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Groundwater Flow and Transport
over Larger Volumes of Rock:
Cross-Hole Hydraulic
and Tracer Testing
Claire Tiedeman and Allen Shapiro, USGS
USEPA-USGS Fractured Rock Workshop
EPA Region 2
14 January 2014
Purpose of Cross-Hole Hydraulic Testing
(also called aquifer testing, pump testing)




Identify flow and transport
pathways and barriers between
boreholes.
Use of this info with geologic
framework helps identify
locations of connected high-K
fractures and lower-K rocks.
This characterization data is
important to developing the site
conceptual model.
Quantitative analysis of test
data yields estimates of K
values.
Fast
Response
Cross-Hole Hydraulic and Tracer Testing
No
Response
2
Expectations: Cross-Hole Hydraulic Tests


In fractured rocks,
hydraulic responses
can travel long
distances in short
times.
Drawdown will not
necessarily decrease
with distance from
pumped well.
Cross-Hole Hydraulic and Tracer Testing
3
Designing Hydraulic Tests

Well spacings and open intervals





Because of high degree of heterogeneity, difficult to predict distances
over which permeable fractures are connected, prior to drilling wells.
Bedding-plane-parting fractures and karst features likely wellconnected over longer distances than fractures in crystalline rocks.
Use multiple criteria when selecting locations of new wells – value for
characterizing contamination as well as for hydraulic testing.
Multilevel wells important for understanding vertical connections.
Testing


Monitor water levels in as many wells as possible.
Pump & treat system provides excellent opportunity for using
remediation activities to further characterize hydrogeology.
Cross-Hole Hydraulic and Tracer Testing
4
Using Existing Pump & Treat System for
Cross-Hole Hydraulic Testing

Procedure:





Shut down pump in one well of the P&T
system.
Monitor water-level rises in obs wells.
Start with relatively short tests (run test
during the day, with overnight recovery)
Repeat for all wells of system
Advantages


No additional contaminated water
withdrawn
Relatively short tests limit effect of
shutdown on offsite contaminant
migration
Cross-Hole Hydraulic and Tracer Testing
5
Well Shutdown Testing in
Sedimentary Rocks: An Example
rain
1st Day: Shut down 45BR
2nd Day: Shut down 56BR
3rd Day: Shut down 15BR
Cross-Hole Hydraulic and Tracer Testing
6
Well Shutdown Testing in
Sedimentary Rocks: An Example
24BR open to bed ‘301’
Cross-Hole Hydraulic and Tracer Testing
7
Analyzing Cross-Hole Hydraulic Test Data
In heterogeneous
fractured rock aquifers,
analytical solutions for
estimating K or T from
hydraulic tests have
limited applicability.
 Need to use numerical
model (e.g. MODFLOW)
so that heterogeneity can
be properly represented.

Cross-Hole Hydraulic and Tracer Testing
8
Value of Modeling for Interpreting
Hydraulic Test Data

Process of developing and calibrating
gw flow model helps advance the 3D
hydrogeologic conceptual model.

Enables consistent synthesis of site
characterization data – geology,
geophysics, hydraulics.

Provides a tool for understanding the
3D effects of changes to the flow
system, such as increased pumping.

Can be updated as new data collected

Starting point for transport modeling
Hydraulic Conductivity Representation
Cross-Hole Hydraulic and Tracer Testing
Simulated Drawdown
9
Cross-Hole Hydraulic Tests in
Fractured Rocks: Final Thoughts

Valuable for identifying:



Interpreting hydraulic test data:



Paths for relatively rapid contaminant transport
Less permeable volumes of rock where slow advection and
diffusion likely dominate transport
Incorporate heterogeneity! Use geology!
Presence of permeable high-angle fractures might be inferred
from data, but can be difficult to identify their locations
Tracer testing provides more definitive characterization
of transport processes
Cross-Hole Hydraulic and Tracer Testing
10
Application of Tracers Tests for
Decision Making at Sites of
Groundwater Contamination

Hydraulic testing monitors fluid pressure responses (hydraulic connections)
Groundwater velocity or processes
affecting chemical residence times and chemical
dilution are difficult to infer from hydraulic
information alone (especially in fractured rock)

