Electrical properties of NAPL contaminated soils:
Final report submitted to the Grand Water
Research Institute
Nimrod Schwartz and Alex Furman
February 25, 2011
Civil and Environmental Engineering, Technion IIT; afurman@technion.ac.il
1
General
This report summarizes work performed primarily by Mr Nimrod Schwartz (initially a
MSc student and now a PhD student). The work is one of the first attempts to understand
the electrical signature of contaminated soils, and primarily of unsaturated ones. The
potential of detecting and quantifying contamination using geophysical equipment has
enormous potential from both environmental and economical aspects.
The work presented here include two phases. The first is work performed at the Technion, primarily using a LCR system. This system provided very interesting results,
sometime with agreement with expectations and sometimes contrary to our expectations. But regardless, results were extremely noisy. We decided to make the effort and
sent Nimrod to conduct some verification measurements at Forschungszentrum Juelich,
Germany using a different, much more accurate system. Results obtained using this
system compose the second phase of the report.
At early stages of the research a proposal was submitted to the ISF and was approved.
Using this financial support we ordered from Forschungszentrum Juelich a new system
similar to the one built there. We believe that with the new system we will be able to
make significant progress in this research.
1
2
Background
As groundwater constitutes around 30% of the world’s total fresh water (Gleick, 1996)
one can’t over estimate the importance of this resource to the well-being of human beings. In the last few decades groundwater has become increasingly vulnerable as a result
of increase in the population, increase in per-capita water consumption, growing use of
chemicals, salinization processes, global warming, etc. (UNESCO., 2009). Of those
causes of concern, non aqueous phase liquids, (NAPLs) such as petroleum products,
solvents, etc. are of a significant concern due to their toxicity and carcinogenic nature
(United states department of health & humen services, 2009) , and because NAPLs is
widely used by humans.
NAPL contamination typically originates at the soil surface or close to it, and becomes
a direct threat once it reaches the groundwater because of its faster transport pace. Early
detection of NAPLs in the vadose zone increases the chance of preventing groundwater contamination as well as potentially lowering costs and simplifying the remediation
possibilities. Most traditional methods for soil contamination surveying require drilling
for soil and water samples and laboratory chemical analysis (Fetter, 1999). This is an
expensive and long process, that by its nature produces a localized map of the pollution
distribution (i.e. pollution can be found only at the specific location of drilling). In addition to the traditional method, there are few others emerging techniques for the detection
of NAPL at the subsurface, for example: detection of volatile organic carbons by soil
gas analysis (Fetter, 1999), detection of NAPL by laser induced fluorescence (Benjamin
et al., 1996), and more. Yet, all those techniques suffer from the same disadvantage
of producing a localized map of the pollution distribution as the sensed volume is very
small.
In recent years there is a growing interest in mapping NAPL contamination, their transport and fate using geophysical techniques. That is because those methods are noninvasive, inexpensive, and potentially produce a spatial map of the pollution distribution
(Snieder et al., 2007). For example, (Sogade et al., 2006) found a good agreement between the location of a NAPL plume in an aquifer, and the electrical response obtained
using time domain induced polarization tomography. Kaufmann and Deceuster (2007)
showed that electrical resistivity tomography has a potential in monitoring fuel leak
from under ground storage tanks. Aal et al. (2006) studied the effect of NAPL biodegradation on the spectral induced polarization response of soils. Those studies, as many
others (Borner et al., 1993; Chambers et al., 2004; Minsley and Morgan, 2008; Naudet
et al., 2003; Atekwana and Slater, 2009) show the potential of geo electrical methods as
a complementary tool to detect NAPL contamination and its fate at the subsurface.
2
2.1
Polarization mechanisms
At low to medium frequencies (up to 1 MHz), there are two major contributions to the
SIP response of soil: electrochemical polarization and interfacial polarization (Lesmes
and Morgan, 2001). Electrochemical polarization is typically attributed to the polarization of the electrical double layer (EDL) (Titov et al., 2002; Chelidze and Gueguen,
1999). According to the Gouy-Chapman-Stern model, the EDL consist of an inherent (in soils usually negative) charge at the solid’s surface surrounded by a contrastingly
charged ions atmosphere which can be divided into two regions - a mono layer of weakly
sorbed charged ions which called the stern layer, and a broader layer that balance the
charge from the solid surface and the stern layer by charge located in what is called
the diffuse layer (Adamson et al., 1997). Application of an external electric field on a
negatively charged EDL, leads to migration of cations in the direction of the field and
to polarization effect (Chelidze and Gueguen, 1999; Shilov et al., 2000). Leroy et al.
