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Geotextiles and Geomembranes 28 (2010) 149–162

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Geotextiles and Geomembranes

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Hydraulic conductivity of geosynthetic clay liners to tailingsimpoundment solutions

Charles D. Shackelford a,*, Gerald W. Sevick b,1, Gerald R. Eykholt c,2

a Department of Civil and Environmental Engineering, 1372 Campus Delivery, Colorado State University, Fort Collins, CO 80523-1372, USAb Foth & Van Dyke, LLC, 2737 S. Ridge Road, Suite 700, P.O. Box 11295, Green Bay, WI 54037-1295, USAc Foth Infrastructure & Environment, LLC, 1402 Pankratz St., Suite 300, Madison, WI 53704, USA

a r t i c l e i n f o

Article history:Received 1 July 2008Received in revised form18 March 2009Accepted 1 October 2009Available online 26 November 2009

Keywords:CompatibilityGeosynthetic clay linersHydraulic conductivityMine waste disposalTailings impoundmentTailings leachate

* Corresponding author. Tel.: þ1 970 491 5051; faxE-mail address: [email protected] (C.D.

com (G.W. Sevick). [email protected] (G.R. Eykholt).1 Tel.: þ1 920 496 6834; fax: þ1 920 497 8516.2 Tel.: þ1 608 242 5921; fax: þ1 608 242 5999.

0266-1144/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.geotexmem.2009.10.005

a b s t r a c t

The results of a comprehensive testing program conducted to evaluate the hydraulic conductivity (k) oftwo geosynthetic clay liners (GCLs) considered as a liner component for a tailings impoundment ata proposed zinc and copper mine are reported. The two GCLs were permeated with a relatively low ionic-strength ground water (GW) from the mine site and two electrolyte solutions, a process water (PW) anda simulated leachate (SL), with chemical compositions consistent with those expected during operationof the impoundment. A total of 22 flexible-wall tests were performed to determine the effects of pre-hydration with the GW, type of GCL, type of permeant liquid, and duration of the back-pressure stage ofthe test. The k values for both GCLs permeated with the GW were 1.7� 10�9 cm/s, which is within therange 1–3� 10�9 cm/s typically reported for GCLs permeated with low ionic-strength liquids, such asdeionized water. However, the mean values of k based on permeation of duplicate specimens of bothtypes of GCL with either PW or SL relative to the values of k based on permeation with GW, or k/kw,ranged from a factor of 200 (2.3 orders of magnitude) to a factor of 7600 (3.9 orders of magnitude). Thus,both tailings impoundment solutions had significant adverse impacts on the hydraulic performance ofboth GCLs. Given the overall range of k/kw values, factors such as prehydration, type of GCL, type ofpermeant liquid, and duration of back pressure, were relatively insignificant. The results of this studyserve to emphasize the need to perform hydraulic conductivity testing using site specific materials.

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1. Introduction

Traditional geosynthetic clay liners (GCLs) are thin (w5 to 10-mm thick), manufactured hydraulic barriers (liners) that consistprimarily of a processed clay, typically sodium bentonite, or otherlow permeability material that is either encased or ‘‘sandwiched’’between two geotextiles or attached to a single polymer membrane(i.e., geomembrane) and held together by needle-punching,stitching, and/or gluing with an adhesive. The hydraulic resistanceof these conventional GCLs that do not include a geomembranecomponent is attributed to the bentonite component of the GCL,which swells in the presence of water to form a tight sealing layer.

The concept behind the use of factory manufactured clayproducts as barriers to migrating liquids was introduced more than

: þ1 970 491 7727.Shackelford). gsevick@foth.

All rights reserved.

30 years ago (Koerner, 2005), and prefabricated GCLs have beenused extensively over the past two decades as barriers or compo-nents of barriers designed and constructed for a wide variety ofhydraulic containment applications, including landfill liners andcovers, surface impoundments (e.g., ponds and lakes, aerationlagoons, fly ash lagoons, and other surface impoundments), canals,storage tanks, and secondary containment of above-grade fuelstorage tanks (e.g., Rowe, 1998, 2005, 2006, 2007; Bouazza, 2002;Koerner, 2005). The first use of GCLs for waste containment ata hazardous waste landfill apparently was in 1986 (Daniel andGilbert, 1996).

The two primary motivations driving the increasingly prefer-ential use of GCLs in such applications relative to alternativebarriers or barrier components, such as compacted clay liners(CCLs) and geomembrane liners (GMLs), are a savings in cost, andestablishment of technical equivalency relative to CCLs(e.g., Koerner and Daniel, 1995). The savings in cost result essen-tially from the ease of installation of GCLs relative to both CCLs andGMLs as well as from the maximization of disposal space due to thethinness of GCLs relative to CCLs. In terms of technical equivalency,there are a number of technical advantages for preferring GCLs

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Table 1Chemical properties of bentonites in geosynthetic clay liners (GCLs).

Parameter Type of GCL

Standard GCL CR GCL

Water soluble elements (mg/kg)Na 3020 2774Mg 40.8 27.0K 23.5 15.0Ca 5.3 5.3Pb 1.05 0.18Cd <0.1 <0.1Cl 48.7 54.5

Exchangeable cations (cmolc/kg)Naþ 46.5 54.1Ca2þ 17.4 19.5Mg2þ 9.7 10.6Kþ 0.65 1.1Pb2þ <0.1 <0.1Cd2þ <0.1 <0.1

Sum 74.3 85.3

C.D. Shackelford et al. / Geotextiles and Geomembranes 28 (2010) 149–162150

relative to CCLs and/or GMLs (e.g., Bouazza, 2002). However, theprimary technical justification probably has been the extremelylow hydraulic conductivity, k, of GCLs when permeated withdeionized water (DIW), which is typically less than approximately3�10�9 cm/s (e.g., Daniel et al., 1997). Other technical aspects thatfavor the use of GCLs are the greater self-healing capability of thebentonite in GCLs relative to CCLs constructed with typically lowerplasticity natural clay soils, and a generally greater ability towithstand relatively large differential settlements compared withCCLs. The greater self-healing capability of GCLs allows GCLs toovercome small defects, such as puncture holes, up to 75 mm indiameter upon hydration (EPA, 2001), and also generally leads togreater resistance of GCLs to increases in k resulting from clima-tological distress due to repeated freezing–thawing and/orwetting–drying cycles. Finally, the ability to seal containmentfacilities by simply overlapping adjacent GCL panels and placingdry bentonite between the panels favors the installation of GCLsrelative to GMLs (Estornell and Daniel, 1992), where such adjacentpanels must be welded together to ensure an intact, continuousseam.

Among the issues affecting the use and performance of GCLs inwaste containment applications, the potential for incompatibilityin k of GCLs and the corresponding effect of such incompatibility onthe long-term performance of GCLs used in basal lining systemscontinue to be relevant (NRC, 2007). Although numerous studieshave been conducted to determine the effects of a variety ofchemical solutions on the hydraulic performance of GCLs (e.g.,Daniel et al., 1993; Rad et al., 1994; Didier and Comeaga, 1997;Petrov and Rowe, 1997; Petrov et al., 1997a,b; Quaranta et al., 1997;Ruhl and Daniel, 1997; Shackelford et al., 2000; Jo et al., 2001, 2004,2005; Egloffstein, 2001, 2002; Vasko et al., 2001; Katsumi et al.,2002, 2004, 2007a,b; Shan and Lai, 2002; Kolstad et al., 2004;Guyonnet et al., 2005; Katsumi and Fukagawa, 2005; Lee et al.,2005; Lee and Shackelford, 2005a,b; Rowe et al., 2006; Touze-Foltzet al., 2006; Brown and Shackelford, 2007; Benson et al., 2008),most of these studies have used simple inorganic salt or organicchemical solutions as the permeant liquids, and very few, if any,studies have focused specifically on the hydraulic performance ofGCLs subjected to permeation with chemical solutions that arerepresentative of actual mine tailings leachates.

Accordingly, the primary purpose of this study was to assess thehydraulic performance of two types of GCLs that were beingconsidered for use as the secondary liner component in a compositebasal liner for a tailings impoundment at a proposed zinc andcopper mine. The assessment of the performance of the GCLsreported herein was based on the measurement of the hydraulicconductivity (k) of the GCLs permeated with three permeantliquids, viz., ground water of relatively low ionic strength recoveredfrom the mining site, a solution with a chemical compositionconsistent with that expected during operation of the impound-ment, and a solution with a chemical composition consistent witha worst-case scenario corresponding to a remote possibility thatoxidation of the impounded tailings after closure of the facilitymay cause the leachate to turn acidic. The results of this studyshould provide useful information regarding the potential use ofGCL as components of basal liners for similar mine tailingsimpoundments.

