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Laboratory study of landfill leachate effect on resistivity in unsaturated soil using cone penetrometer. G.L. Yoon ф M.H. Oh ф J.B. Park. Abstract Electrical ...
Original article

Laboratory study of landfill leachate effect on resistivity in unsaturated soil using cone penetrometer G.L. Yoon Æ M.H. Oh Æ J.B. Park

Abstract Electrical resistivity of soils is dependent upon various factors, including soil type, water content, saturation and pore fluid property. Experimental works have been performed to investigate the relationship between electrical resistivity and unsaturated subsurface conditions with varying physical property and landfill leachate contamination. The moisture density can be the most effective indicator for describing the relationship between electrical resistivity and physical property of unsaturated subsurface. For three different tested soils, the electrical resistivity of soil exponentially decreased as moisture density increased. The adding of leachate having various ions decreased the electrical resistivity. Also, the formation factor can be described by the term of moisture density in unsaturated sand. The formation factor was higher when soil and pore water were contaminated by higher concentration of leachate than when soil and pore water are uncontaminated, since ionic movement is restrained by electro-chemical interactions between soil particles and leachate constituents. Keywords Electrical resistivity Æ Landfill leachate Æ Moisture density Æ Formation factor

Received: 18 March 2002 / Accepted: 10 June 2002 Published online: 22 August 2002 ª Springer-Verlag 2002 G.L. Yoon Coastal and Harbor Engineering Research Laboratory, Korea Ocean Research and Development Institute, Sa-dong, Ansan-si, Kyunggi-do 425–744, Korea M.H. Oh Æ J.B. Park (&) School of Civil, Urban and Geosystem Engineering, Seoul National University, Shinlim-dong, Gwanak-gu, Seoul 151–742, Korea E-mail: [email protected]

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Introduction The evaluation of groundwater and soil properties has become increasingly important for site characterization when more industrial wastes and domestic solid refuse come into contact with groundwater and soils and cause subsurface contamination. One of the emerging techniques to assess the contaminated media is the electrical resistivity method, which can be performed rapidly and nondestructively in situ. Every soil possesses a natural resistivity within certain limits; deviations may suggest possible pollutions as the contaminants may influence the bulk resistivity of soil (Abu-Hassanein and others 1996). Alternatively, a resistivity cone penetration test (RCPT), which is a modification of the cone penetration test (CPT) equipment as shown in Fig. 1, can be employed at a relatively low cost for delineation of subsurface contamination in situ, and then be supplemented with a minimum confirmatory sampling and laboratory testing program. The use of the resistivity cone is particularly appealing because the cone can measure resistivity to a higher resolution and at the same time note change in lithology (Campanella and Weemees 1990). At first, the measurement of electrical resistivity (or conductivity) was developed to evaluate in situ density or porosity of sands. Previous efforts in geoenvironmental site investigation using RCPT, as reported in the literature, have shown some applicability of the resistivity measurement method (Campanella and Weemees 1990; Piccoli and Benoit 1995). Kokan (1990) and Strutynsky and others (1991) reported reasonable success in delineating light non-aqueous phase liquid (LNAPL) hydrocarbon. Additionally, Okoye and others (1995) confirmed the applicability of the RCPT method in delineating creosote contamination in saturated soil. Although the electrical resistivity measurement can give a good indication of a contaminant plume, quantitative correlation between electrical resistivity and the concentration of contaminant in the subsurface has not been fully established. Only a few studies have been conducted on the electrical resistivity characteristics of soil with the change of the subsurface condition in unsaturated soil (McCarter 1984; Abu-Hassanein and others 1996). As the factors affecting electrical resistivity of soils are very important for the characterization of contaminated subsurface, parametric studies based on laboratory tests have been performed in this paper. The objectives of this paper are twofold: first, to examine the relationship