(2008) argue that in porous media where there is grain to grain contiguity the diffuse
layer does not polarized, and the only contribution to the polarization effects resulted
from the polarization of the stern layer.
In addition to electrochemical polarization, application of an external electrical field on
a multiphase system (such as soil) leads to charge build-up at the interface between the
phases differing in electric conductivity and/or dielectric constant in what is known as
interfacial (or Maxwell-Wagner) polarization (Alvarez, 1973; Chelidze and Gueguen,
1999; Sen et al., 1981). The accumulation of charge leads to enhancement of the bulk
dielectric permittivity of the porous media (Chen and Or, 2006). This process is usually
modeled using an effective medium theory approach where an inclusion of one phase
(e.g. water) is incrementally added as inclusion to the background (e.g. solid) phase
(Hanai, 1960; Sen et al., 1981; Chen and Or, 2006; Chelidze and Gueguen, 1999). Despite the different nature of the two polarization processes, they act simultaneously but
with different weight for each frequency: At low frequencies (in the order of tens of Hz)
the dominant mechanism is the electrochemical while at higher frequencies interfacial
polarization controls the electrical response of the soil (Leroy et al., 2008; Chen and Or,
2006).
2.2
The effect of fresh NAPL on the electrical properties of soil
The low electrical conductivity of NAPL compare with that of fresh water (5-6 order
of magnitude) lead researches to relate NAPL content and the electrical conductivity of
soils (e.g. Archie, 1942; Waxman and Smits, 1968). The basic assumption with those
works is that the replacement of the relatively conductive water with the practically
non conductive NAPL reduce the apparent DC electrical conductivity of the media in
3
what is known as the “insulating layer theory” (Sauck, 2000). Using this approach
applications and research in the field of oil reservoir engineering (e.g. Tsallis et al.,
1992) and contaminated hydrology (e.g. Kaufmann and Deceuster, 2007; Olhoeft, 1987)
were implement.
In addition to the effect of NAPL on the DC conductivity of the porous media, several
researchers also investigate the effect of NAPL on the dielectric properties of the soil.
For example, Olhoeft (1985) report that at low frequencies (up to 100 Hz), the phase of
oil contaminated clayey sample is lower than the phase of the clean sample and related
this phenomena to inhibition of the cation exchange processes as a result of attachment
of organic molecule to the clay surface. Borner et al. (1993) investigate the effect of
numerous organic pollutant on the real and imaginary part of the complex conductivity
(1mHz - 1KHz) for both clay and sand sample. They report that for the sandy samples,
σ 0 and σ 00 decrease as a result of the addition of NAPL, and that for the clayey soil only
σ 0 decrease with no change in the behavior of σ 00 . Similar results to thous of Borner
et al. (1993) are reported at Vanhala (1997) which also demonstrate an increase in the
phase spectra of the contaminated sample with time. More recently, Martinho et al.
(2006) investigate the effect of different NAPLs on the chargeability of clayey soils and
found that a decrease in chargeability for low NAPL content (up to 10%) is followed
by an increase in chargeability at high NAPL content. Cassiani et al. (2009) conducted
an SIP measurement (10mHz - 45 KHz) on partially saturated sand contaminated with
NAPL and documented a decrease in σ 0 for the contaminated sample. They attribute
the decrease in σ 0 to NAPL segregation in the column, resulted from the density difference between the NAPL and the water phase. Schmutz et al. (2010) studied the effect
of NAPL (at different saturation) on the SIP response of sand (1mHz - 45KHz) and reported that an increase in the phase is correlate to an increase in the NAPL saturation
(which in this experiment lead to a decrease in the water content). In addition they also
observed a decrease in conductivity with an increase in the NAPL saturation. The authors were able to explain those results by using Revil and Florsch (2010) model, which
relate between the hydraulic permeability and the SIP signal, i.e. their explanation is
mostly based on the effect of water content on the SIP signal. In summary, despite
the cumulative effort in the last few centuries and the potential application of detecting
NAPL in the subsurface using electrical measurements, the mechanisms in which NAPL
affect the electrical properties of porous media, especially in the vadose zone is still not
clear.
