real (ε’) and imaginary (ε”) components, with ε *=ε’-iε” and i   1 .
In most applications we consider the real part of the dielectric only.
When the capacitor is connected to an oscillator to form a tuned electrical circuit, changes in
soil moisture can be detected through changes in operating frequency. This basic frequency
domain theory is applied in capacitance and frequency domain reflectometry (FDR) sensors.
In capacitance sensors the dielectric permittivity of a medium is determined by measuring the
charge time of a capacitor. In FDR sensors the oscillator frequency is modulated within a
certain range to find the resonant frequency (greatest amplitude) that is related to soil water
content.
A soil-specific calibration is recommended because the operating frequency of these devices
is generally below 100 MHz. At these low frequencies the bulk permittivity may be affected
by soil minerals. Furthermore effects of temperature, salinity, bulk density, and clay content
are more pronounced than for high frequency techniques (e.g., TDR).
Commercially available capacitance sensors include ECH2O probes (Decagon Devices, Inc.),
CS616-L Water Content Reflectometer and CS620 HydroSense® probe (Campbell
Scientific), HYDRA probe (Stevens Water Monitoring Systems, Inc.).
Figure 8: (a) HYDRA probe (Stevens Water Monitoring Sys.Inc.); (b) CS620
HydroSense® probe (Campbell Sci. Inc); (c) ECH2O (Decagon Devices Inc.)
There is a group of related sensors termed Frequency Domain Reflectometry (FDR) sensors
including the Sentry 200-AP probe (Troxler, NC, USA) that was evaluated by Evett and
Steiner (1995), and the EnviroScan sensor (Sentek) evaluated by Paltineanu and Starr (1997).
Figure 9: Sentek EnviroSCAN sensor for profiling water content along an access tube.
The measurement range and accuracy vary considerably among this family of sensors, for
example, Evett and Steiner (1995) found about 3 times larger measurement error with the
Sentry 200AP relative to comparable measurements using neutron probe.
3.1.2.3 Impedance Sensors (Amplitude Domain Reflectometry)
When an electromagnetic wave (energy) travelling along a transmission line (TL) reaches a
section with different impedance (which has two components: electrical conductivity and
dielectric constant), part of the energy transmitted is reflected back into the transmitter. The
reflected wave interacts with the incident wave, producing a voltage standing wave along the
TL, i.e., change of wave amplitude along the length of the TL. If the soil/probe combination
is the cause for the impedance change in the TL, measuring the amplitude difference will
give the impedance of the probe (Gaskin and Miller, 1996; Nakashima et al., 1998). The
influence of the soil electrical conductivity is minimized by choosing a signal frequency so
that the soil water content can be estimated from the soil/probe impedance. Impedance
sensors use an oscillator to generate a sinusoidal signal (electromagnetic wave at a fixed
frequency, e.g., 100 MHz), which is applied to a coaxial TL that extends into the soil through
an array of parallel metal rods, the outer of which forms an electrical shield around the
central signal rod. This rod arrangement acts as an additional section of the TL, having
impedance that depends on the dielectric constant of the soil between the rods (Fig.10).
Figure 10: Theta Probe (Delta-T Devices Ltd) and a bank of probes in a soil profile.
3.1.2.4 Phase Transmission Sensors
After travelling a fixed distance, a sinusoidal wave will show a phase shift relative to the
phase at the origin. This phase shift depends on the length of travel, the frequency, and the
propagation velocity. Since propagation velocity is related to soil moisture content, for a
fixed frequency and length of travel, soil water content can be determined based on the phase
shift. The probe uses a particular waveguide design (two concentric metal, opened rings), so
that phase-measuring electronics can be applied at the beginning and ending of the
waveguides (Fig.11).
Figure 11: VIRRIB phase transmission probe
3.1.2.5 Ground Penetrating Radar (GPR)
GPR is a high-resolution geophysical technique for non-invasive imaging of the shallow
subsurface (Davis and Annan, 1989). A transmitter antenna pulses low energy
electromagnetic (EM) waves into the ground and a receiver antenna records time delays and
signal strength of returning waves. By moving the antenna along the ground surface, a crosssection of reflection times to subsurface features can be recorded (Fig.12a). GPR reflections
originate at discontinuities of dielectric permittivity induced by textural variations, and more
often by spatial variations in soil water content (Van Dam and Schlager, 2000). GPR and
TDR are complementary. Both measure time delays and amplitudes of EM waves
propagating through the subsurface. TDR delivers high precision point measurements while
GPR detects lower resolution spatial variability. An excellent introduction to GPR in
hydrogeological applications is available in Davis and Annan (1989).
Figure 12: (a) Illustration of Ground Penetrating Radar (GPR) components and
measurement setup; (b) GPR image (traces) of a subsurface formation, and
(c) Noggin 1000 with 1 GHz antenna and control and data acquisition unit
(www.sensoft.on.ca ).
Two important aspects of GPR are resolution and depth penetration. GPR resolution is
determined by the period of the emitted pulse, which is controlled by the frequency
bandwidth of the GPR system. Because impulse radar systems are designed to achieve
bandwidths that are about equal to the center frequency, the resolution of GPR increases with
increasing center frequency (Davis and Annan, 1989). Depth penetration of GPR
measurements is strongly controlled by the soil electrical conductivity combined with the
center frequency of the GPR system. In low-conductivity media, such as dry sand and gravel,
low-frequency GPR systems (e.g., 50- or 100-MHz antennas) can achieve penetration up to
several tens of meters, and high-frequency systems (e.g., 450- or 900-MHz antennas) achieve
penetration of one to several meters. For silty sands and clays, depth penetration will be
significantly less. It is important to realize that this high sensitivity to soil texture and
electrical conductivity reduces the range of soils where GPR can successfully be applied.
Several methods are used to estimate water content from reflected wave travel time data. The
first class uses a single antenna separation for water content estimation (e.g., soil water
content estimation from scattering objects and traditional GPR sections). The second class
contains the methods that require multiple measurements with different antenna separations.
Additionally, certain applications involve borehole or cross-borehole profiling by lowering
an transmitter antenna into a borehole and recording either with surface receiver or adjacent
borehole receiver antenna (Fig.13). Recently, GPR equipped with suspended horn antenna
have been used for hydrologic processes monitoring without direct contact with the surface.
Variance of such technology is available as stand alone horn antenna sensors
Figure 13: Cross borehole GPR measurement layout and comparison between GPR
and neutron probe measurements from the same domain (modified from
Majer et al., 2002, and Ferre et al., 2003)
Figure 14: (Top) Comparison of GPR and neutron probe measurements within the
same rock formation; and (bottom) horn antenna GPR, TDR, and direct
sampling for water content determination in a drying silt loam soil.
Measurement Range:
 Entire range of water content (non saline formations)
Accuracy:
 Unknown at this time (evidence suggests similar to neutron probe) more likely ±
volumetric water content at best.
Applicability for PC Plan:
 Monitoring near-field and formation of drift shadow through variations in water content
 Assist subsurface characterization efforts for feature identification and selection of sensor
deployment locations.
 Use for cross borehole water content mapping (low resolution exhaustive coverage).
3.1.2.6 Electrical Resistivity Methods for Water Content Monitoring
Changes in the electrical resistivity of soils with changes in water content (and with soluble
ionic constituents) have been used to develop simple and cheap sensors to infer soil water
status. These sensors usually consist of concentric or flat electrodes embedded in a porous
matrix and connected to lead wires for measurement of electrical resistance within the
sensor’s porous matrix. The commonly used term ‘gypsum block’ arises from early models
which were in fact made of gypsum (Bouyoucos and Mick, 1940), and from the practice of
saturating the matrix of many sensors made from alternative materials with gypsum to buffer
local soil ionic effects. The sensor is embedded in the soil and allowed to equilibrate with the
soil solution. The matric potential of water in the sensor is determined from the measured
electrical resistance through previously determined calibration of the sensor itself (i.e.,
electrical resistance vs. matric potential). Under equilibrium conditions the sensor matric
potential is equal to the soil water matric potential (to be discussed shortly), however the
sensor water content may be different than the soil. Hence these measurements are often used
to infer soil water matric potential from which the soil water content may be estimated based
on a known relationship between these quantities (Gardner, 1986). With proper calibration
for a particular soil the sensor could be used to infer soil water content directly (Kutilek and
Nielsen, 1994).
The primary advantages of electrical resistance sensors are their low cost and simple
measurement requirements. Measurements may be obtained using a simple resistance meter,
or more conveniently acquired automatically using a data logger. On the other hand, the
usual requirement for specific calibration of each sensor and for each soil to obtain
acceptable accuracy, and lack of sensitivity under wet conditions, render this measurement
method appropriate mostly as a qualitative indicator of soil water status (Spaans and Baker,
1992).
Figure 15:(a) 253-L Watermark soil matric potential block for multiplexer use
(Irrometer Company Inc.); (b) 223-L Delmhorst Soil Matric Potential Block
(Delmhorst Instrument)
3.2 Water Potential - Definitions
Water held in rock or soil pores is subjected to several force fields, the combined effects of
which result in a deviation in potential energy relative to the reference state, called the total
soil water potential (T) defined as: “The amount of work that an infinitesimal unit quantity
of water at equilibrium is capable of doing when it moves (isothermally and reversibly) to a
pool of water at similar standard (reference) state, i.e., similar pressure, elevation,
temperature and chemical composition”. It should be emphasized that there are alternative
definitions of soil water potential using concepts of chemical potential or specific free energy
of the chemical species water (which is different than the soil solution termed ‘soil water’ in
this chapter). Some of the arguments concerning the definitions and their scales of
application are presented by Corey and Klute (1985), Iwata et al. (1988), and Nitao and Bear
(1996). Recognizing that these fundamental concepts are subject to ongoing debate, we have
opted to present simple and widely accepted definitions which are applicable at macroscopic
scales and which yield an appropriate framework for practical applications.
The primary forces acting on soil water held within a rigid soil matrix under isothermal
conditions can be conveniently grouped (Day et al., 1967) as: (i) matric forces resulting from
interactions of the solid phase with the liquid and gaseous phases; (ii) osmotic forces owing
to differences in chemical composition of soil/rock solution across semi-permeable
membrane (will be ignored in this report); and (iii) body forces induced by gravitational and
other (e.g., centrifugal) inertial force fields.
The thermodynamic approach whereby potential energy rather than forces are used is
particularly useful for equilibrium and flow considerations. Equilibrium would require the
vector sum of these different forces acting on a body of water in different directions to be
zero; this is an extremely difficult criterion to deal with in soils. On the other hand, potential
energy mathematically defined as the negative integral of the force over the path taken by an
infinitesimal amount of water when it moves from a reference location to the point under
consideration is a scalar (not a vector) quantity. Subsequently, we can express the hydraulic
potential (total potential ignoring osmotic competent) as the algebraic sum of the component
potentials corresponding to the different fields acting on soil water as:
 h = m + p + z
(9)
m is the matric potential resulting from the combined effects of capillarity and adsorptive
forces within the soil matrix. The primary mechanisms for these effects include: (i)
capillarity caused by liquid-gas interfaces forming and interacting within the irregular soil
pore geometry; (ii) adhesion of water molecules to solid surfaces due to short-range Londonvan der Waals forces and extension of these effects by cohesion through hydrogen bonds
formed in the liquid; and (iii) ion hydration and water participating in diffuse double layers
(particularly near clay surfaces). There is some disagreement regarding the practical
definition of this component of the total potential. Some consider all contributions other than
gravity and solute interactions (at a reference atmospheric pressure). Others use a device
known as a tensiometer (to be discussed later) to measure and provide a practical definition
of the matric potential in a soil volume of interest in contact with a tensiometer’s porous cup
(Hanks, 1992). The value of m ranges from zero when the soil is saturated to increasingly
negative values as the soil becomes drier (note that m=0 mm is greater than m=-1000 mm;
in analogy, a temperature of 0oC is greater than -100oC).
p is the pressure potential defined as the hydrostatic pressure exerted by unsupported water
that saturates the soil and overlays a point of interest. Using units of energy per unit weight
provides a simple and practical definition of p as the vertical distance from the point of
interest to the free water surface (unconfined water table elevation). The convention used
here is that p is always positive below a water table, or zero if the point of interest is at or
above the water table. In this sense non-zero magnitudes of p and m are mutually
exclusive: either p is positive and m is zero (saturated conditions), or m is negative and p
is zero (unsaturated conditions), orp = m = 0 at the free water table elevation. Note that
some prefer to combine the pressure and matric components into a single term, which
assumes positive values under saturated conditions and negative values under unsaturated
conditions. Based on operational and explanatory considerations, we prefer to adopt the more
commonly used separate components protocol.
z is the gravitational potential which is determined solely by the elevation of a point
relative to some arbitrary reference point, and is equal to the work needed to raise a body
against the earth's gravitational pull from a reference level to its present position. When
expressed as energy per unit weight, the gravitational potential is simply the vertical distance
from a reference level to the point of interest. The numerical value of z itself is thus not
important (it is defined with respect to an arbitrary reference level) - what is important is the
difference (or gradient) in z between any two points of interest. This value is invariant of
the reference level location.
Total soil water potential and its components may be expressed in several ways depending on
the definition of a "unit quantity of water". Potential may be expressed as (i) energy per unit
of mass; (ii) energy per unit of volume; or (iii) energy per unit of weight. A summary of the
resulting dimensions, common symbols, and units are presented in Table 1.
Table1: Units, Dimensions and Common Symbols for Potential Energy of Soil Water
Units
Symbol Name
Dimensions* SI Units
Energy/Mass

