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Geotextiles and Geomembranes 27 (2009) 321–331

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Geotextiles and Geomembranes

journal homepage: www.elsevier .com/locate/geotexmem

Performance-based indicators for controlling geosynthetic clay liners inlandfill applications

Dominique Guyonnet a,*, Nathalie Touze-Foltz b, Veronique Norotte c, Catherine Pothier d, Gerard Didier c,Helene Gailhanou a, Philippe Blanc a, Fabienne Warmont e

a BRGM, BP 36009, 45060 Orleans Cedex 2, Franceb Cemagref, BP 44, 92163 Antony Cedex, Francec INSAVALOR, Coulomb 3, 34 Avenue des Arts, 69621 Villeurbanne Cedex, Franced INSA Lyon – LGCIE, Coulomb 3, 34 Avenue des Arts, 69621 Villeurbanne Cedex, Francee CRMD, UMR 6619 – CNRS, 1b rue de la ferollerie, 45071 Orleans Cedex 2, France

a r t i c l e i n f o

Article history:Received 7 July 2008Received in revised form28 January 2009Accepted 1 February 2009Available online 14 March 2009

Keywords:Geosynthetic clay linersCation exchangeHydraulic conductivity

* Corresponding author.E-mail address: d.guyonnet@brgm.fr (D. Guyonne

0266-1144/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.geotexmem.2009.02.002

a b s t r a c t

The results of a project aimed at identifying performance-based indicators that can be used by landfilloperators to check the suitability of GCLs for bottom barrier applications are presented. The generalmethodology consisted of performing detailed characterization of the prevalent GCLs used in France forlandfill barrier applications, before and after prolonged contact with several fluids during oedo-per-meameter tests. Results of mineralogical analysis illustrate the variety of composition of the testedbentonites, which in addition to smectite clay contain a large number of accessory minerals. For one ofthe GCLs tested, the proportion of smectite was lower than 30 wt%, which highlights the limitations ofthe generic designation ‘‘bentonite’’ when referring to GCLs destined to landfill applications. Results alsounderline the correlation between cation exchange capacity (CEC) and smectite content, the correlationbetween free swell volume and proportion of exchangeable sodium and the influence of the bentonite’scalcium carbonate fraction on hydraulic conductivity. Transmission electron microscopy (TEM) photo-graphs illustrate the effect of cation exchange on clay microstructure, with the formation of clay particleswhich lead to increased hydraulic conductivity. The exchange is also documented by exchangeable cationanalyses. Results of isotopic analyses indicate that information provided by suppliers with respect to the‘‘natural’’ versus ‘‘activated’’ nature of the bentonite, may sometimes be arbitrary and related to factorsthat are very difficult to check in practice, even by the suppliers themselves. This further underlines theneed for performance-based indicators, rather than generic designations, to provide objective infor-mation regarding GCL suitability for landfill applications. Several performance-based indicators areselected in order to provide practical tools for checking the suitability of sodium-bentonite GCLs inbottom barrier applications and limit values are proposed.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Geosynthetic clay liners (GCLs) typically employ granular orpowdered bentonite between two layers of geotextiles. For landfillbarrier applications, the bentonite is generally sodium (Na)bentonite, meaning that sodium ions predominantly constitute theexchangeable cations of the bentonite. Sodium as the exchangeablecation may be the result of natural geological processes, as forWyoming sodium bentonite, or the result of an activation process,whereby calcium bentonite is mixed with soda ash to force the

t).

All rights reserved.

Ca–Na exchange. Because the term ‘‘bentonite’’ is an industrial andnot a mineralogical term, the quality of bentonites used in GCLs forlandfill applications may vary. Bentonite is a mixture of a variety ofminerals, the predominant mineral being smectite clay. It is thesmectite (mainly montmorillonite) which confers the bentoniteits swelling properties and low hydraulic conductivity. Becausesmectite has a stronger thermodynamic affinity for divalent cations(primarily calcium and magnesium) than for sodium, the bentoniteof a GCL may exchange its sodium for other cations if they arepresent in the fluids with which the GCL comes into contact.

In recent years, several researchers have investigated thisexchange mechanism and its consequences on the hydraulicconductivity of GCLs. For example Petrov and Rowe (1997) andRowe (2007) reported the effect of the electrolyte concentration of

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331322

NaCl solutions on GCL hydraulic conductivity. Ruhl and Daniel(1997) addressed changes in both electrolyte concentration andnature of the contact-solution cation. Petrov and Rowe (1997) andRowe et al. (2004) note that GCL prehydration with a chemicallynon-aggressive fluid, prior to contact with divalent cation-richfluids, helps to maintain low values of hydraulic conductivity. Leeand Shackelford (2005) showed that the effect of prehydrationdepends on the concentration of the permeation fluid and may beinsignificant in the case of permeant liquids containing relativelylow concentrations of inorganic solutes. Shackelford et al. (2000)and Jo et al. (2001) stress the importance of allowing for sufficientequilibration time during hydraulic conductivity tests in order toobtain representative hydraulic conductivities. Extremely largenumbers of pore volumes (up to 686) were reached by Jo et al.(2005), who in some cases observed variations of hydraulicconductivity after more than 100 pore volumes. Jo et al. (2006) usedmodeling to estimate times required to establish chemical equi-librium in GCLs in contact with divalent cation-rich fluids andobserved general agreement with their experimental data. Kolstadet al. (2004a) used an indicator (designated RMD) of the relativeabundance of monovalent versus divalent cations in leachate, topredict the likelihood of hydraulic conductivity increase in GCLs inbottom barriers. For typical documented leachates they suggestthat significant increases are not likely to occur. Guyonnet et al.(2005) investigated the correlation between surface chemistry,microstructure and permeability, on a natural sodium-bentoniteGCL and a sodium-activated calcium-bentonite GCL. They foundthat the natural sodium-bentonite GCL consistently displayedsuperior hydraulic performance compared to the activatedbentonite GCL during oedo-permeameter tests. They also notedthat the activated bentonite had a calcium carbonate content of10.3 wt% compared to only 3.4 wt% for the natural sodiumbentonite.