2. Materials and methods

2.1. Geosynthetic clay liners

The two geosynthetic clay liners (GCLs) tested in this study wereBentomat� DN GCL, hereafter referred to as ‘‘standard GCL’’, andBentomat� DN CR GCL, hereafter referred to as ‘‘CR GCL’’. Both GCLs

were shipped directly to the testing laboratory from the manufac-turer (Colloidal Environmental Technologies Company, 1500W. Shure Drive, Arlington Heights, IL 60004). The standard GCLconsisted of a layer of Volclay sodium bentonite sandwichedbetween two nonwoven geotextiles and reinforced by needle-punching. The CR GCL was reported to be a standard GCL that wastreated to provide a level of contaminant resistance, although thespecific treatment was not revealed by the manufacturer. The CRGCL was considered for use in the composite liner primarilybecause of the concern that multivalent metals contained in thetailings leachate may replace the sodium that dominates thebentonite portion of the GCL, resulting in an increase in hydraulicconductivity and an overall decrease in hydraulic performance ofthe GCL component (e.g., see NRC, 2007). Although the currentavailability of the CR GCL is not known, the standard GCL iscurrently available from the manufacturer.

Since the focus of the study was to evaluate the suitability of twoGCL products for an actual mine tailings impoundment, a morecomprehensive assessment of the physical and chemical propertiesand mineralogical composition of the bentonites in the GCLs wasnot deemed to be essential. However, some chemical properties ofthe bentonites from the two GCLs are provided in Table 1.

The pore waters of both bentonites were dominated by sodium(Na), and exchangeable sodium cation (Naþ) dominated theexchange complexes of both bentonites. Therefore, both bentoniteswere sodium bentonites. The sums of the exchangeable cations forthe two bentonites are indicative of the cation exchange capacities(CECs) for the two bentonites, i.e., 74.3 cmolc/kg for the bentonitefrom the standard GCL and 85.3 cmolc/kg for the bentonite from theCR GCL.

2.2. Permeant liquids

The three permeant liquids used in the tests were an on-siteground water (GW), a process water (PW), and a simulated leachate(SL). The on-site GW was used in this study both as the prehydra-tion water, when the testing protocol required the GCL to be pre-hydrated before permeation with either the PW or the SL, and asa permeant liquid. Prehydration was considered because, prior tothe start of tailings placement into the disposal cells of theimpoundment, the disposal cell bottom was to be flooded with GW,resulting in prehydration of the underlying GCL due to defects in anoverlying HDPE geomembrane. Non-prehydration also wasconsidered as possible along the side slopes of the tailingsimpoundment, since the disposal cells were not planned to be

Table 3Chemical composition of process water used as permeant liquid.

Parameter Source compound Value

Target Measureda

pH Ca(OH)2 9–10 6.9 – 9.8Calcium CaSO4$2H2O 330 mg/L 360 – 570 mg/LPotassium K2SO4 10 mg/L Not measuredSodium Na2SO4 110 mg/L Not measuredSulfate – 1200–1500 mg/L 680–1100 mg/LIonic strengthb – 44–50 mM 32–51 mM

a Based on two samples.b Approximate value, estimated using I¼½Scizi

2, where I is ionic strength, ci ismeasured molar concentration of ionic species i, and zi is principal charge of ionicspecies i.

Table 4Chemical composition of simulated leachate used as permeant liquid.

Parameter Source compound Value

C.D. Shackelford et al. / Geotextiles and Geomembranes 28 (2010) 149–162 151

totally flooded with GW. The PW was considered as a permeantliquid because the leachate resulting during the operation period ofthe facility was expected to be similar to the PW. Finally, the SL wasconsidered as a permeant liquid because of a worst-case scenariocorresponding to a remote possibility that oxidation of theimpounded tailings after closure of the facility may cause theleachate to turn acidic.

The GW was collected from an on-site well and was expected tohave characteristics similar to the water to be placed in tailingsimpoundment prior to commencement of tailings deposition. Themeasured chemical composition of the GW is shown in Table 2. Asshown in Table 2, the pH of the GW was relatively neutral (w7.24)and the ionic strength, I, was low (w3.2 mM).

The process water (PW) used in this study as a permeant liquidwas prepared to have the chemical composition expected for theprocess water to be used at the site. The PW was prepared using thesulfate salts of the desired constituent metals and calciumhydroxide (Ca(OH)2) for pH adjustment. The target and resultingmeasured chemical compositions of the PW are given in Table 3.The range of estimated values of I for the PW based on themeasured solute concentrations (i.e., 32 mM� I� 51 mM) wasfrom 10 to 16 times greater than that for the GW, and the measuredvalues of pH for the PW ranged from neutral (6.9) to basic (9.8).

The simulated leachate (SL) used as a permeant liquid in thisstudy was prepared using the sulfate salts of the desired constit-uent metals and sulfuric acid (H2SO4) for pH adjustment. Theresulting measured chemical composition of the SL is given in Table4. As indicated in Table 4, the SL was a complex chemical solutionwith a low pH (2.5) characteristic of an acid drainage solution, veryhigh concentrations of sulfate (6900 mg/L), zinc (1800 mg/L), andmagnesium (1400 mg/L), moderately high concentrations of iron(410 mg/L), calcium (270 mg/L), manganese (180 mg/L), copper(51 mg/L), and aluminum (31 mg/L), and an estimated I based onmeasured concentrations of 350 mM that was approximately 110times greater than that for the GW and approximately 8 timesgreater on average than that for the PW. Thus, even though themeasured concentrations for the SL were lower than the target(desired) concentrations (Table 4), the SL was still expected to havethe greatest potential for a detrimental effect on the hydraulicconductivity of the GCLs among the three permeant liquids.

2.3. Hydraulic conductivity tests

The hydraulic conductivity tests were conducted in flexible-wallpermeameters and, with some exceptions, in general accordancewith ASTM D 5084 (Standard Test Method for the Measurement of the

Table 2Chemical composition of ground water used as permeant liquid.

Parameter No. of samples Measured value(s)

Range Mean

Temperature (field) 8 7.5–10.9 �C 9.3 �CpH (field) 8 6.52–7.88 7.24Conductance (field) 8 237–548 mS 335.6 mSAlkalinity (as CaCO3) 8 142–160 mg/L 152.4 mg/LHardness (as CaCO3) 7 36–160 mg/L 131.7 mg/LTotal dissolved solids 8 164–220 mg/L 182.5 mg/LCalcium 8 35.4–38.3 mg/L 36.5 mg/LMagnesium 8 14.2–16.0 mg/L 15.0 mg/LManganese 8 0.17–0.48 mg/L 0.25 mg/LSulfate 8 5.4–11.0 mg/L 8.0 mg/LChloride 5 1.0–2.0 mg/L 1.2 mg/LNitrateþ nitrite (as N) 4 0.728–0.982 mg/L 0.800 mg/LIonic strengtha – 3.1–3.5 mM 3.2 mM

a Approximate value, estimated using I¼½Scizi2, where I is ionic strength, ci is

measured molar concentration of ionic species i, and zi is principal charge of ionicspecies i.

Hydraulic Conductivity of Saturated Porous Materials Using a FlexibleWall Permeameter) and ASTM D 6766 (Standard Test Method forEvaluation of Hydraulic Properties of Geosynthetic Clay LinersPermeated with Potentially Incompatible Liquids). The GCL specimenswere trimmed to a nominal diameter of 102 mm and assembled inthe permeameters similar to the procedure described by Danielet al. (1997) to prevent short-circuiting through the geotextiles atthe edge of the GCL. In general, this procedure involves trimmingthe specimen from a GCL panel using a cutting ring and sharp razorknife, and applying the back-pressure liquid lightly around the edgeof the specimen during trimming to prevent loss of bentonite.