DOI 10.1007/s00254-002-0649-1

Original article

nella and Weemees 1990). Electrical conduction in electrolytic solutions, moist soils, and water-bearing rocks occurs as a result of the movement of ions. The capability to transmit ions governs the electrical resistivity, which is a basic property of all materials (Abu-Hassanein and others 1996). The electrical resistivity method works on the principle that the measured voltage drop across a pair of electrodes at a certain current is proportional to the electrical resistivity of the soil. All materials, including soil and rock, have an intrinsic property of resistivity, which governs the relationship between the current density and the gradient of the electrical potential. Typical measurement systems include the Wenner type and the Schlumberger type. Details on electrode array systems are described in the literature (Kearey and Brooks 1991; Kalinski and Kelly 1994; Yoon and Park 2001). Factors affecting electrical resistivity of soil Electrical resistivity of soil is known to depend on the soil type and the electrical resistivity and temperature of pore fluid. Soil properties affecting the resistivity of the soil also include porosity, degree of saturation, water content, grain size distribution, particle shape and orientation, and pore structure(Abu-Hassanein and others 1996; Yoon and Park 2001). A pioneering work was carried out by Archie (1942) who derived the first general relationship of the electrical reFig. 1 sistivity of saturated soil (qsat) to the electrical resistivity Schematic diagram of RCPT (Shinn and others 1998) of its pore fluid (qw). Archie’s formula assumes that bulk resistivity is directly related to pore fluid resistivity and the between electrical resistivity and the unsaturated subsurgeometry of the pore structures in the subsurface (Archie face condition; and second, to investigate the variation of 1942; Campanella and Weemees 1990). A term commonly electrical resistivity and formation factor of unsaturated used to relate bulk resistivity to pore fluid resistivity is the soils due to landfill leachate based on the laboratory-scaled formation factor. Archie’s formula is as follows: experiments using the resistivity cone penetrometer. q FF ¼ sat ¼ anm ð1Þ qw

Background Fundamentals of electrical resistivity Electric current can be conducted in three different ways, i.e. ohmic conduction, electrolytic conduction, and dielectric conduction. The electrons flow through the crystalline structure of some materials in ohmic conduction. Ohmic conduction occurs most readily in metals. Electrolytic conduction is carried out by ions dissolved in groundwater, through the interconnected pores of soil, unconsolidated sediment, or rock. In dielectric conduction, an alternating electrical field causes ions in a crystalline structure to shift cyclically. Even though there is no actual flow of charged particles, the cyclic change in the positions of ions is a movement that can be viewed as an alternating current. Dielectric conduction can occur in electric insulators, which are materials that otherwise do not carry electric current (Robinson and Coruh 1988). In RCPT, conduction takes place in two modes: (1) ohmic conduction across the metallic electrodes and (2) electrolytic conduction through the pore fluid (Campa-

where FF is the formation factor, n is porosity, and a and m are constants depending on the type of soils. The constants a and m are parameters whose values are assigned arbitrarily to make the equation fit a particular group of measurements. Keller and Frischknecht (1966) suggest that a is less than unity for rocks with intergranular porosity and greater than unity for those with joint porosity. The constant m is referred to as the cementation factor, and it varies between 1.4 for quartz sand and over 3.0 for sodium montmorillonite (Archie 1942; Atkins and Smith 1961; Jackson 1975). m is dependent on the shape of the particles, increasing as they became less spherical, while variations in size and spread of sizes appeared to have little effect. Thus, m is a measure of pore tortuosity (Jackson and others 1978). Equation (1) shows that the electrical resistivity of saturated soils is sensitive to the porosity, electrical resistivity of the pore fluid and the structure of the pores (AbuHassanein and others 1996). Archie’s formula has been used extensively for determination of porosity in sediments and is valid under the condition that the pore fluid

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resistivity is relatively low and there is small content of clay minerals present in saturated soil (Campanella and Weemees 1990). The electrical resistivity also depends on the degree of saturation. The electrical resistivity of unsaturated soils (q) can be related to that of saturated soils (qsat) as follows (Keller and Frischknecht 1966; McNeill 1990): q ¼ qsat SB

ð2Þ

where S is degree of saturation and B is an empirical parameter. This equation was developed for porous media having fixed pore structure and variable degree of saturation. Abu-Hassanein and others (1996) reported that parameter B falls between 0.64 and 2.29 for ten clays, all of which have been used to successfully construct compacted clay liners having field-scale hydraulic conductivity of less than 1·10–7 cm/s. Equation (2) demonstrates that increasing the degree of saturation results in lowering electrical resistivity. However, the degree of saturation in the field may not be directly obtained by measuring the electrical resistivity using RCPT, because the resistivity of saturated soil in Eq. (2) needs to be determined through on extra experiment. The extra experiment for evaluating the resistivity of saturated soil is time-consuming and difficult. Therefore, to rapidly obtain the properties of the unsaturated subsurface in situ, the direct relationship between the resistivity in the unsaturated soil and the soil property of unsaturated subsurface, which is more practical for field application, needs to be developed.