Effects of ion exchange on the geotechnical properties of GCLswere investigated by Bouazza et al. (2007) who studied Atterberglimits and noted that chemical equilibrium was not typically ach-ieved during Atterberg limit tests. Bouazza et al. (2006) note also aneffect of ion exchange on the gas permeability of GCLs subject towet-dry cycles. Touze-Foltz et al. (2006) studied load-penetrationcurves of GCLs subject to contact with cation-rich fluids. The effectsof ionic exchange on solute diffusion coefficients have also beeninvestigated (Melkior et al., in press; Malusis and Shackelford,2002) while the compatibility of GCLs with fluids other than cation-rich solutions, such as landfill leachates, has been studied in thecase of volatile organic compounds (Lake and Rowe, 2004) or jetfuel (Rowe et al., 2006, 2008).

The influence of cation exchange on the deterioration ofhydraulic performance has also been documented by field inves-tigations which, for reasons of ease of access, relate nearly

Table 1Some characteristics of the tested GCLs.

Code Type ofbentonitea

Mass perunit areaa

(kg/m2)

Measured massper unit area(wet) (kg/m2)

Measuredwatercontentwt%

Measured massper unit area(dry) (kg/m2)

LX1 Natural Na 5 5.1 14.8 4.5LX2 Natural Na 5 7.0 14.3 6.1LX3 Natural Na 5.3 6.3 9.1 5.7LX4 Na-activated Ca 5 5.2 10.5 4.7LX5 Na-activated Ca 5 5.7 19.4 4.7LX6 Natural Ca 5 5.4 11.5 4.9LX7 Na-activated Ca 4.5 5.2 8.7 4.8LX8 Natural Ca 10 10.3 9.5 9.4

a Based on GCL supplier information.

exclusively to GCLs in landfill covers rather than in bottom barriers(e.g. Meer and Benson, 2007; Melchior, 2002) or to GCLs in waterlagoons (Benson et al., 2004). While some authors (e.g. Egloffstein,2001) have suggested that a soil cover of at least 0.75 m thickness isrequired to protect a GCL (Egloffstein et al., 2002 recommend atleast 1 m), few indications regarding the type and chemistry of theprotective soil have been provided in order to reduce the likelihoodof cation exchange. According to Benson et al. (2007), who alsoinvestigated a landfill final cover, a surface layer of 0.75 m isunlikely to protect conventional GCLs from damage caused bycation exchange and dehydration. These authors recommend theuse of geomembranes or geofilms to protect the GCLs from cation-rich infiltrating rainwater.

In order to attenuate the effects of ion exchange, modifiedbentonites are currently being developed (Katsumi et al., 2008;Lorenzetti et al., 2005; Kolstad et al., 2004b). However, theeconomical aspects of their implementation still need to beaddressed. In previous research, detailed information regarding themineralogical composition of the bentonite, and in particularaccessory minerals that accompany the smectite (e.g. Andrejkovi-cova et al., 2008; Dananaj et al., 2005; Kolstad et al., 2004a) is oftenlacking. Yet this appears to be one of the primary factors influencingGCL hydraulic performance. Egloffstein et al. (2002) recommendsmeasuring the calcium carbonate content of the bentonite. Morerecently, Benson et al. (2008) report accessory mineral contents (inparticular calcite but also feldspar, cristobalite and quartz) of testedbentonites, in studies of GCL chemical compatibility and attributeobserved increases in hydraulic conductivity to electrolyteconcentration and granule size effects, but suggest also thatdissolution of calcite may have played a role.

Geosynthetic clay liners may present significant advantages forlandfill barrier applications; e.g. ease of implementation comparedto compacted clay liners, competitive cost, etc. (Bouazza, 2002,presents a comparison of advantages and disadvantages), butlandfill operators need tools to help them check whether a givenproduct is suitable for a given application, following delivery of theproduct to the landfill site. For obvious reasons of time constraints,it is not technically feasible to use oedo-permeameter tests in orderto check the hydraulic conductivity between the moment it isdelivered in the field and the moment it is installed. Operators needtests that are cost-effective and fast to implement. The objective ofthe LIXAR2 project (Guyonnet et al., 2008) was to identify whichindicators should be tested, based on basic characterization testsand oedo-permeameter tests performed with different contactfluids. This paper presents the methods used, the results andrecommendations with respect to indicators of the potential suit-ability of a GCL for landfill bottom barrier applications.

2. Materials and methods

Eight GCLs were provided by the main GCL suppliers operatingin France. The products were selected to be representative of GCLsused in bottom barrier applications, but some products were morerepresentative of landfill cover liners. Six of the GCLs were sodium-bentonite GCLs, while two were calcium-bentonite GCLs. To ensurerepresentativeness, the products were selected by the leadingauthor directly on the production site (i.e., Poland and Germany)among a choice of at least 10 GCL rolls. The first three meters of theselected roll were discarded. Nine additional meters were unrolled,weighed, marked and conditioned for transport to the testinglaboratories in France. Some basic characteristics of the eight GCLsare presented in Table 1. In this table, some information wasprovided by the GCL supplier. As will be seen in the following, forone product, the information concerning the type of bentonite wascontradicted by independent analysis.

Collection vial

Fuidoutlet

Fixationscrew

NormalstressDisplacement

gauge

Purge

Purge

Mariotte tube

Fluidentry

Porous plates

GCL

O-ring

h

Piston

Base

Fig. 1. Schematic of the oedo-permeameter cell (from Guyonnet et al., 2005).

Table 2Confinement conditions during the oedo-permeameter tests.

Swelling phase Percolation phase

s (kPa) 10 25 50 100

Tests with HF HF HF HF HFTests with SL HF SL SL SLTests with RL HF RL RL RL

Notes: HF¼ hydration fluid, SL¼ synthetic leachate, RL¼ real leachate.

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331 323

The overall testing procedure consisted of detailed character-ization of the bentonites, before and after prolonged contact withthree different fluids (see below) during oedo-permeameter tests,with the objective of correlating the results to possible modifica-tions of bentonite chemical and/or structural characteristics. Globalchemical analyses of the bentonites were performed using X-rayfluorometry. These results were used to determine the proportionsof the various minerals associated within the bentonites, frommodal calculations (Blanc et al., 2006). They also guided the inter-pretation of XRD patterns, using methods of automated adjustmentof simulated patterns, as described in Blanc et al. (2007). Chemicalanalyses were performed on eluents obtained from leaching testsperformed according to standard AFNOR (2002a) at a liquid-to-solid ratio of 25. Carbonate analyses were performed according tostandard AFNOR (1996) which uses the acid attack and CO2

measurement method. The cations occupying the clay exchangesites were analyzed according to Remy and Orsini (1976), wherebya mass of clay is placed in contact with a concentrated solution ofcobaltihexamin trichloride (Co(NH3)6Cl3; 4 g/L) for one hour. Thequantity of cobaltihexamin adsorbed is directly converted intoa cation exchange capacity (CEC) taking into account the massesand volumes used. As cobaltihexamin is a colored molecule, the ionexchange capacity is measured by spectrocolorimetry as thedifference between the initial solution and the final solution afteradsorption. The cations displaced into solution by cobaltihexaminwere analyzed using capillary electrophoresis. For the analysis ofexchangeable ammonium (NH4

þ), a modified method was usedbased on cesium as the exchanger, as the cobaltihexamin intro-duces a bias for this cation.

