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VOLATILE ORGANIC COMPOUND (VOC) TRANSPORT THROUGH

COMPACTED CLAY

By Jae Y. Kim,1 Associate Member, ASCE, Tuncer B. Edil,2 Member, ASCEand Jae K. Park3

ABSTRACT: Movement of volatile organic compounds (VOCs) through compacted clay liners was investigatedusing laboratory-scale column and tank tests. Hydraulic conductivity of the compacted clay was not significantlyimpacted by the introduction of VOCs in concentrations up to 20 mg/L. Soil-water partition coefficients of theseven VOCs tested had a strong logarithmic relationship with the octanol-water partition coefficient. Partitioncoefficients from batch tests were in good agreement with those measured directly on soil samples at thetermination of the column/tank tests. The VOCs were degraded in the clay, with estimated half-lives rangingfrom 2 to 116 days. Mechanical dispersion was not significant in the range of the hydraulic conductivities ofthe test specimens (i.e., <1027 cm/s). Effective molecular diffusion coefficients were mostly in 1026 cm2/s andgenerally decreased with increasing aqueous solubility. Mass transport parameters of VOCs in clay liners canbe estimated from laboratory batch tests and properly prepared small-scale column tests. However, accountingfor degradation of VOCs and minimizing the number of transport parameters that are simultaneously estimatedfrom a single response-time record are important considerations for accurate determination of transport param-eters.

INTRODUCTION

Leachates from municipal solid waste (MSW) and hazard-ous waste landfills contain a wide range of volatile organiccompounds (VOCs) (Friedman 1988; Gibbons et al. 1992;Krug and Ham 1995). VOCs have been shown to migrate andcontaminate the surrounding environment and impair the useof ground water (Plumb and Pitchford 1985; Nelson and Book1986; Friedman 1988; Battista and Connelly 1994; Rugge etal. 1995). Therefore, the effectiveness of modern landfill linersystems to minimize migration of VOCs is of concern.

Most modern landfills employ a composite liner consistingof a geomembrane overlying a compacted clay liner or a geo-synthetic clay liner. The geomembrane is often believed to bethe primary barrier to contaminant transport. However, forVOCs, the clay component usually controls the rate of trans-port, since VOCs diffuse through geomembrane at appreciablerates (Park and Nibras 1993; Sakti 1993). For example, a re-cent study by Foose (1997) has shown analytically that break-through and mass flux of VOCs from composite liners underrealistic field conditions depends on the characteristics (e.g.,thickness, sorptive capacity) of the clay liner component.Therefore, transport of VOCs through clay liners merits scru-tiny.

To evaluate VOC transport through compacted clay liners,mass transport parameters such as retardation factor, hydro-dynamic dispersion coefficient, degradation rate, and seepagevelocity must be known. Limited laboratory tests have beenconducted to characterize and quantify the mass transport pa-rameters of VOCs in natural clay deposits (Barone et al. 1992;Myrand et al. 1992). Previous laboratory tests have been per-formed using small-scale columns with specimen lengths of25 mm (Barone et al.) to 100 mm (Myrand et al.). These tests

1Asst. Prof., Div. of Civ., Urban, and Geosys. Engrg., Seoul Nat. Univ.,151-742, Seoul, Korea.

2Prof., Dept. of Civ. and Envir. Engrg., Univ. of Wisconsin-Madison,Madison, WI 53706 (corresponding author).

3Assoc. Prof., Dept. of Civ. and Envir. Engrg., Univ. of Wisconsin-Madison, Madison, WI.

Note. Discussion open until July 1, 2001. To extend the closing dateone month, a written request must be filed with the ASCE Manager ofJournals. The manuscript for this paper was submitted for review andpossible publication on July 6, 1999. This paper is part of the Journalof Geotechnical and Geoenvironmental Engineering, Vol. 127, No. 2,February, 2001. qASCE, ISSN 1090-0241/01/0002-0126–0134/$8.00 1$.50 per page. Paper No. 21335.

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were short-term, covering a period of less than a month. Thesesmall-scale column tests may not represent the conditions inthe field and preclude an understanding of the long-term be-havior of VOCs in clay liners. In this study, a series of small-scale column tests and large-scale tank tests were conductedto estimate the mass transport parameters of VOCs under hy-draulic gradients like those occurring in the field and in long-term conditions.

This paper is the last paper in a series of papers publishedwith the objective of defining the transport parameters for typ-ical VOCs through compacted clay liners (Edil et al. 1994,1995; Kim et al. 1997a). As such, the paper updates earlierinformation regarding partitioning of VOCs onto clay andpresents estimates of degradation rates and hydrodynamic dis-persion coefficients for VOCs in compacted clay.

MATERIALS

Liner Soil

The soil used in this study is Kirby Lake till, obtained fromthe Outagamie County Landfill in Appleton, WI, where it isused as the landfill liner material. Kirby Lake till typicallyclassifies as low plasticity clay (CL), according to the UnifiedSoil Classification System. The Kirby Lake till sample used inthe experiments had a specific gravity of solids of 2.7, a liquidlimit of 33%, and a plasticity index of 17. The standard Proctormaximum dry unit weight (ASTM D-698) was 17.76 kN/m3

at an optimum moisture content of 15.5% (Heim 1992). Sieveand hydrometer tests (ASTM D-422) indicated D10, D30, andD60 were approximately 0.0005, 0.002, and 0.011 mm, respec-tively (Wambold 1993). Its fraction passing the No. 200 sievewas 83% and the fraction less than 5 mm was 47%. It had 494mg/kg of exchangeable cations (i.e., Ca12, Mg=2, K1, andNa1), and 1.06% organic carbon mass fraction.