After preparation of a GCL specimen, the specimen was placed ina flexible-wall permeameter, the permeameter was assembled, andthe specimen was back pressured to achieve a high initial degree ofsaturation before permeation. The test specimens were back pres-sured using either the on-site GW for prehydrated GCL specimensor the PW or SL for non-prehydrated GCL specimens to a loweffective stress of 27.6� 3.5 kPa (4.0� 0.5 psi) to simulate theinitial condition expected at the start up of the tailings impound-ment. The back-pressure procedure consisted of initially applyinga cell-water pressure of 34.5 kPa (5.0 psi) and a back pressure atboth ends of the specimen of 6.9 kPa (1.0 psi) to establish the27.6 kPa (4.0 psi) difference required as the final initial effectivestress at the end of the back-pressure stage of the test (i.e., beforepermeation). Both the cell-water and back pressures wereincreased incrementally thereafter in either 34.5-kPa (5.0-psi) or

Target Measureda

pH H2SO4 2.5 2.5Aluminum Al2(SO4)3$18H2O 30 mg/L 31Arsenic As2O3 1.0 mg/L 0.6Cadmium 3CdSO4$8H2O 5.0 mg/L 4.3Calcium CaSO4$2H2O 300 mg/L 270Cobalt CoSO4$xH2O (x¼ 6–7) 3.0 mg/L 1.3Copper CuSO4$5H2O 65 mg/L 51Iron FeSO4$7H2O 450 mg/L 410Lead PbSO4 1.2 mg/L BDb

Magnesium MgSO4$7H2O 1500 mg/L 1400Manganese MnSO4$H2O 180 mg/L 180Nickel NiSO4$6H2O 2.0 mg/L 1.5Sulfate – 12,000 mg/L 6900Zinc ZnSO4$7H2O 2400 mg/L 1800Ionic strengthc – 485 mM 350 mM

a Based on one sample.b BD¼ below detection.c Approximate value, estimated using I¼½Scizi

2, where I is ionic strength, ci ismeasured molar concentration of ionic species i, and zi is principal charge of ionicspecies i.

Table 5Statistics of measured electrical conductivity (EC) and pH of permeant liquids.

Statistic Permeant liquid

Groundwater

Processwater

Simulatedleachate

Parameter EC (mS/cm) pH EC (mS/cm) pH EC (mS/cm) pHNo. of measurements 6 160 104Mean 0.320 6.71 3.95 8.25 27.5 2.45Standard deviation 0.021 0.12 0.268 0.83 0.549 0.097Maximum 0.417 6.93 4.66 9.60 28.6 2.63Minimum 0.267 6.62 3.38 6.24 26.4 2.31Range 0.150 0.31 1.28 3.36 2.26 0.32


69-kPa (10-psi) increments until final cell-water and back pres-sures of 345 kPa (50 psi) and 317 kPa (46 psi), respectively, wereachieved. The cell-water and back pressures resulting after eachincremental increase in the pressures typically were maintained fora period of 3–4 h. As a result, the entire procedure associated withincreasing incrementally the cell-water and back pressures lastedfrom one to two days. The final cell-water and back pressures of345 kPa (50 psi) and 317 kPa (46 psi), respectively, were main-tained for the duration of the back-pressure stage of the test. Twodifferent back-pressure durations were evaluated: (1) a short-termback-pressure duration of 2 d, and (2) a long-term back-pressureduration ranging from 21 to 29 d.

At the end of the back-pressure stage of the test, the specimenswere permeated with GW, PW, or SL. Permeation was establishedby increasing the pore-water pressure at the bottom or inflow endof the specimen from 317 kPa (46 psi) to 331 kPa (48 psi), anddecreasing the pore-water pressure at the top or outflow end of thespecimen from 317 kPa (46 psi) to 304 kPa (44 psi). As a result, anaverage effective stress of 27.6� 3.5 kPa (4.0� 0.5 psi) was main-tained in the specimen during permeation, with the nominal 27.6-kPa (4.0-psi) difference between the headwater pressure and thetailwater pressure establishing the hydraulic gradient for flowthrough the specimen.

Although both the headwater and tailwater levels changedduring the tests, resulting in falling headwater-rising tailwaterconditions as described in ASTM D 5084, the change in elevationhead due to the difference in the liquid levels was always less thanone percent of the change in pressure head. As a result, calculationsof the hydraulic conductivity, k, in accordance with both the fallingheadwater-rising tailwater equation and the constant-head equa-tion presented in ASTM D 5084 resulted in essentially the samecalculated value for k (i.e., to 2 significant digits) due to the negli-gible effect of the difference in elevation head.

The difference in pressure head across the specimens of 27.6 kPa(4.0 psi) resulted in hydraulic gradients on the order of 430�100,depending primarily on the differences in specimen thicknesses.These magnitudes of hydraulic gradients were substantially higherthan that specified in ASTM D 5084, where the recommendedmaximum hydraulic gradient for specimens with k< 1�10�7 cm/sis 30. However, as described by Shackelford et al. (2000), the use ofsuch high hydraulic gradients is typical for hydraulic conductivitytesting of GCLs, and generally does not have a significant effect onthe hydraulic conductivity of GCLs.

Because the tests reported in this study were conducted toevaluate the hydraulic conductivity of two GCLs for possible use inan actual tailings impoundment liner facility, the criteria used forterminating the tests were based primarily on the practical need toprovide as reliable and accurate assessment of k as possible withina reasonably short period. For this reason, the work plan called forperformance of the tests until the general termination criteriaestablished in ASTM D 5084 had been achieved. However, for testsinvolving permeation with the PW and SL leachates in which theGCL was deemed to be compatible with the leachate, correspondingto a measured k value of �3�10�9 cm/s, the tests were to becontinued until such time that chemical equilibrium based on theratios of outflow to inflow electrical conductivity (EC) and pH, orECout/ECin and pHout/pHin, respectively, were within 1.00� 0.15.These additional termination criteria were consistent with,although somewhat less stringent than, those recommended byShackelford et al. (1999) and ASTM D 6766 based on the need toestablish chemical equilibrium between the outflow and inflowbefore terminating hydraulic conductivity tests involving non-standard permeant liquids (i.e., liquids other than water). Inaccordance with these required termination criteria, the EC and pHof numerous samples of all three permeant liquids were measured

to establish the values for ECin and pHin, and the resulting statisticsof these measurements are reported in Table 5. Based on the valuesof EC and pH for the GW shown in Table 5, which are consistentwith those of a very low ionic-strength liquid, the EC and pHtermination criteria were not considered applicable in the case oftests involving permeation with GW, i.e., since no adverse impactfrom permeating the GCL specimens with GW was anticipated.

2.4. Testing program

As shown in Table 6, the testing program consisted of a total of12 test series and 22 flexible-wall hydraulic conductivity tests. Thefirst 2 tests (Test Series 1 and 2) were performed to establish thebaseline k values for each type of GCL based on permeation withthe GW. Test Series 3–6 were established to evaluate the effects ofthe PW and SL permeant liquids on non-prehydrated specimens ofeach type of GCL, whereas Test Series 7–10 were established toevaluate the effects of the PW and SL permeant liquids on prehy-drated specimens of each type GCL. Finally, Test Series 11 and 12were established to determine the effect of a shorter back-pressureperiod (i.e., 2 d versus 21–29 d) on the k values of specimens of onlythe CR GCL permeated with PW and the SL. All test series exceptTest Series 1 and 2 involving permeation with GW were performedin duplicate.

3. Results

3.1. Hydraulic conductivity

The measured data from the tests are shown in Figs. 1–4, and thetest results are summarized in Table 7 and Fig. 5. Both arithmeticmean and geometric mean values of the measured values for thehydraulic conductivity, k, are reported in Table 7. The arithmeticmean k values, kAM, represent averages of the actual measured kvalues, whereas the geometric mean k values, kGM, essentiallyrepresent averages of the exponents of the measured k values.

Since only one test was conducted in Test Series 1 and 2, bothmean values for k are the same as the measured values for k,whereas in all other cases (Test Series 3–12), the mean values for kare based on the duplicated test results. In the majority of the casesinvolving duplicated tests, the duplicated tests resulted in similarmeasured k values (Test Series 3–5, 7, and 9–12). In these cases,there is little, if any, difference between the reported values for kAM

versus kGM. However, for Test Series 6 and 8, where the measured kvalues for the duplicated test specimens were not as close, thevalues for kAM are appreciably greater than those for kGM. Thereason for kAM> kGM in these cases is that, since k can vary overorders of magnitude, kAM tends to favor the higher of the twomeasured k values when plotted on a logarithmic scale, whereaskGM tends to represent geometrically the ‘‘middle’’ between the twomeasured k values when plotted on a logarithmic scale. For

Table 6Testing program.

