A review of the performance of geosynthetics for environmental protection

Geotextiles and Geomembranes xxx (2016) 1e17

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A review of the performance of geosynthetics for environmental protection* N. Touze-Foltz a, *, H. Bannour c, 1, C. Barral b, 2, G. Stoltz d, 3 a

Irstea, Antony Regional Center, Hydrosystems and Bioprocesses Research Unit, 92761 Antony, France Conservatoire National des Arts et M etiers, 2, rue Cont e, 75141 Paris cedex 03, France c ISSAT, Sousse, Tunisia d RECOVER Research Unit, Irstea, 3275 Route C ezanne, CS 40061, 13182 Aix-en-Provence cedex 5, France b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 April 2015 Received in revised form 26 March 2016 Accepted 15 May 2016 Available online xxx

This paper, which is based on an Invited Lecture for the 7th International Conference on Environmental Geotechnics, gives an updated overview of the properties of transfer of geosynthetic liner materials used in environmental applications. To begin, the water-retention curves of geosynthetic clay liners (GCLs) are discussed, with the focus being on the high temperatures that can be encountered and the concomitant risk of desiccation. Next, an overview is given of quantifying advective transfer through intact geomembranes (virgin or after exposure on site) and through multicomponent GCLs. Experimental quantification of advective transfer through composite liners is also addressed, whereby geomembranes or the film or coating of a multicomponent GCL is damaged. Finally, based on a literature review including the most recent data, the discussion turns to the diffusion of organic and inorganic species through virgin and aged geomembranes and GCLs. The synopsis of the most recent data presented here in terms of elementary transfer mechanisms, either advective or diffusive, should contribute to improving the quantification of transfer through barrier systems. These four topics were selected as they correspond to the fields of expertise of the co-authors in which they have been publishing in the past 20 years. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Geosynthetic Geomembrane Geosynthetic clay liner Advection Diffusion Measurement

1. Introduction Recent years have seen many advances in the understanding of issues related to the use of geosynthetics such as geomembranes (GMs) and geosynthetic clay liners (GCLs) as contaminant barriers also respectively known as polymeric or bituminous barriers for the first ones and clay geosynthetic barriers for the latter This paper presents an updated version of an Invited Lecture for the 7th International Conference on Environmental Geotechnics (Touze-Foltz et al., 2014) and focuses on the performance of geosynthetics for environmental protection (e.g., landfills or mining applications). The geosynthetics under study are those whose function is to ensure the lining, in other words, GMs and GCLs. In *

Review of this paper was handled by A. Bouazza. * Corresponding author. Tel.: +33 1 40 96 60 39. E-mail addresses: [email protected] (N. Touze-Foltz), hajerbannour@ gmail.com (H. Bannour), [email protected] (C. Barral), guillaume.stoltz@ irstea.fr (G. Stoltz). 1 Tel.: +216 58779292. 2 Tel.: +33 1 40 27 22 91; fax: +33 1 58 80 86 01. 3 Tel.: +33 442666964; fax: +33 442668865.

the Recommended Descriptions of Geosynthetics Functions, Geosynthetics Terminology, Mathematical and Graphical Symbols of the International Geosynthetics Society, GMs are defined as planar, relatively impermeable, polymeric (synthetic or natural) sheets for use in civil-engineering applications. GCLs are defined as an assembled structure of geosynthetic materials and low-hydraulicconductivity earth material (clay) in the form of a manufactured sheet to be used in civil-engineering applications. The EN ISO 10318 standard (AFNOR, 2006a) defines geosynthetic barriers, which may be polymeric, bituminous, or clay geosynthetic barriers (GBR-Cs), according to which component fulfills the barrier function. A GBR-C is defined as factory assembled structure of geosynthetic materials in the form of a sheet which acts as a barrier. The barrier function is essentially fulfilled by clay. As explained below, this definition does not allow the interchangeable use of geosynthetic barriers and GCLs. In fact, multicomponent GCLs are now available on the market. The following proposed definitions are currently being discussed by the ASTM D35 terminology task group and may be added in the future to the ASTM terminology standard D4439 (von Maubeuge et al., 2011). Currently, a multicomponent GCL is defined as a GCL with an attached film, coating, or membrane that

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decreases the hydraulic conductivity, protects the clay core, or both. An adhered GCL is a GCL product in which the clay component is bonded by adhesion to a film or membrane. A coated GCL is a GCL product with at least one layer that consists of a solidified synthetic substance that was applied to the GCL as a fluid (von Maubeuge et al., 2011). In this case, as illustrated in Section 3.2, the coating or attached film may ensure the liner function much more so than the clay, which contradicts the definitions of clay geosynthetic barriers. In the following the words GM or GCL will thus be used instead of barriers. To clearly distinguish between the various materials, the term “multicomponent GCL” is used systematically in the following for a GCL that contains a coating or an attached film. The function of a barrier material is to control contaminant transport and thereby isolate the contaminants from the environment over the long term. To ensure this, methods to quantify contaminant transfer through liner materials were developed (some very recently). This paper discusses quantification in the laboratory of the properties of transfer in geomembranes and GCLs. The elementary transfer modes focused on herein are advection, which is the transport of liquid due to a difference in hydraulic head between the two sides of a liner material, and diffusion, which is transfer of liquid due to different concentrations of a given contaminant on the two sides of a liner material. No attempt is made here to quantify the performance of barrier systems on the field scale by accounting for the combined effect of advection and diffusion. This aspect was previously discussed extensively by, for example, Rowe (2005, 2007; 2012). Instead, the objective here is to discuss the material properties of geomembranes and GCLs and how they can be quantified, with the emphasis on recent innovative developments. The quantified elementary transfer properties can subsequently be used for modeling at the barrier scale to predict contaminant transfer, although this falls outside the scope of this paper. Temperature is a factor that may influence the long-term performance of barrier systems (Rowe, 2005). In waste-containment facilities, heat may be generated in various ways (Ali et al., 2014). For the biodegradation of waste, the temperature may rise to 60
C under normal operation in municipal solid-waste landfills. Singh and Bouazza (2014) mention that, due to the combined effect of solar radiation and brine processing, liners containing brines produced from coal seam-gas wells can be exposed continuously to temperatures close to 70e80
C. One potential consequence of elevated temperatures is the development of thermal gradients across the liner toward the cooler subgrade soil. Such thermal gradients can create a risk of outward moisture movement and possible desiccation of the mineral liner, which may be a GCL (Singh and Bouazza, 2014). Thus, to adequately predict transfer through barrier systems, understanding the hydraulic behavior under unsaturated conditions of GCLs is of vital importance, because their capability to serve as a barrier to fluids is intimately linked to the uptake of moisture by the bentonite and can be affected by temperature. Section 2 of this paper is thus dedicated to providing a synopsis of existing data that elucidates the retention curves of GCLs. Quantifying advective transfer in geomembranes and multicomponent GCLs is addressed in Section 3, where an insight is given into advective transfer through geomembranes after exposure on site. There, data are presented that were collected from hydraulic applications that can be of interest for using geomembranes as, for example, landfill covers. The quantification of advective transfer through a damaged GM, or through a hole in the attached film or coating of a multicomponent GCL appears in Section 4. This section also discusses the role played by the composition of the GM [high-density polyethylene (HDPE) or bituminous], and of the bentonite (sodium or calcium),