Volatile Organic Compounds (VOCs)

Chloroform (CF), ethylbenzene (EB), methylene chloride(MC), toluene (TOL), 1,1,1-trichloroethane (1,1,1-TCA), tri-chloroethylene (TCE), and m-xylene (m-XYL) were selectedfor testing based on the availability of laboratory analyticalmethods, relatively high detection frequency in contaminatedenvironments (e.g., landfill leachate), as well as their reason-ably broad range of solubilities and molecular weights. Prop-erties of these VOCs are listed in Table 1.

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TABLE 1. Properties of Volatile Organic Compounds (VOCs) Tested

VOCs(1)

Molecularformula

(2)

Molecularweight a

(g/mol)(3)

Specificgravity a

(4)

Aqueoussolubility b

(mg/L)(5)

Octanol-waterpartition coefficient,

log Kowb,c

(6)

Free solutiondiffusion coefficient, Do

e

(cm2/s)(7)

Chloroform CHCl3 119.38 1.4832 9,300d 1.97 9.37 3 1026

Ethylbenzene C8H10 106.17 0.8670 152 3.15 8.07 3 1026

MC CH2Cl2 84.93 1.3266 16,700d 1.25 10.97 3 1026

Toluene C7H8 92.14 0.8669 515 2.69 8.47 3 1026

1,1,1-TCA C2H3Cl3 133.40 1.3390 4,400 2.47 8.11 3 1026

TCE C2HCl3 131.39 1.4642 1,100d 2.53 8.61 3 1026

m-Xylene C8H10 106.17 0.8642 200 3.20 6.85 3 1026

aLide and Frederikse (1995).bKow in L/kg.cU.S. EPA RREL treatability database (1990).dAt 257C.eYaws (1995).

METHODS

Batch Isotherm Tests

Batch isotherm tests were conducted to estimate the soil-water partition coefficients for the VOCs. Soil was crushedand ground past the No. 4 sieve (opening = 4.75 mm) to re-move gravel-size particles and stored in a desiccator at roomtemperature before conducting the batch isotherm tests. Thedry soil (2–30 g) was weighed and put in glass tubes (43 or25 mL with Teflon-coated septa) with deionized water for sat-uration. After one day, the glass tubes containing soil saturatedwith deionized water were filled with a solution containing theVOC. The volumes of the deionized water and the VOC so-lution in the tubes were estimated from their respectiveweights and densities. The solid:solution ratio was between0.02 and 0.25.

Solution concentrations were measured initially and peri-odically thereafter using a gas chromatograph. Measured so-lution concentrations stabilized in seven days, indicating thatequilibrium was reached. Tubes were tumbled end-over-endusing a Millipore Rotary Agitator during the experiments. Toprevent bacterial growth, sodium azide was added during eachtest at a concentration of 0.05% (by weight). The headspacewithin the test tubes was kept as small as possible to minimizevolatilization of VOCs. All batch tests were conducted at 207C.The losses are estimated to be less than 1% in the batch tests(Kim 1996).

Column/Tank Tests

The experimental apparatus and procedures used for the col-umn and tank tests were developed specifically to test VOCsand overcome numerous difficulties of testing with such chem-icals. They have been previously described in detail (Edil etal. 1994, 1995; Kim et al. 1997a); however, a brief backgroundis provided herein. Six small-scale column tests and four large-scale tank tests were conducted. Three tanks, T2A, T4, andT6, contained 10, 10, and 2 kg of tire chips on the top of thecompacted clay, respectively. Tire rubber is known to sorbsignificant amount of VOCs (Park et al. 1996, 1997; Kim etal. 1997b). The tire chips, weighing 2 and 10 kg, occupiedapproximately 1 and 5%, respectively, of the influent reservoirvolumes (Fig. 1). T2A also contained a perforated plate of 220g of Styrofoam, which had the same volume as 10 kg of tirechips. The other column and tank tests consisted of only soilspecimens.

The soil was crushed and ground past the No. 4 sieve (open-ing = 4.75 mm), mixed with the appropriate amount of Mad-ison tap water to achieve the optimum or slightly wet of theoptimum moisture content, and allowed to hydrate for 24

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FIG. 1. Design of Tank Used in Tests

hours or more. After hydration, the soil was compacted in thecolumn cells or the tanks using a standardized procedure [seeEdil et al. (1994, 1995) for details]. After preparation, the com-pacted specimens were permeated with tap water (tank tests)or 0.005 N CaSO4 solution (column tests) until steady-stateflow was obtained. Characteristics of the column/tank speci-mens are listed in Table 2. After the hydraulic conductivitiesstabilized, liquid in the influent reservoir and the influent bagwere replaced with the VOC solution. For the tank tests, meth-ylene chloride, toluene, and trichloroethylene were introduced.For the column tests, all seven VOCs listed in Table 2 wereintroduced. Additionally, lithium bromide (LiBr) at a concen-tration of 20 mg/L was incorporated into the influent reservoiras a conservative tracer to determine seepage velocity (Kim etal. 1997a). The initial target concentration of each VOC was16 mg/L (tank tests) or 20 mg/L (column tests). These con-centrations were selected considering typical leachate concen-trations measured in municipal solid waste landfills (Friedman1988; Gibbons et al. 1992; Krug and Ham 1995). All testswere conducted in a 207C constant temperature room. The testdurations varied from 55 to 209 days in the column tests andfrom 719 to 926 days in the tank tests.