4
3
Materials and method
3.1
LCR system
LCR meter is an instrument design to measure various electrical properties of electronic
component. LCR meter induces an alternating electric field and measures the potential
over time. In this work a WAYNE KERR, model 4310 LCR meter was used. This instrument is able to measure electrical parameter such as: phase angle (deg) , admittance
(S) , impedance (Ω) , resistance (Ω) , and reactance (Ω) . The frequency range of
the instrument is 20 Hz to 100 KHz . Four electrode measurements were made using
two circular stainless steel 4 cm in diameter current electrodes, and two Ag/AgCl 7 cm
long and 25 mm in width potential electrodes. The current electrodes were placed at
the column edges (15 cm apart). The potential electrodes were placed at the center of
the column 3 cm apart. To ensure good contact between the potential electrodes and the
soil, the electrodes were inserted from the column top to the bottom (see Figure 3.1).
We choose this configuration (Figure 3.1) based on a work by Ulrich and Slater (2004),
which compare different electrode configuration, and reported that this configuration
leads to high accuracy.
Ag\AgCl
Electrodes
Stainless steel
electrode
Stainless steel
electrode
6 cm
3 cm
15 cm
Figure 3.1: Column configuration at the LCR system
3.2
SIP measurement system
In addition to the LCR meter, we also use the ZEL-SIP04 impedance meter developed
at Forschungszentrum, Juelich, Germany, especially for the measurement of soil SIP
(Zimmerman, 2008). The system comprise of function generator, an amplifier unit, an
AD (analog to digital) card, a PC, and a sample holder with two electrodes for current
5
injection and two electrodes for potential measurement (see Figure 3.2). The signal generator generate a sinusoidal wave at a frequency range between 1mHz to 45KHz. this
wave is injected through the current electrode and the voltage on the potential electrodes
is measured. Following the potential measurement, a signal analysis code is implement
and based on the results, the real and imaginary part of the complex conductivity is calculated. The system is reported to accurately measure the SIP response with a sensitivity
of the phase around 0.1 mrad.
ADC
Amplifier
Unit
PC
RS232
Function
generator
Figure 3.2: SIP measurement system
An important issue in the measurement setup of SIP is electrodes. Based on an experiment with this system (not reported here), we choose to use a brass electrode for
both the potential and current electrodes. To avoid chemical interaction between the
potential electrodes and the soil constitutes (solid, electrolyte, NAPL, etc.), an aggarose
(Bio-Rad, Inc.) bridge is used. To minimized interference to the current flow, only the
surface of the Aggarose is in contact with the soil.
3.3
Soils
Two different soils were used in this work. The first one is a red sandy soil (“Hamra”)
typical to the coastal area of Israel. The soil was air dried and sieved through a 4
mm screen. The cation exchange capacity (CECA) of the soil was also examined by
a standard method (Sumner and Miller, 1996) and presented with some more chemical
properties of the soil in Table 3.1.
Table 3.1: Exchangeable ion concentration and cation exchange capacity (unit of mew/100gr)
of the red sandy soil used in this work
red sandy soil
Ca
2.3
Mg Na
0.7 0.5
6
K CECA
0.3
3.8
The second soil was obtained by mixing a sandy soil (mean diameter of 160 µm) with a
clean Benton clay in a 1:4 ratio by weight (for simplicity, the mixture of the sandy soil
and the clay will be referred to as clay). Note that the electrical measurement of the red
sandy soil is done using the LCR meter, while measurements for the clay soil was done
in the SIP measurement system.
3.4
Mixing procedure
The goal of the mixing experiment is to compare between the electrical properties of
partially saturated soil, to that of partially saturated, NAPL contaminated soil where
the NAPL is homogeneously mixed in the soil. In the mixing experiment, soil, water
solution and NAPL were mixed and then packed in P.V.C columns. Immediately after
the packing, an electrical measurement was performed using the LCR or the SIP system.