Testseries

No. oftests

Type ofGCLa

Prehydration?b Permeantliquid

Back-pressureperiod (d)

1 1 Standard Yes Ground water 3.82 1 CR Yes Ground water 3.83 2 Standard No Process water 234 2 Standard No Simulated leachate 215 2 CR No Process water 236 2 CR No Simulated leachate 217 2 Standard Yes Process water 238 2 Standard Yes Simulated leachate 289 2 CR Yes Process water 2710 2 CR Yes Simulated leachate 2911 2 CR Yes Process water 2.012 2 CR Yes Simulated leachate 2.0

a CR¼ contaminant resistant.b Prehydration with ground water.


example, for Test Series 6, the value for kAM of 1.3�10�5 cm/swould appear to be closer to the measured value for k of2.5�10�5 cm/s for Test No. 6a relative to that of 1.9�10�6 cm/s forTest No. 6b (i.e., when plotted on a logarithmic scale), whereas thevalue for kGM of 6.9�10�6 cm/s would appear to be exactly in themiddle the two measured k values. Although use of kAM valueswould be more conservative in that higher mean k values wouldresult, the kGM values essentially give equal weight to the measuredk values and, therefore, tend to provide a better representation ofthe actual measured k values based on the duplicated test results.

As shown in Fig. 1 and Table 1, the k values for both GCLspermeated with the GW were 1.7�10�9 cm/s, which is within therange 1–3�10�9 cm/s typically reported for GCLs permeated withlow ionic-strength liquids, such as deionized water (e.g., Danielet al., 1997; Shackelford et al., 2000). The reason for these lowmeasured values for k for both GCLs is the low ionic strength (Table2) and low EC (Table 5) of the GW. In contrast, the mean values of kfor all other tests series (Test Series 3–12) involving permeationwith either PW or SL were substantially higher than the k values of1.7�10�9 cm/s based on permeation with GW.

For example, as shown in Table 7 and Fig. 5, the mean values of kbased on permeation with either PW or SL relative to the values of kbased on permeation with ground water, or k/kw, ranged froma factor of 200 (2.3 orders of magnitude) based on kGM for TestSeries 8 to a factor of 7600 (3.9 orders of magnitude) based kAM forTest Series 6. On an individual test basis, the range of values for k/kw

is somewhat greater, ranging from a factor of 45 (Test No. 8b) toa factor of 15,000 (Test No. 6a). Thus, both simulated tailingsimpoundment solutions had significant adverse impacts on thehydraulic performance of both GCLs, regardless of whether or not

10-10

10-9

10-8

10-7

10-6

0 1 2 3 4 5 6 7 8

Standard GCL (1)CR GCL (2)

10-12

10-11

10-10

10-9

10-8

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)

Pore Volumes of Flow

Hydraulic C

onductivity, k (m/s)

Fig. 1. Hydraulic conductivity and volumetric flow ratio versus pore volumes of flow forground water.

the GCL specimens were prehydrated with GW prior to permeationwith either PW or SL. Given the significantly higher ionic strength, I,of the SL (Table 4) relative to the GW (Table 2), such an adverseimpact was not necessarily unexpected in the case of permeationwith the SL. However, the extent of the adverse impact of the PW onthe hydraulic performance of the GCLs was somewhat unexpected,i.e., since the ionic strength of the PW (Table 3) was significantlylower than that of the SL.

3.2. Test durations and termination criteria

As indicated in Table 7, none of the tests was terminated beforethe volumetric flow ratio, Qout/Qin, was within 1.00� 0.15, and onlytwo tests (Test Nos. 6a and 12b) were terminated before Qout/Qin

was within 1.00� 0.10. In addition, none of the tests except for TestNo. 6b was terminated before at least 6.3 pore volumes of flow(PVF) had passed through the specimens, and in several cases (e.g.,Test No. 6b, Test Series 8, 11, and 12), a substantially greater numberof PVF were collected before termination of the tests. However,except for the two tests involving permeation with GW (Test Nos. 1and 2), all of the durations of permeation for the tests involvingpermeation with either PW or SL (i.e., Test Series 3–12) generallywere less than 24 h, and less than 48 h in all cases. Such shortdurations of permeation are not unprecedented (e.g., Lee andShackelford, 2005a,b).

For example, Lee and Shackelford (2005a) permeated non-pre-hydrated specimens of two GCLs containing different qualities ofsodium bentonite with solutions of calcium chloride (CaCl2)ranging in concentration from 5 to 500 mM (15� I� 1500 mM)until chemical equilibrium between the outflow and inflow asevidenced by both EC and solute concentrations had been estab-lished. Similar to the non-prehydrated specimens in the currentstudy, the specimens tested by Lee and Shackelford (2005a) wereexposed to the permeant liquid for 48 h prior to permeation. Thetest durations (excluding the 48-h exposure period) tended todecrease with increasing CaCl2 concentration, and ranged from934 d for a test using a 5 mM CaCl2 solution to only 0.014 h for a testconducted using a 500 mM CaCl2 solution. The test durations alsowere affected by the quality of the bentonite contained in the GCL,with significantly shorter test durations for the GCL containing thehigher quality bentonite (HQB) relative to those for the GCL con-taining the lower quality bentonite (LQB). In particular, for testsinvolving permeation with the 50 mM CaCl2 solution (I¼ 150 mM),the test durations for the tests involving the GCL with the LQBlasted 15 and 16 d, whereas those involving the GCL with the HQBlasted only 2.6 and 4.2 h. Thus, extremely short durations ofpermeation should not necessarily be unexpected, especially when

0

0.5

1

1.5

2

0 1 2 3 4 5 6 7 8

Standard GCL (1)CR GCL (2)

Volu

met

ric F

low

Rat

io, Q

ou

t/Qin


specimens of two geosynthetic clay liners (GCLs) prehydrated and permeated with

10-6

10-5

10-4

10-8

10-7

10-6

0 2 4 6 8 10 12 14

3a3b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)


Hydraulic C


Standard GCLProcess Water

0

1

2

3

4

0

1

2

3

4

0 2 4 6 8 10 12 14

Q (3a)Q (3b)

EC (3a)pH (3a)EC (3b)pH (3b)

Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /E

Cin , or

pH

Ratio, p

Hou

t /p

Hin


10-6

10-5

10-4

10-8

10-7

10-6

0 5 10 15 20

4a4b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)


Standard GCLSimulated Leachate

Hydraulic C

onductivity, k (m/s) 0

1

2

3

4

0

1

2

3

4

0 5 10 15 20

Q (4a)Q (4b)


Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /E

Cin , or

pH

Ratio, p

Hou

t /p

HinPore Volumes of Flow

10-6

10-5

10-4

10-8

10-7

10-6

0 2 4 6 8 10 12

5a5b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)


Hydraulic C


CR GCLProcess Water

0

1

2

3

4

0

1

2

3

4

0 2 4 6 8 10 12

Q (5a)Q (5b)


Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /EC

in , or p

H R

atio, pH

ou

t /pH

in


10-7

10-6

10-5

10-4

10-9

10-8

10-7

10-6

0 2 4 6 8 10

6a6b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)


Hydraulic C


CR GCLSimulated Leachate

0

1

2

3

4

0

1

2

3

4

0 2 4 6 8 10

Q (6a)Q (6b)


Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /E

Cin , or

pH

Ratio, p

Hou

t /p

HinPore Volumes of Flow

Fig. 2. Hydraulic conductivity and termination criteria versus pore volumes of flow for duplicate non-prehydrated specimens of two geosynthetic clay liners (GCLs) permeated witheither process water or simulated leachate.


permeating GCLs containing relatively high qualities of bentonitewith relatively high ionic-strength solutions.

Although not exclusively the case, the permeation stages for thetests conducted on prehydrated specimens (Test Series 7–12) ten-ded to last somewhat longer and result in somewhat greater PVFthan the tests conducted on non-prehydrated specimens (TestSeries 3–6). This observation is consistent with the results of otherstudies involving permeation of both prehydrated and non-pre-hydrated specimens of GCLs with electrolyte solutions (e.g., Lee andShackelford, 2005b), and occurs because prehydration with a lowionic-strength solution tends to establish a lower baseline value for

k than is established without prehydration (i.e., due to a greaterextent of bentonite swelling), such that a greater number of porevolumes of flow with the higher ionic-strength solution and longerdurations of permeation are required before chemical equilibriumbetween outflow and inflow can be established with prehydratedspecimens (Shackelford, 1994).