Schematics of the experimental apparatus are shown in Figs.1 and 2. The only important difference between the columnand the tank is the size. The influent and the effluent weresupplied and collected by means of reservoirs above and belowthe specimen, respectively. This means of collection permitsthe water to flow evenly across the entire soil layer. All partsused were made of stainless steel and brass. Teflon bags were

AND GEOENVIRONMENTAL ENGINEERING / FEBRUARY 2001 / 127

TABLE 2. Characteristics of Column/Tank Specimens

Test designation(1)

Thickness(cm)(2)

Water content(%)(3)

Dry unit weight(kN/m3)

(4)Total porosity

(5)

Effectiveporosity a

(6)

VURb

(cm3)(7)

VLRc

(cm3)(8)

(a) Column

C1 12.3 16.4 18.10 0.32 0.29 1,226 152C2 12.1 16.4 18.05 0.32 N.A.d 1,262 152C3 12.1 16.1 17.77 0.33 0.33 1,266 152C4 11.5 15.6 18.35 0.31 N.A. 1,368 149C5 11.7 15.7 18.32 0.31 0.32 1,335 149C6 11.9 15.3 18.47 0.30 0.30 1,304 149

(b) Tank

T2A 31.8 18.7 16.60 0.37 0.35 153,000 7,300T4 30.5 18.0 17.40 0.34 0.32 162,000 7,300T5 31.0 18.3 17.06 0.36 0.35 170,000 7,300T6 31.2 18.5 16.93 0.36 0.32 168,000 7,300

aEstimated from bromide tracer tests (Kim et al. 1997a).bVLR = volume of lower (effluent) reservoir.cVUR = volume of upper (influent) reservoir.dData are not available.

FIG. 2. Design of Column Used in Tests

connected to the influent and effluent reservoirs to control headand rate of flow with minimal loss of VOCs to atmosphere.All tubing was made of Teflon [see Edil et al. (1994, 1995)for the details of the experimental apparatus].

During the testing period, concentrations of VOCs in theinfluent reservoir and effluent reservoir were monitored peri-odically. When the tank tests were terminated, the soil wascored and sectioned to obtain the concentration-depth profile,which was obtained by analyzing the pore water of each sec-tion. Steel Shelby tubes with a diameter of 68.5 mm were usedfor obtaining specimens from the tanks. The tube specimenswere sectional into 1–2 cm thick slices at the desired depths.Sectioned samples wrapped in Teflon tape and aluminum foilwere stored in a 47C temperature-controlled room until porewater was extracted. A temperature-controlled centrifuge using40 mL Teflon tubes was used to obtain pore water from thesections. The soil samples were centrifuged at 15,000 rpm for20 min at 47C (Kim 1996).

VOC Analytical Method

Gas chromatographic analysis was performed on a Varian3600 gas chromatograph with a 60 3 0.25 mm I.D. Supel-

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cowax-10 megabore column and a flame ionization detector(FID). An aliquot of 1.0 mL liquid sample was directly injectedinto the column using a gas-tight microsyringe or an autosam-pler. Helium was used as the carrier gas. The column temper-ature was programmed to initially hold at 507C for fiveminutes, then to rise to 857C at a rate of 57C/min, to rise againimmediately to 2307C at a rate of 407C/min, and finally tohold for two minutes for the cleaning of any residuals withinthe gas chromatograph column. A Varian 1093 SeptumEquipped Programmable Injector (SPI) was programmed toinitially hold at 507C for 0.5 min and immediately rise to2307C at a rate of 2257C/min, then hold for five minutes. Cal-ibration curves were produced using standards at 0, 10, 20,30, 40, and 50 mg/L. Detection limits were considered to beapproximately 0.5 mg/L for chloroform, methylene chloride,1,1,1-trichloroethane, and trichloroethylene, and 0.1 mg/Lfor ethyl benzene, toluene, and m-xylene, respectively (Kim1996).

MATHEMATICAL MODELING

In a one-dimensional column test, mass transport of a non-decaying solute through a porous medium can be expressed asfollows using the retardation factor, Rf (Hashimoto et al. 1964;Freeze and Cherry 1979):

2­C(z, t) D ­ C(z, t) v ­C(z, t)h z= 2 (1)2­t R ­z R ­zf f

r Kd pR = 1 1 (2)f S Dn

where C(z, t) = aqueous concentration of a nondecaying solute(M/L3); z = distance along the direction of mass transport (L);t = elapsed time (T); Dh = hydrodynamic dispersion coefficient(L2/T); Rf = retardation factor; vz = seepage velocity (L/T); n= total porosity of the porous medium; rd = dry density of thesoil (M/L3); and Kp = soil-water partition coefficient (L3/M).

Organic compounds can degrade during the transportthrough the porous medium. If the degradation can be assumedto be a first-order reaction, the mass transport equation can bewritten as follows:

2­C(z, t) D ­ C(z, t) v ­C(z, t) kh z d= 2 2 C(z, t) (3)2­t R ­z R ­z Rf f f

where kd = first-order degradation constant (1/T).In the one-dimensional flow system, the hydrodynamic dis-

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persion coefficient is represented mathematically as follows(Freeze and Cherry 1979):

D = D* 1 D (4)h L

D = a v (5)L L z

where D* = effective molecular diffusion coefficient (L2/T);DL = longitudinal mechanical dispersion coefficient (L2/T); andaL = longitudinal dispersivity (L).