by the ageing of the bentonite through cation exchange and hydrationedesiccation cycles, and by the use of a multicomponent GCL rather than a more classical GM-GCL composite liner. Finally, section 5 summarizes the parameters that, based on existing data from the literature, govern diffusion through GCLs and GMs for virgin and aged materials. 2. Experimental determination of hydraulic behavior under unsaturated conditions of geosynthetic clay liners When installed in composite liners at the bottom of landfills, GCLs present an as-manufactured water content close to 10%. The suction of the bentonite contained in GCLs depends on the ambient relative humidity and can be as high as 1000 MPa (Beddoe et al., 2010). After installation, GCLs typically hydrate from both the liquid flux through defects in the geomembrane and from the transfer of vapor and liquid water from the underlying soil through the GCL (Azad, 2011; Beddoe et al., 2010). Accurately predicting the hydraulic characteristics of composite liners requires knowledge of both the water-retention curve (WRC) of the GCL and the volumetric changes of such liners during hydration. GCLs hydrate under a compressive stress corresponding to the overburden load generated by waste. Assuming a typical depth of waste deposits of between 20 and 30 m and using a density of 800e1000 kg/m3 for the waste leads to vertical stresses of up to 300 kPa applied to GCLs at the bottom of landfills. As summarized in Table 1, many experimental studies have investigated the water-retention properties of GCLs (Daniel et al., 1993; Barroso et al., 2006a; Southen and Rowe, 2007; Abuel-Naga and Bouazza, 2010; Beddoe et al., 2010, 2011; Hanson et al., 2013). Motivated by the contrasting water-retention characteristics of geotextile and bentonite, these studies focused on determining the WRCs of GCLs under confining stress. The difficulty of determining the WRCs of GCLs could arise from a capillary barrier to water continuity developing between the bentonite and the water-retention equipment, which could affect resulting WRC. The effect on the transport characteristics of confining stress applied along a wetting path was investigated by Abuel-Naga and Bouazza (2010), who applied a stress of 50 kPa by using a modified triaxial apparatus that combines a thermocouple psychrometer and a relative humidity sensor, and by Beddoe et al. (2011), who applied a small stress of 2 kPa by using a high-capacity tensiometer and a capacitive relative-humidity sensor. Siemens et al. (2013) used a numerical simulation done with the SEEP/W finiteelement software (GeoStudio, 2007) to study how confining stresses of 2 and 100 kPa affect water retention in GCLs. More recently, Bannour et al. (2014) developed a new laboratory methodology to determine the WRCs of GCLs under various stresses. Constant vertical stresses of 10, 50, 100, and 200 kPa, corresponding to different waste-layer thicknesses, were applied to GCL specimens placed in controlled-suction oedometers. The suction was set to mimic a wetting path from the initial dry state to zero suction and the capillary barrier effect of the carrier geotextile was circumvented by using the vapor-control technique. To obtain the two largest values of suction (4.2 and 8.5 MPa), they applied the standard technique of controlling relative humidity by using a saturated saline solution. For lower suction (0.1, 0.5, 1, and 2.8 MPa), they adapted the osmotic technique to control vapor by using calibrated concentrations of polyethylene-glycol solutions, ensuring a continuous series of applied suction along the wetting path, down to zero suction. Measurements were complemented by standard saturated oedometer swelling tests with water infiltration at zero suction. The measurements were made on a needlepunched GCL containing sodium bentonite. The experimental water-retention curves obtained under various confining stresses

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Table 1 Published investigations of GCL water-retention curves. Authors

Technique used

Confining stress (kPa) Water cycle

Daniel et al. (1993) Southen and Rowe (2005) Barroso et al. (2006a) Southen and Rowe (2007) Abuel-Naga and Bouazza (2010) Beddoe et al. (2010) Beddoe et al. (2011) Hanson et al. (2013) Bannour et al. (2014)

Thermocouple Psychrometer (SCM)a and vapor equilibrium (MCM) Pressure-plate technique (SCM) Filter Paper (MCM) Pressure plate (SCM) and pressure membrane extractors (SCM) Thermocouple psychrometer (MCM) and a capacitive relative humidity sensor (MCM)r High-capacity tensiometers (MCM) and capacitive relative humidity sensors (MCM) High-capacity tensiometers (MCM) and capacitive relative humidity sensors (MCM) Pressure plate-Filter paper and relative humidity methods Oversaturated salt solution with forced vapor circulation and polyethylene glycol solution with forced vapor circulation

0 0 0 0-0.5-3-100 50 2 2 0 0-10-50-100-200

a

Wetting path Drying Wetting path Drying path Wetting path Drying path Wetting/Drying path Wetting/Drying path Wetting

“SCM” stands for “suction-control method” and “MCM” stands for “moisture-control method.”

allowed the following conclusions to be drawn, which are consistent with conclusions from previous studies:  Increasing vertical stress results in a decrease in water uptake along the wetting path accompanied by a reduction in the swelling capacity and in the saturated hydraulic conductivity of the GCL (Fig. 1).  In the low-suction range, a GCL hydrating at a high confining stress hydrates faster than one at a lower confining stress. At a higher confining stress, GCLs have the benefit of a higher suction where the saturated hydraulic conductivity is achieved and, for a given suction, their saturated degree is higher than that of an unconfined GCL.  Water retention in bentonites depends only slightly on bentonite density, which is attributed to the predominance of physicochemical clayewater interactions.  Two well-known equations for WRCs (van Genuchten, 1980; Fredlund and Xing, 1994) correctly fit the data. Additionally, Bannour et al. (2014) proposed two new expressions that include stress effects for WRCs. The corresponding surfaces were represented in three-dimensional graphs that show how water content depends on stress and suction along a wetting path.