In addition, the test durations noted in Table 7 do not include theback-pressure stage of the testing procedure. In the case of the non-prehydrated specimens, the specimens were back-pressured withthe permeant liquid, which lasted either 21 d or 23 d (Table 6).Thus, the non-prehydrated specimens were exposed to the

10-8

10-7

10-6

10-5

10-10

10-9

10-8

10-7

0 5 10 15 20 25

7a7b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)


Hydraulic C


Standard GCLProcess Water

0

1

2

3

4

0

1

2

3

4

0 5 10 15 20 25

Q (7a)Q (7b)


Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /EC

in , or p

H R

atio, pH

ou

t /pH

in


10-8

10-7

10-6

10-5

10-10

10-9

10-8

10-7

0 5 10 15 20 25 30 35

8a8b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)

Pore Volumes of FlowH

ydraulic Conductivity, k (m

/s)

Standard GCLSimulated Leachate

0

1

2

3

4

0

1

2

3

4

0 5 10 15 20 25 30 35

Q (8a)Q (8b)


Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /EC

in , or p

H R

atio, pH

ou

t /p

Hin


10-8

10-7

10-6

10-5

10-10

10-9

10-8

10-7

0 5 10 15 20

9a9b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)


Hydraulic C


CR GCLProcess Water

0

1

2

3

4

0

1

2

3

4

0 5 10 15 20

Q (9a)Q (9b)


Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /EC

in , or p

H R

atio, pH

ou

t /p

Hin


10-8

10-7

10-6

10-5

10-10

10-9

10-8

10-7

0 5 10 15 20

10a10b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)


Hydraulic C


CR GCLSimulated Leachate

0

1

2

3

4

0

1

2

3

4

0 5 10 15 20

Q (10a)Q (10b)

EC (10a)pH (10a)EC (10b)pH (10b)

Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /EC

in , or p

H R

atio, pH

ou

t /p

Hin


Fig. 3. Hydraulic conductivity and termination criteria versus pore volumes of flow for duplicate specimens of two geosynthetic clay liners (GCLs) subjected to long-term (20–30 d)prehydration with ground water before permeation with either process water or simulated leachate.


permeant liquids for significantly longer periods than exemplifiedby the durations of permeation listed in Table 7.

Since none of the tests involving permeation with either PW orSL resulted in k values� 3�10�9 cm/s, the criteria to continue thetests until such time that ECout/ECin and pHout/pHin, were within1.00� 0.15 was not applicable. Nonetheless, as indicated in Table 7,the final values for ECout/ECin for all tests conducted in Test Series 3–12 ranged from a low of 0.95 (Test No. 8b) to a high 1.15 (Test No.5b), such that all of these tests did comply with the establishedtermination criterion on EC. In terms of pH, only 5 (Test Nos. 3a, 5a,5b, 8b, 11b) of the 20 tests in Test Series 3–12 did not comply with

the established termination criterion of pHout/pHin within1.00� 0.15.

3.3. Trends in outflow EC and pH

For the tests involving permeation with either PW or SL (TestSeries 3–12), the temporal trends in both EC and pH in the per-meant outflow (i.e., normalized with respect to the respective ECand pH values in the inflow) are also shown in Figs. 2–4. In all casesinvolving permeation with PW, the values of ECout/ECin are initiallygreater than unity (i.e., ECout/ECin> 1), but decrease towards unity

10-8

10-7

10-6

10-5

10-10

10-9

10-8

10-7

0 5 10 15 20 25 30 35 40

11a11b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)


Hydraulic C


Process Water0

1

2

3

4

0

1

2

3

4

0 5 10 15 20 25 30 35 40

Q (11a)Q (11b)

EC (11a)pH (11a)EC (11b)pH (11b)

Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /EC

in , or p

H R

atio, pH

ou

t /p

Hin


10-8

10-7

10-6

10-5

10-10

10-9

10-8

10-7

0 5 10 15 20 25 30 35 40

12a12b

Hyd

raul

ic C

ondu

ctiv

ity, k

(cm

/s)

Pore Volumes of FlowH

ydraulic Conductivity, k (m

/s)

Simulated Leachate0

1

2

3

4

0

1

2

3

4

0 5 10 15 20 25 30 35 40

Q (12a)Q (12b)

EC (12a)pH (12a)EC (12b)pH (12b)

Volu

met

ric F

low

Rat

io, Q

ou

t/Qin Electrical C

onductivity Ratio,

EC

ou

t /E

Cin , or

pH

Ratio, p

Hou

t /p

Hin


Fig. 4. Hydraulic conductivity and termination criteria versus pore volumes of flow for duplicated specimens of the contaminant resistant GCL subjected to short-term (2 d)prehydration with ground water before permeation with either process water or simulated leachate.

Table 7Test results.

Test series Test No. Duration ofpermeation

Final flow and chemical ratiosb Measured hydraulicconductivityc

Mean hydraulic conductivityd

Time PVFa Qout/Qin ECout/ECin pHout/pHin k (cm/s) k/kw kAM (cm/s) kGM (cm/s) kAM/kw kGM/kw

1 1 57.0 d 6.7 1.01 NA NA 1.7� 10�9 1.0 1.7� 10�9 1.7� 10�9 1.0 1.02 2 56.1 d 7.0 1.01 NA NA 1.7� 10�9 1.0 1.7� 10�9 1.7� 10�9 1.0 1.03 3a 4.4 h 11.2 1.01 1.03 0.81 9.4� 10�6 5500 1.0� 10�5 1.0� 10�5 5900 5900

3b 2.6 h 12.4 0.99 1.03 0.95 1.1� 10�5 6500

4 4a 1.3 h 17.8 0.98 0.98 1.05 3.8� 10�6 2200 3.9� 10�6 3.9� 10�6 2300 23004b 1.1 h 18.9 1.05 0.98 1.03 4.0� 10�6 2400

5 5a 0.3 h 10.7 1.01 1.01 0.79 1.0� 10�5 5900 9.5� 10�6 9.4� 10�6 5600 55005b 1.1 h 6.3 1.01 1.15 0.78 8.9� 10�6 5200

6 6a 0.1 h 9.6 0.89 0.98 1.12 2.5� 10�5 15,000 1.3� 10�5 6.9� 10�6 7600 41006b 0.2 h 3.5 0.97 0.97 1.15 1.9� 10�6 1100

7 7a 2.0 h 10.6 1.01 1.01 0.85 4.0� 10�6 2400 2.7� 10�6 2.4� 10�6 1600 14007b 36.4 h 23.9 0.94 1.01 0.99 1.4� 10�6 820

8 8a 32.4 h 30.1 0.99 0.96 1.05 1.5� 10�6 880 7.9� 10�7 3.4� 10�7 460 2008b 8.9 d 31.1 0.99 0.95 1.23 7.6� 10�8 45

9 9a 1.2 h 19.5 0.99 1.02 0.87 4.6� 10�6 2700 4.3� 10�6 4.3� 10�6 2500 25009b 1.2 h 17.6 1.00 0.99 0.86 4.0� 10�6 2400

10 10a 0.9 h 17.7 0.99 0.99 1.06 8.0� 10�6 4700 7.2� 10�6 7.1� 10�6 4200 420010b 1.0 h 17.1 1.02 0.97 1.08 6.3� 10�6 3700

11 11a 24.7 h 38.7 0.97 0.99 0.99 3.8� 10�6 2200 3.1� 10�6 3.0� 10�6 1800 180011b 32.3 h 29.1 1.04 0.99 0.81 2.4� 10�6 1400

12 12a 2.9 h 26.1 1.00 0.97 1.08 2.4� 10�6 1400 1.7� 10�6 1.5� 10�6 1000 88012b 9.9 h 36.7 1.13 0.99 1.03 1.0� 10�6 590

a Pore volumes of flow.b Qout/Qin¼ ratio of volumetric outflow to volumetric inflow; ECout/ECin¼ ratio of electrical conductivity (EC) in outflow to EC in inflow; pHout/pHin¼ ratio of pH in outflow to

pH in inflow.c k¼Measured value of hydraulic conductivity; k/kw¼measured k value normalized with respect to k value for same GCL based on permeation with ground water (Test Nos.

1 and 2).d kAM¼Arithmetic mean¼ (k1þ k2)/2; kGM¼ geometric mean¼ (k1k2)0.5; kAM/kw and kGM/kw¼ kAM and kGM values normalized with respect to k value for same GCL based on

permeation with ground water (Test Nos. 1 and 2).