The boundary and initial conditions of the column/tank testsin this study can be expressed as follows (Kim 1996):

t tQ nAD dCh

C(0, t) = C (t) = {C (t) 2 C (t)} dt 1 dtUR IN URE EV V dzUR UR z=00 0

(6)

t tQ nAD dCh

C (t) = {C(L, t) 2 C (t)} dt 2 dt (7)LR LRE EV V dzLR LR z=L0 0

C(z, 0) = 0 (8)

where CUR(t) = concentration of the solute in the upper (influ-ent) reservoir (M/L3); Q = flow rate (L3/T); VUR = volume ofthe upper (influent) reservoir (L3); CIN(t) = influent concentra-tion of the solute from the feeding bag (M/L3); A = cross-sectional area of the specimen (L2); L = length of the porousmedium (L); CLR(t) = concentration of the solute in the lower(effluent) reservoir (M/L3); and VLR = volume of the lower(effluent) reservoir (L3).

Subject to (6), (7), and (8), solutions for (1) and (3) wereobtained numerically by the implicit finite-difference scheme(Kim 1996). Solution of the tridiagonal matrix in the finite-difference scheme was calculated using the Gauss eliminationapproach (Conte and Boor 1980).

The concentration in the influent bag, CIN , could not bemaintained constant over the testing period due to loss ofVOCs in the influent bag. The concentration in the influentbag was monitored and the average value was used as CIN.Average concentrations of VOCs in the influent bag rangedfrom 13 to 15 mg/L, depending on the compound. Deviationsfrom the average VOC concentration in the influent bag wereless than 2 mg/L, and the volume of the influent bag is muchsmaller compared with the volume of the influent reservoir forthis deviation to impact the influent reservoir concentrationsignificantly.

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FIG. 3. Hydraulic Conductivities before and after VOC Spike(Tank T2A)

RESULTS

Hydraulic Conductivity of CompactedClay Specimens

At high concentrations, organic compounds can affect thehydraulic conductivity of clay (Anderson et al. 1985; Fernan-dez and Quigley 1988). To evaluate the effect of the VOCs onthe hydraulic conductivities of the test specimens, the hydrau-lic conductivity was monitored before and after spiking theinfluent with VOCs. Fig. 3 shows the hydraulic conductivitydata of tank T2A as an example. Hydraulic conductivities ofbefore and after the spiking with VOCs are summarized inTable 3. Hydraulic conductivity data after the spike of VOCswere not available for columns C2 and C4, because these testswere pure diffusion tests (i.e., no advective transport).

As shown in Table 3, the ratio of the average hydraulicconductivity before and after the spike of VOCs ranged from0.8 (T4) to 1.2 (T2A). Thus, the VOCs do not appear to affectthe hydraulic conductivity of the column and tank specimens.

Soil-Water Partition Coefficients

The soil-water partition coefficient (L/kg) was calculated asthe ratio of the solid-phase concentration (mg/kg) to the so-lution-phase concentration (mg/L). Soil-water partition coef-ficients from the single- and multisolute batch isotherm tests

TABLE 3. Hydraulic Conductivity Data for Test Columns and Tanks

Column/tank(1)

Hydraulicgradient

(2)

Elapsed Time(days)

Beforea

(3)Afterb

(4)

Hydraulic Conductivity(31028 cm/s)

Before(Kh ,before)

(5)

After(Kh ,after)

(6)(Kh ,after/Kh ,before)

(7)

C1 9.1 418 294 2.5 6 0.1 2.9 6 0.3 1.2C2 0 272 440 2.0 6 0.2c N.A.d N.A.C3 9.3 418 294 3.3 6 0.1 3.9 6 0.4 1.2C4 0 272 440 1.6 6 0.1c N.A. N.A.C5 3.3 258 454 2.2 6 0.1 2.6 6 0.3 1.2C6 3.2 258 454 2.4 6 0.1 2.9 6 0.3 1.2T2A 2.9 113 926 1.1 6 0.1 1.3 6 0.7 1.2T4 3.0 124 823 1.7 6 0.2 1.4 6 0.5 0.8T5 2.9 96 792 1.4 6 0.2 1.7 6 0.6 1.2T6 2.9 78 719 1.6 6 0.2 1.8 6 0.4 1.1

aBased on last 50 days of data before VOCs were spiked.bAfter VOCs were spiked.bMeasured under hydraulic gradient of 3 prior to spiking with VOCs.dData are not available.

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TABLE 4. Measured Soil-Water Partition Coefficients of VOCs

Soil-water partition coefficient(L/kg)

BATCH TESTSDirect Measurements in

Multisolute Tests Single-Solute Tests Column/Tank Tests

VOCs(1)

Average(2)

Standarddeviation

(3)

Number ofobservations

(4)Average

(5)

Standarddeviation

(6)

Number ofobservations

(7)Average

(8)

Standarddeviation

(9)

Number ofobservations

(10)

CF 0.39 0.15 11 —a — — — — —EB 1.22 0.71 20 — — — — — —MC 0.27 0.15 11 0.11 0.03 10 0.21 0.17 56TOL 0.80 0.48 14 1.02 0.53 8 1.86 1.23 771,1,1-TCA 0.55 0.34 11 — — — — — —TCE 0.94 0.59 13 0.97 0.20 10 1.39 0.74 108m-XYL 1.26 0.65 18 — — — — — —

aTests were not conducted.

are summarized in Table 4. For the single-solute condition,only three VOCs were tested.