The solution was prepared by adding 0.5 gr/l of NaCl to distillates water (red sandy soil),
or by adding tap water (EC=0.5 dS/m) in the case of the clayey soil. A constant amount
of solution was added to the soil; 30 and 62 ml for 0.5 Kg of air dry red sandy and clayey
soil respectively. The mixing of the soil with the solution was done using a bench drill,
equipped with a mixing head. Soil and solution were mixed in this instrument for about
3 min, and then the desired amount of NAPL was added. After the addition of NAPL the
mixture was mixed again for 3 more min. At the end of the mixing process, the mixture
was packed in a P.V.C column by adding small portions of the mixture and compressing
it using a match circular wooden plate.
4
Results
In this section we present the results obtain for the two soils (and measurements system)
used in this research. In section 4.1we show the results for the red sandy soil (obtain
using the LCR meter), and in section 4.2 the results for the clayey soil (obtain using the
SIP system) is presented.
4.1
Red sandy soil
In this section a compression between the phase spectra of the contaminated and uncontaminated red sandy soil samples is presented. In general the results suggest that at
low frequencies the phase of the contaminated samples is higher then that of the uncontaminated samples, while at high frequencies the phase of the contaminated samples is
lower then that of the uncontaminated one (see Figure 4.1 and 4.2 respectively). Figure
4.1 shows the electrical response of the sandy soil at low frequencies (up to 100 Hz)
7
Phase (rad)
0.03
0.02
0.01
0
-0.01
Phase (rad)
0.03
0.02
0.01
0
-0.01
Phase (rad)
0.03
0.02
0.01
0
-0.01
Phase (rad)
to different NAPL concentration. The results indicate that addition of NAPL results
in an increase of the phase compared to the uncontaminated samples (see Figure 4.1
treatments H1, H2, H5, H6, H8, H20, H30, H50, and H70). Treatment H7 shows the
opposite trend, i.e. a decrease in phase of the contaminated samples compared to the
uncontaminated one, and in treatment H9 and H10 no change between the contaminated
and uncontaminated samples is observed.
0.03
0.02
0.01
0
-0.01
H1
H2
H5
H6
H7
H8
H9
H10
H20
H30
H50
H70
2
10
Frequency (Hz)
2
10
Frequency (Hz)
2
10
Frequency (Hz)
Figure 4.1: Phase spectrum of the contaminated (red plus signs) and the uncontaminated (blue
stars) red sandy soil samples at low frequencies. The labels H1, H2 etc. refer to the different
treatment of the contaminated samples. Note that the blue stars always refer to treatment H0
From Figure 4.1, it seems that although an addition of NAPL to soil results in an unmistakable phase shift, the effect does not depend on the amount of NAPL added. Similar
conclusion, regarding the effect of the amount of NAPL on the dielectric constant was
reported earlier by Camille et al. (2001) for a saturated soil columns. It is important to
note that the minimum amount of NAPL that was added to the soil is in the order of few
grams per liter, which is much higher then the solubility of NAPL in water. Therefore
we argue that a threshold regarding the effect of NAPL concentration on the phase shift
exists, but this is yet to be validated experimentally.
The results for high frequencies (10 to 100 KHz) are shown in Figure 4.2. In treatments
H1, H2, H6, H7, H10, H20, H30, H50 and H70 a reduction in phase relatively to H0 is
observed. In the other treatments no clear change in phase between the contaminated
and the uncontaminated treatments is observed. Similarly to the low frequencies, it
seems that NAPL concentration does not affect the phase shift. For example, the phase
spectrum of H2 and H50 are almost identical, but they are different from the spectrum of
H5. Despite the relatively high noise in the results (in both the low and high frequencies)
the general trend is clear: increase in the phase of the contaminated samples at low
frequencies and decrease in the phase of the contaminated samples at high frequencies.