K
v   h
~
n ~





Groundwater velocity, v
Estimates of fracture porosity (n) from mechanical considerations are unreliable
Hydraulic conductivity (K) is usually an order of magnitude estimate
What are the errors in estimating the hydraulic gradient (Dh) in highly heterogeneous aquifers ?
If information on residence time and dilution is needed. . . conduct a test
that provides a direct measure of these quantities
Cross-Hole Hydraulic and Tracer Testing
11
Characterizing Residence Time and Dilution
Continuous pumping
CVOC Conc. (micrograms per Liter)
Designing the injection of remediation amendments
Hydraulic communication
between monitoring
locations is a prerequisite
for conducting tracer
studies. . .
Feasibility of
bioaugmentation in
strata between 36BR
and 15BR. . .
What is the travel time
and dilution from 36BR
to 15BR?
Cross-Hole Hydraulic and Tracer Testing
12
Characterizing Residence Time and Dilution
Designing the injection of remediation amendments
Naval Air Warfare Center
West Trenton, NJ
135 feet
Pumping
Simulations demonstrate slow
advection (low K) is responsible
for mass retention at greater
depths in dipping beds
Bromide concentration - February 2008
(7 months after injection)
0.08
simulated
observed
0.06
conc (mg/L)
Tracer injection
0.04
0.02
0.00
0
10
20
30
40
50
time (days)
Cross-Hole Hydraulic and Tracer Testing
13
60
Processes Affecting Chemical Migration
Estimating parameters governing matrix diffusion
Naval Air Warfare Center
West Trenton, NJ
Tracer
Injection
B
Characteristics of breakthrough
curves can be used to identify the
significance of chemical processes
Pumping
B'
160
BRP-03
15BR
150
BRP3
Fill
140
Regolith
130
90 ft
Withdraw
25BR
70BR
71BR
15BR
ALTITUDE, IN FEET
120
Inject
Fractured
Bedrock
110
100
90
80
70
60
50
40
30
20
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
DISTANCE, IN FEET
Simulations for different
effective diffusion coefficients
10
Bromide Recovery at 15BR
1
1.00
Fraction of Bromide Mass Recovered
10
Bromide Concentration (mg/L)
BRP3
Bromide Concentration vs. Time at 15BR
0
-1
10
0.80
0.60
0.40
0.20
-2
10
Storage of tracer solutions
at land surface
1
10
100
1000
Elapsed Time from Start of Tracer Injection (hours)
0.00
0
200
400
600
800
1000
Elapsed time from start of tracer injection (hours)
Cross-Hole Hydraulic and Tracer Testing
14
Tracer Test Design
 Tracer tests can be conducted over various distances (provided hydraulic perturbations are observed
between locations)
Tracer arrival at monitoring
wells (> 5 km from tracer
injection) in the Madison
Limestone
 Single-hole tracer tests can also be designed (point-dilution, push-pull, etc.)
 Tracer tests can be designed over hours, days, months or years
 Various types of tracers are available to quantify processes of interest (inert, reactive, varying free-water
diffusion coefficients, dissolved gases, bacteria, colloids, microspheres, etc.)
 Maintaining the geochemical signature of the ambient groundwater (oxic, anoxic, fluid density)
 How much tracer mass must be added to register and interpreted a response ? Preliminary estimates of
dilution and attenuation are difficult to derive in fractured rock (In some cases, tracer tests are not run;
they are re-run!)
 Effect of fluid density in structured media
 Designing injection apparatus at land surface, and apparatus in injection and monitoring boreholes–
maintaining geochemical conditions of ambient groundwater; volume of fluid in boreholes may be large
relative to fracture volume; borehole volume may dilute tracer responses
Cross-Hole Hydraulic and Tracer Testing
15
Final Thoughts
 Tracer tests can provide a direct measure of fluid velocity, chemical
dilution and attenuation processes
 Tracer tests provide valuable information to (1) conceptualize processes
affecting fate and transport of contaminants, (2) design and implement
remediation strategies
 Tracer tests in fractured rock are likely to be successful under hydraulically
stressed conditions, provided the hydraulic perturbation can be monitored
in cross-hole tests
 Single-hole tracer tests are used primarily to estimate in situ process and
identify local parameter values. . .less reliable in estimating groundwater
velocity
Controls on Fate, Transport, and Remediation of VOCs
16
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