concentration of the heavy metal ion constituents in the sample was extremely low as listed in Table 2. Tap water was used as the pore fluid for the uncontaminated soils. The conductivity of tap water was measured at 25 C by a conductivity meter (ORION 550A, K=0.475; ASTM 1991), and then converted into the electrical resistivity, which was measured as 67 Wm (=0.15 mS/cm). To simulate contaminated soil conditions, landfill leachate collected from the industrial landfill site called the Greater City of Inchon Landfill in Korea was added in the experiments. Leachate is the complex mixture of fluids containing both conductive and insulating constituents. Normally, such mixtures will produce highly conductive plumes as the influence of the conductive contaminants is greater than that of the non-conducting contaminants (Campanella and Weemees 1990). Major heavy metal concentrations, such as Cr, As, Cd, Pb, Cu, Hg and ionic constituent concentrations, were analyzed before performing experiments because the concentrations of heavy metal and ion constituents are the main factors in the resistivity measurements. The constituent concentrations of the Inchon landfill leachate are shown in Table 3. The conductivity of the leachate measured by the conductivity meter was 23.51 mS/cm (0.425 Wm) and measured values of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) for the leachate were 820 and 285 mg/L, respectively. Also, the pH value was 8.76. Inchon landfill leachate used here could be considered as representative for the typical landfill leachate in its constituents and concentration because the constituents and concentration of the Inchon landfill leachate fall into the general range of

Materials and methods Test materials Three different local soils – sandy soil, weathered granite soil, and silty sand – commonly distributed around the Korean peninsula were used to investigate the relationship between electrical resistivity and unsaturated soil properties. Soil index tests and standard proctor compaction tests were performed as preliminary tests. The grain size distribution curves of the test soils are shown in Fig. 2 and the physical properties for the test soils are summarized in Table 1. Note that only the silty sand contains about 7.5% of silty clay particles among the three sampled soils. Also, the heavy metal concentrations of the test soils are shown in Table 2. The effect of heavy metal ions on electrical Fig. 2 resistivity in the soil sample could be negligible since the Grain size distribution of the test soils Table 1 Physical characteristics of the test soils. NP Non-plastic; USCS Unified Soil Classification System; SP poorly graded sand; SP-SM poorly graded silty sand Soils

Sandy soil Weathered granite soil Silty sand

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Index properties

Compaction characteristics

Specific gravity

Passing #200 (%)

Plasticity index

USCS

Maximum dry unit weight (g/cm3)

Optimum water content (%)

2.64 2.68 2.66

1.5 0.7 7.5

NP NP NP

SP SP SP-SM

1.67 1.91 1.89

17.7 12.6 15.9

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Table 2 Heavy metal concentrations (ppb) in soils Heavy metals

Sandy soil

Weathered granite soil

Silty sand

Cr As Cd Pb Cu Hg

0.8 5.0 0 17.0 0 0

0.8 5.0 0 14.8 0 0.3

1.0 5.0 0 12.0 0 0

the reported leachate data in the literature. In the laboraFig. 3 tory experiment, 5, 10 and 30% (by volume) of the leachate was mixed with deionized water and used as contaminated The dummy cone pore fluid to analyze the sensitivity of the leachate concentration in the soil phase to electrical resistivity. The conductivity of the diluted leachate solutions were measured at 25 C. Test equipment and methods Laboratory test methods for measuring the resistivity of soils include the use of a manufactured resistivity dummy cone as shown in Fig. 3. The dummy cone consists of a four-electrode array, which minimizes the effect of polarization, as the current drawn through the measurement electrodes is very small, thus, there is no appreciable buildup of ions at the electrodes (Campanella and Weemees 1990). Current is passed between the two outer electrodes and the potential or voltage drop is measured between the two inner electrodes. The Wenner array, comprised of four evenly spaced electrodes, was used with the dummy cone having electrode spacing of 50 mm. The STING R1/SWIFT resistivity measuring system ‘Memory Earth Resistivity Meter’ (Advanced Geosciences, Inc.) was used to induce the current and measure the voltage drop. Laboratory experiments were performed using a circular chamber made of polyvinylchloride (PVC), which is an electrical insulator (electrical resistivity 1014 Wm), with a diameter of 500 mm and height of 700 mm, as shown in Fig. 4. To convert from resistance to resistivity, a laboratory calibration was made for the dummy cone. The chamber was filled with potassium chloride solution. Then the resistivity of solution was noted with a conductivity meter (ORION 550A), and the values were compared with the resistance measured by the dummy cone used in this study. A linear relationship between the measured resistance and the resistivity was obtained from the calibration. To investigate the electrical resistivity under the same conditions for the test soils and consider water content more or less than the optimum water content, four

Fig. 4 Laboratory experimental setup with dummy cone and cylinder chamber

different dry unit weights with three different water contents on the basis of the standard proctor compaction tests were used (Table 4) because the variation in soil resistivity with increasing water content is very different before and after the optimum water content (Abu-Hassanein and others 1996). The water contents decreased slightly during the tests due to evaporation. In this study, the water

Table 3 Constituent concentrations in Inchon landfill leachate Heavy metals

Concentration (mg/L)