In the tank tests, the solid and liquid phase concentrationsof VOCs were directly measured at the termination of the tanktests on tube samples. Edil et al. (1995) calculated the soil-water partition coefficients of methylene chloride, toluene, andtrichloroethylene from the column and tank tests of KirbyLake till using direct measurements. At the termination of thetests, tube samples were obtained and sectioned for analysis.Pore water samples were squeezed from the sections obtainedfrom the tube samples with a press and concentrations of or-ganic chemicals in the liquid phase were determined. A carbondisulfide (CS2) extraction technique was then used to extractthe organic chemicals partitioned on the soil solids of thesections. All the soil-partition coefficients calculated by Edilet al. (1995) were combined with the additional direct mea-surements made in this study. All of the data are compiled inTable 4.

The soil-water partition coefficients of the three VOCs fromthe direct measurements appear to be slightly greater thanthose from the batch tests. The presence of other organic com-pounds, as was the case in the direct measurements, may affectthe sorption of a given organic compound onto soil due to thepotential competition or change in activity of the organic com-pound. Thus, a paired t-test was conducted with a significancelevel of 0.05 to assess whether the partition coefficients in themultisolute and single-solute batch tests were significantly dif-ferent from each other. The hypothesis that the mean partitioncoefficient in the multisolute test is equal to that from thesingle-solute test was not rejected. That is, the mean partitioncoefficients are not statistically different from each otherwithin the concentration ranges that were used.

The soil-water partition coefficient, Kp, for an organic com-pound is often estimated using the organic compound/organiccarbon partition coefficient, Koc, of the organic compound(Briggs 1973, 1981; Karickhoff et al. 1979; Chiou et al. 1983;Piwoni and Banerjee 1989). For soils with varying organiccarbon content, foc, the soil-water partition coefficient for agiven VOC can be obtained from

K = f K (9)p oc oc

Koc was shown to be a reasonably constant quantity (68%)for a large number of soils with foc in the range of 0.1–6%(Kile et al. 1995); thus (9) can be used to estimate Kp. Toobtain Koc, the measured soil-water partition coefficients forKirby Lake till were divided by its organic carbon mass frac-tion, foc, of 1.06%. Fig. 4 shows the relationship between theKoc and the octanol-water partition coefficient, Kow, of theVOCs tested (in a range of log Kow from 1.25 to 3.25) whichis expressed as

130 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

FIG. 4. Relationship between Soil Organic Carbon–OrganicCompound Partition Coefficients (log Koc) and Octanol-WaterPartition Coefficients (log Kow) of VOCs Tested

2log K = 0.92 1 0.36 ? log K (R = 0.93) (10)oc ow

Since Kow of most organic compounds are readily available(USEPA RREL Database Program 1997), (9) and (10) providea very convenient basis for estimating Kp for soils with foc inthe range of 0.1–6%.

The soil-water partition coefficients determined from batchtests in this study agree with those from previous studies(Briggs 1973, 1981; Chiou et al. 1983; Piwoni and Banerjee1989), except those from Karickhoff et al. (1979). The soil-water partition coefficients estimated using Karickhoff’s em-pirical relationship, which is based on bed sediments, arehigher than those determined in this study. Depending on thetype of organic compound, the linear relationship between theKow and the Koc (and Kp) may be affected. Also, it was recentlyproposed that there is significant difference between the Koc ofbed sediments (river, lake, and marine) and normal soils (ter-restrial soils from the A horizon) (Kile et al. 1995). Thus, theobserved difference between Karickhoff’s relationship and theother studies conducted with normal soils can be explained.

Degradation of VOCs

Fig. 5 shows the concentration of VOCs in the influent res-ervoir of a pure diffusion column test (column C2). The upper(influent) reservoir concentrations (CUR) decreased as a func-tion of time even though the influent bag concentrations (CIN )were kept nearly constant. For most of the VOCs, the concen-tration decreased faster in the influent reservoir as the soil-water partition coefficient increased. However, the concentra-

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FIG. 5. Observed VOC Concentrations in Influent Reservoir ofColumn C2

tion of 1,1.1-trichloroethane and to an extent chloroform inthe influent reservoirs of all column tests decreased unreason-ably fast when compared with the other compounds.

Inorganic disinfectants—mercuric chloride and sodiumazide—were injected into reservoirs of column/tank tests andtheir feeding solutions to prevent biodegradation of VOCs.Several factors, however, suggested that VOC degradation wassignificant even though disinfectants were added to the influ-ent. First, gas generation was observed in the influent reser-voirs of the tank tests. Second, methylene chloride disappearedcompletely in two of the tank tests (T2A and T4). Furthermore,in the column tests, the influent reservoir concentrationchanges and the breakthrough curves did not fit well with themodel without consideration of degradation. Figs. 6 and 7show the observed upper (influent) reservoir concentrationchanges and effluent breakthrough data along with the pre-dicted curves with and without consideration of degradation.In the case of 1,1,1-trichloroethane in column C1, the hydro-dynamic dispersion coefficient based on the influent reservoirconcentrations should result in significant levels of break-through (Fig. 6). However, there has been no evidence ofbreakthrough for 294 days. All of the VOCs in this study ex-hibited these phenomena, although the degree varied. Thus,for the mass transport parameter estimation in column tests,the degradation term was introduced in the mathematicalmodel.