Chemical Potential
L2/t2
J/kg
Energy/Volume

Soil Water
Potential, Suction,
or Tension
M/(Lt2)
N/m2 (Pa)
Energy/Weight
h
Pressure Head
L
m
* L is length, M is mass, and t is time
Only  has actual units of potential;  has units of pressure, and h of head of water.
However, the above terminology (i.e., potential energy expressions rather than units of
potential, per se) is widely used in a generic sense in the soil and plant sciences. The various
expressions of soil water energy status are equivalent, with:
=

= gh
w
(10)
where w is density of water (1000 kg/m3 ) and g is gravitational acceleration (9.81 m/s2).
3.2.1 Matric Potential Measurement
3.2.1.1 Tensiometer
A tensiometer consists of a porous cup, usually made of ceramic (or sintered metal) with very
fine pores, connected to a vacuum gauge through a water-filled tube (Fig.16).
The porous cup is placed in intimate contact with the bulk soil at the depth of measurement.
When the matric potential of the soil is lower (more negative) than inside the tensiometer,
water moves from the tensiometer along a potential energy gradient to the soil through the
saturated porous cup, thereby creating suction sensed by the gauge. Water flow into the soil
continues until equilibrium is reached and the suction inside the tensiometer equals the soil
matric potential. When the soil is wetted, flow may occur in the reverse direction, i.e., soil
water enters the tensiometer until a new equilibrium is attained.
Figure 16: Illustration of typical tensiometers for matric potential measurement using
vacuum gauges and electronic pressure transducers.
Measurement Range
A tensiometer measurement range is limited on the one hand by air entry pressure into the
porous cup and by spontaneous cavitation of water under tension on the other. Porous cups
for tensiometers are selected such that air entry value occurs at suction values in excess of 10 m. Under ambient pressure and temperature conditions, as water pressure approaches -8 to
-10 m, impurities and entrapped gas bubbles serve as nuclei for cavitation and break up of the
continuous water column connecting soil water and tensiometer gauge leading to failure of
tensiometric measurements. Despite efforts to extend the range of tensiometric measurements
by delaying onset of cavitation, the practical range for these measurements remain limited to
suction values (negative matric potential) of less than 100 kPa, i.e., 1 bar or 10 m head of
water. Therefore other means are needed for matric potential measurement under drier
conditions.
Installation
Before installation, tensiometers should be thoroughly tested under controlled laboratory
conditions. It is also important to saturate the porous cups for 24 to 48 hrs prior installation.
Before tensiometers can be put in place a hole with a diameter slightly larger than the
tensiometer diameter is cored down to the intended sampling depth. Soil material collected
close to the bottom of the hole is sieved and mixed to a slurry that is poured back to refill the
first 10 to 20 cm. Now the tensiometer is gently pushed into the slurry that establishes tight
hydraulic contact between the saturated porous cup and the surrounding soil. For exact
sampler placement it is advantageous to mark the sampler and auger beforehand. In rocks
silica flour can be used instead of the ambient soil to improve hydraulic contact. Especially in
expansive soils caution should be taken to prevent water from seeping through gaps between
sampler and auger hole. In such cases it is recommended to pour and compact a bentonite
collar around the top portion of the sampler. Note that tensiometers can be installed in any
desired direction. Macropores or highly structured coarse soils may cause significant
problems for the application of tensiometers.
Measurement Range:
 Typically from 0 to -10 m of pressure head (could measure positive pressures)
Accuracy:
 Dependent on pressure gauge and response time, typically within ±mm
Limitations:
 Frequent maintenance (partial solution by newly designed advanced tensiometers
depicted in Fig.17)
 Limited measurement range
 Small measurement volume.
Advantages:
 Repetitive and non destructive measurements at a location
 Most direct measurement of capillary/matric potential (at appropriate range)
 Automation and remote monitoring and service (advanced design)
 Low cost
Applicability for PC Plan:
 Could be used to quantify deep percolation flux at top boundary (combined with
NP or TDR)
 Perhaps the most sensitive sensor for in-situ assessment of local hydraulic
gradients associated with capillary diversion around drifts.
Figure 17:
A schematic of advanced tensiometer for large depth monitoring installed in
PVC guide pipe (McElroy and Hubbell, 2004).
3.2.1.2 Heat Dissipation Sensors
The rate of heat dissipation in a porous medium is dependent on the medium’s specific heat
capacity, thermal conductivity, and density. The heat capacity and thermal conductivity of a
porous matrix is affected by its water content. Heat dissipation sensors contain heating
elements in line or point source configurations embedded in a rigid porous matrix with fixed
pore space. The measurement is based on application of a heat pulse by applying a constant
current through the heating element for specified time period, and analysis of the temperature
response measured by a thermocouple placed at a certain distance from the heating source
(Phene et al., 1971; Bristow et al., 1993). With the heat dissipation sensor buried in the soil,
changes in soil water matric potential result in a gradient between the soil and the porous
ceramic matrix inducing water flux between the two materials until a new equilibrium is
established. The water flux changes the water content of the ceramic matrix which, in turn,
changes the thermal conductivity and heat capacity of the sensor. Typical useful matric
potential range for such sensors is -10 kPa to -1000 kPa.
A line source sensor is depicted in Fig.18 with a fine-wire heating element axially centered in
a cylindrical ceramic matrix having a radius of 1.5 cm and length of 3.2 cm. A thermocouple
is located adjacent to the heating element at mid-length. Both the heating wire and the
thermocouple are contained in the shaft portion of a hypodermic needle. Because the
thermocouple is located adjacent to the heating element, as the soil dries and water moves out
of the ceramic, the temperature change during a given heating period will increase due to the
reduced thermal conductivity. The magnitude of the temperature increase is often linearly
related to the natural logarithm of matric potential.
Line-Source Heat Dissipation Sensor
Porous
Matrix
Heating
Element
Thermocouple
Figure 18: A scheme of CSI 229 heat dissipation sensor. (Source: Campbell Scientific
Inc., Logan, UT).
Figure 19: Water potential dynamics measured by heat dissipation sensor (HDS),
tensiometer, and psychrometer during a laboratory experiment (Reece,
1996)
Measurement Range:
 Typical matric potential range from -0.01 to -1 MPa (in some studies claims were
made for upper range of -100 MPa, highly unlikely for many soils)
Accuracy:
 Measurement sensitivity is proportional to matric potential value. The data of
Flint et al. (2002) suggest 20% absolute in the range of -0.01 and -35 MPa (other
data place the value around ±%)
Limitations:
 Limited accuracy, slow response time.
 Hydraulic decoupling with surrounding porous media (dry conditions)
 Indirect measurement of matric potential requiring calibration.
Advantages:
 Simple installation, low maintenance, remote monitoring and automation.
 Low cost
Applicability for PC Plan:
 A potential backup sensor for tensiometers under dry conditions
3.2.1.3 Psychrometers
Psychrometric measurements are based on equilibrium between liquid water and water vapor
in the ambient pore space. Water potential in the gaseous phase is related to relative
humidity, RH, through the Kelvin equation (Or and Wraith, 2002)
RH  e e0  expM w gh RT 
(11)
where e is water vapor pressure, e0 is saturated vapor pressure at the same temperature, Mw is
the molecular weight of water (0.0018 kg mol–1), g is the gravitational acceleration (9.81 m s–
2
), R is the ideal gas constant (8.31 J K–1 mol–1), and T is the absolute temperature (K). The
relative humidity of the air can be determined from the dew point temperature, using a
chilled mirror.
Typically, psychrometers measure the difference between a dry bulb and wet bulb
temperature. The dry bulb is at the temperature of the surrounding soils, the wet bulb at the
temperature of an evaporating surface. The lower the humidity, the higher will be the rate of
evaporation from the wet bulb, and thus the temperature depression below ambient. Since air
is an effective diffusion barrier for most solutes, the corresponding water potential includes
the osmotic and the matric potential. Rearranging Eq.11 and taking a log-transformation
leads to:
h
RT
ln e e0 
Mwg
(12)
For the range of e e0 near 1, which is usually encountered in soils in humid climates, Eq.12
can be simplified to:
h

RT  e
  1  0.471  10 6
M w g  e0

e

T   1
 e0

(13)
for h in meter.
A thermocouple psychrometer consists of a fine-wire chromel-constantan or other bimetallic
thermocouple. A thermocouple is a double junction of two dissimilar metals. When the two
junctions are subject to different temperatures, they generate a voltage difference (Seebeck
effect). Conversely, when an electrical current is applied, the junction is heated or cooled,
depending on the direction of the current (Peltier effect). For typical soil use, one junction of
the thermocouple is suspended in a thin-walled porous ceramic or stainless screen cup buried
in the soil, while another is embedded in an insulated plug to measure the ambient
temperature at the same location (Fig.20). By an electrical current, the suspended
thermocouple is cooled below the dew point until water condenses on the junction. The
cooling current then stops, and as water evaporates, it draws heat from the junction,
depressing it below the temperature of the surrounding air until it attains wet bulb
temperature. The difference in temperatures between the wet and dry bulb is related to the
relative humidity by the psychrometer equation
 s 
e
 1  
e0
 e0