Ions

Cr

As

Cd

Pb

Cu

Hg

K+

Na+

NH4+

Mg2+

Ca2+

Fe2+

Cl)

SO42)

1.015

0.215

0.030

0.440

0.030

0.005

1,400

1,716

935

238

243

26

4,214

152

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Table 4 Summary of experimental conditions for test soils Sandy soil 3

Dry unit weight (g/cm ) Porosity Water content (%)

1.55 0.41 19 14 10

1.50 0.43 19 14 10

Weathered granite soil 1.45 0.45 19 14 10

1.40 0.47 19 14 10

1.75 0.35 19 14 11

contents at the end of the experiments were used to analyze the experimental results. To analyze the sensitivity of leachate concentration in soil, 5, 10 and 30% (by volume) of leachate mixed with fresh water was added to each soil. All measurements were conducted for less than a few minutes because the applied current causes overheating of the specimen (Shang and others 1995; Kaya and Fang 1997). Boundary effect of the test set up The electrical resistivity of natural lake water was measured by dummy cones for investigating the boundary effect due to the PVC cylinder chamber, assuming the natural lake water is a homogeneous material. The electrical resistivity of natural water was measured using dummy cones in the lake, which is in Seoul, Korea, where it can be regarded as in a semi-infinite condition. The natural water, which was sampled at the same point in the lake, filled the cylinder chamber used in this experiment. The electrical resistivity of the lake water was measured by the dummy cone in the same way. Table 5 shows that, in the case of measuring in a semiinfinite condition, the measured electrical resistivity of the lake water was about 17.7 Wm. In the case of measuring at the cylinder chamber, the electrical resistivity of the lake water is about 18.7 Wm. The electrical resistivity measured using a dummy cone having 50-mm electrode spacing showed a small difference between the semi-infinite condition and the circular chamber; however, this difference can be negligible. The electrode spacing can be practically assumed to be the distance of observation in the electrical resistivity measurement system. This assumption is supported by previous research output such that 70–90% of the injected current flows through depths at less than the electrode spacing. Moreover, that distance can be practically assumed to be the depth of current intrusion (Keller and Frischknecht 1966; Van Nostrand and Cook 1966; Kalinski and Kelly 1994). It is evident that the electrical resistivity measured Table 5 Comparison of resistivity measurement (Wm) under semi-infinite condition and cylinder chamber condition Measured resistivity

Average

Test no. 1 Test no. 2 Test no. 3 Semi-infinite condition Cylinder chamber condition

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17.7 18.7

17.7 19.3

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17.6 18.1

17.7 18.7

1.70 0.37 19 14 11

1.65 0.39 19 14 11

Silty sand 1.55 0.42 19 14 11

1.70 0.36 18 14 10

1.60 0.40 18 14 10

1.50 0.44 18 14 10

1.40 0.47 18 14 10

by the cone with 50-mm electrode spacing is not affected by the PVC cylinder chamber with 500-mm diameter, because the radius of the cylinder chamber (250 mm) is larger than the effective distance of the resistivity measurement.

Results and analysis Effects of moisture density on resistivity It has been recognized that water is the dominant factor in the electrical resistivity of soil, except in the case of high specific surface minerals in high resistivity fluids (Curtis and Narayanan 1998; Saarenketo 1998; Oh and Park 2000). Figure 5 shows that the electrical resistivity of soils decreases with increasing gravimetric water content. However, the electrical resistivity varies with the change of porosity (or dry unit weight) at a particular water content. The decrease of porosity (i.e. the increase in dry unit weight) enhances the connectivity of pore fluid and soil particles, which results in increasing electric current path and, consequently, resistivity decreases. Therefore, gravimetric water content alone cannot be used as a criterion on which to base the resistivity of soil because soil samples may have identical water contents but different porosity (i.e. different dry unit weight). McCarter (1984) reported that the decrease in the air– void ratio would reduce resistivity at a particular water content. The electrical resistivity is primarily influenced by the weight of water per unit volume of soil mass rather than the gravimetric water content defined as the ratio of weight of water to weight of dry soil. The former is termed moisture density in grams per cubic centimeter. The moisture density, md, is calculated as follows (Selig and Mansukhani 1975): md ¼

Ww Ww Ws ¼ ¼ xcd Vt Ws V t

ð3Þ

where Ww is the weight of pore water, Vt is the volume of soil sample, Ws is the weight of solid phase, cd is the dry unit weight of soil in grams per cubic centimeter, and x is the conventional gravimetric water content. The moisture density reflects the changes of gravimetric water content and dry unit weight. If the unit weight of fluid is 1 g/cm3, moisture density has the same value as volumetric water content, and can be expressed by times of porosity(n) and degree of saturation(S):