Degradation was found to follow a first-order reaction,which is reasonable, since most processes are first order, i.e.,concentration dependent (Howard et al. 1991). Degradationappeared to happen only within the clay specimen, not in thereservoirs. There was little biomass available in the reservoirsamples and biomass settled down on the surface of the soil.The average hydraulic conductivity after the spike of VOCs,the effective porosities as obtained from the bromide tracertests in the same columns (Kim et al. 1997a), and the soil-water partition coefficients obtained from the batch tests undera multisolute condition were used as inputs to the model. Thesoil-water partition coefficients obtained from the batch testswere used because they were available for all of the VOCstested and also had smaller standard deviations, reflectinggreater control of potential losses compared with the directmeasurements from the end-of-test tube samples of the clayspecimens. The hydrodynamic dispersion coefficient and thefirst-order degradation constant were estimated from the influ-ent and effluent reservoir concentration-time data using a leastsquares method. The estimated first-order degradation con-stants for the VOCs are listed in Table 5.

The estimated first-order degradation constants varied de-pending on the compound. The estimated half-lives ranged

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FIG. 6. Observed and Predicted Concentration Changes of1,1,1-Trichloroethane in Influent and Effluent Reservoirs of Col-umn C1

FIG. 7. Observed and Predicted Concentration Changes of m-Xylene in Influent and Effluent Reservoirs of Column C2

from about 2 to 116 days. Of the seven VOCs tested, 1,1,1-trichloroethane, m-xylene, trichloroethylene, ethylbenzene,chloroform, toluene, and methylene chloride degraded in de-scending order and roughly in order of decreasing molecularweight.

Analyses of gas samples collected from the influent reser-voirs in the tank tests showed that the gas contained N2, CH4,and CO2 at 58.3, 10.0, and 1.9% (by volume), respectively.Dissolved oxygen concentrations in the influent reservoirswere below 1 mg/L. Thus, the degradation of VOCs observedin tank tests seems to be an anaerobic biodegradation, whichis one of the most common degradation processes in soil-watersystems (Howard et al. 1991). The estimated first-order deg-radation half-lives given in Table 5 are generally comparableto the range reported for anaerobic degradation (Howard et al.1991) for most of the compounds, except for 1,1,1-TCA, andto a lesser extent for TCE and EB. No further investigationwas made regarding degradation by-products or the type ofmicroorganisms. It should be noted that the degradation ratevalues reported herein include potential loss, chemical/physi-cal decomposition, as well as biological degradation.

Hydrodynamic Dispersion Coefficients

The ratios of the estimated hydrodynamic dispersion coef-ficients to the free solution diffusion coefficients (see Table 1)with and without consideration of degradation are listed inTable 6. The hydrodynamic dispersion coefficients estimatedwithout considering degradation are greater than those withdegradation, which is expected. Because of the strong evi-dence of degradation in the tests, the hydrodynamic dispersion

L AND GEOENVIRONMENTAL ENGINEERING / FEBRUARY 2001 / 131

TABLE 5. Estimates of First-Order Degradation Constants

Columns(1)

CF(2)

EB(3)

MC(4)

TOL(5)

1,1,1-TCA(6)

TCE(7)

m-XYL(8)

(a) First-Order Degradation Constant (1/Day)

C1 0.019 0.043 0.006 0.012 0.530 0.059 0.055C2 0.020 0.015 0.016 0.010 0.071 0.021 0.024C3 0.016 0.071 0.019 0.025 0.290 0.078 0.081C4 0.027 0.031 0.034 0.017 0.100 0.016 0.037C5 0.035 0.035 0.020 0.030 0.075 0.050 0.040C6 0.061 0.033 0.030 0.024 0.400 0.034 0.035Average 0.030 0.038 0.021 0.019 0.244 0.043 0.045Standard deviation 0.017 0.019 0.010 0.008 0.194 0.024 0.020

(b) First-Order Degradation Half-Life (Days)

C1 36.5 16.1 115.5 57.8 1.3 11.7 12.6C2 34.7 46.2 43.3 69.3 9.8 33.0 28.9C3 43.3 9.8 36.5 27.7 2.4 8.9 8.6C4 25.7 22.4 20.4 40.8 6.9 43.3 18.7C5 19.8 19.8 34.7 23.1 9.2 13.9 17.3C6 11.4 21.0 23.1 28.9 1.7 20.4 19.8Average 28.5 22.5 45.6 43.2 5.2 21.9 17.7Standard deviation 11.8 12.4 35.3 21.1 3.9 13.6 6.9

Anaerobic degradationhalf-life (days)a 7–28 176–228 28–112 56–210 560–1,092 98–1,647 28–112aHoward et al. (1991).