Carbon and oxygen isotope analyses were performed on thecarbonate fraction of the bentonites using the method described byLandis and von Maubeuge (2004). Following the origin of the water(marine or not) that circulated through the pyroclastic depositswhich led to the formation of the bentonite, the water temperature,the mode of calcium carbonate precipitation, etc., the isotopiccomposition of the carbon and oxygen in the CaCO3 present as anaccessory mineral in the bentonite will vary. The isotopic ratios areexpressed with respect to an international standard; PDB (Pee Dee

Belemnite, a fossil belemnite found in the Cretaceous Pee DeeFormation in North Carolina, USA). This method is used routinely inFrance to check the natural versus activated nature of bentonitesused in GCLs for landfill applications. Unpublished blind tests,performed by the authors of this paper, suggest that the method isreliable, except for complex mixtures of bentonites from differentorigins. Ratios are calculated as (Rundel et al., 1989):

d13C ¼13C=12Csample � 13C=12C

standard13C=12Cstandard

� 1000 (1)

and:

d18O ¼18O=16Osample � 18O=16O

standard18O=16Ostandard

� 1000 (2)

Images of hydrated bentonite microstructures, ranging froma few micrometers to less than a nanometer, were obtained usingtransmission electron microscopy (TEM). The observations werecarried out with a PHILIPS CM20 microscope operating at 200 kVwhich has a point resolution of 0.14 nm. Samples were preparedusing the method described by Srodon et al. (1990) which involvesthe successive replacement of the clay-water by methanol andfinally L.R. White resin. This procedure allows the preservation ofthe microstructure of hydrated clay particles during the embeddingprocess (Elsass, 2006; Tessier, 1984; Spurr, 1969). Sections of around70 nm thickness were cut with a diamond knife, using a Reicher-

Table 3Composition of the real leachate; RL (ionic strength¼ 0.1 M/L).

pH Cond(mS/cm)

Fe tot(mg/L)

Fe2þ

(mg/L)DIC(mg C/L)

DOC(mg C/L)

CH3COO�

(mg/L)Cl�

(mg/L)Br�

(mg/L)

7.25 9930 4 4 1136 562 <0.25 1382 7.96

NO3�

(mg/L)SO4

2�

(mg/L)S2O3

2�

(mg/L)PO4

3�

(mg/L)Naþ

(mg/L)NH4þ

(mg/L)Kþ

(mg/L)Mg2þ

(mg/L)Ca2þ

(mg/L)

0.97 37.4 1.78 <0.25 1068.8 1080.8 751.6 113.2 62.4

Al(mg/L)

Cr(mg/L)

Co(mg/L)

Ni(mg/L)

Cu(mg/L)

Zn(mg/L)

As(mg/L)

Cd(mg/L)

Pb(mg/L)

375 110 29 140 430 5 39 1 8

Notes: Cond¼ electrical conductivity, DIC¼ dissolved inorganic carbon,DOC¼ dissolved organic carbon.

Table 4Composition (mg/L) of the synthetic leachate; SL (Ionic strength¼ 0.21 M/L).

pH Ca2þ Kþ Mg2þ Naþ NH4þ Cl� SO4

2� COT

7 1042 665 365 690 720 3474 480 1441

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331324

Jungt Ultracut E microtome, and deposited on carbon-coveredcopper TEM grids.

Swell tests were performed according to standard AFNOR(2002b), whereby two grams of dried and ground bentonite aredropped into 100 mL of fluid and the volume occupied by thebentonite is measured after 24 h. Methylene blue tests were alsoperformed. Twenty four oedo-permeameter tests were performedaccording to standard AFNOR (2008); a testing standard designedfor measuring the hydraulic conductivity of GCLs using a rigid-walloedo-permeameter. A characteristic of this standard is that thedimensions of the permeameter should allow a GCL testing surfaceof at least 0.2 m in diameter. It is therefore well suited for testingneedle-punched but also stitch-bonded GCLs, as this surface should

Table 5Salt concentrations (moles/L) used to generate the synthetic leachate.

Ca(OH)2 CaCl2 MgCl26(H2O) KOH NH4Cl MgSO47(H2O) CH3COOH CH3COONa

0.007 0.019 0.010 0.017 0.040 0.005 0.030 0.030

Table 6Results of mineralogical analysis of the bentonite compositions (wt%).

Minerals LX1 LX2 LX3 LX4 LX5 LX6 LX7 LX8 M

Albite 1.0 3.6 2.9 0.0 0.9 0.0 0.0 1.4 GAnorthite 0.0 4.4 4.4 0.0 1.7 0.0 0.0 0.0 BaMicrocline 0.2 2.5 0.8 0.0 0.0 0.0 0.0 0.8 PyOligoclase 0.0 0.0 0.0 7.6 3.4 0.0 3.2 0.0 To

Totalfeldspath 1.2 10.5 8.1 7.6 6.0 0.0 3.2 2.3 DC. Org. 0.4 0.2 0.1 0.0 0.1 0.1 0.1 0.0 AAragonite 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 FlAnkerite 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 HCalcite 1.8 0.0 1.0 7.0 1.5 0.4 1.6 0.9 HDolomite 0.0 1.2 0.0 0.9 0.0 0.0 0.0 0.0 MSiderite 0.5 0.0 1.4 0.8 1.4 0.0 1.1 0.0 A

Totalcarbonates 2.3 1.2 2.4 9.2 2.9 0.4 2.7 0.9 RuCristobalite 0.0 4.5 0.7 1.0 0.0 8.7 1.4 1.9 IllOpal 5.1 5.2 2.4 4.1 2.9 5.6 3.8 2.9 KQuartz 4.4 8.3 11.5 1.3 4.8 9.0 5.6 7.3 MTot SiO2 9.5 18.0 14.6 6.4 7.7 23.4 10.9 12.1 To

include a sufficient number of discontinuities due to stitches inorder to be representative of the product’s hydraulic behaviour.Furthermore, one advantage of the rigid-wall versus flexible-walloedo-permeameter is that a soil layer in contact with the GCL canbe included (this was not the case in this study). The standard isalso well suited for a detailed monitoring of the thickness of theGCL during the swelling phase, using a displacement sensor, ratherthan measuring thickness only once the sample has been retrievedfrom the permeameter.