They also discussed the possible validity of the state-surface concept applied to GCLs under stress and concluded that the validity of this concept would likely be confirmed by constant-suction compression measurements on hydrated bentonites. These results indicate the importance of rapidly confining GCLs once they are installed so as to make them more rapidly operational and hydrated in barrier systems. In addition, a resistant bonding structure is also recommended to naturally confine the GCL, even under low stress. 3. Quantification of advective transfer through geomembranes and multicomponent geosynthetic clay liners 3.1. Quantification of advective transfer through geomembranes 3.1.1. Background A GM is a nonporous medium, which means that the material contains no voids, but only free spaces of size similar to that of a solvent molecule. Transport in GMs thus occurs at the molecular level (Lambert and Touze-Foltz, 2000). However, gases and liquids can migrate through the intact GMs by an activated diffusion process that differs from the liquid convection process that occurs in the pores of porous soils (Barroso, 2005). Different driving forces

Fig. 1. Fits made by using van Genuchten's expression in a two-dimensional representation to experimentally determined water-retention curves for various vertical stresses.

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Fig. 2. Photograph of stainless-steel cell and pressure volume controllers of type-B device for measuring flow rate in geomembranes.

Bitumen 3.5 mm

inflow outflow

PVC-P 1 mm

EPDM 1.5 mm

PVC-P 1 mm

HDPE 1.5 mm

HDPE 2.5mm

HDPE 1.5 mm

HDPE 1,5 mm

HDPE 0.75 mm

Structured HDPE 0.75 mm

FPO 3 mm

HDPE0.5 mm

FPP 1 mm FPP 1 mm

FPP 1 mm

10-7

FPO 2 mm

10-6

Bitumen 5.5 mm

10-5

FPP 1 mm

may cause diffusion; for example, concentration, hydraulic, or temperature gradients. Diffusion has also been shown to occur even if there is no gradient: this phenomenon is called self-diffusion (Eloy-Giorni, 1993). An instrument was first developed in France (Eloy-Giorni, 1993; Pelte, 1993; Durin et al., 1998; Lambert and Touze-Foltz, 2000) to quantify flow rates through GMs under the influence of a hydraulic gradient. This development gave rise to a French standard and, following this, to a European standard (EN 14150; AFNOR, 2006b) for measuring steady-state liquid flow through a geomembrane under a hydraulic pressure difference of 100 kPa between opposite sides of the geomembrane. The test method and apparatus described in EN 14150 allow flows to be accurately measured down to 106 m3 m2 d1. The two-part cell described in this standard (see Fig. 2) is made of stainless steel because the cell must resist oxidation during long-term immersion. In each part of the cell, a cavity allows hydraulic pressure to be applied. A porous disc placed in the downstream cavity prevents deformation of the geomembrane. The cell is designed to clamp the specimen with no leaks. No tightening system is required because clamping between flat surfaces is usually sufficient. For bituminous geomembranes, a bitumen rubber sealant can be used. The minimum diameter of the measuring chambers is 0.2 m. The cell is equipped with a liquid inlet on the upstream part, a liquid outlet on the downstream part, and flushing valves on each part. Volume measurements can be done by using capillary tubes (Type-A devices) or pressure-volume controllers (type-B devices). Fig. 3 summarizes the measurements done with a type-B device. This device allows the application of a constant pressure when measuring the volume and consists of a cylinder in which a piston slides. An automatically controlled motor applies the required pressure by moving the piston. Finally, a pressure sensor included in the system measures the pressure. Displacement of the piston corresponds to a variation of the volume of the liquid. A type-B device uses at least three temperature transducers, with one placed on each pressure-volume controller and one on the cell. Temperature measurements are then used to correct variations in volume. The validity of a measurement is determined by comparing upstream and downstream flow rates. Although these values should be equal in theory, in practice this is rarely the case for GMs. For flow rates greater than or equal to 106 m3 m2 d1, upstream

Flow rate (m3/m2/d)

4

10-8 0

2

4

6

8

10

12

14

16

18

Test number Fig. 3. Synopsis of measurements of flow rate through geomembranes made as per EN 14150 with Type-B device (from Touze-Foltz and Zanzinger, 2009) (FPP: flexible polypropylene, EPDM: ethylene-propylene-diene terpolymer, HDPE: high-density polyethylene, PVC-P: plasticized polyvinyl chloride, FPO: flexible polyolefine).

and downstream flow rates are considered to be equal if they differ by less than 10% of the flow rate measured on the upstream side. In particular circumstances where testing according to the described test method gives values for a GM that lie below the threshold of sensitivity of the test method, then the liquid flow is assumed to be less than 106 m3 m2 d1. 3.1.2. Flow rates for virgin geomembranes Most flow rates obtained for virgin GMs are below the threshold, as can be seen from Fig. 3. In France, the definition of a GM as per the standard NF P84-500 (AFNOR, 2013) states that the flow rate measured according to EN 14150 must be less than 105 m3 m2 d1. 3.1.3. Flow rates for geomembranes after on-site exposure The testing device described by EN 14150 has also been used to quantify the evolution of the hydraulic properties of GMs used in hydraulic applications. To the best of our knowledge, no such data exist for environmental applications. Data obtained in hydraulic applications are thus briefly reported here because they could be useful; for example, for landfill covers. Touze-Foltz et al. (2010a,b) reported the case of six oxidized bituminous geomembranes. They showed that, when an oxidized bituminous GM remains exposed for ten to fifteen years, the hydraulic performance decreases to the level of protection similar to what would be provided by a 1-m-thick clay layer, with flow rates measured in the range 2.9  105 to 1.5  103 m3 m2 d1 for a hydraulic head of 0.5 m. One oxidized bituminous GM that was covered during the 30 years since its installation had a flow rate identical to that of virgin bituminous GMs, which means that no hydraulic evolution was detected. A recent study (Touze-Foltz et al., 2015) presents the results obtained from an elastomeric bituminous GM and an oxidized bituminous GM after 15 years of exposure in a pond, during which time neither GM was covered. The flow rate through the elastomeric bituminous GM was one order of magnitude lower than that through the oxidized bituminous GM. The flow rate through the elastomeric bituminous GM, whose bitumen had not been altered, was close to 1.9  106 m3 m2 d1. Both of these studies concluded by recommending that oxidized bituminous GMs not be left exposed after installation.