TABLE 6. Estimates of Hydrodynamic Dispersion Coeffi-cients

Columns(1)

CF(2)

EB(3)

MC(4)

TOL(5)

1,1,1-TCA(6)

TCE(7)

m-XYL(8)

(a) Dh/Do Estimated without Consideration of Degradation

C1 1.75 2.02 0.68 1.33 7.80 1.26 2.45C2 1.77 0.83 0.25 0.61 4.40 0.69 0.80C3 1.30 1.57 0.37 1.03 5.50 0.88 1.90C4 2.50 0.70 0.34 0.58 7.30 0.60 0.68C5 1.90 1.77 0.55 1.11 7.40 1.24 1.87C6 3.00 1.42 0.48 1.10 9.10 1.26 1.66

(b) Dh/Do Estimated with Consideration of Degradation

C1 1.16 1.33 0.55 1.10 1.16 0.65 1.51C2 1.19 0.67 0.11 0.48 2.92 0.46 0.57C3 0.88 0.84 0.20 0.71 0.95 0.39 0.97C4 1.78 0.45 0.10 0.39 4.70 0.43 0.41C5 0.99 1.19 0.23 0.62 5.00 0.61 1.19C6 1.70 0.96 0.16 0.68 3.30 0.74 1.09

coefficients estimated by considering degradation were ac-cepted and analyzed. The hydrodynamic dispersion coeffi-cients were in the range of 1026 to 1025 cm2/s except for 1,1,1-TCA and CF, which were generally larger than 1025 cm2/s.

The effective molecular diffusion coefficient and longitu-dinal dispersivity can be estimated using the hydrodynamicdispersion coefficient determined in the column/tank tests andthe seepage velocity using (4) and (5) (i.e., the intercept andslope of the hydrodynamic dispersion coefficient versus seep-age velocity, respectively, for a given VOC). The results arelisted in Table 7. The longitudinal dispersivities of VOCs es-timated in this study ranged approximately from 2 to 4 cmexcept 1,1,1-trichloroethane. The magnitude of estimated lon-gitudinal dispersivities of VOCs implies that the seepage ve-locity in the testing range does not significantly affect the hy-drodynamic dispersion coefficients of VOCs; in other words,mechanical dispersion was not significant in the tests and themolecular diffusion governed.

The correlation of the effective molecular diffusion coeffi-cients with readily available physical parameters of the VOCssuch as average molecular diameter and aqueous solubility wasinvestigated. A correlation with molecular diameter was re-ported for pure Fickian diffusion of organic solutes in plasticpipe materials (Berens 1985; Park et al. 1991). However, no

132 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENG

TABLE 7. Average Effective Molecular Diffusion Coefficients,Apparent Tortuosity, and Longitudinal Dispersion Coefficients

VOCs(1)

Moleculardiffusion

coefficient (D *)(cm2/s)

(2)

Apparenttortuosity

(ta)(3)

Longitudinaldispersion

coefficient (aL)(cm)(4)

Chloroform 8.60 3 1026 0.92 2.55Ethylbenzene 6.02 3 1026 0.75 3.05Methylene chloride 1.38 3 1026 0.13 2.55Toluene 4.13 3 1026 0.49 3.501,1,1-Trichloroethane 2.37 3 1025 2.92 9.97Trichloroethylene 4.39 3 1026 0.51 1.99m-Xylene 4.86 3 1026 0.71 3.99

FIG. 8. Relationship between Average Effective Molecular Dif-fusion Coefficients and Aqueous Solubilities of VOCs Tested

such correlation was found for this data, perhaps reflecting thatsoil pores, unlike plastic pipe pores, are large enough for mo-lecular diameter not to be a significant factor. There seemed,however, to be a general inverse relationship between the av-erage molecular diffusion coefficient and the aqueous solubil-ity of the VOCs, as shown in Fig. 8. This makes sense, sincediffusion depends on chemical potential, which in turn de-

INEERING / FEBRUARY 2001

pends on the activity coefficient of the organic compound inwater at saturation, i.e., aqueous solubility (Schwarzenbach etal. 1993). Fig. 8 shows some grouping of organic compoundsthat is probably related to their structure. EB, m-XYL, TOL,and TCE have p binding relative to the other compounds.Solution characteristics of 1,1,1-TCA and CF may be differentthan the others and similar to each other due to the identicaltrichloromethyl group they have in their structure.

Apparent tortuosity, ta (Shackelford and Daniel 1991) is theratio of the effective molecular diffusion coefficient, D*, tothe free solution diffusion coefficient, Do. The apparent tor-tuosity is primarily a characteristic of the porous medium. Theapparent tortuosities estimated in this study are given in Table7. Unreasonably high apparent tortuosities for 1,1,1-TCA andCF indicate that the diffusion estimates may not be reliablefor these two compounds.

DISCUSSION

As previously mentioned, the number of key parameters inthe advective-dispersive mass transport is four, i.e., the hydro-dynamic dispersion coefficient, the retardation factor (or par-tition coefficient), the seepage velocity (or effective porosity),and the degradation constant. It is not easy to estimate thesefour parameters all simultaneously from the concentration-timecurves obtained from a column test, and this traditional ap-proach makes these parameters codependent. Thus, in thisstudy the maximum number of unknowns in the curve fittingfor the parameter estimation is reduced to two. The hydrody-namic dispersion coefficient and the degradation constant werethe unknowns, because the seepage velocity and retardationfactor were independently estimated from bromide tracer testand batch isotherm tests, respectively.