The apparatus used in this study was a rigid-wall oedo-per-meameter composed of two main parts; a piston and a base asdescribed in Fig. 1. The piston is connected to a loading devicewhich allows the application of a confining pressure while a Mari-otte bottle allows the application of a constant hydraulic head(Fig. 1). Because the research program was focused on bottombarrier applications, a testing procedure was defined in order to‘‘mimic’’ confining conditions in bottom liners. Tests started witha saturation phase, under 10 kPa, using a ‘‘hydration fluid’’ (notedHF) and no pressure head. The hydration fluid was a 10�3 M NaClsolution, i.e., a low electrolyte concentration. The composition andconcentration of this fluid are such that it should not be at allchemically aggressive with respect to the tested bentonites. Thisdilute NaCl solution was preferred for prehydration to a calcium ormagnesium solution (e.g. Vasko et al., 2001) because it was antic-ipated that the possible influence of calcite dissolution, duringpercolation, on GCL hydraulic conductivity would be easier toidentify. When at least 90% of equilibrium swelling had beenreached, as described in Norotte et al. (2004), the test fluid wasreplaced (for those tests involving other fluids than the hydrationfluid; HF) and confining pressure was increased gradually, witha first step at 25 kPa, one at 50 kPa and finally one at 100 kPa(Table 2). The rationale for this procedure was to attempt toreproduce field situations, where waste is progressively landfilledand hence confining stress gradually increases. Tests were notperformed on non-prehydrated samples because, due to thenumber of tests, it was preferred to select test conditions that werecharacteristic of ‘‘normal’’ landfill conditions rather than alteredconditions. Under normal conditions the GCL, which is protected bya geomembrane and in contact with the subgrade for some timebefore waste is landfilled, is expected to hydrate before it comesinto contact with landfill leachate.

Consistent with NF P 84-705 (AFNOR, 2008), during eachconfining pressure step, three steps of hydraulic head were used:

inerals LX1 LX2 LX3 LX4 LX5 LX6 LX7 LX8

ypsum 0.4 0.5 0.0 0.0 0.6 0.0 0.1 0.0rite 0.1 0.0 0.4 0.8 0.0 0.5 0.0 0.0rite 0.1 0.3 0.3 0.1 0.0 0.0 0.2 0.0tal S 0.6 0.8 0.6 0.9 0.6 0.5 0.3 0.1

iopside 0.0 0.0 0.0 0.0 0.0 0.8 0.0 1.2patite 0.1 0.0 0.0 0.1 0.2 0.1 0.2 0.0uoroapatite 0.8 0.0 0.1 0.1 0.2 0.1 0.2 0.0alite 1.8 0.0 0.0 0.0 0.9 0.0 0.0 0.0ematite 0.3 0.0 0.0 0.0 2.2 0.0 0.9 0.0aghemite 0.0 0.0 0.0 0.0 4.2 0.0 2.4 0.0

natase 1.2 0.0 0.0 0.6 0.9 0.0 0.0 0.1

tile 0.0 0.0 0.0 0.0 0.0 0.7 0.6 0.0ite 0.0 0.5 0.0 0.0 0.4 8.9 0.0 2.5aolinite 0.0 0.0 0.0 0.0 1.1 35.5 0.0 4.2uscovite 0.6 0.0 2.1 0.0 1.3 0.0 3.7 0.0t smectite 76.5 68.8 71.3 75.0 71.5 29.6 74.8 76.8

Table 7Some results of initial characterization.

Sample pHa Electricalconductivitya

(mS/cm)

Chloridea (mg/L) Calcite wt% Smectite wt% CEC (meq/100 g) Exchangeable cations (% exchangesites)

Ca K Mg Na

LX1 9.75 1134 2856 2.1 76.5 88.0 7.6 0.5 7.8 84.1LX2 9.55 525 113 1.5 68.8 77.2 27.5 1.2 6.2 65.1LX3 9.8 563 101 1.4 71.3 80.6 27.8 1.8 3.4 66.9LX4 10 980 340 4.9 75.0 82.8 5.1 1.4 6.0 87.5LX5 9.95 961 2236 2.7 71.5 87.0 10.6 0.5 9.9 79.1LX6 7.7 122 107 0.5 29.6 38.1 67.6 1.2 24.8 6.4LX7 9.85 1016 1621 2.8 74.8 83.5 12.6 1.2 5.3 81.0LX8 8.7 231 328 0.4 76.8 80.1 60.3 0.6 27.9 11.3

Notes: CEC¼ cation exchange capacity.a Measured in eluent from leachate tests.

Cation exchange capacity (meq/100 g)

0

20

40

60

80

100

Smec

tite

con

tent

(w

t)

a

0 20 40 60 80 100 0 10 20 30 40 50

Methylene blue value (g/100 g)

b

This study

Kaufhold et al. (2002)

Fig. 2. Cation exchange capacity versus smectite content (a); Methylene blue value versus smectite content (b). Note: smectite data from Kaufhold et al. are reported with anuncertainty of �5 wt%.

-20

-15

-10

-5

0

5 Wyoming natural Na-bentonite Indian activated bentonite

Greek activated bentoniteGreek natural Ca-bentonite

18O

vs

PD

B

South AF & Austr. activatedbentonite

32

75 1

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331 325

0.1, 0.3 and 0.6 m. For each step, the specific volumetric flux (m3/m2/s) under constant head conditions was measured. Specific fluxwas plotted against hydraulic gradient in order to check thata straight line was obtained and hence the validity of Darcy’s lawfor the given experimental conditions. In addition to the dilute NaClsolution, two other test fluids were used: a synthetic leachate(noted SL) and a real leachate (noted RL) collected from the(covered) leachate pond of a French domestic solid waste landfillwith stabilized methanogenic conditions. The composition of thereal leachate is presented in Table 3. The composition of thesynthetic leachate was selected based on a review of leachatecompositions in France (SITA-FD, pers. comm.) and of literaturedata (Kjeldsen et al., 2002), with the objective of obtaininga leachate composition that was characteristic of a ‘‘young’’leachate, rich in divalent cations and hence potentially ‘‘aggressive’’with respect to a sodium bentonite. The synthetic leachatecomposition is presented in Table 4, while Table 5 shows the saltconcentrations used to generate this composition. It should benoted that while the aim was to mimic a ‘‘young’’ and, therefore,more aggressive leachate, the fluid described in Table 4 is far lessaggressive than the 0.1 M CaCl2 solution used in Guyonnet et al.(2005) Lee et al., (2005) and others.