Original article

Fig. 6 Relationship between electrical resistivity and moisture density of reconstituted soil

The results are shown in Fig. 6. Good correlation was observed between moisture density and measured electrical resistivity of soils. Although three types of tested soils have different ranges of electrical resistivity with moisture density, it is shown that the electrical resistivity of soils decreases and gradually converges as the moisture density increases for all soils. The increase of moisture density means the increase of dry unit weight at a particular water content or the increase of water content at a certain dry unit weight. It is evident that the increase of water content resulted in the decrease of electrical resistivity of bulk soil. Also, the increase of dry unit weight means the increase of the degree of saturation at a certain water content and the increase of connectivity of pore fluid. Consequently, these effects cause decreasing electrical resistivity. Effects of soil type on resistivity In Fig. 6, sandy soil and weathered granite soil showed relatively higher resistivity values than silty soil. This can be explained by applying the fact that sandy soil and weathered granite soil contain more coarse-grained soil particles than silty soil (Table 1). The coarse-grained particles are primarily quartz and feldspars that have high electrical resistivity (Keller and Frischknecht 1966). Also, the decreasing rate in resistivity with increasing moisture density in sandy soil and weathered granite soil was higher than that of silty sand, which means there was not much variation in resistivity values with increasing moisture density in silty soil. As moisture density increased from 0.1 to 0.45 g/cm3, the electrical resistivity decreased from 459 Fig. 5 to 87 Wm for sandy soil, from 371 to 45 Wm for weathered Effect of water content and porosity (dry unit weight) in resistivity granite soil, and from 111 to 27 Wm for silty sand, remeasurements spectively. The electrical resistivity of silty sand is lower than that of the others due to the presence of 7.5% fine particles in silty Ww Vw cw Vv Vw md ¼ ¼ ¼ ¼ nS ð4Þ sand (Fig. 2; Table 1). Soils with more fines often contain a Vt Vt Vt Vv higher percentage of conductive particles, and have higher where cw is the unit weight of fluid, Vv is the volume of the specific surface which improves surface conduction (Kwader 1985). The presence of fine particles in silty sand pores and Vw is the volume of pore water. Environmental Geology (2002) 43:18–28

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would affect the resistivity in two ways: (1) increased fines content will decrease porosity, which has the effect of increasing the resistivity; and (2) the presence of fines in the soil may indicate the presence of conducting clay minerals, which would result in a decrease in the resistivity. In clayey soil, electrical conduction occurs in the pores and on the surfaces of electrically charged clay particles, known as surface conduction (Rhodes and others 1976; Urish 1981). For clays in high resistivity fluids, surface conduction can be a significant factor affecting the bulk electrical resistivity of the soil (Mitchell 1993; Sadek 1993). Also, the higher the cation exchange capacity, the lower the resistivity of the soil. In general, the movement of ions within the pore fluid is related to the cation exchange capacity of the soil. For a given pore fluid, the cation exchange capacity is generally highest for clayey soils, lower for silty soils, and lowest for granular soils because of mineralogical factors. The electrical resistivity is low in clayey soil and high in granular soils for the same pore fluid (Okoye and others 1995). Therefore, the electrical resistivity of silty sand, which contains 7.5% fine particles, is lower than that of sandy soil and weathered granite soil because of the effects of the surface conduction of fine particles. Effects of landfill leachate contamination on resistivity The electrical resistivity of soil is also affected by the electrical resistivity of pore fluid (Abu-Hassanein and others 1996; Kaya and Fang 1997; Saarenketo 1998). To investigate the effects of the electrical resistivity of pore fluid due to landfill leachate contamination by adding 5, 10 and 30% (by volume) of leachate mixed with deionized water to three sampled soils, the variations of electrical resistivity measured in the three soils were observed with changing moisture density. Also, the electrical resistivity of soils was measured for four different dry unit weights with three different water contents on the basis of Table 4. The conductivity of leachate dilute solutions was measured by the conductivity meter, and then the resistivity was calculated as shown in Fig. 7. The electrical resistivity of

Fig. 8 Relationship between electrical resistivity of soils mixed with Inchon landfill leachate dilute solution and moisture density

Fig. 7 Electrical conductivity and resistivity of leachate dilute solution (at 25 C)

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leachate dilute solution ranges from 7.9 Wm for 5% leachate solution to 1.4 Wm for 30% leachate solution, and the electrical resistivity of dilute solution decreases with increasing volumetric leachate proportion of solution. The conductivity of the leachate diluted solution linearly increases with the concentration of the leachate, as shown