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Noval et al. (2015) also recently studied the evolution over 21 years of an ethylene propylene diene terpolymer (EPDM) GM installed in a pond in the Canary Islands. After 21 years, the flow rate remained below 106 m3 m2 d1. Further joint studies based on samplings from ponds are ongoing between CEDEX (Spain), IFSTTAR (France), and Irstea (France) to better understand the temporal evolution of the properties, including the hydraulic properties, of HDPE and plasticized polyvinyl chloride (PVC-P) GMs. 3.2. Quantification of advective transfer through intact multicomponent geosynthetic clay liners Multicomponent GCLs have recently been introduced to the market to seal landfills, dams, dikes, ponds etc. Because these GCLs combine a GCL and a coating or film, their hydraulic properties fall between GMs and GCLs. Very recently, instruments have been developed to characterize transfer through these materials. Lucas (2002) studied hydraulic flows through a multicomponent GCL composed of a uniform layer of granular sodium bentonite encapsulated between a slit-film woven geotextile and a virgin staple fiber nonwoven geotextile. Tests in accordance with ASTM D5084 (ASTM, 2010) conducted on this multicomponent GCL estimated a hydraulic conductivity of about 5.0  1012 m/s. However, Lucas also suggested the possibility that sidewall leakage occurred during the tests. Cleary and Lake (2011) estimated the hydraulic conductivity of the same multicomponent GCL by using three different types of permeameters. All results, except for the specimens tested in the constant-head, fixed-wall, double-ring permeameter, gave hydraulic conductivities below 1.0  1011 m/s. In these studies, the mass per unit area of the coating in the multicomponent GCL was less than 100 g/m2, which resulted in a nonuniform coating, so the watertightness was ensured by the clay core. Touze-Foltz et al. (2012a,b) and Barral and Touze-Foltz (2012) developed a procedure whereby devices from NF EN 14150 (AFNOR, 2006b) designed to measure flow rate through GMs are combined with a rigid-wall permeameter from NF P84-705 (AFNOR, 2008), which is designed to measure flow rate through GCLs, to measure flow rates through multicomponent GCLs (see Fig. 4). Indeed, the regular measurement device used in NF P84-705 to measure flow rate through GCLs is not able to measure small flow rates in multicomponent GCLs. It was thus required to use a measuring device for precise measurements of flow rates in GMs combined with the rigid-wall permeameter. Barral et al. (2014) extended their study to include three GCLs and five different multicomponent GCLs from several manufacturers. Two multicomponent GCLs were coated with a coating with areal density below 100 g/m2 and above than 200 g/m2, respectively. Three multicomponent GCLs were laminated with a film having various different thicknesses and bonding modes. The features of the GCLs are presented in Table 2 and the features of the multicomponent GCLs appear in Table 3. Fig. 5 shows the results obtained for the three multicomponent GCLs for which the thickness of the film coating exceeded 0.2 mm (M1 to M3, see Table 3). The results show that flow rates in multicomponent GCLs are one order of magnitude greater than those usually found for virgin GMs (i.e. 106 m3 m2 d1). For the two multicomponent GCLs with the light coating (M4) and with perforations by needling (M5), flow rates were measured according to NF P84-705, which is the standard used to quantify flow rate and hydraulic conductivity in regular GCLs. The flow rates obtained (without any modification of the GCL standard) are presented in Fig. 6 and range from 1.4  1011 to 2.2  1011 m/s for a hydraulic head of 0.1e0.6 m. These flow rates are closer to GCL flow rates than to flow rates in GMs, which are known for virgin GMs to be around 106 m3 m2 d1. The flow rate through multicomponent

Fig. 4. Experimental device for measuring flow rates through multicomponent GCLs.

GCLs with an adhesive bounded film thicker than 0.2 mm or with a coating with a density exceeding 200 g/m2 is closer to the flow rate in GMs than to the flow rate in GCLs, which implies that, in such products, flow rate is controlled by the coating or attached film rather than by the bentonite. Thus these GCLs cannot be considered as clay geosynthetic barriers. They cannot be presented as GMs because the flow rate through them exceeds the allowable flow rate through GMs established in NF P84-500 (i.e., 105 m3 m2 d1). Thus, the terminology standard for geosynthetics, EN ISO 10318, lacks the appropriate vocabulary to label some multicomponent GCLs. 4. Quantification of flow rates in composite liners with a GCL and multicomponent GCLS with damaged film or coating 4.1. Flow rates and interface transmissivity in composite liners with a geosynthetic clay liner 4.1.1. Case of virgin geosynthetic clay liners containing sodium bentonite The work done over the past years regarding the characteristics of GCLs that are part of a composite liner mainly focused on the situation where the GCL (which contains sodium bentonite) is located under a hole in a HDPE GM. As indicated by Brown et al. (1987), the flow through a defect in the GM depends on the contact between the GM and the underlying soil liner. According to these authors, if the contact is not perfect, fluid that has migrated through the defect spreads laterally within the gap (i.e., the interface) between the GM and the underlying soil. The area covered by this interface flow is called the “wetted area.” Finally, the liquid migrates into and through the soil liner.

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Table 2 Characteristics of GCLs. GCL

Type of bentonite

Cover GTXa

Carrier GTX

Thickness under 10 kPa (mm)

Measured total dry mass per unit area in specimen (kg/m2)

G1 G2 G3

Sodium/Powder Sodium/Granular Sodium/Powder

Woven Non-woven Woven with surface coating and fleece

Non-woven Woven Woven with surface coating

5.7 7.4 9.2

4.8 6 5.9

a

“GTX” stands for “geotextile.”