-20 -15 -10 -5 0 513C vs PDB

Fig. 3. Results of carbon and oxygen isotope analyses. Data from this study (numbers)are compared to data (symbols) from Landis and von Maubeuge (2004) and Decheret al. (1996).

3. Results

Results of mineralogical analyses, from XRD and global chemicalanalyses, are presented in Table 6. Although not shown, results for

the individual varieties of smectite suggest a predominance of Na-Montmorillonite for all sodium bentonites, with varying propor-tions of Mg-Montmorillonite and Na-Nontronite. One of thesamples (LX6) has a smectite content of less than 30 wt%. The mostabundant clay in this sample is kaolinite; yet it is neverthelessreferred to by its supplier as ‘‘bentonite’’. This reflects the fact that

0

5

10

15

20

25

30

35

40

LX1 LX2 LX3 LX4 LX5 LX6 LX7 LX8

Sample

Fre

e sw

ell i

ndex

(cm

3/2g

)

HF (fresh samples) SL (fresh samples)

RL (fresh samples) DI (HF+SL samples)

DI (HF+RL samples)

Fig. 4. Results of swell tests. HF (fresh samples)¼ swell test on fresh bentonite samples with hydration fluid (HF); SL (fresh samples)¼ ditto with synthetic leachate (SL); RL (freshsample)¼ ditto with real leachate (RL); DI (HFþ SL samples)¼ swell tests with deionised water on samples retrieved from oedo-permeameter tests during which prehydrated GCLswere permeated with synthetic leachate; DI (HFþ RL)¼ ditto on samples from oedo-permeameter tests during which prehydrated GCLs were permeated with real leachate.

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331326

the term ‘‘bentonite’’ is not a mineralogical term and may be usedincorrectly to refer to materials of extremely diverse mineralogicalcompositions. It also further stresses the need for indicators thatrelate to hydraulic performance. Table 7 summarizes some basicmineralogical and chemical characteristics of the samples. Acorrelation was observed between the cation exchange capacityand the smectite content, as shown in Fig. 2(a), where the datacollected in this study are plotted along with data collected onGerman, Slovakian and Greek bentonites presented in Kaufholdet al. (2002; their Table 1). The correlation was much better thanwith the methylene blue test, which showed more scatter(Fig. 2(b)) due in particular to the difficulty, with strongly sorbingmaterials, to precisely identify the moment when sorption ofmethylene blue ceases. Such results suggest that CEC might be usedas a useful indicator of smectite content for controlling GCLs in fieldsituations, as it is relatively fast and cheap to implement, particu-larly compared to quantitative interpretation of XRD patterns: inthe case of relatively complex mineralogical compositions such asthose of bentonites, reliable XRD analysis requires correlating XRDpattern modeling with global chemical analyses.

Fig. 3 shows results of the isotopic analyses performed on thesodium bentonites. In this figure, each LIXAR2 sample is repre-sented by its sample number. The data are compared to those ofLandis and von Maubeuge (2004) and Decher et al. (1996). Results

Table 8Hydraulic conductivities measured during oedo-permeameter tests at 100 kPa confining

HF SL

k (m/s) Durationa (days) nv k (m/s) D

LX1 2.4� 10�11 14/148 3.4 2.5� 10�11 1LX2 1.2� 10�11 8/158 2.1 1.0� 10�10 2LX3 1.4� 10�11 29/136 1.0 3.3� 10�11 1LX4 3.5� 10�11 19/91 2.4 1.1� 10�10 1LX5 1.8� 10�11 24/160 3.5 3.7� 10�11 1LX6 4.5� 10�9 27/41 1.3 2� 10�10 1LX7 1.1� 10�11 8/145 2.4 4.5� 10�11 2LX8 4.7� 10�10 12/70 15.1 1.8� 10�9 3

Note: nv¼ number of pore volumes, i.e., volume of effluent divided by volume of void ia First number is duration of saturation phase under 10 kPa, second number is total d

suggest that: samples LX2 and LX3 are natural Na-bentonite fromWyoming, sample LX4 is a Greek activated bentonite, whilesamples LX1, LX5 and LX7 are activated bentonites. The analysesconfirm the information provided by the suppliers for all samplesexcept sample LX1. The activated nature of sample LX1 is furtherconfirmed by the very high electrical conductivity of bentonitesuspensions and high chloride concentration measured in theeluent during leachate tests (Table 7), indicating salt contentstypical of an activation process. The similarity between character-istics of samples LX1 and LX5 suggests that they most probablycome from the same deposit. The initial information provided bythe GCL supplier, who is not a bentonite manufacturer, that sampleLX1 was a natural sodium GCL, simply reflected informationdirectly provided by his bentonite supplier. Such information isdifficult to check as it depends on events occurring at the scale ofthe bentonite deposit (in this case in India). This shows that thedesignation ‘‘natural’’ versus ‘‘activated’’ sodium bentonite maysometimes be subjective and further underlines the need forperformance-based criteria.

Results of swell tests are presented in Fig. 4 which shows datafrom tests performed on two different types of bentonite samples:fresh samples and samples retrieved from the oedo-permeametercells at the end of testing. Swell tests were performed on the freshsamples using the three test fluids (HF, SL, RL; see Fig. 4). Tests on

pressure.

RL

urationa (days) nv k (m/s) Durationa (days) nv

9/97 1.8 1.0� 10�11 11/122 3.28/126 6.6 1.7� 10�11 13/141 4.18/97 1.6 1.0� 10�11 10/108 1.98/165 9.1 4.4� 10�11 19/112 10.88/80 3.0 1.0� 10�11 11/112 3.95/81 14.2 1.1� 10�10 11/111 9.78/171 13.2 1� 10�11 22/87 2.64/147 37.2 6.0� 10�10 31/144 21.2

n the specimen.uration of percolation phase.