Consideration of the degradation of VOCs in analyzing thecolumn test data seems important, and ignoring it may resultin hydrodynamic dispersion/diffusion coefficients that are toolow or partitions coefficients that are too high, especially ifthe test duration is long. The previous studies of VOC trans-port did not consider any degradation of organic compoundsin column tests (Barone et al. 1992; Myrand et al. 1992). Bar-one et al. (1992) estimated the partition and diffusion coeffi-cients of organic compounds by the curve fitting techniqueusing the influent reservoir concentration-time data in pure dif-fusion tests. They did not consider any degradation, and theestimated values of the partition coefficients were 2–4 timesgreater than those from batch tests. Thus, there is some un-certainty with regard to the parameters reported from columntests. The approach adopted herein is aimed at reducing thisuncertainty by considering degradation and determining asmany parameters as possible by independent methods otherthan using the column/tank test data.

In this study, the partition coefficients estimated from thebatch isotherm tests were used for estimation of the degrada-tion constant and hydrodynamic dispersion coefficient. Eventhough statistically there was no significant difference betweenthe partition coefficients from the batch isotherm tests and thedirect measurements made on the end-of-test tube samples ofthe clay specimens of the column/tank tests, the degradationconstant and hydrodynamic dispersion coefficient were alsoestimated using the partition coefficient from the direct mea-surements. This showed that parameter estimation can be sig-nificantly affected by the value of partition coefficient used. Ifthere are enough effluent breakthrough data, the values of par-tition coefficient used in parameter estimation can be evaluatedindependently.

CONCLUSIONS

Based on a laboratory investigation of VOC transport in acompacted clay liner material using long-term tests in small-

JOURNAL OF GEOTECHNICA

scale columns and large-scale tanks, the following conclusionscan be drawn:

1. The hydraulic conductivities of column and tank speci-mens were not significantly affected by the introductionof VOCs within the range of concentrations used in thetests, i.e., up to 20 mg/L. This range is typical for VOCsin municipal solid waste landfill leachate.

2. The soil-water partition coefficients of VOCs were notsignificantly affected by the presence of other VOCs inthe range of concentrations tested. The soil-water parti-tion coefficients from batch tests were not statisticallydifferent from those determined by direct measurementon the end-of-test soil specimens in the column/tanktests. The soil-water partition coefficient of a VOC canbe predicted using the available octanol-water partitioncoefficient of the VOC and organic carbon mass fractionof the soil (up to 6% organic carbon mass fraction). Thisprovides a convenient means of estimating the soil-waterpartition coefficient of a VOC, since octanol-water par-tition coefficients of VOCs are readily available.

3. The VOCs degraded during the testing period, and thehalf-lives ranged from 2 to 116 days. Of the seven VOCstested, 1,1,1-trichloroethane had the highest first-orderdegradation constant, followed by m-xylene, trichloro-ethylene, ethylbenzene, chloroform, toluene, and meth-ylene chloride in descending order.

4. Recognition of degradation of VOCs during laboratorytests is important. Realistic tests require relatively lowhydraulic gradients and long testing periods. IgnoringVOC degradation may result in estimating lower disper-sion coefficients and higher partition coefficients forVOCs.

5. The effective molecular diffusion coefficients and appar-ent tortuosities of the VOCs tested generally decreasedwith the aqueous solubility; however, the values seemedto be influenced by the structure of each compound.Within the testing conditions of this study, any significanteffect of mechanical dispersion on the hydrodynamic dis-persion, as would be expected, was not shown.

6. Insofar as possible, use of independent means of mea-suring transport parameters, e.g., determining partitioncoefficient from batch isotherm tests and seepage veloc-ity or effective porosity from conservative tracer trans-port tests, is important, for obtaining unique values ofhydrodynamic dispersion coefficients from the columntests.

ACKNOWLEDGMENT

This research was funded by the University of Wisconsin SystemGroundwater Research Advisory Council and the Wisconsin Departmentof Natural Resources Waste Tire Management or Recovery Grant Pro-gram. Wayne S. Wambold and David P. Heim contributed to the devel-opment of column and tank tests procedures. The writers thank Dr. CraigH. Benson for his review of the manuscript and valuable suggestions.

APPENDIX I. REFERENCES

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APPENDIX II. NOTATION

The following symbols are used in this paper:

A = cross-sectional area of column specimen (L2);a, b = constants;

C(z, t) = aqueous concentration of nondecaying solute (M/L3);CIN(t) = influent concentration of solute from feeding bag

(M/L3);CLR(t) = concentration of solute in lower (effluent) reservoir

(M/L3);CUR(t) = concentration of solute in upper (influent) reservoir

(M/L3);Dh = hydrodynamic dispersion coefficient (L2/T);D 9L = longitudinal mechanical dispersion coefficient (L2/T);Do = diffusion coefficient in free solution (L2/T);D* = molecular diffusion coefficient (L2/T);foc = organic carbon mass fraction;Kh = hydraulic conductivity (L/T);

Koc = organic compound/organic carbon partition coefficient(L3/M);

Kow = octanol-water partition coefficient (L3/M);Kp = medium-water partition coefficient of solute (L3/M);kd = first-order degradation constant (1/T);L = length of specimen (L);n = total porosity of porous medium;

ne = effective porosity of porous medium;Q = flow rate (L3/T);Rf = retardation factor;t = elapsed time (T);

VLR = volume of lower (effluent) reservoir (L3);VUR = volume of upper (influent) reservoir (L3);

vz = seepage velocity (L/T);z = distance along direction of mass transport (L);

aL = longitudinal dispersivity (L);rd = density of soil (M/L3); andta = apparent tortuosity.

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