Original article

Fig. 9 Variations of electrical resistivity of bulk soil with leachate concentration

in Fig. 7. As the solution is relatively homogeneous and thus the movement of ions is free, the conductivity of the solution is proportional to the ionic concentration, which gives the electrical resistivity or conductivity method great potential for detecting contaminants, quantitatively. The conductivity or resistivity measurement method is frequently applied to water analysis to obtain a rapid estimate of the dissolved solids content of a water sample (Sawyer and others 1994). However, the ground is not homogeneous and the electrical resistivity of bulk soil is affected by many factors, for example, shape and orientation of soil particles and pore structure. Nevertheless, the experimental results show that the electrical resistivity of bulk soil decreases as the proportion of leachate increases (Fig. 8). The values of electrical resistivity of soils are plotted as a function of moisture density in Fig. 8. At all moisture densities, there were significant drops in resistivity for the three sampled soils after adding leachate compared with those values in Fig. 6. More electrical conduction occurred as a result of the movement of ions in the leachate. As the leachate contains various ions (Table 3), the electrical resistivity was affected and dropped more significantly than that of the uncontaminated (leachate-free) condition. Table 6 Results of regression analysis

Figure 8 also shows that the higher the concentration of leachate in pore fluid, the lower the electrical resistivity. It is noted that all the tested soils have similar values of electrical resistivity with moisture density. As the leachate dilute solutions have very low electrical resistivity in the range 1.4 to 7.8 Wm, the electrical resistivity of the soil particles is negligible compared to that of the pore fluid. The variations in electrical resistivity of bulk soil with the proportion of leachate are plotted at a moisture density of 0.25 in Fig. 9. The uncontaminated soils have the resistivity value of 160 Wm for sandy soil, 130 Wm for weathered granite soil and 50 Wm for silty sand. At this point, the important finding shown in Fig. 8 was that the measured resistivity was significantly changed by adding only 5% proportion of leachate. Although the decrease of electrical resistivity is not distinct at level of leachate concentrations greater than 5%, the results in this study indicate that the electrical resistivity method using a resistivity cone has potential in the surveying of existing leachate contamination of the subsurface and can be suitable for leachate leakage monitoring of pre-existing waste landfill. Effect of leachate contamination on formation factor A regression analysis was performed to evaluate quantitatively the electrical resistivity variation with changing moisture density. The regression analysis results for uncontaminated and leachate contaminated soil (curves in Figs. 6and 8) are tabulated in Table 6. The regression equations of the three tested soils are determined in the same form of Eq. (5). This equation provides a good fit to the experimental data of uncontaminated and contaminated cases, as evidenced by the high determination coefficients. q ¼ Amm d

ð5Þ

where q is the electrical resistivity of soil mass, md is the moisture density, and A and m are constant. The exponent m can be referred to as the cementation factor, which measures pore tortuosity and degree of cementation as quoted in the literature. The constant A can be assumed the function of pore fluid resistivity, based on Archie’s law assuming that bulk resistivity is directly related to pore fluid resistivity. Therefore, Archie’s formula

Soil type

Leachate proportion (%)

Regression curve equation

R2

Sandy soil

0 (tap water) 5 10 30 0 (tap water) 5 10 30 0 (tap water) 5 10 30

q=21.33md–1.53 q=2.00md–1.62 q=0.71md–1.99 q=0.34md–1.87 q=21.40md–1.25 q=1.92md–1.73 q=1.04md–1.75 q=0.33md–2.07 q=11.64md–1.20 q=1.80md–1.84 q=0.41md–2.46 q=0.10md–3.09

0.9716 0.9611 0.9809 0.9691 0.8241 0.9887 0.9924 0.9560 0.8158 0.9621 0.9592 0.9391

Weathered granite soil

Silty sand

FF=0.32md–1.53 FF=0.25md–1.62 FF=0.18md–1.99 FF=0.25md–1.87 FF=0.32md–1.25 FF=0.24md–1.73 FF=0.27md–1.75 FF=0.24md–2.07 FF=0.17md–1.20 FF=0.23md–1.84 FF=0.11md–2.46 FF=0.07md–3.09

Environmental Geology (2002) 43:18–28

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The formation factor is dependent on the intricate geometry of the pore channel, and therefore describes the manner in which the particles are arranged, if soil particles are assumed insulators (Jackson and others 1978). The formation factor in an unsaturated condition was described by moisture density as shown in Fig. 10 and the regression analysis results for the formation factor are tabulated in Table 6. In Fig. 10, the formation factor did not show the unique curve for a given soil. The formation factor tends to increase as the leachate concentration of pore fluid increases (i.e. the electrical resistivity of pore fluid decreases). These results could be explained by the restraint in the chargecarrying ionic movement due to electro–chemical interactions, such as cation exchange reactions and chemisorption on aluminosilicates and oxides between soil particles and leachate constituents. However, it would be considered that completion time of these reactions might be usually significantly longer than that of laboratory experimental duration. To establish the effects of electro– chemical interactions on electrical resistivity or formation factor, more specific research is needed. The results showing the higher value of the formation factor in higher leachate concentration is more significant in silty sand than any other soil. The result comes from a higher