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

LX1 LX2 LX3 LX4 LX5 LX6 LX7 LX8

Sample

Hyd

raul

ic c

ondu

ctiv

ity

(m/s

)

HF + HF HF + SL

HF + RL

Fig. 5. Hydraulic conductivities measured at the end of testing under 100 kPaconfining pressure. HFþHF¼ hydration fluid used for the swelling and percolationperiods; HFþ SL¼ hydration fluid used for the swelling period, synthetic leachate usedfor the percolation period; HFþ RL¼ hydration fluid used for the swelling period, realleachate used for the percolation period.

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331 327

the oedo-permeameter samples were performed using deionizedwater (standard AFNOR, 2002b) and are noted HFþ SL for thesamples extracted following oedo-permeameter tests performedwith synthetic leachate on prehydrated GCLs and are noted HFþ RLfor samples from oedo-permeameter tests performed with realleachate on prehydrated GCLs. Note that swell tests performed onfresh samples with deionised water or the prehydration fluidshowed quasi-identical results. As seen in Fig. 4, swell indices forfresh sodium bentonites (whether natural or activated) are verylow when the bentonites are put in direct contact with thesynthetic leachate. Direct contact with the real leachate hada significant but lesser influence. Swell tests with DI water per-formed on Na-bentonites extracted from the oedo-permeametersshowed much smaller effects of the synthetic and real leachates,due most probably to prehydration with the HF fluid during theoedo-permeameter tests. This was especially the case for LX1 andLX3, and to a lesser extent for the other sodium bentonites. As couldbe expected, the Ca-bentonites (LX6 and LX8) showed much lowerswell indices.

Results of hydraulic conductivity measurements are shown inTable 8 along with test durations and the number of pore volumesachieved during the tests (defined as the volume of effluent divided

Time since start of test (days)

1E-12

1E-11

1E-10

1E-9

Hyd

raul

ic c

ondu

ctiv

ity

k (m

/s)

a

0 50 100 150 2

Fig. 6. Results of oedo-permeameter tests with sample LX5; (a): as a function of time sincecollected out of cell.

by the volume of voids in the specimen). Fig. 5 shows results ofhydraulic conductivities measured at the end of the tests whileFig. 6 gives an example of observed variations of hydraulicconductivity as a function of time as well as pore volumes. Giventhe number of tests performed and due to time constraints and lowhydraulic conductivities, for some of the tests, the number of porevolumes achieved is low (see Table 8). Also, considering results of Joet al. (2001) and others, chemical equilibrium was probably notattained during most of the tests, as suggested by monitoring ofelectrical conductivity and pH. Fig. 7 shows measured electricalconductivity and pH during several tests: while electrical conduc-tivities are relatively stable at the end of testing, pH values showfluctuations which suggest chemical disequilibrium.

However, notwithstanding the issue of chemical equilibrium,results in Fig. 5 suggest that while all sodium-bentonite GCLsmaintained low values of hydraulic conductivity, values for thecalcium-bentonite GCLs were significantly higher. The syntheticleachate was found to increase hydraulic conductivities of thesodium-bentonite GCLs (see also Fig. 6), compared to valuesmeasured with HF, although increases were not dramatic. The realleachate, on the other hand, generally led to slightly lower values ofhydraulic conductivities compared to the values measured with HF.It is hypothesized that this decrease may be due either to precipi-tation of salts (see Benson et al., 2008; Lake et al., 2007) or toblockage due to colloidal or particulate organic matter (the realleachate had a measured suspended matter content of 250 mg/L).In the case of the calcium-bentonite GCLs, hydraulic conductivity isseen in Fig. 5 either to decrease with SL or RL (LX6) or else to stayvirtually the same (LX8).

Fig. 8 illustrates some examples of transmission electronmicroscopy (TEM) photos taken for sample LX7. Fig. 8(a) shows themicrostructure (scale¼ 100 nm) of the bentonite hydrated at150 wt% with the hydration fluid. The photo shows a typical ‘‘gel-type’’ structure (see Tessier, 1984; Guyonnet et al., 2005), withindividual clay layers or particles made of just a few clay layers,separated by large distances. Such a structure leads to very lowhydraulic conductivities because the water is bound by electrostaticforces. Contact with the synthetic leachate during the oedo-per-meameter tests (Fig. 8(b)) led to the formation of particles made upof many clay layers (40–60 or more). This structure, which isreferred to as a ‘‘hydrated solid’’, has a higher hydraulic conduc-tivity because the water located between the particles is moremobile. Contact with the real leachate during oedo-permeametertests yields an intermediate picture (Fig. 8(c)), with gel phase and

00 0 1 2 3 4

Pore volumes out

b Hydration fluid

Synthetic leachate

Real leachate

start of test (including saturation phase); (b): as a function of number of pore volumes

Pore volumes out

6

8

10

12

14

16

18

20

Ele

ctri

cal c

ondu

ctiv

ity

(mS/

cm)

a

0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20

Pore volumes out

7

8

9

10

pH

b

LX4

LX7

LX8

Fig. 7. Example of electrical conductivity (a) and pH (b) monitoring: oedo-permeameter tests performed with synthetic leachate on prehydrated samples of GCLs LX4, LX7 and LX8.

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331328

some particles. Transmission electron microscopy (TEM) images forall samples are presented in Guyonnet et al. (2008) which can beaccessed on the BRGM’s website. They suggest that the influence ofthe synthetic leachate on the clay microstructure depends on itsinitial structure following hydration with the hydration fluid. If,following hydration, there is a good development of gel phase, thencontact with the synthetic leachate leads to less development ofhydrated solid phase.

The trends observed in the TEM photographs are also seen in theresults of exchangeable cation analyses. Fig. 9 shows results ofexchangeable cation analyses performed on sample LX7. Thedesignation ‘‘initial’’ refers to an analysis performed on the freshbentonite hydrated at 150 wt% with deionised water. The othercolumns refer to analyses performed on the GCLs retrieved fromthe oedo-permeameters, following contact with the different fluids(HF, SL, RL). Fig. 9 shows that, following contact with the syntheticleachate, the proportion of sodium in the clay of sample LX7 issignificantly reduced compared to the initial value or valuesmeasured following contact in the oedo-permeameters with eitherthe hydration fluid or the real leachate. Results of exchangeablecation analyses for other samples are summarized in Table 9. Theincreases of the proportions of calcium, with respect to the initialproportions, observed in four of the six sodium bentonitesfollowing percolation with the prehydration fluid, suggest somedissolution of the calcite associated with the bentonite.