T

(14)
where s is the slope of the saturation water vapor pressure curve (s=de0/dT),  is the
psychometric constant (~0.067 kPa K–1 at 20°C), and T is the temperature difference (K)
between the dry and wet bulb.
An accurate determination of the temperature difference plays a critical role in psychrometric
water potential determinations In order for water potential measurements to be accurate to ~
104 m, temperature difference measurements need to be accurate to 0.005 °C. Psychrometers
are therefore highly susceptible to thermal gradient effects and do not perform well at
shallow soil depths. The necessity of equilibrium of different phases further causes a
relatively slow response time. When the osmotic potential is negligible, the soil water
potential measured by a psychrometer is nearly equal to the soil matric potential. In principle,
soil psychrometers may be buried in a soil and left for long periods, although corrosion is a
problem in some environments.
Recently introduced water activity measurement devices (Decagon Inc., Pullman) rely on a
chilled mirror sensor to measure water potential following equilibrium between liquid and
vapour phases of water in a sample within a closed chamber. A thermoelectric (Peltier)
cooler controls the mirror temperature. A beam of infrared light is directed onto the mirror
and reflected back to a photodetector, which detects change in reflectance when condensation
occurs on the mirror (wet bulb temperature). A thermocouple attached to the mirror
accurately measures the dew-point temperature.
Figure 20: Schematics of (A) screen-caged sensor and (B) ceramic-cupped sensor.
(C) Photograph of commercially available sensors (from Andraski and
Scanlon, 2002).
Measurement Range:
 Andraski and Scanlon (2002) state that the upper measurement limit with
psychrometers is about −0.03 to −0.2 MPa. The lower limit of water potential
measurements with wet-loop sensors is about −300 MPa (were a large drop of water is
placed on the sensing junction) due to more stable readings for a longer time following
water application than Peltier cooling sensors. The lower limit of routine measurements
made with Peltier sensors is about −8 MPa. At lower water potentials, the dew-point
temperature is more than 0.6°C below ambient temperature and the efficiency of Peltier
cooling is no longer sufficient to condense sufficient water on the sensing junction for
stable readings. Typical range for field sensors -0.1 MPa to -10 MPa (e.g., Wescor
PST-55).
Accuracy:
 Dependent on RH or matric potential value ±(at best -0,01 MPa) 
Limitations:
 Limited measurement range at the wet end and coarse resolution
Advantages:
 Good in situ measurement capability at the dry (low RH) range.
Applicability for PC Plan:
 Extends the range of in-situ matric potential measurement supports water content
measurement in drying regions.
3.3 Soil Pore Water Solution Extraction Methods
The importance of collecting soil solution for EC measurements and environmental studies in
general has been recognized long time ago by Joffe (1932), who described the soil solution as
the “blood circulating in the soil body”. Soil scientists, hydrologists, geochemists, ecologists,
engineers, and health safety specialists have major interests in the chemical composition of
the soil solution, as it provides crucial information regarding distribution of plant nutrients
and hazardous chemicals in the soil profile. Water quality monitoring below waste disposal
sites, for example, is important for detection of contaminant plumes migrating from leaking
liners towards the groundwater table, and allows early initiation of remedial measures to
prevent extended pollution of aquifers. In order to determine the chemical composition of the
soil solution a wide variety of extraction techniques and devices were developed throughout
the last decades. In the following sections we will focus on the equipment and techniques
used to extract solution.
3.3.1 Suction Cups
Briggs and McCall (1904) were among the first to introduce a soil-water extraction method
through porous ceramic cups. Numerous modifications to the initial design of the suction cup
were developed since its invention almost one century ago. Among those modifications was
the introduction of automated soil solution samplers by Cole (1968). Chow (1977) developed
a vacuum sampler that automatically shuts down after collecting a specific volume of soil
solution. Further improved samplers were introduced by Parizek and Lane (1970), Wood
(1973), and Stone and Robl (1996), who designed a heavy duty device to withstand soil
compaction due to farm equipment.
The most commonly applied devices for collection of solution from unsaturated soils are
vacuum soil water samplers (Rhoades and Oster, 1986), such as suction cups, or suction
lysimeters. These instruments operate under the same principle where a porous material (cup
or plate) is brought in hydraulic contact with the surrounding soil, and evacuation of the
sampler to a pressure slightly below the soils matric potential induces a pressure gradient and
flow of solution into the sampler and collection containers. It is important to manually or
automatically adjust the applied vacuum based on tensiometer measurements to prevent high
gradients and the development of preferential flow paths towards the cup. Therefore soil
water samplers are commonly installed in combination with tensiometers, or sampler and
tensiometer are combined in one instrument as discussed shortly. The potential field
developing around a suction cup was measured with tensiometers by Krone et al. (1951).
The time requirement for collection of soil solution depends on the volume necessary for
chemical analysis, the hydraulic conductivity and water content (matric potential) of the soil,
and the applied gradient (Rhoades and Oster, 1986). A sandy soil close to field capacity will
provide sufficient sample volume within a few hours. Note that automated sampling stations
(Cepuder and Tuller, 1996) enable continuous sampling within limitations discussed below.
A typical soil water collection system contains three main functional units, the suction cups
or plates, sampling bottles, and a vacuum container connected to a vacuum pump (Fig.21).
Figure 21: Sketch of typical setup of soil water samplers (suction cups).
The applicable range of soil water samplers is limited to suction values (vacuum) of less than
10 m head of water due to air bubbling pressure (air entry value) of porous materials and
onset of cavitation in the metastable solution at subatmospheric pressure.
The primary differences between various suction soil water samplers are shape and size of
the devices and the chemical and physical properties of the porous materials used to establish
hydraulic contact with the surrounding soil. A large number of porous materials such as
ceramic (widely used), polytetrafluoroethylene (PTFE), polyethylene (PE), stainless steel,
nylon, PVC, PP, PVDF, Teflon, or glass, may be used for suction cups or plates. These are
often selected based on costs, durability, and to minimize chemical interactions with
components in pore water. There is conflicting evidence concerning the applicability of
ceramic samplers for collecting solution for chemical analyses. These are probably attributed
to differences in chemical composition and physical properties of ceramic materials used for
their construction and differences in chemical composition of soil solution. Installation of
suction cups closely follows procedures discussed for tensiometers.
3.3.2 Combined Solution Sampling – Tensiometer Probes
Characterization of solute transport in soils requires measurement of spatial and temporal
changes of the soil solute concentration and soil water status (matric potential), which is
commonly achieved with suction samplers, such as introduced in the previous section, and
tensiometers. Due to a similar basic design of tensiometers and suction samplers it is
convenient to combine these devices into one individual probe.
Moutonnet et al. (1989) introduced a modified tensiometer termed Tensionic that allows
measurement of matric potential and extraction of soil solution (Fig.22a). The ceramic cup is
sealed immediately after entering the PVC shaft portion of the tensiometer with two tubes
guided to the soil surface for priming and sample extraction, and a third tube leading to a
sealed tensiometer compartment in the upper portion of the shaft (Moutonnet et al., 1993).
Following installation the tensiometer is primed with de-aired and deionized water. After
equilibration of the pressure inside the tensiometer with the matric potential of the
surrounding soil the Tensionic operates in tensiometer mode, and the matric potential is
manually or automatically recorded using vacuum gauges or pressure transducers in
combination with dataloggers. At the same time, ions present in the soil solution diffuse
through the porous cup, and after some time (Moutonnet et al., 1993 report 8 to 10 days for
NO3-) the water inside the tensiometer is in chemical equilibrium with the surrounding pore
water. After chemical equilibrium is attained the sample is extracted, and the tensiometer
flushed and refilled with de-aired and deionized water. Moutonnet et al. (1993) and
Moutonnet and Fardeau (1997) used automated Tensionic probes to determine concentrations
of NO3-N, NO2-N, and NH4-N under maize. Similar devices were applied by Morrison et al.
(1983) and Rehm et al. (1986) to characterize contaminant migration under waste disposal
sites. Major drawbacks of the Tensionic are the lack of control over the sample volume that
is largely dependent on soil water content (Saragoni et al., 1990), and uncertainty regarding
the required time for chemical equilibration.
Essert and Hopmans (1998) took a different design approach when combining tensiometer
and soil water sampler. They separated the porous cup with an acrylic barrier into two
compartments (Fig.22b). The bottom compartment is used for sample extraction, while the
top compartment operates as a tensiometer. Interactions between the two compartments and
operation modes may occur due to (1) reduction of the matric potential caused by solute
extraction, and (2) change of solute concentration due to diffusion of solutes between soil
and tensiometer compartment. To overcome theses biases Essert and Hopmans (1998)
recommend separating the compartments with a 10 cm spacer. Though this distance is
somewhat arbitrary it is supported by theoretical considerations (Warrick and AmoozegarFard, 1977).
Figure 22: Sketch showing (a) the Tensionic probe (Moutonnet et al., 1993), and (b) a
probe with separated sampling and tensiometer compartments (Essert and
Hopmans, 1998).
3.3.3 Suction Lysimeters
Lysimeter studies date back to the late 18th century where scientists investigated the fate of
precipitation in soils (Joffe, 1932). The term ‘lysimeter’ originating from the Greek words
“lysi” (loosening) and “meter” (measuring) is somewhat misused today. Lysimeters were
originally developed to study the complex soil-plant-atmospheric relationships with solute
transport being only one component. In its original definition a lysimeter is a large soil block
surrounded by a casing with its lower boundary separated from the parent material
(Bergström, 1990) and commonly mounted on a large balance for monitoring evaporation or
evapotranspiration as a function of atmospheric conditions and soil water status. Numerous
different lysimeter designs and sizes with varying boundary conditions and application areas
are reported in literature. In this section we will briefly describe lysimeters designed for
collection of soil solution and refer interested readers to additional publications when
appropriate.
We distinguish between two basic types of lysimeters that either contain a soil monolith
(undisturbed) or are filled with (disturbed) soil matrix. They may reach from the soil surface
to depths of 2-3 meters or may be buried (Cepuder and Tuller, 1996) or installed from the
sidewall of a trench. They may be isolated from the surrounding soil via impermeable
sidewalls or in hydraulic contact with the parent material. Casings may be round, square, or
rectangular, and made of concrete, steel, fiberglass or PVC (ASTM, 1998; Best and Weber,
1974; Furth, 1985; Weber, 1995).
Collection of soil solution is commonly achieved by means of gravity drainage or through a
porous material via suction, similar to the previously discussed suction samplers (note that
suction samplers are sometimes referred to as lysimeters).
Suction lysimeters (Fig.23) as well as gravity drainage lysimeters were applied in numerous
studies, ranging from monitoring transport of agrochemicals and water movement
(Bergström, 1990; Jemison and Fox, 1994; Tyler and Thomas, 1977; Winton and Weber,
1996; Joffe, 1932; Kilmer et al., 1944; Kohnke et al., 1940; McMahon and Thomas, 1974;
Karnok and Kucharski, 1982; Dolan et al., 1993; Cepuder and Tuller, 1996; Moyer et al.,
1996) to colloid facilitated transport of organic compounds and heavy metals (Thompson and
Scharf, 1994), and fate and cycling of N15 (Reeder, 1986).
Figure 23: Sketch showing a refilled suction lysimeter installed in combination with a
tensiometer for monitoring soil matric potential. A similar design was used
by Cepuder and Tuller (1996) for nitrate leaching studies.
3.3.4 Passive Capillary Samplers
Passive capillary samplers (PCAPS) utilize tension exerted by a hanging wick to passively
extract solution from the soil above a sampling pan (Fig.24). PCAPS that were first
introduced by Brown et al. (1986) show distinct advantages, such as potential for measuring
water flux density, when compared to suction cups or lysimeters (Selker, 2002). Recent
progress of the PCAPS technique includes the development of advanced wick selection
design equations (Boll et al., 1992; Knutson and Selker, 1994; Rimmer et al., 1995), and
wick pre-treatment methods (Knutson et al., 1993).
PCAPS are designed for long term operation using environmentally stable, nonadsorbing
materials such as stainless steel, fiberglass, and HDPE (Topp and Smith, 1992). The major
component is a fiberglass or HDPE container that supports a stainless steel or HDPE top
panel that is divided into multiple compartments with a circular opening for the wick in the
center of each section (Louie et al., 2000). The wick is cut to the desired length and one end
is separated into individual strands and cleaned by kiln combustion according to Knutson et
al. (1993). The wick is guided through the center hole and the filaments of the open end are
spread out radially on the top of the panel and the ends are glued into place with silicone
(Fig.24)
Figure 24: Sketch showing the setup of a passive capillary sampler
The wicks applied for PCAPS are custom products for furnace isolation. They are available
in a wide range of dimensions, weaves, and densities that can be optimized for a wide range
of soil textures (Selker, 2002). The applicable wick materials show exponential relationships
between hydraulic conductivity and pressure that were tabulated by Knutson and Selker
(1994). From these tabulated values and the determined unsaturated hydraulic conductivity of
the parent soil design criteria for PCAPS (e.g., length, type, and number of wicks for
sampling a given area) can be computed. Some caution regarding the wick material is
required (Selker, 2002). Binding agents (e.g., starch) applied during the manufacturing
process may reduce the wettability of wicks and lead to problems with inducing tension to
the soil water. Kiln combustion at 450 oC was found to be the most effective procedure for
coating removal (Knutson et al., 1993). This however might induce contamination of the
wick surface with ash, which is undesirable especially if the collected solution is intended to
be analyzed for trace elements. As for all other samplers throughout laboratory testing and
cleaning of the wick with acid and deionized water is imperative before field installation.
PCAPS are commonly installed from trenches. A tunnel only slightly larger as the sampling
device is excavated perpendicular to the trench at the desired sampling depth. After filling
the top panel with slightly compacted native soil the PCAP is carefully pushed into the tunnel
to its desired location and elevated with wedges to achieve tight hydraulic contact between
the soil layer on top of the sampler and the tunnel ceiling (Louie et al., 2000). A bentonite
layer is applied to hydraulically isolate the sampler from the trench. After guiding the tubing
for sample extraction to the soil surface the trench is refilled and compacted. The soil
solution collected on the bottom of the sampler is extracted with a manual or battery operated
vacuum pump at predetermined time intervals or dependent on monitored matric potential or
soil water content.
Passive capillary samplers were successfully tested in laboratory experiments (Knutson and
Selker, 1996; Rimmer et al., 1995), and applied in a number of field trials (Brandi-Dohrn et
al., 1996; Louie et al., 2000; Holder et al., 1991; Boll et al., 1991).
3.3.5 Capillary Absorbers
When two porous materials with differing water potential energy (e.g., filter paper and soil)
are brought in close hydraulic contact, water will flow from the medium with higher potential
energy to the medium with lower potential energy in pursuit of equilibrium state. The driving
force is the gradient due to differences in matric and osmotic potentials between soil and
filter paper. This physical phenomenon is utilized in capillary absorbers, where a porous
membrane (absorber) is brought in close hydraulic contact with the wall of a borehole
(Fig.25). Due to the difference in water potential soil solution is wicked into the absorber.
After allowing equilibration with the surrounding soil the membrane is retrieved and the
solution extracted for chemical analyses (Keller and Hendrickx, 2002).
The membrane-soil system effective permeability controls the flow rate of the liquid into the
absorbing material. This leads to low transfer rates and extended equilibration time under dry
conditions (Keller and Travis, 1993). In moist soils with high hydraulic conductivity on the
other hand the absorber may quickly wick a sample. To estimate equilibration time it is
useful to attach a wire pair to the absorber and monitor changes in resistance as the absorber
wicks the soil solution. From the slope of the resistance-time relationship one can
approximately estimate when equilibrium is reached (Keller and Hendrickx, 2002).
A number of issues require consideration for successful operation of capillary absorbers. It is
crucial to establish tight contact between soil and absorber to prevent evaporation losses
during equilibration, which could cause an increase in solute concentration. Furthermore it is
important to prevent evaporation losses and cross-contamination during absorber retrieval.
Figure 25: Deployment sequence of capillary absorbers: (a) canister placed on surface
casing, and (b) membrane lowered into the borehole and pushed against the
borehole wall using pressurized air. (Koglin et al., 1995)
An advanced installation method that eliminates potential problems with absorber placement,
borehole isolation, and absorber retrieval employs an impermeable balloon-shaped liner with
the absorber material glued to the liner surface (Keller and Hendrickx, 2002; Koglin et al,
1995). (Note that absorbers may be organized as circular or annular patches or cover the
entire liner surface.). The inverted liner and attached tether (i.e., turned outside in) is wound
onto a reel (Fig.25a). When the liner unrolls from the reel as it is lowered into the borehole
with the tether it is everted so that the absorber faces the borehole wall (Fig.25b). Pressurized
air or in certain cases water is used to establish tight contact between absorber and wall and
to isolate individual absorbers if patches are used. When retrieving the absorber the liner is
inverted again. This avoids cross-contamination and exposure of personnel to hazardous
chemicals.
3.3.6 Solution Extraction from Soil & Rock Samples
For completeness this section contains a brief discussion regarding laboratory solute
extraction from soil samples collected in the field by means of coring or simple excavation.
Note that a distinction is made between gravimetric solute content (mass of solute related to
mass of oven-dry soil [kg kg-1]) obtained from disturbed samples, and volumetric solute
content (mass of solute related to volume of soil in kg m-3) determined from undisturbed
samples with known volume (e.g., soil cores). A vast variety of single and sequential
extraction techniques, including column displacement and centrifugation, or a combination of
both are described in literature (Adams et al., 1980; Fuentes et al., 2004; Martens, 2002;
Pueyo et al., 2003; Villar-Mir et al., 2002). The selection of method is mainly based on the
chemical species under investigation and the sample volume required for a particular
analyzes technique.
Adams et al. (1980) compare the ionic composition of solutions extracted from loam and clay
soils by means of column displacement with a CaSO4 – KCNS solution, centrifugation of
moist soil with carbon tetrachloride (CCl4) added, and simple centrifugation of moist soil.
They conclude that the composition of the extracted solution was not affected by the
employed method. Alberts et al. (1977) and Villar-Mir et al. (2002) describe extraction
techniques suitable for NO3-N determination. Fuentes et al. (2004) and Pueyo et al. (2003)
present single and sequential extraction techniques to determine heavy metals in sewage
sludge and contaminated soils.
3.4
Indirect Methods for Monitoring Bulk EC and Saturation Distribution
3.4.1 Electrical Resistivity Methods
The basic principle of electric resistivity measurements is best explained by means of the
relationship between electric resistance (R) and resistivity () of a wire that is given as:
R
L
A
(15)
where R is the electric resistance of the wire (), L is the wire length (m), and A is the crosssectional area of the wire (m2). Substituting Ohm’s law that relates the electromotive force V
(volts) to current flow I (amperes) (V = I x R) and rearranging Eqn.15 yields an explicit
expression for resistivity:

A V

L I
 m
(16)
Equation 17 illustrates that the resistivity is a function of the ratio of voltage drop to current,
and the dimensions of the conductor (wire). This principle can be applied to measure
resistivity of soil that acts as a conductor between two or more electrodes. The resistivity of
natural porous media is highly dependent on water content, solute concentration, texture, and
structure. In general, increasing water content leads to a decrease in resistivity as water fills
up the pore space and displaces air. Course textured soils usually have higher resistivity than
fine textured soils at the same water content due to smaller grain to grain contact area.
First soil electrical resistivity measurements date back to geophysical prospecting in the
1920s, where two current electrodes (outer pair) and two potential electrodes (inner pair)
were brought in contact with the soil and lined to form an array. The inner pair is used to
measure the potential while constant current is passed through the outer pair (Fig.26). Two
basic array configurations are commonly employed; the Wenner Array (WA) with equally
spaced electrodes and the Schlumberger Array (SA) where the distance between the two
potential electrodes is small in comparison with the distance of the current electrodes. The
measured resistivity is a function of electrode spacing (a, b) (Fig.26) given as:
WA:   2aR
(17)
SA:    (a 2  ab) R
(18)
The bulk soil electrical conductivity ECa is the inverse of the resistivity as illustrated for the
Wenner Array:
ECa 
1
2aR
(19)
The depth of current penetration for the WA probe configuration is approximately equal to
the inner electrode spacing (Rhoades and Ingvalson, 1971; Rhoades and van Schilfgaarde,
1976; Rhoades, 1978). Nadler (1980) theoretically and experimentally determined the soil
volume influenced by the applied electrical field and found that the depth sensed with a WA
is deeper than the inner electrode spacing for most field situations especially under highly
variable conductivity conditions with depth, caused by varying water content and solute
concentrations.
As with all other methods that measure the soil bulk electrical conductivity (e.g., TDR,
Electromagnetic Induction) a calibration relationship is required to determine solution ECw.
Frame mounted electrical resistivity systems equipped with GPS and dataloggers are
commercially available from a number of companies (e.g., GEOSCAN Research, VERIS
Technologies, GEONICS Limited) but can be easily assembled from scratch (Rhoades and
Halvorson, 1977). The basic components consist of a battery powered constant current source
resistance meter with a range from 0.1 to 1000 , four metal electrodes, and connecting wire.
The electrodes are pushed into the soil to a depth of approximately 2.5 cm, lined, and spaced
based on the applied configuration scheme (Fig.27). The separation distance is determined
based on the desired depth and volume of influence (Rhoades, 1978; Nadler, 1980). For
large-scale monitoring with fixed electrode spacing it is advantageous to mount the
electrodes, the current source resistance meter and datalogger on a non-conducting frame.
This allows rapid repositioning and substantial time savings when a large number of
measurements are required.
By successively increasing the inner electrode separation distance around a point of interest,
ECa can be determined for discrete depth intervals (Halvorson and Rhoades, 1974).
Assuming that the penetration depth is equal to the separation distance, the volume measured
may be considered as a uniform lateral layer to which deeper layers are added successively as
the separation distance increases. These layers may be treated as parallel resistors, and the
bulk electrical conductivity (ECx) for each layer calculated as (Barnes, 1952):
ECx  EC( a i a i 1 ) 
ECa i  ai  ECa i 1  ai1
ai  ai1
(20)
where ai is the sampling depth, and ai-1 is the prior sampling depth.
An alternative to the surface-based electrical resistivity method for determination of depth
dependent ECa was introduced by Rhoades and van Schilfgaarde (1976). They developed a
single probe with four equally spaced electrodes mounted as annular rings. The probe that is
pushed into the soil to the desired depth provides higher measurement resolution. A
drawback however is the small sampling volume that requires a large number of
measurements to obtain representative values of ECa. Therefore application of this probe is
advantageous when a precise measurement of salinity within a small localized region is
required. To determine a soil salinity index for extended areas surface positioned probes are
better suited.
Figure 26:Sketch showing the measurement principle and basic electrode
configurations for surface electrical resistivity measurements
Figure 27: Field setup of a Wenner array configuration (Rhoades and Oster, 1986).
Observations of soil electrical conductivity down to larger depths can be achieved with
borehole electrical resistivity tomography (ERT), where arrays of current and potential
electrodes (pole-dipole) mounted on cables are lowered into boreholes. Low frequency
electrical current is injected into the subsurface, and the resulting potential distribution is
measured for a vast number of different current and potential electrode orientations. Robust
regularized nonlinear inverse methods (Binley et al., 2002) allow the reconstruction of 2-D
and 3-D electrical resistivity distributions within the soil volume between two or more
boreholes (cross-borehole ERT). In special cases the current and potential electrodes are
placed in a single borehole (in-line ERT). While ERT was successfully applied to
qualitatively study flow and transport in porous (Daily et al., 1992, 1995) and fractured
porous (Slater et al., 1997) media, quantitative assessment of transport characteristics in soils
and rocks from ERT data is poorly documented (Binley et al., 1996; Slater et al., 2000).
3.4.2 Electromagnetic Induction Methods
Another widely applied method to measure soil apparent electrical conductivity is based on
electromagnetic induction. In contrast to the electrical resistivity methods discussed in the
previous section current is applied to the soil through electromagnetic induction, thus no
direct contact with the soil surface is required (Corwin and Lesch, 2003). Common
instruments consist of a transmitter coil that, when energized with alternating current (AC) at
audio frequency, produces an electromagnetic field (Fig.28). The time-varying
electromagnetic field emitted from the transmitter coil induces weak circular eddy current
loops in the conducting soil, which in turn generate a secondary electromagnetic field that
differs in amplitude and phase from the primary field (McNeill, 1980). The magnitude of
amplitude and phase differences between primary and secondary field preliminary depends
on soil properties such as texture, structure, water content, and solute concentration as well as
spacing between transmitter and receiver coil, distance between coils and soil surface, and
coil orientation (parallel or perpendicular to the soil surface). The effect of magnetic
permeability of the soil seems to be negligible according to De Jong et al. (1979). The
primary and secondary fields are sensed as apparent conductivity at the receiver coil in
Siemens per meter according to McNeill (1980):
m
 Hs 