Fig. 10 Variation of formation factor–moisture density relationship due to leachate contamination in unsaturated sand

can be modified to Eq. (6) including the term of moisture density, and thus the formation factor can be derived by following Eq. (6): q FF ¼ b ¼ amm ð6Þ d qw where qb is the bulk resistivity of soil mass and a is constant. 26

Environmental Geology (2002) 43:18–28

Fig. 11 Variations of constant a and cementation factor m in Eq. (5) with electrical resistivity of pore fluid

Original article

3. The adding of landfill leachate having various ions content of fine grain soils with the capacity of electro– results in decreasing the electrical resistivity. The chemical binding in silty sand. measured resistivity was significantly changed by To investigate the effects of the electrical resistivity of pore adding only 5% proportion of leachate. It is concluded fluid on the constant a and the cementation factor m of that the resistivity measurement method using RCPT Eq. (6), the relationships between the constants a and m can be used for detecting leachate in an unsaturated values and the electrical resistivity of pore fluid are plotted subsurface. in Fig. 11. This shows that the electrical resistivity of pore 4. It is known that the formation factor in unsaturated fluid affects the constant a and cementation factor m of sand which is dependent on leachate concentration can Eq. (6). In Fig. 11a, the variation of a value in sandy soil be described in terms of moisture density. The formaand weathered granite soil shows similar tendency except tion factor is also dependent on leachate concentration. in the case of leachate at 10%, but silty sand shows a quite The formation factor was higher when soil and pore different tendency from the other soils. However, a specific water are contaminated by high concentration of leactendency cannot be described as shown in Fig. 11a. In hate than when soil and pore water are uncontamiFig. 11b, the cementation factor m tends to slightly denated. This could be explained by the restraint in the crease as the electrical resistivity of pore fluid increases for charge-carrying ionic movement due to electro–chemall tested soils. However, the variations of cementation ical interactions between soil and leachate constituents. factor m values were not significant in both sandy soil and To establish the quantitative relationship between forweathered granite soil, but were remarkable in silty sand. mation factor and leachate concentration, more work The decrease of cementation factor means that the elecunder field conditions is required. trical resistivity of the unsaturated soil at low electrical resistivity of pore fluid is less sensitive to moisture density 5. Further research will be fruitful on the resistivity and the formation factor variations of soil depending on the than at higher electrical resistivity of pore fluid. These interaction between soil particles and various individfacts suggest that low electrical resistivity pore fluids reual contaminants. duce the effect of moisture density on electrical resistivity of bulk soil because the influence of the conductive fluid is dominant in resistivity or conductivity measurements (Campanella and Weemees 1990). Although the electrical resistivity of pore fluid is known to affect constants a and m, more experimental data to derive the quantitative References equations are needed.

Conclusions The following conclusions are drawn from the analysis of electrical resistivity variation for unsaturated soils based on experimental laboratory works: 1. For three different sands tested, a unique relationship exists between the electrical resistivity of soils and moisture density, although individual soils have different resistivity values. The electrical resistivity of soils exponentially decreases as moisture density increases. Moisture density is the most effective indicator for describing the relationship between electrical resistivity and unsaturated subsurface condition. 2. Silty sand has relatively lower resistivity values than sandy soil or weathered granite soil. The electrical resistivity of silty sand was in the range of 27 to 111 Wm, while the electrical resistivity of sandy soil varied between 87 and 459 Wm and the values for weathered granite soil were between 45 and 371 Wm. This can be explained by surface conduction through 7.5% fine particles in silty sand. Soils with more fines contain a higher percentage of conductive particles. In contrast, electrical conductions in sandy soil and weathered granite soil occur primarily in liquid contained in the pores.