4. Discussion

The data presented herein confirm results by Kolstad et al.(2004a) and Guyonnet et al. (2005) suggesting that, due in partic-ular to the presence of cations other than divalent cations (Table 3),real leachates from domestic solid waste landfills might not have

Fig. 8. Transmission electron microscope images of sample LX7 following contact with (a)followed by real leachate. Scale¼ 100 nm.

dramatic detrimental influence on the hydraulic conductivity ofGCLs. It can be expected that detrimental effects of cation exchangemay be far more dramatic in landfill covers than in bottom barriers,especially if the GCL is not protected by a membrane (Benson et al.,2007). Rainwater being very dilute, it will dissolve carbonates in thenatural cover materials, putting into solution divalent cations thatare then available for exchange.

Kolstad et al. (2004a), and (2006), used the RMD as an indicatorof the relative abundance of monovalent versus divalent cationsand associated this indicator with ionic strength to examine theeffects of leachates on hydraulic conductivity:

RMD ¼ MMffiffiffiffiffiffiffiffiMD

p (3)

where MM¼ total molarity of monovalent cations and MD¼ totalmolarity of divalent cations. They preferred this indicator to the SARindicator (McBride, 1994) as the latter only accounts for two divalentcations. From the data in Table 3, we obtain a RMD¼ 1.6 M1/2 for thereal leachate (with an ionic strength I¼ 0.10 M), a value which isquite high compared to the data reported for several leachates inKolstad et al. (2004a). A question, however, is whether these authorstook into account ammonium (NH4

þ) and potassium. Ammonium isabundant in domestic waste landfill leachate and although thismonovalent cation is certainly less hydratable than sodium, itsinfluence on clay structure is far less detrimental than calcium ormagnesium. It is therefore proposed to include NH4

þ (and Kþ) in thecalculation of RMD. If we consider only sodium as a monovalentcation, we obtain RMD¼ 0.59 M1/2 which is more in the range ofvalues reported in Kolstad et al. (2004a). Application of Eq. (3) to thecomposition of the synthetic leachate (Table 4) yieldsa RMD¼ 0.43 M1/2 if all cations are considered and RMD¼ 0.15 M1/2

if only Na is considered.

hydration fluid, (b) hydration fluid followed by synthetic leachate, (c) hydration fluid

20

0

10

30

40

50

60

70

80

90

100

Initial HF + HF HF + SL HF + RL

Pro

port

ion

of c

atio

ns

Na NH4 Mg K Ca

Fig. 9. Sample LX7: results of exchangeable cation analyses. HFþHF¼ sampleretrieved following oedo-permeameter test performed on prehydrated samplepermeated with prehydration fluid; HFþ SL¼ ditto for prehydrated sample permeatedwith synthetic leachate; HFþ RL¼ ditto for prehydrated sample permeated with realleachate.

Table 10Changes in log hydraulic conductivity predicted from application of Eq. (4) fromKolstad et al. (2006). RMD1 considers all monovalent cations while RMD2 considersonly sodium.

I (M) RMD1 (M1/2) log Kc/log KDI RMD2 (M1/2) log Kc/log KDI

Real leachate 0.10 1.6 1.00 0.59 0.92Synthetic leachate 0.21 0.43 0.80 0.15 0.78

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331 329

Kolstad et al. (2006) propose an equation, based on datacollected on non-prehydrated GCLs, for relating changes inhydraulic conductivity to RMD and ionic strength:

log Kc

log KDI¼ 0:965� 0:976I þ 0:0797RMDþ 0:251I2RMD (4)

where: Kc¼ hydraulic conductivity to the inorganic chemicalsolution; KDI¼ hydraulic conductivity to deionized water. Applica-tion of this equation to the real leachate and the synthetic leachateyields results in Table 10. Although the oedo-permeameter testsperformed herein were all for prehydrated GCLs, these resultsconfirm that the synthetic leachate is far more likely to inducesignificant changes in hydraulic conductivity than the real leachate.

Table 9Summary of proportions of exchangeable cations (%).

Initial After HF After SL After RL Initial After HF After SL After RL

LX1 LX2Ca 7.6 22.4 18.6 13.8 27.5 25.4 35.8 17.4K 0.5 0.7 2.8 8.8 1.2 1.4 11.3 8.3Mg 7.8 16.0 14.7 7.9 6.2 7.7 25.7 6.4NH4 0.0 0.0 3.9 17.0 0.0 0.0 18.6 1.9Na 84.1 60.9 60.0 52.5 65.1 65.5 8.6 65.9

LX3 LX4Ca 27.8 28.6 31.1 18.5 5.1 13.5 18.1 22.5K 1.8 2.0 3.5 7.3 1.4 1.3 7.5 16.6Mg 3.4 14.9 15.8 11.7 6.0 10.5 26.8 16.3NH4 0.0 0.0 4.8 11.4 0.0 0.0 10.9 0.0Na 66.9 54.5 44.9 51.0 87.5 74.7 36.7 44.6

LX5 LX6Ca 10.6 22.7 22.5 12.9 67.6 68.4 43.9 24.9K 0.5 0.6 3.3 9.9 1.2 1.2 9.9 16.9Mg 9.9 15.5 21.2 11.8 24.8 22.7 20.0 15.3NH4 0.0 0.0 4.0 11.8 0.0 0.0 18.8 17.6Na 79.1 61.2 49.1 53.6 6.4 7.7 7.5 25.3

LX7 LX8Ca 12.6 18.6 25.6 15.9 60.3 68.0 39.1 33.9K 1.2 1.2 8.4 6.3 0.6 0.3 9.3 19.3Mg 5.3 8.8 27.8 8.2 27.9 27.9 23.3 19.9NH4 0.0 0.0 14.9 1.1 0.0 0.0 21.0 1.5Na 81.0 71.3 23.3 68.4 11.3 3.8 7.2 25.4

With respect to the results of the oedo-permeameter tests,although the numbers of pore volumes reached at the end oftesting were often limited, due to time constraints, results suggestthat all the sodium-bentonite GCLs tested displayed low values ofhydraulic conductivity (<5�10�11 m/s) in presence of hydrationfluid and real leachate, whether they contained natural sodiumbentonite or sodium-activated calcium bentonite. This differs fromresults reported in Guyonnet et al. (2005) where the naturalsodium-bentonite GCL systematically showed lower values ofhydraulic conductivity compared to the activated bentonite GCL.However, in this previous study, the activated bentonite GCL, witha hydraulic conductivity k¼ 1.7�10�10 m/s under 10 kPa, hada calcite content of 10.3 wt%, compared to only 3.4 wt% for thenatural bentonite GCL (k¼ 1.0�10�11 m/s under 10 kPa). Theexchangeable cation analyses reported by Guyonnet et al. (2005)clearly demonstrated dissolution of the calcite in the activatedbentonite, with an exchange of sodium for calcium ions. As shownin Table 7, all the sodium bentonites tested in LIXAR2 had relativelylow calcite contents. The largest value is measured for LX4, i.e.,4.9 wt%. This sample also happens to be the sodium bentonite thatshowed the highest values of hydraulic conductivity (Fig. 5). Suchresults underline the importance of calcite content for bentonitehydraulic performance. As calcite is a soluble mineral, when thebentonite is in contact with a dilute fluid, this fluid will tend todissolve the calcite, freeing calcium ions that may exchange for thesodium in the clay.