Since from Figures 4.1 and 4.2 it seems that the different NAPL concentration have no
effect on the phase shift we calculate the arithmetic mean of the phase of the contaminated samples hθH i , and compare it to the mean of the uncontaminated samples θH0
8
Phase (rad)
Phase (rad)
Phase (rad)
Phase (rad)
. The results are shown in Figures 4.3 and 4.4, where Figure 4.3 simply show the full
phase spectrum of H0 and hθH i , and Figure 4.4 shows the difference between them,
i.e. hθH i − θH0 . The standard deviations shown in Figure 4.4 represent the variation
between the different phase difference of the contaminated treatments and the uncontaminated one. Note that the transition from higher phases value of the contaminated
samples at low frequency to lower values at high frequency occurs around the first maximal value of H0 (around 1 KHz - see Figure 4.3 ). This area in the phase spectrum was
already emphasized, since it separate between two IP mechanisms: electrochemical polarization, and interfacial polarization (Leroy et al., 2008). Therefore, at low frequencies
the relative increase in phase of the contaminated samples is likely to be related to interaction of the NAPL with the solid phase, while at high frequencies the relative decrease
in phase is likely related to interfacial phenomena between the different component of
the soil (solids, water, NAPL, and air).
H1
H2
H5
H6
H7
H8
H9
H10
H20
H30
H50
H70
0.2
0.1
0
0.2
0.1
0
0.2
0.1
0
0.2
0.1
0
2
4
6
8 10
4
x 10
Frequency (Hz)
2
4
6
8 10
4
x 10
Frequency (Hz)
2
4
6
8 10
4
x 10
Frequency (Hz)
Figure 4.2: Phase spectrum of the contaminated (red plus signs) and the uncontaminated (blue
stars) red sandy soil samples at high frequency. The labels H1, H2 etc. refer to the different
treatment of the contaminated samples. Note that the blue stars always refer to treatment H0
A decrease in the phase difference (hθH i − θH0 ) with frequency is observed in Figure
4.4. At low frequencies (up to few hundred Hz) the phase of the contaminated treatments
is higher then the phase of the uncontaminated one, around 1 KHz the phase difference
is approaching zero, further increase in frequency resulted in a decrease of the phase
difference with a maximum negative value at 100 KHz.
9
0.2
Phase (rad)
0.15
0.1
0.05
0
-0.05
10
2
3
10
Frequency (Hz)
10
4
10
5
Figure 4.3: Full phase spectrum of uncontaminated soil (H0: blue stars) and the average phase
of the contaminated samples (H1 to H70: red diamonds). Note the transition zone (around 1
KHz) between the higher phase values of the contaminated samples compare to the uncontaminated at low frequencies to lower phase values of the contaminated samples compare to the
uncontaminated at high frequencies
0.04
0.03
Phase difference (rad)
0.02
0.01
0
−0.01
−0.02
−0.03
−0.04 1
10
2
10
3
10
Frequency (Hz)
4
10
5
10
Figure 4.4: Difference between the average phases of the contaminated samples and the uncontaminated ones (i.e. hθH i − θH0 ). Note the transition zone from positive values to negative
values around 1 KHz, and the relatively high standard deviation at high frequencies
4.2
Clayey soil
To examine our system we measure the electrical properties of water using the the same
experimental setup (described above). In addition to the measurement we calculated the
complex conductivity of water using the Debye relaxation model and found an excellent
fit (up to 1KHz) between measured data and calculated values (see Fig. 4.5). From 1
KHz to 45 KHz, the results indicate the the system is not able to accurately measure the
complex conductivity, probably due to polarization of the measurement system itself.
Therefore in the following we only show results up to a frequency of 1KHz where the
system accuracy is high.
10
500
5
σ' (μS∙cm-1)
measured σ'
measured σ''
calculated σ'
calculated σ''
300
3
200
2
100
1
σ'' (μS∙cm-1)
4
400
0
0
10
-1
10
0
1
2
10
10
10
Frequency (Hz)
3
10
4
10
5
Figure 4.5: Measured (dots) and calculated (line) electrical properties of water
180
170
160
150
140
130
120
110
100
2
a
σ'' (μS/cm)
170
160
150
140
130
120
110
100
1.5
clean
5% diesel
15% diesel
b
clean
5% motor oil
15% motor oil
d
1
0.5
0
2
c
σ'' (μS/cm)
σ' (μS/cm)
σ' (μS/cm)
In Fig. 4.6 the real and imaginary part of the complex conductivity for clean soil, and
diesel and motor oil contaminated soil (5% and 15%) are shown. For both the diesel
fuel and the motor oil, σ 0 increase as a result of addition of the NAPLs (panel a&c
in Fig. 4.6). Interestingly addition of 5% diesel to the soil lead to a 40% increase in
σ 0 , while addition of 15% diesel resulted in a 20% increase in σ 0 . This trend is also
observed for the motor oil (52% and 45% increase in σ 0 for the 5 and 15% treatment,
respectively) but to lesser extent. Because in this experiment NAPL replace air, both
possessing a very low electrical conductivity (compare to the electrical conductivity of
the soil solution both NAPL and air can be consider as electrical insulator), the increase
in σ 0 as a result of the addition of the NAPLs is not trivial. Further, the decrease in σ 0
as a result of further addition of NAPL is even less trivial. To revolve the mechanisms
governing the observed effect of NAPL on σ 0 further work is required.