0 s 2  H p 
4
(21)
where  is the angular operating frequency (radians per second) of the instrument, 0 is the
permeability of free space (1.2566 x 10-6 H m-1; H = Henries), s is the coil spacing (m), and
Hs and Hp are the sensed intensities of the primary and secondary fields at the receiver coil
(A m-1). Note that the linear relationship in Eqn.21 is only valid under the assumptions that
the distance between the coils and the soil surface is zero, the soil is homogeneous and has
uniform ECa, and the induction number NB is much smaller than 1 (NB << 1). The induction
number is defined as:
NB 
s
(22)

where s is the coil spacing (m), and  is the skin depth, defined as the depth where the
primary magnetic field has been attenuated to 1/e (i.e., 37%) of its original strength (e is the
base of the natural logarithm) (Hendrickx et al., 2002).
Assuming that NB << 1, depth-dependent bulk soil electrical conductivity ECa(z) can be
calculated for horizontal and vertical coil orientation by solving the following Fredholm
integral equations of first kind (McNeill, 1980; Borchers et al., 1997; Hendrickx et al., 2002):

m H (h)    H ( z  h) ECa ( z ) dz
(23)
0
with the sensitivity function H(z) given as:
 H ( z)  2 
4z
(4 z  1)1 2
2
(24)
and

m (h)    V ( z  h) ECa ( z ) dz
V
(25)
0
with the sensitivity function V(z) given as:
 V ( z) 
4z
(4 z  1)3 2
2
(26)
where superscripts H and V indicate horizontal and vertical coil orientation respectively, h is
the distance between the coils and the soil surface (Fig.28), and m(h) is the instrument
reading (S m-1). The sensitivity functions represent the relative contribution of the electrical
conductivity at depth z to the instrument reading m(h). Note that in practice it might be
difficult to solve the inverse problem given in Eqs.23-26, because the continuous functions
m(h) are not known (only a finite sets of measurements at different heights h are available),
and small variations in m(h) might lead to large changes in ECa(z). Borchers et al. (1997)
applied a second-order Tikhonov regularization method to solve the inverse problem for ECa
profiles in layered soils.
For cases where the assumption NB << 1 is not applicable Hendrickx et al. (2002) present a
nonlinear model based on solutions of Maxwell’s equations in the frequency domain to relate
electromagnetic induction measurements to depth-dependent ECa.
An important issue is whether the physical models that are discussed above and describe the
electromagnetic response for homogeneous media are applicable for heterogeneous field
soils. As stated in Hendrickx et al. (2002), at this point it is not entirely clear whether the
attempts to use electromagnetic measurements (EM) for determination of vertical ECa
distributions are only thwarted by the problem of non-uniqueness inherent to inverse
procedures or also by the lack of understanding of physical relationships between the vertical
distribution of soil EC and the response of electromagnetic induction EM ground
conductivity meters under heterogeneous field conditions.
Electromagnetic induction meters are commercially available from a vast number of
geophysical instrumentation companies. A comprehensive literature review indicates that the
most commonly applied systems in vadose zone hydrology and soil science are the
GEONICS Limited EM-31 and EM-38 (Fig.29) ground conductivity meters. The EM-31 has
a coil spacing of 3.66 m, which results in a penetration depth of approximately 3 m when the
coils are oriented parallel to the soil surface (horizontal), and 6 m when the coils are
perpendicular to the surface (vertical orientation). The coil spacing of the EM-38 is exactly 1
m, which leads to penetration depths of 0.75 and 1.0 m respectively, when operated in
horizontal and vertical mode (Fig.29). Note that the horizontal mode is obtained by simply
turning the instrument 90o. Both instruments are lightweight and can be easily operated by a
single person. For large-scale salinity surveys it might be more convenient to mount the
instrument on a sledge that is pulled by an all-terrain vehicle as shown in Corwin and Lesch
(2003).
Recent applications of the electromagnetic induction method in soil and environmental
science are reported in Hendrickx et al. (1992), Triantafilis et al. (2000), Hendrickx et al.
(2002), Lesch and Corwin (2003), Corwin and Lesch (2003), and Sudduth et al. (2003).
Figure 28: Sketch illustrating the basic principle of electromagnetic induction
measurements
Figure 29: Handheld Geonics EM-38 ground conductivity meter with horizontal coil
orientation (top) and vertical coil orientation (bottom) (Corwin and Lesch,
2003).
3.4.3 Fiber Optic Sensors
Rapid advancements in fiber optic sensor applications offer a promise for expanding
capabilities in monitoring transport of dissolved constituents (solutes and tracers), as well as
yet to be developed multipurpose probes that could simultaneously measure relative
humidity, moisture content, temperature, and CO2, and to detect fluorescent tracers in soils
and fractured rock (see also fiber optic thermometry and chilled mirror techniques below).
The fiber optic technique is based on directing a constant light beam through optical fibers
(input leg) to a target location within the soil matrix where it is partially adsorbed and
partially reflected back into the probe. The reflected light is guided through a separate fiber
bundle (output leg) from the probe to a photo detector that quantifies its intensity and
converts the optical to an electrical signal that is recorded with a computer or datalogger (Fig.
30). Narrow and broad band filters are employed to condition the outgoing and reflected light
beams respectively (Ghodrati, 1999). Given that the intensity of the ingoing light remains
constant with time, the intensity of the reflected beam will be constant if the system under
investigation is in equilibrium (Krohn, 1988). Perturbations of the equilibrium state will
cause a change in output light intensity that can be analyzed to quantify the perturbation
causing process based on calibration relationships that need to be established for each probe
and the process of interest (e.g., change in solute concentration or water content).
Figure 30: Schematic illustration of a fiber optic miniprobe measurement system
(Ghodrati, 1999)
Though fiber optic sensors where previously applied to measure soil water content (Alessi
and Prunty, 1986; Garrido et al., 1999) the most promising application for soil and
environmental science is the characterization of solute transport phenomena in soils via
fluorescent tracers (Ghodrati, 1999).
Assuming rigid and stable soil matrix (no particle rearrangement or swelling) Garrido et al.
(2000) provide calibration procedures for laboratory and field experiments that relate tracer
concentration to output light intensity. The conventional calibration procedure for laboratory
experiments consists of stepwise leaching a soil column of interest with several pore volumes
of tracer with known concentration. After the photo detector indicates stable output intensity
for a certain tracer concentration the column is flushed with CaCl2 and the procedure
repeated for the next tracer concentration until a few points are established on the calibration
curve. A second order polynomial function may be fitted to the measurements to establish a
continuous calibration relationship Ghodrati (1999).
Due to the presumably large amount of tracer that would be required this procedure is not
practical for field application. Therefore Garrido et al. (2000) developed a point calibration
device that allows site specific calibration of fiber optic sensors. The device consists of a
stainless steel tube that either forms a jacket around or is attached to the outside of the
miniprobe, and allows injection of a small amount of tracer directly into the soil in front of
the fiber optics. For tracer injection the tube is connected to a peristaltic pump or a syringe.
The calibration curve is constructed in the same manner as for the conventional method after
the concentration – light intensity relationship is established for a few points. A comparison
of both methods in the laboratory shows good agreement (Garrido et al., 2000).
Though studies by Kulp et al. (1988), Nielsen et al. (1991), Campbell et al. (1999), Ghodrati
(1999), and Ghodrati et al. (2000), and a comprehensive review of fiber optic sensors and
applications for environmental monitoring by Rogers and Poziomek (1996) reveal great
potential of this technique extensive testing and calibration under field conditions and further
sensor development is required to make it feasible for continuous real time monitoring of
environmental parameters.
3.5 Temperature Measurement
Temperature can be measured via a diverse array of sensors. All of them infer temperature by
sensing some change in a physical characteristic. The most common types that might be
employed in porous media are: thermocouples, resistive temperature devices (RTDs and
thermistors), and fiber-optic sensors.
3.5.1 Thermocouples
A thermocouple consists of a double junction of two dissimilar metals and provides a simple
and efficient means of measuring temperature. When the two junctions are subjected to
different temperatures they generate a voltage difference explained by the Seebeck effect
(Seebeck, 1821). This voltage trop can be read using an analog to digital converter (or any
voltmeter), and the temperature can be inferred from standard calibration tables.
In principle, a thermocouple can be made from almost any two metals. In practice, several
thermocouple types have become standard because of desirable qualities such as linearity of
the voltage drop as a function of temperature and large voltage to temperature ratio. The four
most common types are E, J, K, R, S, and T. Each type has a different temperature range and
environment, although the maximum temperature varies with the diameter of the wire used in
the thermocouple.
Table2: Measurement range and calibration coefficients for E, J, K, R, S, and T-type
thermocouples.
Figure 31: Thermocouple array connected to datalogger prior installation
3.5.2 Resistance Temperature Detectors (RTD)
Resistance Temperature Detectors (RTD) are sensors used to measure temperature by
correlating the resistance of the RTD element with temperature. Most RTD elements consist
of fine coiled wire of known length wrapped around a ceramic or glass core. The element is
usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD
element is made from a pure material whose resistance at various temperatures has been
documented. The material has a predictable change in resistance as the temperature changes;
it is this predictable change that is used to determine temperature. Common Resistance
Materials for RTDs are Platinum (most popular and accurate), Nickel, or Copper.
The RTD is one of the most accurate temperature sensors with a measurement range from
-200 to 850oC. Not only does it provide good accuracy, it also provides excellent stability
and repeatability. RTDs are also relatively immune to electrical noise and therefore well
suited for temperature measurement in industrial environments, especially around motors,
generators and other high voltage equipment.
Figure 32: Rugged RTD probe suitable for installation in soils and rocks
The RTD is a more linear device than the thermocouple, but it still requires curve-fitting. The
Callendar-Van Dusen equation is commonly used to approximate the RTD temperature
response:
3