0

1000

2000

3000

4000

5000

6000

7000

8000

3 4 5 6 7 8 9 10 11 12

a

Arithmetic Mean k

Geometric Mean k

k/k

w

Test Series

0

1

2

3

4

5

6

3 4 5 6 7 8 9 10 11 12

b

Arithmetic Mean k

Geometric Mean k

log

(k/k

w)

Test Series

3.83.4

3.73.83.4

3.7 3.93.6

3.2

2.7

3.43.6

3.33.03.1

2.3

3.43.6

3.32.9

Fig. 5. Ratio of hydraulic conductivity based on permeation with process water orsimulated leachate, k, to hydraulic conductivity based on permeation with groundwater, kw: (a) arithmetic scale; (b) logarithmic scale.


EC

ou

t/E

Cin

, or pH

R

atio

, pH

ou

t/p

Hin

Elec

trica

l Con

duct

ivity

Rat

io,

0

1

0

ECin < ECsoil

pHin < pHsoil

ECin > ECsoil

pHin > pHsoil

Trend Line

Trend Line

Fig. 6. – Schematic trend lines in electrical conductivity and pH ratios versus porevolumes of flow for permeation of geosynthetic clay liners with chemical solutions(after Shackelford et al., 1999).


(i.e., ECout/ECin / 1) as the number of PVF increases, whereas thevalues for pHout/pHin are relatively constant and typically slightlyless than unity throughout permeation. As illustrated schematicallyin Fig. 6, these trends are typical in cases where the EC of thepermeant liquid is lower than the initial EC of the soil pore water(i.e., ECin< ECsoil), and the pH of the permeant liquid is approxi-mately the same as that initially in the soil pore water (i.e.,pHin z pHsoil) (Shackelford et al., 1999).

In contrast, in all cases involving permeation with SL, the valuesof pHout/pHin are initially greater than unity (i.e., pHout/pHin> 1), butdecrease towards unity (i.e., pHout/pHin / 1) as the number of PVFincreases, whereas the values for ECout/ECin are relatively constantand typically approximately unity (ECout/ECin z 1) throughoutpermeation. These trends are consistent for permeation with anacidic solution of relatively high ionic strength, such as the SL usedin this study, such that pHin< pHsoil and ECin z ECsoil (see Fig. 6).Thus, the temporal trends in both outflow EC and pH resulting formthe hydraulic conductivity tests performed in this study areconsistent with those expected on the basis of the permeant liquidsused in the study.

4. Discussion

4.1. General basis for interpretation of test results

A good explanation of the basis for the effect of the chemicalcomposition of a permeant liquid on the hydraulic conductivity (k)of bentonite is provided by Jo et al. (2001). In general, the effect isgoverned primarily by the amount of swelling associated with themontmorillonite (smectite) mineral component of the bentonite.The amount of swelling depends on the valence (charge) of thecations and the ionic concentration between the crystalline layerscomprising the montmorillonite. If the cations in this interlayer

region are primarily monovalent (e.g., Naþ), ‘‘osmotic’’ swellingoccurs, and most of the water in the bentonite is retained electro-statically between the layers and is hydraulically immobile, suchthat the remaining ‘‘free water’’ (i.e., water capable of flow) residesin small pore spaces where the flow and transport paths are con-stricted and highly tortuous resulting in a relatively low k. However,when the cations in the interlayer region are predominantlymultivalent (e.g., Ca2þ and Mg2þ), only ‘‘crystalline’’ swellingoccurs, and the interlayer region is hydrated with at most fourlayers of water molecules, resulting in a significant reduction inswelling. Consequently, little constriction of the pore space exists,and the resulting k is higher. As a result, when liquid containingmultivalent cations passes through the bentonite, the multivalentcations tend to exchange with the native monovalent cations on thenegatively charged mineral surfaces, due to their higher charge. Asthe fraction of multivalent cations in the interlayer space increases,the interlayer region gradually contracts, resulting at the macro-scale in decrease in swell volume and an increase in k.

This exchange process and the resulting change in k also iscontrolled by the rate at which multivalent cations diffuse from thebulk pore water into the interlayer space (Jo et al., 2005, 2006).Therefore, the concentration of multivalent cations in the permeantliquid affects not only the magnitude of the change in k, but also therate at which this change in k occurs, because the rate of diffusion isgoverned by the concentration gradient between the bulk porewater and the interlayer space. Thus, all other factors being thesame, the higher the concentration of multivalent cations in thepermeant liquid, the greater the expected change in magnitude ofk, and the more rapid the expected change in k.

In terms of prehydration, the results of several studies haveindicated that prehydration of GCLs or soil mixtures containingbentonite (e.g., compacted sand–bentonite mixtures) with waterprior to permeation with actual permeant liquids can havea significant effect on the measured hydraulic conductivity (e.g., seeLee and Shackelford, 2005b and references cited therein). Thepotential impact of this effect, commonly referred to as the pre-hydration or the first exposure effect, is that specimens that are firstprehydrated with water prior to permeation with actual permeantliquids tend to result in lower final k values and require longer testdurations to achieve chemical equilibrium between the effluentand influent than specimens that are not prehydrated. However, asdescribed by Lee and Shackelford (2005b), this impact tends to bedependent on the concentration of multivalent cations in the actualpermeant liquid.

For example, for permeation with liquids containing lowerconcentrations of multivalent cations (�20 mM CaCl2 for Lee and


Shackelford, 2005b), non-prehydrated specimens may requiregreater time to achieve chemical equilibrium and result in essen-tially the same final k relative to prehydrated specimens, whereasthe opposite is true when the specimens are permeated with per-meant liquids containing the higher concentrations of multivalentcations (50 mM and 100 mM CaCl2 for Lee and Shackelford, 2005b).In the case of the tests involving lower multivalent concentrations,hydration and osmotic swell dominate the initial behavior of thespecimen, whereas both the rate and effect of cation exchange isrelatively minor due to the low concentration of multivalentcations. As a result, the final, steady-state k of the non-prehydratedspecimen is expected to be essentially the same as that for theprehydrated specimen, but the non-prehydrated specimen is likelyto require longer to achieve steady state because the specimenmust establish equilibrium with respect to hydration and swell.However, as the concentration of multivalent cations in the per-meant liquid increases, the rate of cation exchange begins todominate the ability of the non-prehydrated GCL to hydrate andswell, resulting in reduced swell, higher k, and shorter testdurations. In contrast, for prehydrated GCL specimens, cationexchange must overcome the effect of swelling resulting fromprehydration, typically resulting in longer times required to achievechemical equilibrium for the prehydrated specimens and lowerfinal k values.

4.2. Effect of prehydration

As indicated in Fig. 7, prehydration tended to result in a lowermeasured k value relative to non-prehydration, with values for kbased on non-prehydrated specimens relative to those based onprehydrated specimens, or kNP/kP, ranging from 1.8 to 4.9 based onkAM values and from 1.0 to 11 based on kGM values. However, theeffect due to prehydration tended to vary depending on type of GCLand permeant liquid.

For example, there was less effect for permeation of the CR GCLrelative to permeation of the standard GCL with either PW or SL(1.0� kNP/kP� 3.0), and permeation of the CR GCL with the SLshowed virtually no effect (1.0� kNP/kP� 1.8), whereas permeationof the standard GCL with SL showed the greatest effect (4.9� kNP/kP� 11). Thus, although all of the measured values of k based onpermeation of both the standard GCL and the CR GCL with eitherPW or SL were significantly greater than the measured k value foreach respective GCL based on permeation with GW, the k valuesfor non-prehydrated specimens tended to be greater than those forprehydrated specimens, and the difference was more significant in

0

2

4

6

8

10

12

Std GCL-PW Std GCL-SL CR GCL-PW CR GCL- SL

Arithmetic Mean kGeometric Mean k

kN

P/k

P

Basis for Comparison

3.7 4.24.9

11

3.02.2 1.8

1.0

Fig. 7. Ratio of hydraulic conductivity of non-prehydrated specimens, kNP, to that ofprehydrated specimens, kP, for specimens of the standard (Std) and contaminantresistant (CR) geosynthetic clay liners (GCLs) permeated with either process water(PW) or simulated leachate (SL).

the case of the standard GCL permeated with SL relative to the CRGCL permeated with SL.