Table 3 Characteristics of multicomponent GCLs. Type of bentonite Cover GTX Multi component GCL

Carrier GTX

Thickness Measured total Bonding type under 10 kPa (mm) dry mass per unit area of bentonite in specimen (kg/m2)

Film or coating Film or coating thickness (mm) measured total dry mass per unit area (kg/m2)

M1 M2 M3 M4 M5

Non-woven Woven Non-woven Non-woven Woven/Surface coating

5.2 6.6 5.8 5.1 9.6

0.4 < ef < 0.7 ~0.25 ~0.25 _a 0.2 < ef < 0.3

a

Sodium Sodium Sodium Sodium Sodium

Powder Granular Granular Powder Powder

Woven Non-woven Slit film woven Slit film woven Woven/Surface coating and fleece

4.58 5.28 4.41 4.13 5.7

Coated Adhered Adhered Coated Stitch-bonded

0.25 < mf < 0.4 ~0.2 ~0.2 <0.1 ~0.1

Not measurable.

upstream flow rate (m3/m2/d)

1,E-03 10-3

1,E-04 10-4

M1; differential pressure 50 kPa

M1; average from latest 7 days

M2; differential pressure 50 kPa

M2; average from latest 7 days

M3; differential pressure 50 kPa

M3; average latest 7 days

1,E-05 10-5

-6

1,E-06 10 0

1

2

3

4

5

6

7

8

Time (Days) Fig. 5. Synopsis of upstream flow rates obtained for multicomponent GCLs for the latest seven days of measurements with a differential pressure of 50 kPa [adapted from TouzeFoltz et al. (2012a,b)].

Contact between the GM and the GCL was quantified in terms of the flow rate through the composite liner and in terms of interface transmissivity. Various situations were tested to evaluate the flow through a smooth GM in contact with a GCL (Harpur et al., 1993; Barroso et al., 2006b, 2010). Harpur et al. (1993) verified that, under steady-state conditions, the most significant fraction of the flow occurs along the interface between the GM and the cover geotextile of the GCL, through the cover geotextile, and along gaps between the cover geotextile of the GCL and the bentonite. A less significant amount of fluid percolates through the bentonite and below the GCL. Barroso et al. (2006b; 2010) examined how hydraulic head, pre-hydration of the GCL, and confining stress affects the GM-GCL interface transmissivity. The results show that it is difficult to identify general trends for the influence of hydraulic head, prehydration, and confining stress on the interface transmissivity. Nevertheless, for flow rate, accounting for both the initial water

content of the specimen and the confining stress appears to be important (Barroso et al., 2006b). The effect of confining stress on flow rate depends on the initial water content of the specimen. In fact, the flow rate in pre-hydrated GCLs is about one order of magnitude larger for a confining stress of 50 kPa than for a confining stress of 200 kPa. For non-pre-hydrated specimens, the flow rates for both confining stresses are similar under steadystate-flow conditions (Barroso et al., 2006b). 4.1.2. Influence of geomembrane on flow rate Barroso et al. (2008) investigated the case of a textured HDPE geomembrane in contact with the GCL. Three different textured geomembranes were used in these experiments. The results show that the measurements are reproducible: the initial measurements give a larger flow rate for smooth geomembranes than for textured geomembranes; however, further measurements show that texture

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7

upstream flow rate (m3/m2/d)

1,E-03 10-3

1,E-04 10-4

1,E-05 10-5

-6

10 1,E-06 0

1

M4; 60 cm; 10 kPa

M4; average from latest 7 days

M5; 60 cm; 10 kPa

M5; average from latest 7 days

G1; 60 cm; 10 kPa

G1; average from latest 7 days

G2; 60 cm; 10 kPa

G2; average from latest 7 days

G3; 10 cm; 10 kPa

G3; average latest 7 days

2

3

4

5

6

7

8

Time (Days) Fig. 6. Synopsis of upstream flow rates obtained for GCLs and multicomponent GCLs for the latest seven days of measurements.

has only a small impact on steady-state flow rates. This suggests that, in the early phases of such measurements, water flows more easily at the interface of smooth GMs. A texture seems to reduce the space available at the interface for water flow. However, with time, the sodium bentonite in the GCL swells, resulting in a better contact between the GM and the GCL. Bannour et al. (2013a) studied the effect of bituminous GMs in contact with a GCL instead of a HDPE GM. This practice is not recommended for environmental applications because doubts exist about the chemical compatibility of the bituminous GM with leachate for example; however, it works fine for hydraulic applications or landfill covers where no environmental stakes are involved. Bannour et al. reported no significant difference in terms of flow rates compared to the combination of a bituminous GM or a HDPE GM, independently of which bituminous geomembrane face (rough or smooth) is in contact with the GCL. 4.1.3. Influence on flow rate of concrete in contact with geosynthetic clay liner Rowe and Hosney (2014) examined the performance of four GCLs for use as a hydraulic barrier below concrete-lined sewagetreatment lagoons. Their research was based on a series of laboratory-scale measurements designed to detect the change over a 14 month period in the interface transmissivity, q, between the GCLs and a 0.1-m-thick cast-in-place concrete above the GCL. The four GCLs contained either untreated bentonite, polymer-enhanced bentonite, granular bentonite, or powder bentonite. An increase in the wastewater head from 1 to 2.5 m results in a decrease in the interface transmissivity. For the GCL with untreated granular bentonite, q ¼ 1.5  1011 m2/s (under 1 m) and decreases by about one order of magnitude under 2.5 m. The change in interface transmissivity of the GCL with polymer-enhanced granular bentonite is greater than or similar to that for the GCL with untreated granular bentonite. For the GCL with untreated powder bentonite and with 1280 g/m2 of bentonite in the cover geotextile, q ¼ 1.8  1012 and 3.5  1013 m2/s at 1 and 2.5 m head, respectively. 4.1.4. Influence on flow rate of composition of bentonite in geosynthetic clay liner The relationship between the composition of the bentonite in the GCL (i.e., sodium or calcium bentonite) and flow rates in the GCL