 T
 T 
 T
 T  
RT  R0  R0  T    
 1
 1
  
 
 100  100 
 100  100  

(27)
where RT is the resistance at a given temperature T, R0 is the resistance at 0oC, and a, d, and b
are temperature coefficients determined from testing the RTD at four temperatures and
solving the associated Callendar-Van Dusen equations.
3.5.3 Thermistors
Like the RTD, the thermistor is also a temperature sensitive resistor. The major difference is
the measurement sensitivity. The thermistor exhibits by far the largest parameter change with
temperature when compared to RTDs and thermocouples. Thermistors are generally
composed of semiconductor materials. Although positive temperature coefficient units are
available, most thermistors have a negative temperature coefficient (TC); i.e., their resistance
decreases with increasing temperature. The negative T.C. can be as large as several percent
per degree Celsius, allowing the thermistor circuit to detect minute changes in temperature
which could not be observed with an RTD or thermocouple circuit. The price that is paid for
this increased sensitivity is loss of linearity. The thermistor is an extremely non-linear device
which is highly dependent upon process parameters. Consequently, manufacturers have not
standardized thermistor curves to the extent that RTD and thermocouple curves have been
standardized.
An individual thermistor curve can be closely approximated with the Steinhart-Hart equation:
1
 A  B ln R  C (ln R) 3
T
(28)
where R is the resistance at temperature T, and A, B, and C are coefficients determined from
a three-point calibration.
Figure 33: Thermistor with epoxy and stainless steel casing
3.5.4 Fiber Optic Thermometry
Fiber optic thermometry is based on measuring the decay time of an inorganic (ceramic)
photoluminescent sensor material (i.e. a “phosphor”). The phosphor sensor is attached to the
end of a quartz fiber, which is cabled with Teflon sheath to ensure high dielectric integrity.
The sensor is subjected to excitation by a light pulse, generated by a high intensity LED at
the appropriate wavelength, which produces hundred of pulses per second.
When stimulated with red light from the LED, the phosphor sensor emits light over a broad
spectrum in the near infrared region (Fig.34a). The time required for the fluorescence to
decay is dependent upon the sensor’s temperature (Fig.34b). After the LED is turned off, the
decaying fluorescent signal continues to transmit through the fiber to the instrument, where it
is focused onto a detector. The signal from the detector is amplified and sampled after the
LED is turned off.
The measured decay time is then converted to temperature using a calibrated conversion
table. Different calibration tables are used depending on the temperature range and
application, but the overall temperature range capability of this optical sensor technology is
currently –200oC to 330oC.
Because the excitation light signal and the fluorescent decay signal pass along the same
optical path fiber optic probes can be designed relatively small (Fig.35).
Figure 34: (a) Wavelength of LED excitation light pulse and sensor emitted light. (b)
Decay of sensor emitted fluorescent signal.
Figure 35: Rugged probe with spiral wrap and probe tip detail
3.6 In situ Measurement of Relative Humidity in Soils and Fractured Rock
In previous sections we discussed measurement of RH as related to determination of water
potential of water held in the porous medium. Additionally, the PC plan requires RH
measurements in drifts and other subterranean cavities to assess conditions giving rise to
corrosion, microbial activity, and potential for condensation and dripping.
Although measurements in porous media using psychrometers provide direct estimates of RH
values, the extent of mass exchange and ventilation requires other methods for RH
determination directly in the gaseous phase of a tunnel or drift.
3.6.1 Psychrometers
These sensors were introduced in section 2.4.1 above, and here we provide a brief recap and
extension. Psychrometers are used to infer the relative humidity from the difference between
dry bulb and wet bulb temperatures. The dry bulb is the temperature of the ambient air (nonevaporating surface), and the wet bulb is the temperature of an evaporating surface which is
generally lower than the dry bulb temperature because of latent heat loss in the evaporation
process.
For typical use in porous media, one junction of the thermocouple psychrometer is suspended
in a thin-wall ceramic or stainless screen cup embedded in the porous material (Fig.36),
while the other is embedded in an insulated plug to measure the ambient temperature at the
same location. In psychrometric mode, the suspended thermocouple is cooled below the dew
point by means of an electrical current until pure water condenses on the junction. This is
called Peltier cooling. The cooling then stops, and as water evaporates it draws heat in the
form of latent heat of vaporization from the junction, depressing it below the temperature of
the surrounding air until it attains a wet bulb temperature. The warmer and dryer the
surrounding air, the higher the evaporation rate and the greater the wet bulb depression. The
difference in temperatures between the insulated dry bulb and the wet bulb thermocouples is
measured and used to infer the relative humidity or relative vapor pressure using the
psychrometer equation:
RH 
e
s
 1
T
e0
e0
(29)
where s is the slope of the saturation water vapor pressure curve (s=deo/dT),  is the
psychrometric constant (about 0.067 kPa K-1 at 20oC), and T is the temperature difference
(K). The slope s is temperature-dependent and can be approximated from (Brutsaert, 1982):

d e0 373.15 e0

13.3185  3.952 t R  1.9335 t R 2  0.5196 t R 3
2
dT
T

(30)
where tR=1-373.15/T. The saturated vapor pressure eo is also temperature dependent and is
estimated from the integral of Eq.26:
e0  101.325 exp 13.3185 t R 1.9760 t R
2
 0.6445 t R 3  0.1299 t R 4

(31)
Figure 36: Psychrometer for porous media applications
3.6.2 Chilled Mirror Hygrometers (Dew-Point Technique)
A chilled mirror hygrometer (CMH) makes a direct measurement of the dew point
temperature of a gas by allowing a sample of gas of unknown water vapor content to
condense on an inert, chilled, mirror-polished metal surface. Thermoelectric modules
(Peltier) are typically used to chill the surface. A beam of light, emitted from an LED, is
reflected from the surface onto a photodetector (Fig.37).
Figure 37: Chilled mirror hygrometers (CMHs) detect dew point by cooling a
reflective condensation surface until water begins to condense. The
condensed fine water droplets are detected optically by components such as
shown here
With a properly designed feedback system, the mirror is maintained at the temperature at
which the rate of dew condensation exactly equals the rate of the dew layer's evaporation. In
this state, the mass of the dew layer is neither increasing nor decreasing, and the deposit is in
dynamic equilibrium with the water vapor pressure of the surrounding gas sample, thus
defining the dew point temperature of the sample. Under such conditions, the surface
temperature of the metallic condensation surface represents the saturation temperature for the
water vapor in the gas under measurement. A second detector is sometimes used to monitor
the polarization of the scattered light, and allows automatic determination of the phase of the
condensate, i.e., dew point or frost point.
A typical CMH, in contrast to many other humidity sensors, can be made very inert,
rendering it virtually indestructible and minimizing the need for recalibration. A full-range
dew point sensor is capable of handling dew points from >100ºC to as low as -70ºC. The gas
sample contacts only inert materials: a glass or quartz lens, a Teflon O-ring, and a stainless
steel housing and metallic condensation surface. Among the inert mirror materials are gold,
chromium-plated silver or copper, and titanium nitride. A common design for the mirror for
use in harsh environments is either a copper or silver mass for the mirror, covered by a thin,
polished stainless steel sheath.
The chilled mirror hygrometer (CMH) has several distinct advantages over other water vapor
sensing technologies:
 A CMH provides one of the few truly direct physical measurements of humidity. It is
recognized as the most precise method of determining the water vapor content of a gas
above 5% RH.
 The CMH optical sensor is a totally inert device. The sample gas contacts glass and nonreactive metals. Thus, it can be easily cleaned and can last indefinitely.
 Unlike polymer RH sensors, lithium chloride dew cells, and other chemically-based
sensors, a CM sensor does not lose its calibration.
 The dew/frost point temperature defines the saturation point for the water vapor in the gas.
From this unique equilibrium temperature, all other reporting formats of gas humidity can
be derived.
Figure 38: Chilled mirror dewpoint sensor for environmental applications (GE
General Eastern Instruments, www.gesensing.com )
3.6.3 Capacitive Humidity Sensors
Capacitive relative humidity (RH) sensors (Fig.39) are widely used in industrial, commercial,
and weather telemetry applications. They consist of a substrate on which a thin film of
polymer or metal oxide is deposited between two conductive electrodes. The sensing surface
is coated with a porous metal electrode to protect it from contamination and exposure to
condensation. The substrate is typically glass, ceramic, or silicon. The incremental change in
the dielectric constant of a capacitive humidity sensor is nearly directly proportional to the
relative humidity of the surrounding environment. The change in capacitance is typically
0.2–0.5 pF for a 1% RH change, while the bulk capacitance is between 100 and 500 pF at
50% RH at 25°C. Capacitive sensors are characterized by low temperature coefficient, ability
to function at high temperatures (up to 200°C), full recovery from condensation, and
reasonable resistance to chemical vapors. The response time ranges from 30 to 60 s for a
63% RH step change.
State-of-the-art techniques for producing capacitive sensors take advantage of many of the
principles used in semiconductor manufacturing to yield sensors with minimal long-term
drift and hysteresis. Thin film capacitive sensors may include monolithic signal conditioning
circuitry integrated onto the substrate. The most widely used signal conditioner incorporates
a CMOS timer to pulse the sensor and to produce a near-linear voltage output.
The typical uncertainty of capacitive sensors is ±2% RH from 5% to 95% RH with two-point
calibration. Capacitive sensors are limited by the distance the sensing element can be located
from the signal conditioning circuitry, due to the capacitive effect of the connecting cable
with respect to the relatively small capacitance changes of the sensor. A practical limit is <10
ft. Direct field interchangeability can be a problem unless the sensor is laser trimmed to
reduce variance to ±2% or a computer-based recalibration method is provided. These
calibration programs can compensate sensor capacitance from 100 to 500 pF.
Figure 39: Capacitive relative humidity sensor (ROTRONIC Instrument Corp.)
3.6.4 Resistive Humidity Sensors
Resistive humidity sensors measure the change in electrical impedance of a hygroscopic
medium such as a conductive polymer, salt, or treated substrate. The impedance change is
typically an inverse exponential relationship to humidity.
Resistive sensors usually consist of noble metal electrodes either deposited on a substrate by
photoresist techniques or wire-wound electrodes on a plastic or glass cylinder. The substrate
is coated with a salt or conductive polymer. When it is dissolved or suspended in a liquid
binder it functions as a vehicle to evenly coat the sensor. Alternatively, the substrate may be
treated with activating chemicals such as acid. The sensor absorbs the water vapor and ionic
functional groups are dissociated, resulting in an increase in electrical conductivity. The
response time for most resistive sensors ranges from 10 to 30 s for a 63% step change. The
impedance range of typical resistive elements varies from 1 k to 100 M
Most resistive sensors use symmetrical AC excitation voltage with no DC bias to prevent
polarization of the sensor. The resulting current flow is converted and rectified to a DC
voltage signal for additional scaling, amplification, and linearization. Nominal excitation
frequency is from 30 Hz to 10 kHz.
The “resistive” sensor is not purely resistive in that capacitive effects >10–100 M makes the
response an impedance measurement. A distinct advantage of resistive RH sensors is their
interchangeability, usually within ±2% RH, which allows the electronic signal conditioning
circuitry to be calibrated by a resistor at a fixed RH point. This eliminates the need for
humidity calibration standards, so resistive humidity sensors are generally field replaceable.
The accuracy of individual resistive humidity sensors may be confirmed by testing in an RH
calibration chamber or by a computer-based DA system referenced to standardized humidity-
controlled environment. Nominal operating temperature of resistive sensors ranges from –
40°C to 100°C.
A drawback of some resistive sensors is their tendency to shift values when exposed to
condensation if a water-soluble coating is used. Resistive humidity sensors have significant
temperature dependencies when installed in an environment with large (>10°F) temperature
fluctuations. Simultaneous temperature compensation is incorporated for accuracy.
3.6.5 Thermal Conductivity Humidity Sensors
These sensors measure the absolute humidity by quantifying the difference between the
thermal conductivity of dry air and that of air containing water vapor. When air or gas is dry,
it has a greater capacity to “sink” heat, as in the example of a desert climate. A desert can be
extremely hot in the day but at night the temperature rapidly drops due to the dry atmospheric
conditions. By comparison, humid climates do not cool down so rapidly at night because heat
is retained by water vapor in the atmosphere.
Thermal conductivity humidity sensors (or absolute humidity sensors) consist of two
matched negative temperature coefficient (NTC) thermistor elements in a bridge circuit; one
is hermetically encapsulated in dry nitrogen and the other is exposed to the environment.
When current is passed through the thermistors, resistive heating increases their temperature
to >200°C. The heat dissipated from the sealed thermistor is greater than the exposed
thermistor due to the difference in the thermal conductively of the water vapor as compared
to dry nitrogen. Since the heat dissipated yields different operating temperatures, the
difference in resistance of the thermistors is proportional to the absolute humidity.
A simple resistor network provides a voltage output equal to the range of 0–130 g/m3 at
60°C. Calibration is performed by placing the sensor in moisture-free air or nitrogen and
adjusting the output to zero. Absolute humidity sensors are very durable, operate at
temperatures up to 575F (300°C) and are resistant to chemical vapors by virtue of the inert
materials used for their construction, i.e., glass, semiconductor material for the thermistors,
high-temperature plastics, or aluminium.
3.7
In situ Measurement of Water Flux
3.7.1 Water Flux Meter
Flux meters are designed to use a conical collector (e.g., funnel) filled with soil. This soil
captures flow from a predetermined area and converges it into the restricted channel (funnel
neck) occupied by a fiberglass wick capable of applying a capillary suction. Water flux is
measured directly by placing a transducer at or near the distal end of the wick. The top 15 cm
of the wick material is separated into single strands, which are used to line the interior of the
collector. To prevent soil from filtering through the funnel and the rope, a thin layer of
diatomaceous earth is placed in the bottom of the funnel above the rope. The wick, which
extends vertically 60 cm below the collector, is analogous to a hanging water column
applying a suction of 60 cm at the base of the collector. Figure xx shows a cross-sectional
view of a typical water fluxmeter with divergence control.
Figure 40: A scheme and a photograph of a fluxmeter proposed by Gee et al. (2005)
3.7.2 Heat Pulse Sensors – Water Content, Thermal Properties, and Water Flux
The dual probe heat pulse method was first proposed by Campbell et al. (1991) for
measurement of soil water content and thermal properties from determination of soil
volumetric heat capacity (linearly related to volumetric water content). Campbell et al.
(1991) proposed a sensor with two parallel, cylindrical probes. One probe contained a
thermocouple and the other contained enamel-coated resistance wire used to introduce a heat
impulse. Considering the sensor as an instantaneous and infinitely long heat line source in
isothermal, homogeneous soil, they developed an expression relating the maximum
temperature rise at the temperature probe and volumetric heat capacity of the medium
C 
q
er 2Tm
(32)
where C is the volumetric (bulk) heat capacity (J m-3C-1), q is the heat input per unit length
of heater (J m-1), and Tm is the maximum temperature rise (0C) observed at a radial distance r
from the heat source (m), and e is the base of natural log (e(1)=2.71828). The value of r is as
apparent rather than actual probe spacing obtained by taking measurements in a medium of
known heat capacity and calculating r. Hence, for know q only measurement of Tm is needed
to calculate C. Recently, Knight and Kluitenberg (2004) presented an improved expression
for heat capacity that considers the time interval of heating:
C 
q
er 2Tm
 2
1 
8