4.3. Effect of type of GCL

The effect of type of GCL on the hydraulic conductivity of theGCLs for tests performed using long-term back-pressure durationsis illustrated in Fig. 8 in terms of the value of k for the CR GCLrelative to the value of k for the standard GCL, or kCRGCL/kStdGCL, forboth non-prehydrated and prehydrated specimens permeated witheither PW or SL. As indicated in Fig. 8, except for the case of non-prehydrated specimens permeated with PW, the CR GCL actuallyperformed worse than the standard GCL, with only a relativelyslight difference (1.6� kCRGCL/kStdGCL� 1.8) for the prehydratedspecimens permeated with PW, but a significant difference(9.1� kCRGCL/kStdGCL� 21) for the prehydrated specimens perme-ated with the SL. Explanation of the effect of type of GCL is difficultsince the nature of the treatment process for the CR GCL isproprietary. However, Ryan (1987) also reported a poorer hydraulicperformance for a ‘‘contaminant resistant’’ bentonite permeatedwith a hazardous waste leachate relative to that of an untreatedbentonite. Thus, these results emphasize the need to performhydraulic conductivity tests using the actual materials versusrelying on perceived benefits.

4.4. Effect of type of permeant liquid

The effect of type of permeant liquid on the hydraulic conduc-tivity of the GCLs for tests performed using long-term back-pres-sure durations is illustrated in Fig. 9 in terms of the ratio of the kbased on permeation with SL to k based on permeation with PW, orkSL/kPW, for both non-prehydrated and prehydrated specimens ofboth types of GCL. In terms of the standard GCL, permeation with SLconsistently resulted in a lower k than permeation with PW(0.14� kSL/kPW, �0.39), regardless of whether or not the specimenswere prehydrated. In contrast, the results for the CR GCL are mixed,indicating that permeation with the SL resulted in a higher k thanpermeation with PW based on arithmetic mean k values and thegeometric mean k value for the prehydrated specimens (1.4� kSL/kPW, �1.7), whereas kSL/kPW¼ 0.73 based on the geometric mean kvalue for the non-prehydrated specimens. In general, the higherionic strength and lower pH of the SL relative to the PW wasexpected to result in higher k values (i.e., kSL/kPW, >1), such thatthese results were unexpected. However, the overall effect of SLrelative to PW is less than an order of magnitude, and, given that

0

5

10

15

20

25

NP-PW NP-SL P-PW P-SL

Arithmetic Mean k

Geometric Mean k

kC

R G

CL/k

Std

GC

L


0.95 0.94

3.31.8 1.6 1.8

9.1

21

Fig. 8. Ratio of hydraulic conductivity of contaminant resistant geosynthetic clay liner(GCL), kCRGCL, to that of standard GCL, kStdGCL, for both non-prehydrated (NP) andprehydrated (P) specimens permeated with either process water (PW) or simulatedleachate (SL).

1

Arithmetic Mean k


both PW and SL resulted in significantly higher k values relative tothe k values based on GW, the relative effect of SL to PW is minor.

0

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Process Water Simulated Leachate

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Fig. 10. Ratio of hydraulic conductivity of prehydrated specimens of the contaminantresistant geosynthetic clay liner based on relatively short back-pressure duration, kSBP,to that based on relatively long back-pressure duration, kLBP, prior to permeation witheither process water (PW) or simulated leachate (SL).

4.5. Effect of back-pressure duration

A comparison of the hydraulic conductivity values of prehy-drated CR GCL specimens based on the two different back-pressuredurations is provided in Fig. 10 in terms of the ratio of the k basedon permeation of prehydrated specimens of the CR GCL subjectedto a relatively short back-pressure duration (Test Series 11 and 12)relative to k based on permeation of prehydrated specimens of theCR GCL subjected to a relatively long back-pressure duration (TestSeries 9 and 10), or kSBP/kLBP. As indicated in Fig. 10, the longer back-pressure duration actually resulted in higher k values (i.e., kSBP/kLBP< 1) regardless of whether PW or SL was used as the permeantliquid, with only a slight effect in the case of permeation with PW(0.70� kSBP/kLBP� 0.72), and a somewhat more significant effect inthe case of permeation with SL (0.24� kSBP/kLBP� 0.21). Theseresults are inconsistent with expected behavior, since a longerback-pressure duration should allow for greater bentonite swellingand a subsequently lower k value. However, similar to the effect oftype of permeant liquid, the overall effect of back-pressure durationis less than an order of magnitude, and neither the longer nor theshorter back-pressure duration provided sufficient resistance toadverse chemical effects from permeation with either the PW or SL.Thus, from an overall hydraulic performance viewpoint, the lengthof back-pressure duration was largely inconsequential.

An interesting observation is that the temporal trends in themeasured k values for the tests involving GCL specimens back-pressured for only 2 d and permeated with PW (i.e., Test Series 11)are different than those for the tests involving GCL specimens back-pressured for only 2 d and permeated with SL (i.e., Test Series 12),as well as those for the tests involving GCL specimens back-pres-sured for the longer durations (27 or 29 d) and permeated witheither PW or SL (Test Series 9 and 10). In the case of the GCLspecimens back-pressured for only 2 d and permeated with PW, themeasured k values tend to initially decrease with permeation,before subsequently increasing prior to establishment of steady-state k (see results for Test Nos.11a and 11b in Fig. 4), whereas in theother tests, no such initial decrease in k is apparent (see results forTest Nos. 9a, 9b, 10a, and 10b in Fig. 3 and Test Nos. 12a and 12b inFig. 4). These temporal trends in measured k are consistent with theaforementioned competing effects of hydration and swelling versuscation exchange as described by Shackelford (2005, 2008).

0

0.5

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2

NP-Std GCL NP-CR GCL P-Std GCL P-CR GCL

Arithmetic Mean kGeometric Mean k

kS

L/k

PW


0.39 0.39

1.4

0.73

0.290.14

1.7 1.7

Fig. 9. Ratio of hydraulic conductivity based on permeation with simulated leachate,kSL, to that based on permeation with process water, kPW, for both prehydrated (P) andnon-prehydrated (NP) specimens of both standard (Std) and contaminant resistant(CR) geosynthetic clay liners (GCLs).

For example, in the case of the GCL specimens back-pressuredfor only 2 d and permeated with PW (Test Nos. 11a and 11b), thespecimens are not full hydrated after only 2 d, and the ionicstrength of the PW is relatively low such that hydration and swelldominate the early behavior in k, such that k decreases as thebentonite in the specimens continues to hydrate and swell uponpermeation with PW. Eventually, cation exchange begins to domi-nate the behavior of the specimens, such that swelling is reducedand k increases until a final, steady-state value of k is achieved. Thisbehavior is not evident in the case of the GCL specimens back-pressured for only 2 d and permeated with SL (Test Nos. 12a and12b), because the ionic strength of the SL is sufficiently greater thanthat of the PW, such that cation exchange plays a more dominantrole relative to hydration and swell in the temporal behavior of themeasured k values for these GCL specimens. As a result, additionalhydration and swell upon permeation with SL is prevented orminimized, such that the initial k values in these tests are some-what higher than those in the tests based on permeation with PW.In the case of the tests involving GCL specimens back-pressured forthe longer durations (27 or 29 d) and permeated with either PW orSL (Test Series 9 and 10, Fig. 3), the longer back-pressure durationsapparently were sufficient to allow the bentonite in the GCL spec-imens to become fully hydrated, such that the initial decrease in kdue to additional hydration and swell is not evident.

4.6. Predicted hydraulic conductivity

Kolstad (2000) correlated the k of non-prehydrated GCLspermeated with inorganic chemical solutions with two chemicalparameters: (1) the ionic strength of the solution, I, and (2) the ratioof the molar concentrations of monovalent cations (MM) to divalentcations (MD) in the solution, or RMD (MM/MD

0.5). The correlation wasbased on reported results using leachates derived from municipalsolid waste, construction and demolition waste, fly ash, and minewaste. The resulting correlation, which is shown in Fig. 11a, wasbased on the premise that higher I and a lower RMD would result inless swelling of the bentonite, such that the k of GCLs wouldincrease with increasing I and decreasing RMD. Previous indepen-dently performed analyses performed by Lee and Shackelford(2005b) and Brown and Shackelford (2007) based on permeatingspecimens of GCLs with simple CaCl2 solutions and a simulatedanimal waste solution, respectively, showed good correlationbetween the measured k values and the predicted k values based onthe correlation for RMD¼ 0 shown in Fig. 11a.