Abu-Hassanein ZS, Benson CH, Blotz LR (1996) Electrical resistivity of compacted clays. J Geotech Eng 122(5):397–406 Archie GE (1942) The electrical resistivity log as an aid in determining some reservoir characteristics. Trans AIME 146:54–62 ASTM (1991) Standard test methods for electrical conductivity and resistivity of water. D 1125–91. American Society for Testing and Materials, Philadelphia, Pennsylvania Atkins ER, Smith GH (1961) The significance of particle shape in formation factor–porosity relationship. J Petrol Technol 13:285– 291 Campanella RG, Weemees I (1990) Development and use of an electrical resistivity cone for groundwater contamination studies. Can Geotech J 27:557–567 Curtis J, Narayanan R (1998) Effects of laboratory procedures on soil electrical property measurements. IEEE Trans Instrument Meas 47(6):1474–1480 Jackson PD (1975) An electrical resistivity method for evaluating the in-situ porosity of clean marine sands. Mar Geotechnol 1(2):91–115 Jackson PD, Taylor Smith D, Stanford PN (1978) Resistivity– porosity–particle shape relationships for marine sands. Geophysics 43(6):1250–1268 Kalinski RJ, Kelly WE (1994) Electrical-resistivity measurements for evaluating compacted-soil liners. J Geotech Eng 120(2):451– 457 Kaya A, Fang HY (1997) Identification of contaminated soils by dielectric constant and electrical conductivity. J Environ Eng 123(2):169–177 Kearey P, Brooks M (1991) An introduction to geophysical exploration, 2nd edn. Blackwell, Oxford, 176 pp Keller GK, Frischknecht FC (1966) Electrical methods in geophysical prospecting. Pergamon Press, London

Environmental Geology (2002) 43:18–28

27

Original article

Kokan MJ (1990) Evaluation of resistivity cone penetrometer in studying groundwater quality. BA Sc Thesis, Department of Civil Engineering, University of British Columbia, Vancouver Kwader T (1985) Estimating aquifer permeability from formation-resistivity factors. Groundwater 23(6):762–766 McCarter WJ (1984) The electrical resistivity characteristics of compacted clays. Geotechnique 34(2):263–267 McNeill J (1990) Use of electromagnetic methods for groundwater studies. Geotech Environ Geophys 1:191–218 Mitchell J (1993) Fundamentals of soil behavior, 2nd edn. Wiley, New York, 249 pp Oh MH, Park JB (2000) Laboratory tests for the development of the contaminant leakage detection system in soil. In: Proc GeoEng 2000, Int Conf on Geotechnical and Geological Engineering, Melbourne, Australia, Publ 2, p 286 Okoye CN, Cotton TR, O’Meara D (1995) Application of resistivity cone penetration testing for qualitative delineation of creosote contamination in saturated soils. In: Proc Conf Geoenvironment 2000, Geotechnical Spec Publ 46, ASCE, New York, pp 151–166 Piccoli S, Benoit J (1995) Geo-environmental testing using the envirocone. In: Proc Conf Geoenvironment 2000, Geotechnical Spec Publ 46, ASCE, New York, pp 93–104 Rhodes J, Raats P, Prather R (1976) Effect of liquid-phase electrical conductivity, water content and surface conductivity on bulk soil electrical conductivity. Soil Sci Soc Am J 40:651–655 Robinson ES, Coruh C (1988) Basic exploration geophysics. Wiley, New York, 446 pp Saarenketo T (1998) Electrical properties of water in clay and silty soils. J Appl Geophys 40:73–88

28

Environmental Geology (2002) 43:18–28

Sadek M (1993) A comparative study of the electrical and hydraulic conductivities of compacted clays. PhD Diss, Department of Civil Engineering, University of California at Berkeley, Berkeley, California Sawyer CN, McCarty PL, Parkin GF (1994) Chemistry for environmental engineering, 4th edn. McGraw-Hill, New York, 75 pp Selig ET, Mansukhani S (1975) Relationship of soil moisture to the dielectric property. J Geotech Eng Div 101(GT8):755–770 Shang JQ, Lo KY, Inculet II (1995) Polarization and conduction of clay–water–electrolyte systems. J Geotech Eng 121(3):243–248 Shinn JD, Timian DA, Morey RM, Mitchell G, Antle CL, Hull R (1998) Development of a CPT deployed probe for in situ measurement of volumetric soil moisture content and electrical resistivity. Field Anal Chem Technol 2(2):103–109 Strutynsky AI, Sandiford RE, Cavalier D (1991) Use of piezometric cone penetration testing with electrical conductivity measurements (CPTU-EC) for detection of hydrocarbon contamination in saturated granular soils. Current practices in groundwater and vadose zone investigations. American Society for Testing and Materials, Philadelphia, Pennsylvania, pp 72–79 Urish D (1981) Electrical resistivity–hydraulic conductivity relationships in glacial outwash aquifers. Water Resour Res 17(5):1401–1408 Van Nostrand RG, Cook KL (1966) Interpretation of resistivity data. US Geological Survey, Professional Paper 499. USGS, Washington, DC Yoon GL, Park J (2001) Sensitivity of leachate and fine contents on electrical resistivity variation of sandy soils. J Hazard Materials 84(2–3):147–161