was determined by Mendes et al. (2010), who concluded that the composition of the bentonite and the manufacturing process of the GCLs studied do not affect the transmissivity across the GM-GCL interface under steady-state flow. They also noticed that, for holes in the GM with diameters ranging from 4 to 10 mm, the diameter has no significant influence on the flow rate through the GM-GCL composite liner. The expansion of the sodium bentonite is effective in blocking the puncture in the geomembrane, leading to a significant reduction in flow rate. These results suggest that GCLs initially containing sodium bentonite, whose hydraulic conductivity increases due to cation exchange, can maintain low transmissivity at the GM-GCL interface and low flow rate through the composite liner when used in a composite liner. This question will be addressed in the next section. 4.1.5. Influence of bentonite ageing on flow rate During its service life in a landfill barrier system, the bentonite in the GCL is continuously subjected to cation exchange, whereby sodium cations, which initially are between the bentonite platelets, are replaced by multivalent cations that originate either in the cover or in the bottom liner and that transfer upon contact with the leachate or soil liner. Cation exchange leads to a decrease in GCL swelling capacity (Lin and Benson, 2000; Barral et al., 2012) and water absorption (Melchior, 2002) and to an order-of-magnitude increase in hydraulic conductivity compared with virgin GCLs (Egloffstein, 2001; Benson, 2013). Finally, as pointed out by Egloffstein (2001), complete cation exchange occurs after one to two years when the GCL is used in unsaturated conditions. To simulate this situation, Rowe and Abdellaty (2012, 2013) made measurements that show that the steady-state flow rate in GM-GCL composite liners remains similar to that of virgin GCLs containing sodium bentonite despite an increase in the hydraulic conductivity of the GCL of the composite liner due to permeation by a highly concentrated NaCl solution that results in cation exchange. However, the GCL can also be subjected to wetedry cycles due to moisture or temperature gradients generated across the whole barrier by climatic conditions, especially in landfill covers and dams. Wet-dry cycles damage GCLs; for example, desiccation with shrinkage cracks leads to preferential flow paths when the GCL hydrates (Melchior, 2002). The effect of cation exchange and wetedry cycles on the hydraulic performance of GCLs has been studied previously and is highly documented, especially as regards

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landfill covers (Lin and Benson, 2000; Egloffstein, 2001, 2002; Melchior, 2002; Southen and Rowe, 2005; Benson et al., 2007; Bouazza et al., 2007; Meer and Benson, 2007; Zanzinger and Touze-Foltz, 2009; Touze-Foltz et al., 2010b; Barral et al., 2012; Benson, 2013). This effect represents the primary mode of degradation for bentonite in GCLs. In fact, the combination of cation exchange and wetedry cycles more strongly affects the swelling capacity of the bentonite and causes a greater increase in the hydraulic conductivity of the GCL than does cation exchange alone, to the point that the GCL no longer acts as a hydraulic barrier (Egloffstein, 2001; Melchior, 2002; Meer and Benson, 2007; Benson et al., 2007). In fact, after a number of wetedry cycles, shrinkage cracks, which occur after desiccation, may not fully heal when the bentonite hydrates. Cation exchange combined with wetedry cycles occurring over the service life of GCLs lead to a significant increase (four to five orders of magnitude) in the hydraulic conductivity of the GCL. This raises the question of how the increase in hydraulic conductivity affects the hydraulic characteristics of a GM-GCL composite liner when the GM covering the GCL has a hole. Bannour et al. (2014) used laboratory measurements to address the question of how cation exchange combined with wetedry cycles affects the flow rate and interface transmissivity of a GM-GCL composite liner. Three of the GCLs measured were exhumed from a dam and a fourth GCL was exhumed from a landfill. These exhumed GCLs had endured cation exchange combined with wetedry cycles, which had led to an increase in their hydraulic conductivity and a decrease in their swell index. The flow rates of these exhumed GCLs were compared with that of a composite liner containing virgin GCLs: although the increase in hydraulic conductivity of the GCL renders it permeable as a single liner, steadystate flow rates and interface transmissivities for composite liners containing GCLs that were pre-exposed to cation exchange and wetedry cycles are of the same order of magnitude as for composite liners containing virgin GCLs. Thus, the flow rate through composite liners containing GCLs that were subjected to cation exchange and wetedry cycles is not linked to hydraulic conductivity, even if the hydraulic conductivity of GCLs exhumed from field sites has increased by four to five orders of magnitude with respect to virgin GCLs. Thus, ageing of GCLs is not a concern when they are used in a composite liner. 4.1.6. Summary of results Fig. 7 gives an overview of the various interface transmissivity data obtained from the studies discussed above. All data were obtained under the conditions of geomembrane-GCL contact defined by Barroso (2005), which means that the interface transmissivity may be related to the hydraulic conductivity kGCL of the GCL as follows:

log q ¼ 2:2322 þ 0:7155 log kGCL :

(1)

In Equation (1), q is the interface transmissivity and kGCL is the hydraulic conductivity of the GCL. Recently, Bannour et al. (2015) defined the additional contact condition given by Equation (2) (see Fig. 7) for composite liners containing GCLs whose hydraulic exceeds 1010 m/s. This contact condition is valid for GCLs pre-exposed to cation exchange and wetedry cycles and can also be extended to GCLs containing calcium bentonite. Therefore, the GM-GCL contact condition initially given by Barroso (2005) for effective GCLs (i.e., kGCL less than 1010 m/s) is enhanced and readjusted for all GCLs, whatever their composition or field history:

log q ¼ 8:5965 þ 0:1476 log kGCL :

(2)