Accordingly, the measured k values in this study, kmeasured, basedon the non-prehydrated specimens (Test Series 3–6) of both types


of GCLs are plotted versus the predicted k values, kpredicted, inFig. 11b. All values for kpredicted were based on RMD¼ 0, even thoughthe chemical compositions for the permeant liquids noted in Tables2–4 indicate that RMD> 0, because RMD¼ 0 would result in themost conservative (highest) possible values for kpredicted, and theactual value for RMD for the permeant liquids used in this study isunknown given the range of measured concentrations associatedwith the GW (Table 2) and PW (Table 3), and the uncertaintyassociated with measured concentrations based on only onesample for the SL (Table 4) Also, the mean I of 3.2 mM was used tocalculate the kpredicted value based on the GW (Table 2), but the mostconservative (highest) values of I of 51 mM and 485 mM were usedto calculate the kpredicted values based on PW (Table 3) and SL (Table4), respectively.

As shown in Fig. 11b, the correlations between kpredicted andkmeasured based on permeation with GW and SL are excellent.However, the correlations between kpredicted and kmeasured based onpermeation with PW are not only poor but also unconservative (i.e.,

10-10

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0 200 400 600 800 1000

0 0.2 0.4 0.6 0.8 1

a

Pred

icte

d H

ydra

ulic

Con

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ivity

, k

pre

dic

te

d

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/s)

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Precdicted Hydraulic C

onductivity, k

pre

dic

te

d (m/s)

Ionic Strength, I (M)

Upper Bound (RMD = 0):log k (cm/s) = -8.374 +

0.006747I (mM)

Lower Bound (RMD > 0):log k (cm/s) = -9.544 +

0.005849I (mM)

10-10

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bGW-Standard GCLGW-CR GCLPW-Standard GCLPW-CR GCLSL-Standard GCLSL-CR GCL

Mea

sure

d H

ydra

ulic

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ivity

, k

me

asu

re

d

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/s)

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(cm/s)

Measured H

ydraulic Conductivity,

km

ea

su

re

d (m/s)

Predicted Hydraulic Conductivity, kpredicted

(m/s)

1:1

Fig. 11. Predicted versus measured hydraulic conductivity (k) for non-prehydratedspecimens of two geosynthetic clay liners permeated with ground water (GW), processwater (PW), and simulated leachate (SL): (a) correlation between predicted k and ionicstrength of solution based on Kolstad (2000); (b) predicted versus measured k.

kpredicted< kmeasured). These results provide some insight into thespecific mechanism responsible for the adverse impact of the SL onthe hydraulic conductivity of the two GCLs.

For example, as described by Shackelford (1994), chemicalsolutions with low pH, such as the SL used in this study, canadversely affect the hydraulic conductivity of clays in two ways.First, low pH can result in direct dissolution of the clay mineralstructure resulting in the creation of larger pores and increasing k.Second, a decreasing pH also correlates with a greater extent ofdissolution of metals salts, resulting in increasing ionic strength ofthe pore liquid and a concomitant increase in k. In terms of directdissolution of the clay minerals, the lack of any observed murkinessin the permeation outflow from these tests, and the fact that the pHof the SL of 2.5 was slightly higher than the threshold value of 2.0below which the onset of clay mineral dissolution is expected(Shackelford, 1994), suggest that clay mineral dissolution was notprevalent, although such indicators alone may not be adequate toexclude this possibility altogether. In addition, although the pH waslow for the SL, the influence of the low pH on exchange propertiesof the bentonite was expected to be minor relative to the role ofionic strength and RMD. For reference, the equivalent concentrationof magnesium in the SL was approximately 115 meq/L, whereasa pH of 2.5 for the SL would translate to an equivalent concentrationof only 3.2 meq/L. Finally, the fact that the measured k values basedon permeation with SL are close to the predicted k values basedprimarily on the estimated ionic strength of the SL suggests thationic strength was the dominant mechanism controlling thehydraulic performance of the GCLs permeated with the SL. None-theless, without further detail on the differences in compositionbetween the inflow and outflow, or a post-test chemical charac-terization of the GCL, a more conclusive statement regarding themechanisms controlling the increasing in k of the two GCLspermeated with the SL is not warranted.

Thus, although the majority of the results shown in Fig. 11bprovide further evidence that correlation among k, I and RMDdeveloped by Kolstad (2000) provides reasonable estimates of the kof non-prehydrated GCLs permeated with electrolyte solutions, theresults based on the PW suggest that further evaluation of thevalidity of the correlation developed by Kolstad (2000) is neededbefore more broad-based usage can be promoted. Indeed, prudencedictates that the established correlation among k, I and RMD berelied upon only for preliminary estimates of the potential effect ofa given electrolyte solution on the k of a given GCL; hydraulicconductivity testing performed using the actual GCL and permeantliquid is still recommended.

5. Summary and conclusions

The results of a comprehensive testing program conducted toevaluate the hydraulic performance of two geosynthetic clay liners(GCLs) considered as a liner component for a tailings impoundmentat a proposed zinc and copper mine are reported. The two GCLswere permeated with three permeant liquids, viz., ground water(GW) of relatively low ionic strength recovered from the miningsite, process water (PW) with a chemical composition consistentwith that expected during operation of the impoundment, anda simulated leachate (SL) with a chemical composition consistentwith a worst-case scenario corresponding to a remote possibilitythat oxidation of the impounded tailings after closure of the facilitymay cause the leachate to turn acidic. The two GCLs consisted ofa ‘‘standard GCL’’ and a ‘‘contaminant resistant GCL’’ (CR GCL). Thetesting program consisted of a total of 22 flexible-wall hydraulicconductivity, k, tests that were performed to evaluate the effects ofprehydration with the GW, type of GCL, type of permeant liquid,and duration of the back-pressure stage of the test.


The k values for both GCLs permeated with the GW were1.7�10�9 cm/s, which is within the range 1–3�10�9 cm/s typi-cally reported for GCLs permeated with low ionic-strength liquids,such as deionized water. However, the k values for both types ofGCL were substantially greater upon permeation with either thePW or the SL, regardless of whether or not the GCL was prehydratedwith GW and regardless of the duration of back-pressure. Forexample, the mean values of k based on permeation with either PWor SL relative to the values of k based on permeation with groundwater, or k/kw, ranged from a factor of 200, corresponding to 2.3orders of magnitude, to a factor of 7600, corresponding to 3.9orders of magnitude. On an individual test basis, the range of valuesfor k/kw was somewhat greater, ranging from a factor of 45 toa factor of 15,000. Thus, both simulated tailings leachates hadsignificant adverse impacts on the hydraulic performance of bothGCLs, regardless of whether or not the GCL specimens were pre-hydrated with GW prior to permeation with either PW or SL.

In terms of the other factors evaluated in the study, prehydra-tion of the GCL specimens with GW prior to permeation with eitherPW or SL tended to result in lower k values, which is consistent witha greater degree of bentonite swelling prior to permeation witheither PW or SL. However, in terms of type of GCL, type of permeantliquid, and duration of back pressure, the overall results werecontradictory to expected behavior in that the CR GCL performedworse than the standard GCL. Permeation with SL generally resul-ted in either a lower k than permeation with PW or only a slightlyhigher k than permeation with PW, and a longer back-pressureduration tended to result in higher measured k values for prehy-drated specimens of both GCLs permeated with either PW or SLrelative to that for a shorter back-pressure duration. Nonetheless,given the overall range of increase in k upon permeation with PW orSL relative to permeation with GW, factors such as prehydration,type of GCL, type of permeant liquid, and duration of back pressure,were relatively insignificant.

Finally, the measured k values in this study for non-prehydratedspecimens of both GCLs were compared to predicted k values basedon an established correlation between k of the GCL and the ionicstrength, I, and ratio of monovalent-to-divalent cations (RMD) ofthe permeant liquid. The predicted k values were close to themeasured k values based on permeation with either GW or SL, butthe predicted values of k based on permeation with PW were notonly poor but also unconservative (i.e., too low). Thus, prudencedictates that the established correlation among k, I and RMD berelied upon only for preliminary estimates of the potential effect ofa given electrolyte solution on the k of a given GCL, such thathydraulic conductivity testing performed using the actual GCL andpermeant liquid is still recommended.

The results of this study are consistent with those of otherstudies showing that permeation of GCLs with relatively strongelectrolyte solutions (i.e., high ionic strength and/or low values ofRMD) tend to cause significant increases in the k of the GCL relativeto that based on relatively weak liquids, such as deionized water orthe ground water used in this study. The most surprising outcomeof the study was the significant adverse effect of the PW on the k ofthe two GCLs, since the PW, although significantly stronger than theGW, was also significantly weaker than the SL. Overall, the results ofthis study further emphasize the need to actually conduct hydraulicconductivity testing using site specific materials.

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