1
 1  5 7 


    
 3 8 2 3 
3
(33)
where ε is to/tm, to is the duration of the heat pulse, and tm is the time from the initiation of
heating to the occurrence of the maximum temperature rise.
Campbell et al. (1991) also suggested that measurements of C might be useful for
measuring soil water content. Upon assuming that the heat capacity of the soil gas phase is
negligible, C becomes a weighted sum of the heat capacities of soil water and soil solid
constituents yielding (Knight and Kluitenberg, 2004):
v 
C   b C S
C W
(34)
where b is the soil bulk density, Cs is the specific heat capacity of solid constituents, and
(C) W is the volumetric heat capacity of water. The expression assumes that soil bulk
density and solid specific heat capacity remain constant.
Figure 41: Schematic diagram of dual probe heat pulse sensors for in-situ monitoring
of water content, thermal properties, and potentially water flux (Heitman
et. al., 2003)
3.8 In situ Measurement of Gaseous Fluxes
The primary methods for gaseous measurements within the soil include soil air sampling at
different depths (Buyanowski and Wagner, 1983), or laboratory analysis of soil core samples
(Cortassa et al., 2001). Measurements of surface CO2 flux are typically based on the “closedchamber method" whereby surface flux is determined from changes in gas concentration
within an enclosure volume on the soil surface (de Jong et al., 1979; Cropper et al., 1985;
Drewitt et al., 2002). Commonly used chambers are portable devices such as the LiCor Li6200 or Li-6400 systems (LiCor Inc., Lincoln, NE) capable of measuring soil CO2 fluxes
using high accuracy research-grade instrumentation (Dugas, 1993).
Among the primary limitations of soil chamber measurements are the lack of continuous
observations, manual setup, and impact on soil surface boundary conditions that could alter
the nature of the diffusive flux (Davidson et al., 2002). Attempts to improve temporal
coverage (de Jong et al., 1979; Cropper et al., 1985; Freijer and Bouten, 1991) by continuous
air pumping from the enclosure to a gas analyzer resulted in significant alteration of the soilatmosphere boundary conditions due to variations in air pressures within the chamber (Lund
et al., 1999) and perturbation of natural conditions on the soil surface (gas concentration
gradients, precipitation, radiation, etc.).
Fig. 42: (a) detailed view of sensor arrangement showing the combination of CO2 - O2
sensors, and the thermocouple inserted in the soil profile (Turcu et al., 2005); (b)
gas flux measurement system Li-8500 with special surface chamber
(www.licor.com/env/).
Recently, automated surface chamber designs for capturing short-term changes in soil
respiration have been proposed. Such systems were developed on customized experiments
(Ambus and Robertson, 1998) or by specialized companies (i. e. LiCor 8500 system, LiCor
Inc., Lincoln, NE). However, these quasi-continuous systems still present short-time surface
boundary-condition changes and biases due to air pumping and short-time pressure
differences between soil and the chamber, corroborated with difficulty of calibration.
Moreover, surface chamber information is limited to surface CO2 fluxes lacking details
regarding subsurface CO2 dynamics. Therefore, the urgent need for accurate determination of
soil CO2 flux and associated concentration profiles for extended periods is widely recognized
as key to reliable integration of total CO2 exchange between soil and the atmosphere
(Ouyang and Boersma, 1992).
3.9 Monitoring of Deep Percolation in Fractured Rock (incomplete)
A crucial component for performance confirmation (PC) is the installation of sampling
networks and application of tracers to monitor potential preferential flow pathways and
associated travel times (velocities) of infiltrating precipitation.
This can be achieved by placing boreholes at strategic locations around the storage tunnel. To
be able to distinguish between matrix and fracture sections the boreholes should be initially
mapped based on air permeability measurements. Small increments of the borehole are
sequentially sectioned of by inflatable packers Fig,xxx
Figure 43: Sketch illustrating a potential setup for air permeability mapping of
boreholes
Tracers
An ideal water tracer has the following characteristics [Kaufman and Orlob, 1956; Church,
1974; Davis et al., 1980; McLaughlin, 1982]:

The tracer is conservative in behavior. The tracer moves in a manner similar to water,
that is, (1) without sorption to soils, sediments, or rocks and (2) without degradation
during the time frame of interest.

The tracer has low background concentration. The tracer is clearly discernible from
the background of the system.

The tracer is insensitive to changes in solution chemistry. The tracer’s fate and
transport behavior are unaffected by changes in pH, alkalinity, or ionic strength of the
aqueous solution.

The tracer is detectable either by chemical analysis or by visualization.

The tracer generates a low toxicological impact on the study environment.
Tracer Application
Suitable Tracers for PC Plan
Many different dyes have been tested or used as vadose zone tracers. The most prominent
vadose zone tracers are shown in Figurexxx. Some of these tracers are also frequently used in
groundwater or surface water investigations. In contrast to groundwater tracers, vadose zone
tracers are often employed to visualize the spatial flow patterns of water or solutes. Therefore
a tracer’s visibility in soils and other subsurface materials is of paramount importance.
Methylene Blue has excellent visibility in soils and has been extensively used to visualize
macropore flow in soils. Methylene Blue is a cation and sorbs strongly to most soil minerals.
This characteristic renders the dye a strong coloring agent but limits its mobility compared to
anionic dyes. Screening tests have been performed to find the most suitable vadose zone
tracers, resulting in different dye recommendations including Pyranine [Reynolds, 1966],
Erio Floxine [Corey, 1968], Lissamine Yellow FF [Smettem and Trudgill, 1983], Rhodamine
WT [Kung, 1990], and Brilliant Blue FCF [Flury and Flühler, 1995]. Rhodamine WT and
Brilliant Blue FCF are used most frequently. Because of its blue color, Brilliant Blue FCF is
often more visible in soil media than the red-colored Rodamine WT.
Many of the hydrological tracers are acid dyes. Acid dyes are usually anionic and highly
water soluble and contain one or more sulphonic acid or other acidic groups [The Society of
Dyers and Colourists, 1999]. These characteristics make this group of dyes particularly
suitable as tracers. Acid dyes are found in many of the chemical classes of dyes. Some tracer
dyes are basic dyes. Basic dyes form cations in aqueous solutions [The Society of Dyers and
Colourists, 1999] and strongly sorb to most subsurface media.
Figure 44

How should we separate fracture and matrix flows? Water sampling and other
measurements.

Solution samplers based on segmented sampling conditioned upon a prior air or water
permeability characterization step to establish fracture vs. matrix domains.

Proactive verification through placement of chemical tracers on the surface to mark
the water migrating towards the repository. These could be designed at intervals of 510 years to enable clear separation of center of mass, considering either entire
footprint or large enough surfaces for application.
Such non-reactive and
environmentally benign markers would shift the burden from detailed geochemical
signatures to straightforward and quantifiable analyses of travel time and pathways.
3.10 Sensor pairing for in-situ characterization and monitoring
Certain characterization and monitoring activities must rely on multiple sensors measurement
within the same volume of rock or porous medium. This is particularly important for in-situ
determination of various transport properties such as hydraulic conductivity, and liquid
retention characteristics and for continuous monitoring of fluxes.
Sensor pairing such as TDR probes and tensiometers are used for determining soil water
content and matric potential simultaneously and within the same volume. The limitations of
most sensor pairing techniques stem from: (i) differences in the soil volumes sampled by
each sensor, e.g. large volume averaging by a neutron probe vs. a small volume sensed by
heat dissipation sensor or psychrometer; (ii) while many in-situ water content measurement
methods are instantaneous, matric potential sensors require time for equilibrium; hence the
two measurements may not be indicative of the same wetness levels; and (iii) limited ranges
and deteriorating accuracy of different sensor pairs; this often results in limited overlap in
retention information and problems with measurement errors within the range of overlap (Or
and Wraith, 1999a).
A visual summary of the methods available for matric potential measurement and their range
of application is presented in Fig.43 (Or and Wraith, 1999a). The figure illustrate that most
available techniques have a limited range, often they do not overlap. Many of the methods
shown are laboratory methods unsuitable for in situ field applications.
Figure 45: A summary of measurement methods of matric potential
representing their effective measurement range.
The limited measurement range illustrated above is confounded by widely variable accuracy
among the various methods and combinations of sensor pairs, adding to the complexity of
data interpretation and information quality. The reliance on poorly defined porous media
parameters to determine quantities of interest to PC plan such as deep percolation flux (relies
on unsaturated hydraulic conductivity) or gaseous fluxes (dependent on unsaturated gaseous
diffusion coefficient). For example, attempts reported by Hubbell et al. (2004) to estimate
liquid flux in the deep vadose zone (30-70 m) based on the Darcian approach were hindered
by large uncertainty in the values of unsaturated hydraulic conductivity. The liquid flux was
estimated by combining in situ water potential measurements with laboratory estimates for
the unsaturated hydraulic conductivity. Despite remarkably stable tensiometer data for nearly
30 months that confirmed existence of a unit hydraulic gradient in the formation, flux
estimates (Fig.44) ranged over 4 orders of magnitude. The highest flux value was about 500
times the mean precipitation of 22 cm/yr!
Figure 46: Flux estimates from 34-m with horizontal bars representing the range of
water potentials measured at a location, with the solid dot placed at the
mean. The vertical bars represent the range of hydraulic conductivity
K() estimated from those values. The dashed lines represent the
generic curves developed in earlier studies (Hubbell et al., 2004).
Sensor pairing for characterizations vs. monitoring:
In situ characterization of various transport properties invariably relies on use of sensor pairs
measuring properties and dynamics within the same volume and conditions within the porous
medium. The following are examples:


Neutron probe or TDR are often coupled with tensiometer to simultaneously record
water content and matric potential. The information is used to delineate an important
region of the water characteristic curve in situ (albeit in the narrow range of matric
potentials 0 to -10 m). Such sensor pairs may also be used for monitoring percolation
flux and determination of unsaturated hydraulic conductivity by the so-called
instantaneous profile method. Figure 48 illustrates the type of spatial and temporal
data obtained from sensor pairs and used to deduce the unsaturated hydraulic
conductivity from a transient water flow experiment by the instantaneous profile
method. Note the figure illustrates inherent noise in data and the need for averaging
and integration to obtain useful quantities.
Figure 47: TDR-Tensiometer sensor pairing for monitoring soil water dynamics
in plant root zone.
Figure 48: A sketch of the instantaneous profile method for measurement of
hydraulic conductivity in situ using water content and matric potential sensor
pairs.
.
4. Potential Additions of VZ Activities to Repository Performance
Confirmation Plan
The repository PC plan could benefit from consideration of the following:
1. None of existing sensors and monitoring methods is sufficiently robust for multiyear
monitoring activities. This presents a challenge for sensor selection, deployment
methods, reliability of information stream, maintenance and retrievability procedures.
The design of the PC monitoring network should be based on smaller but robust sets
of sensors that would form the backbone of the system where durability, sustained
performance, and remote observability are the key characteristics for sensor selection.
2. Additionally, considering the duration, limited accessibility and harsh conditions
(e.g., near-field during thermal accelerated period), we recommend an approach
similar to that employed by NASA for sensor selection, and testing (treat monitoring
and information gathering as taking place on a different and inaccessible planet!)
3. Special sensors and placement and retrieval protocols must be developed for VZ
monitoring and PC plan.
4. In developing the PC plan we must make a clear distinction between characterization
and monitoring tasks – these two, sometimes very similar activities may require
different approaches, planning horizons, data streams, and often involve entirely
different experimental setups.
In addition we propose incorporation of the following activities into a revised PC plan.
4.1
Tracers and Fractured Rock Water Monitoring (Markus move these here)
4.2
Quantification of Deep Percolation Flux
The importance of quantifying deep percolation flux was discussed above. This quantity may
be estimated from atmospheric flux measurements and soil water balance closure. For
atmospheric-based estimate we supplement precipitation measurements by direct
measurements of actual evapotranspiration using the eddy covariance technique. The
difference between these two quantities on an annual basis provides an estimate for the
annual deep percolation flux. The error in this estimation method is about 20% due to
limitations of the eddy covariance technique. The accuracy of this estimate may be enhanced
with the aid of shallow soil water balance measurements as illustrated by the bank of
instruments in Fig. 49 (right). The associated errors in deep percolation estimates with direct
measurements shown in Fig. 49 are about 10%. Additionally, unlike ET measurements
spanning a large footprint, water balance measurements provide “point” values with limited
spatial representation that could vary considerably with soil type and vegetation cover.
Figure 49: A sketch of the shallow tensiometer and neutron probe instrument bank (right),
and lysimeter with deep tensiometers for direct flux measurement (left).
Another approach to quantification of deep percolation flux relies monitoring of hydraulic
gradients well below the influence of surface processes and plant roots using deep
tensiometers (or psychrometers for drier conditions) as described by Hubbell et al. (2004).
The limitations of this approach are related to the independent estimate of unsaturated
hydraulic conductivity that presents a challenge. To overcome this difficulty, especially for
locations with deep alluvial cover, we could use on deep lysimeters (hydrologically isolated
blocks of soil or rock) as illustrated in Fig. 49 (left). The actual hydraulic gradients within the
soil or rock mass in the lysimeter are monitored using tensiometers (or psychrometers for
drier conditions), and water flux is intercepted and directly measured (drainage from the
lysimeter). In addition to high costs associated with installation and maintenance of a
lysimeter especially in fractured rock, the accuracy of this method is strongly dependent on
the extent of disturbance to the natural hydrological setting. The use of direct hydraulic
gradient monitoring and hydraulic conductivity function could be used to constrain values
obtained from atmospheric flux measurements thereby bracketing the errors associated with
each of these estimates of deep percolation flux.
Finally, a different approach to estimation of deep percolation flux relies on release of inert
and stable tracers flowed by subsequent sampling of pore water for quantification of path and
travel time as discussed above (section 3.9).
Summarizing, the value of deep percolation is a critical parameter for PC plan as it represents
the higher bound on water carrying capacity in case of failure, and provides an important
input parameter for assessment of capillary diversion. Estimation of this important (and
elusive) hydrologic quantity must consider a combination of approaches of various
accuracies and costs, representing estimates relevant to different spatial scales (large
footprint of a few hundreds of meters for atmospheric measurements, to a few square meters
for soil based measurements).
4.3
Confirmation and Quantification of Capillary Diversion
An important aspect of repository design and waste isolation in the vadose zone is the
potential for formation of capillary barrier along the walls of the drifts and subsequent
diversion of percolation flux around the subterranean drifts. The process of capillary
diversion illustrated in Figure 50 is critical for repository performance, and it is implicitly
assumed in many of the predictive models, yet no PC activity is directed towards its
verification and quantification. In addition to verification of conditions for diversion and
onset of seepage into the drift, the PC plan should verify that the presence of the drift indeed
diverts and modifies flux pathways. This could be established by placing banks of
instruments for measurement of water content and matric potential to monitor conditions at
different locations relative to the drift cross section. Such measurements would be aimed at
establishing the hydrologic signature of the unperturbed deep percolation flux above the drift
(with typical water content and matric potential), and the impact of flux diversion and
concentrated flows with potentially higher water contents and possibly lower (less negative)
matric potentials where the flux is diverted (side of the drift). Another aspect of capillary
diversion is related to the formation of “drift shadow” were relatively drier conditions
(relative to measurements above the drift) may develop underneath the bottom of the drift.
Depending on the magnitude of background flux at the repository level, the hydraulic
properties of the rock, existing models can be used to calculate magnitude and distribution of
enhanced or reduced water contents due to diverted flux that would guide sensor selection,
calibration and emplacement. Hypothetical measurement banks are illustrated on Figure 50.
An alternative scenario would explain flux diversion or onset of seepage as a results of
preferential pathways formed by the fracture network as discussed in a recent study
conducted in Yucca Mountain (Salve, 2005) “Observations from this field experiment
suggest that isolated conduits, each encompassing a large number of fractures, develop
within the fractured rock formation to form preferential flow paths that persist if there is a
continuous supply of water. In addition, in fracture welded tuffs the propensity for fracturematrix interactions is significantly greater than that suggested by existing conceptual models,
in which flow occurs along a section of fracture surfaces. An overriding conclusion is that
field investigations at spatial scales of tens of meters provide data critical to the fundamental
understanding of flow in fractured rock.
Figure 50: Potential monitoring banks for confirmation of capillary diversion and
formation of drift shadow. (a) a plane ~5 m above the drift ceiling; (b) access boreholes
or a bank of permanently installed water content and matric potential sensors extending
outwards (5-10 m); and (c) similar bank for verification and quantification of formation
of drift shadow.
5. Summary and Recommendations
Our primary objective in this report was to provide an overview of available sensors and
measurement methods for vadose zone characterization and hydrological process monitoring.
Focusing on methods and sensors relevant to repository performance confirmation plan
stipulated by the U.S. Nuclear Regulatory Commission (NRC) and aimed at confirming that
the actual subsurface conditions and potential changes in these conditions during
construction and waste emplacement operations in Yucca Mountain are within the limits
assumed in the licensing review.
Preliminary evaluation of vadose zone (VZ) characterization and monitoring activities
proposed in the repository performance confirmation (PC) plan developed by DOE reveal
several shortcomings related to (1) incomplete evaluation of suitability of ongoing activities
and methods that would be transitioned to the PC plan; (2) omission of several critical VZ
processes from the PC plan (deep percolation and capillary diversion); (3) lack of clear
distinction between characterization and monitoring activities. The lack of robust sensors and
reliable long term monitoring methods for a resilient monitoring network further confound
the PC challenge. Typical hydrological monitoring networks and sensors are often
constructed for short term and relatively high maintenance operation (with daily or weekly
service schedule). Most sensors reviewed in this work and proposed in DOE’s PC plan are
not sufficiently robust for deployment at depths and the natural environment surrounding the
repository, nor designed for the duration of uninterrupted operation required for PC plan and
beyond. We conclude that a reliable and robust VZ hydrological monitoring network for
confirmation of waste isolation function of the repository must rely on redesigned suite of
sensors, following rigorous and thorough testing protocols. The design must incorporate
inherent redundancy and supplemented by detailed maintenance, upgrading and replacement
procedures.
For near-term VZ monitoring (while robust sensors and protocols are being developed), we
propose several additional key activities to improve quantification of deep percolation fluxes
by near surface monitoring of matric potential and even direct flux interception. Additionally,
we propose installation of banks of instruments above, at the plane, and below waste
emplacement drifts to measure the onset and extent of capillary diversion. This could be
accomplished by coupling matric potential (tensiometers and psychrometers) and water
content (neutron probe and TDR) measurement devices collocated. Finally, rather than
relying on natural tracers for dating water flux and potential pathways as proposed in the PC
plan, we propose enhancing these capabilities by active marking of water through the use of
well-defined and nonreactive tracers released at intervals of a decade or more. Such well
marked events would provide a traceable hydro-chemical signature marking rates and
providing quantifiable means for resolving actual flux rates and travel pathways the VZ.
Recommendations

Development of selection criteria for VZ monitoring activities involving resilient
sensors and monitoring network - what is the minimum set of variables and
observation points? borehole characterization, (robust variables and dependable
sensors, inherent redundancy, maintenance, upgrading and replacement plans)




Develop a protocol for sensor selection and testing – considering long term stability,
calibration, maintenance frequency, rugged and suitable for ambient conditions (e.g.,
high temperatures, high humidity), installation into rock or borehole.
Use existing sensors and technologies for accelerated characterization as resilient
sensors/technologies for long term monitoring are being developed.
Consider the periodic release of inert and stable tracers as markers of fluxes and
pathways. Establishing a well defined record and hydro-dating.
(Markus I’ll finish this later tonight)
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7. Appendix A: Manufacturers
Adcon's Electrotensiometer (http://www.adcon.com/) tensiometers with standard ceramic
cups and sensor signal conditioned to be compatible with their data acquisition units.
Adcon also supply telemetry solutions and offers other sensors.
AUTOMATA Inc. (http://www.automata-inc.com/) Automata manufactures complete
system solutions including telemetry, software, and sensors used in a wide variety of
industrial and environmental applications.
Campbell Scientific Inc. (www.campbellsci.com) manufactures dataloggers, data
acquisition systems, and measurement and control sensors known for dependability in
harsh, remote environments.
Decagon Devices Inc. (http://www.decagon.com/)
elta-T Devices Ltd. (http://www.delta-t.co.uk/) offer a range of electronic, pressure
transducer tensiometers, including miniature and rugged-use models. Typical usage is
in multiple arrays, automatically recorded by a field data logger. They measure soil
water potential to an accuracy of ±0.2 kPa over the range +100 to -85 kPa. These
sensors can also monitor water table height when submerged (and the overburden, if
present).
Earth Systems Solutions (http://www.earthsystemssolutions.com/) are the American
distributors for various SDEC (French) tensiometers including accessories and
electronic transducers for continuous logging.
GE Industries (http://www.gesensing.com/) offers a variety of industrial and environmental
humidity and moisture sensors.
GEONICS Limited (http://www.geonics.com/) offers a wide variety of sensors, TDR
systems, resistivity probes, and conductivity meters (e.g., EM38)
Geoscan Research (http://www.geoscan-research.co.uk/) offers a variety of resistivity
instruments.
Irrometer (http://www.irrometer.com/). The Original American Suppliers since 1951. They
also produce the Watermark sensor.
LI-COR Biosciences (http://www.licor.com/env/): Automated CO2 flux measurement
systems
Onset Computer (http://www.onsetcomp.com/) produces a variety of miniature dataloggers,
humidity and temperature probes.
ROTRONIC Instrument Corp. (http://www.rotronic-usa.com/): Humidity sensors for
industrial and environmental applications.
SDEC's TENSIONICS ( http://www.sdec-france.com/us/index.html). A French company
that also makes a capacitance sensor. They also have a "Tensimeter" (electronic
readout unit) which is designed for use as a portable gauge for use with tensiometers.
SoilMoisture Equipment Corporation (http://www.soilmoisture.com/) supply tensiometers
as well as many other devices for monitoring soil and plant water potential.
Soil Measurement Systems SMS (www.soilmeasurement.com) develops instruments for
determining hydraulic properties (tension infiltrometer, flow cells), soil density
(penetrometer), solute transport properties (column leaching apparatus), vadose zone
and ground water sampling (lysimeters), and water management (tensiometers,
tensimeter).
UMS Munich, Germany (http://www.ums-muc.de/) produces a variety of high quality
tensiometers, including the new self-filling type TS1.
VAISALA (http://www.vaisala.com/) offers a variety of humidity, dewpoint, and CO2
sensors.
VERIS Technologies (http://www.veristech.com/) designs instruments for large scale EC
monitoring.
WESCOR (http://www.wescor.com/environmental/) Dataloggers, RH and soil moisture
sensors