4.2. Flow rates in multicomponent geosynthetic clay liners when coating or film is damaged A study was recently undertaken to determine whether the flow rates through a multicomponent GCL with damaged coating or attached film fall in the range obtained for more classical composite liners with a GCL (Bannour et al., 2013b). Indeed, the results obtained in the steady state do fall within the range of values obtained for geomembrane-GCL interfaces. To address a limitation in the measurements performed by Bannour et al. (2013b) Bannour and Touze-Foltz (2015) extended their work by making measurements on the meter scale, so that edge effects become negligible (see Fig. 8). For all multicomponent GCLs characterized, the coating or attached film was less than 0.7 mm thick. Steady-state results indicate that the flow rates range from 4.61  1012 to 3.01  1011 m3/s with interface transmissivities ranging from 1.20  1011 to 7.59  1011 m2/s, which are broadly in line with flow rates obtained from conventional GMGCL composite liners. Consequently, when the coating or attached film is damaged, the thickness and rigidity of the coating or attached film appears not to affect the steady-state flow rate and interface transmissivity, which leads to good contact at the interface. The swell index and mass per unit area of bentonite in multicomponent GCLs influence the flow rate when a film is attached (i.e., glued) to the cover geotextile of the GCL. It is thus important that the mass of bentonite in the GCL be sufficient so that the swelling capacity of samples leads to better contact at the interface and thus to better performance of the multicomponent GCL. 5. Diffusion in lining materials 5.1. Composition of leachate from landfills Besides gaseous emissions, and in particular that of methane and trace gases caused by the degradation of organic wastes, leachate emissions from landfills pose the main potential long-term environmental threat (Kjeldsen et al., 2002). Traditionally, leachate analyses as a part of regular landfill monitoring have focused on nitrogen content, oxygen consumption measured in the form of biological oxygen demand or chemical oxygen demand, heavy metals, and “classic” persistent organic pollutants such as dioxins or polychlorinated biphenyls (Van Praagh et al., 2011). Metals and metalloids are still recognized as priority pollutants. In contrast with most organic pollutants, metals and metalloids do not degrade in landfills. They are thus maintained in landfills and are mobilized in the liquid or gaseous phases. The literature reports considerable concentrations of metals and metalloids in leachate; for example, Cd appears at a concentration of 0.2e20 mg/L, Cr at 5e600 mg/L, Mn at 0.01e70 mg/L, and Fe at 0.3e220 mg/L (PinelRaffaitin et al., 2006). Landfills also contain micropollutants with toxic effects (acute toxicity, genotoxicity, reproductive toxicity, etc.) (Sisinno et al., 2000; Takigami et al., 2002). The presence of organic contaminants in leachate from municipal solid-waste landfills has been clearly demonstrated in several countries (Oman and Hynning, 1993; Ahel and Tepic, 2000; Robinson et al., 2001; Hiroshi et al., 2002). Van Praagh et al. (2011) reviewed the Swedish and international literature on analytical techniques and on the results of analysis and quantification of several compounds or groups of emerging organic pollutants in leachate. In eleven studies of organic substances in landfill leachate in various countries, a total of 592 different compounds were identified. The major fraction of the substances identified may be categorized into the following groups

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9

Mendes et al. (2010) -6

10 1E-06

Barroso et al (2006) Barroso et al (2008)

-7

Interface transmissivity, θ(m2/s)

10 1E-07

Barroso et al. (2010) Rowe and Abdellaty (2012)

-8

10 1E-08

Bannour et al. (2013a) Excellent

-9

10 1E-09

Bannour et al. (2014b)

-10

10 1E-10 -11

10 1E-11 -12

10

1E-12

GM- GCL (after cation exchange and wetting drying cycles)

GM- GCL (virgin)

-13

10

1E-13 10-12 1E-12

-11 10 1E-11

-10 10 1E-10

-9 10 1E-09

-8 10 1E-08

-7 101E-07

10-61E-06

10-51E-05

Hydraulic conductivity (m/s) Fig. 7. Synopsis of transmissivity taken from the literature for GCLs in contact with geomembranes and for GCLs after cation exchange and wet-dry cycles.

Bannour and Touze-Foltz (2014) GCL 1 (m) Bannour and Touze-Foltz (2014) GCL 2 (m) Bannour and Touze-Foltz (2014) GCL 3 (m)

Flow rate, Q (m3/s)

Bannour et al. (2013b) GCL 1 (dm) Bannour et al. (2013b) GCL 2 (dm) Bannour et al. (2013b) GCL 3 (dm)

0

200

400

600 Time (h)

800

1000

1200

Fig. 8. Comparison of decimeter- and meter-scale flow-rate dynamics along multicomponent-GCL interfaces.

(in descending order of number of detections): phenolic compounds, aromatic hydrocarbons, heterocyclic substances, carboxylic acids, phthalates, anilines, aliphatic acids, phenoxy acids, organo phosphorous substances, terpenoids, and triazines, some of which are used as pesticides. Quantifying the transfer to the surrounding environment of organic pollutants from compounds emerging from leachate is thus of primary importance. 5.2. Quantifying properties of transfer of chemicals through liner materials 5.2.1. Theory of diffusive transfer through geosynthetic clay liners Rowe and Booker (1987) developed a model for predicting the one-dimensional transport of contaminants through soils of finite thickness. The model accounts for realistic landfill parameters such

as dynamic surface-boundary concentrations, as is the case for municipal solid-waste landfills. Lake and Rowe (2004), Rowe et al. (2005), Rosin-Paumier et al. (2011), and Mendes et al. (2013; 2014a) used this model to predict the one-dimensional transport of contaminants through a saturated GCL for a single reactive solute without degradation. The model is based on Equation (3):

n

vC v2 C vC ¼ nDe 2  rd Kd vt vt vz

(3)

Where C is the concentration in the GCL at depth z and time t, n is the total porosity of the GCL, De is the effective diffusion coefficient, rd is the dry density, and Kd is the sorption coefficient. Diffusion coefficients can be estimated by solving Equation (3) in combination with finite-mass boundary conditions (see Rowe et al., 2004) for the measurement setup. Note that “finite mass” refers to the fact that the concentration changes in time at both

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boundaries because of mass transfer through the GCL and sampling for the measurements. The sorption coefficient Kd is first determined from batch adsorption measurements (Lake and Rowe, 2004; Rowe et al., 2005; Ganne et al., 2008; Ahari et al., 2011). Based on knowledge of the sorption coefficients of the various components of a GCL (geotextiles, geotextile fibers in the bentonite, bentonite), an equivalent sorption coefficient Kdeq for the entire GCL may be calculated by using the method suggested by Rowe et al. (2005), which accounts for sorption in the geotextile, the bentonite, and geotextiles fibers in the bentonite. An overall diffusion coefficient for the entire GCL can then be calculated for each experiment and each contaminant, treating the entire GCL as a homogeneous material.