A result with practical implications is evidence that the desig-nation ‘‘natural’’ versus ‘‘activated’’ may be relatively arbitrary andrelated to factors that are very difficult to control by the GCLsupplier, as they depend on events occurring directly at thebentonite extraction site. One quarry operator might consider as‘‘natural’’ a bentonite which another might consider ‘‘activated’’;one difficulty being the absence of a clear definition of whatconstitutes an ‘‘activation’’. It is therefore suggested that the term‘‘natural sodium bentonite’’ is far from sufficient to qualify a GCL forbottom landfill barrier applications and other criteria must beconsidered. Considering information available to-date, it may beargued that a sodium-activated calcium bentonite with a lowcalcium carbonate content would appear preferable to a naturalsodium bentonite with a high calcium carbonate content. While itis fully recognized that Wyoming natural sodium bentonite is oneof the best bentonites in the world, the term ‘‘natural sodium’’ is notsynonymous with Wyoming bentonite, at least in the context of

Table 11Proposed performance-based indicators for GCLs in landfill applications.

Indicator Value Comments

Free Swell Index �24 cm3/2 g This value typically appearsin product sheets

Cation ExchangeCapacity (CEC)

�70 meq/100 g Correlated to smectite content

CaCO3 content �5% weight This CaCO3 content has the potential,upon dissolution, to saturate a CECof 75 meq/100 g with Ca2þ ions

Carbon and oxygenisotope analysis

– If one needs to know the originof the bentonite

0 5 10 15 20 25 30 35

Free swell volume (cm3/2g)

0.0

0.2

0.4

0.6

0.8

1.0

Pro

port

ion

of N

a

Jo et al. (2004)

This study

y = 0.032 x - 0.27

Cutoff value

Fig. 10. Free swell volume versus proportion of exchangeable sodium.

D. Guyonnet et al. / Geotextiles and Geomembranes 27 (2009) 321–331330

GCLs in landfill bottom barrier applications. As suggested above, inthis context the term ‘‘natural sodium bentonite’’ may sometimesbe misleading.

5. Summary and conclusions

In this study, eight GCLs among those most frequently used inlandfill barrier applications in France, were analyzed with a varietyof techniques in order to help identify indicators for a betterselection and control of GCLs. Based on findings obtained duringthis project and also those of previous authors referenced in thispaper, the indicators of Table 11 are proposed to complement moreclassical indicators used in the context of landfill barriers, such asmass per unit area for example. These indicators are proposed forcontrolling sodium-bentonite GCLs in landfill barrier applications:in the case of calcium-bentonite GCLs, additional information onother products would be required in order to propose relevantindicators, although the value for CEC would seem relevant also forcalcium-bentonite GCLs, i.e., since the CEC for constant surfacecharge deficiency, typical of isomorphic substitution in smectite,should be independent of the dominant exchangeable cation.

The values in Table 11 were selected primarily to discard GCLsthat are clearly unsuitable for landfill bottom barrier applications,such as LX6. The value of free swell index is taken equal to the onewhich is frequently reported in product sheets: the data presentedin Fig. 4 suggest that this is a suitable value. The cation exchangecapacity (CEC) was found to be a good indicator of the proportion ofsmectite, as suggested by Fig. 2 and is relatively easy to measurecompared to reliable XRD analysis. It is expected that for bentonitesin GCLs used in landfill applications in France, the value of CECproposed in Table 11 should guarantee a smectite content of at least60 wt%. Also, one might suggest that to demonstrate the sodicnature of a bentonite, the proportion of exchangeable sodiumshould be measured. However, this proportion is highly correlatedto the swell index, as indicated in Fig. 10 where data obtained in thisstudy (swell tests with DI water on fresh bentonite samples) areplotted together with data from Jo et al. (2004). The best-fit linearregression equation in Fig. 10 has a determination coefficientR2¼ 0.94. The data suggests that a free swell index value of at least24 cm3/2 g should guarantee a proportion of sodium in excess of40–50%.

The limit value for calcite weight percent is proposed on thebasis that upon total dissolution, this proportion of calcite has thepotential to liberate enough calcium cations to saturate the entirecation exchange capacity of a bentonite with a CEC on the order of75 meq/100 g. The time required for total dissolution of the calcitefraction of the bentonite is largely unknown, as it depends ona variety of factors including the characteristics of the contact fluid,the cristallinity of the calcite, the seepage velocity, etc. But based onresults of this study and of previous research, it is felt that thecalcite weight percent is an indicator of potential difficulties thatmay occur in the long run. Finally, carbon and oxygen isotopeanalyses are proposed in Table 11 for the case where the nature ofthe bentonite (‘‘natural’’ versus ‘‘activated’’) is required.

Performance-based indicators, such as those proposed in Table11 may contribute to pulling market quality upwards, by helpinglandfill operators distinguish between good products and productsthat are not suitable for landfill barrier applications. As the GCLmarket is very competitive, it is the opinion of the authors of thispaper that such criteria are needed in order to avoid that someproducts find their way into landfills simply for reasons of attrac-tive pricing rather than product quality. It is, of course, possible thatGCLs which do not comply with the criteria of Table 11, willnevertheless display good performance and durability, but in thiscase it would be necessary to demonstrate such performance ona case-by-case basis, using in particular oedo-permeameter tests.

Acknowledgements

This work was part of the LIXAR2 project performed by BRGM,Cemagref and INSA-Lyon, with the support of the French Environ-mental Agency (ADEME), CETCO, HUESKER, NAUE, SITA and Veolia-Proprete. We would like to thank the anonymous reviewers fortheir significant help in improving the initial manuscript.

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