10-2
10-1
100
101
102
Frequency (Hz)
103
1.5
1
0.5
0
10-2
10-1 100 101 102
Frequency (Hz)
103
Figure 4.6: Real and imaginary part of the complex conductivity for diesel (a&b), and for motor
oil (c&d).
11
Figure 4.6 panels b&d shows σ 00 of the clean and diesel/motor oil contaminated soil.
In all cases (diesel/oil at different concentration) up to a frequencies of few tens of
Hz, addition of NAPL result in a decrease of σ 00 (up to 60% after the addition of 15%
of motor oil). The decrease is in accordance to the NAPL concentration where high
concentration resulted in a more significant decrease of σ 00 . In addition, Fig. 4.7 shows
the difference between the spectral response of clean and contaminated samples. The
results indicate an increase in the σ 00 difference (clean higher than contaminated) up to
70 Hz which is followed by a decrease in the difference with increasing frequencies.
Interestingly the difference maximum for both the diesel and the motor oil occur at
the same frequency (70 Hz) which indicate that similar mechanisms for both the diesel
and the motor oil governing the decrease in σ 00 (which directly related to the system
polarization).
0.25
clean-diesel
clean-oil
σ''clean-σ''NAPL (μS/cm)
0.2
0.15
0.1
0.05
0
0.01
0.1
1
10
Frequency (Hz)
100
1,000
Figure 4.7: Spectral response of clean minus contaminated (15%) sample for both the diesel
(black) and the oil (red)
Because the low frequency polarization of porous media is usually associated to electrochemical processes and the solid-fluid interface (Leroy et al., 2008; Lesmes and Morgan, 2001), analysis of the σ 00 results (Fig. 4.6 and 4.7) should be based on the effect
of NAPL on that interface. Assuming that some of the NAPL adsorbed to the mineral
surface, and because the NAPL molecules are bigger than the ions usually attached to
the mineral surface, it is reasonable to assume that under the influence of an external
electric field the organic covered porous media will polarized to lesser extent than the
clean media.
12
5
Summary
The electrical signature of soils contaminated by non aqueous phase liquids was studied
in laboratory setting. In an era where NAPL contamination is common and detection
methods are typically invasive, the ability to indestructively and noninvasively detect
and quantify the location of NAPL in the subsurface has enormous environmental and
economical importance.
Three measurement setups were employed: signal generator and oscilloscope based,
LCR meter based, and an advanced SIP system. Generally speaking all systems were
capable of identifying the electrical signature of the NAPL, but the SIP system was the
only one to provide reliable and repeatable measurements at all times. We have learned
that the addition of NAPL to unsaturated soil generally increases the true conductivity,
σ 0 , and slightly reduces the imaginary conductivity, σ 00 . The signature is frequency
dependent, especially with regards to the imaginary conductivity. As could be expected,
different NAPLs have different signatures. Interestingly, while the addition of NAPL
increases the real conductivity, further addition of NAPL somewhat reduced the real
conductivity. At the same time the imaginary conductivity continues to drop, but not
significantly.
This research opened the way for more sophisticated investigations regarding the mechanisms that lead to these phenomena. Possibilities include geometrical effects (due to
difference in surface tension of the different liquids), NAPL ion content, soluble NAPL,
the formation of additional interfaces in multi-phase system, and more. These will be
investigated in the near future using ISF fund that could not have been received without
the support of the Grand Water Research Institute.
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