5.2.2. Theory of diffusive transfer through geomembranes The diffusion of vapor or aqueous permeants through geomembranes occurs in three steps: adsorption, diffusion, and desorption. First, the contaminant is partitioned between the source medium and the adjacent geomembrane surface. Second, driven by chemical potential, the compound diffuses through the geomembrane. Finally, the compound partitions between the outer geomembrane surface and the receiving medium (Sangam and Rowe, 2001). When a GM is immersed for sufficient time in a fluid containing a contaminant of interest, equilibrium is reached between the concentration cg in the GM and the concentration cf in the fluid. These concentrations are related by Henry's law:

cg ¼ Sgf cf

(4)

Where Sgf is the partition coefficient, which can be calculated from batch sorption measurements (Sangam and Rowe, 2001). The diffusion of organic compounds through a GM can be modeled by using Fick's first law:


f ¼ Dg dcg =dz ;

(5)

Where f is the mass flux or permeation rate per unit area, Dg is the diffusion coefficient of organic compounds through the geomembrane, cg is the concentration of compound in the geomembrane, and z is the distance parallel to the direction of diffusion. According to Fick's second law, the change in contaminant concentration with respect to time t at any point in the GM is governed by the following differential equation:

vcg v2 cg ¼ Dg 2 vt vz

(6)

Desorption is also described by Henry's law. The partition coefficient into the GM is usually equal to the partition coefficient out of the GM when the source and receptor fluid are the same (Sangam, 2001). Because measuring the concentration of contaminant inside the GM is difficult, the concentration in the fluids on either side of the geomembrane is used to infer the permeation characteristics of the GM. The flux associated with the diffusion process can also be expressed as:

f ¼ Pg

dcf dz

(7)

Where Pg is the permeation coefficient or mass-transfer coefficient (m2/s). The diffusion coefficient can be obtained from doublecompartment tests (Islam and Rowe, 2009; McWatters and Rowe, 2008, 2010; Sangam and Rowe, 2001, 2005; Touze-Foltz et al., 2012b) or immersion tests (Park et al., 2012).

5.3. Absorption onto components of geosynthetic clay liners 5.3.1. Case of organic pollutants 5.3.1.1. Adsorption onto geotextiles. To investigate the possible use of geotextiles to retain pesticides in agricultural watersheds, Boutron et al. (2009) measured batch adsorption and desorption of diuron, isoproturon, and azoxystrobin onto and from commercially available geotextiles. Similar studies were made of the adsorption and desorption of polyamide, polyester, and polypropylene onto and from polymeric fibers. Polyamide exhibited a high ability to sorb diuron, with little desorption. The adsorption onto polypropylene and polyester was less significant but still non-negligible (15% of the initial mass of pesticide in water for isoproturon and 30% for azoxystrobin). Lake and Rowe (2004), Rowe et al. (2005), and Ganne et al. (2008) addressed the potential for geotextiles to retain volatile organic compounds (VOCs) from GCLs. The sorption isotherms are linear for VOCs and the sorption coefficients range from 7 to 20 mL/ g for 1,2-dichloroethane, 79e102 mL/g for trichloroethylene, 20e41 mL/g for benzene, 87e135 mL/g for toluene, 229e248 mL/g for ethylbenzene, 263e298 mL/g for m&p-xylene, and 163e192 mL/g for o-xylene. These values were obtained at 22
C. Rowe et al. (2005) showed that diffusion and sorption depend on temperature and that both parameters are lower at 7
C than at 22
C for benzene, toluene, ethylbenzene, m&p-xylene, and oxylene. Ahari et al. (2011) studied the sorption of 13 phenolic compounds: phenol, o-cresol, p-cresol, 2-chlorophenol (2-CP), 4chlorophenol (4-CP), 2,4-dimethylphenol (2,4-DMP), 3,4dimethylphenol (3,4-DMP), 2,4-dichlorophenol (2,4-DCP), 2,4,6trichlorophenol (2,4,6-TCP), 2,3,5,6-tetrachlorophenol (2,3,5,6TeCP), 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP), pentachlorophenol (PCP), and bisphenol A (BPA). They noticed that the adsorption isotherms are nonlinear, contrary to what is observed for VOCs (Rowe et al., 2005; Ganne et al., 2008). Therefore, Ahari et al. (2011) used the Freundlich model to study adsorption. However, for the sake of comparing with previous results obtained from VOCs, they also calculated Kd under the hypothesis of linear sorption (Ahari et al., 2011). For the compounds studied, all results for Kd range from 2.7 to 8.9 mL/g. These values for the sorption coefficient remain small compared with those obtained for VOCs. An important observation is that the amount of chlorophenols sorbed by geotextiles increases as the number of chlorine atoms in the molecule increases. 5.3.1.2. Adsorption onto bentonite. Lake and Rowe (2004) also studied the potential of bentonites to retain VOCs from GCLs. Very small partition coefficients are reported in the literature (less than 1 mL/g). Recently, several studies addressed the potential for sorption of phenolic compounds onto bentonites or organobentonites (Banat et al., 2000; Yoo et al., 2004; Hameed, 2007; Richards and Bouazza, 2007; Malusis et al., 2010; Ahari et al., 2011). As stated above, Ahari et al. (2011) obtained nonlinear adsorption curves for bentonite, so they used the Freundlich model to model bentonite. The results obtained are consistent with previous results from Banat et al. (2000) for phenol. To compare with other published data on linear adsorption isotherms, they also determined the adsorption coefficient Kd. To study the adsorption of phenol onto bentonite, they assumed a linear isotherm. The results obtained under this assumption range from 2.5 to 2.6 mL/g, which are well within the range given by Richards and Bouazza (2007) (Kd ¼ 1e5 mL/g) and by Haijian et al. (2009) for phenol (Kd ¼ 1.2e3.3 mL/g). According to the results for geotextiles, the adsorption of chlorophenols increases as the number of chlorine

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atoms in the molecule increases. This study also shows that the difference between the amount of phenolic compounds adsorbed by the geotextile and that absorbed by the bentonite is smaller than for VOCs.

5.3.2. Case of inorganic pollutants Lange et al. (2004) examined the migration of various metals (Al, Fe, Mn, Ni, Pb, Cd, Cu, Zn) through GCLs exposed to a synthetic municipal solid-waste leachate. The GCLs are found to retard the migration of the metals, although only under specific pH conditions. The migration of Mn is the least attenuated. The migration of Al, Fe, and Cu are strongly retarded, so these metals are retained within the clay. The migration of Ni, Zn, and Cd is moderately attenuated. In addition, Ca may have been responsible for the lack of metal retention of the leachate species. Due to the higher retention at higher pH and the release of metals at lower pH, adsorption of hydrolyzed species in addition to cation exchange are hypothesized to be the mechanisms that contribute the most to metal retention.

5.4. Diffusive transfer through geosynthetic clay liners 5.4.1. Diffusion of inorganic species through geosynthetic clay liners Lake and Rowe (2000) showed that, for void ratios ranging from 1.5 to 3, the diffusion coefficient of sodium and chloride increases linearly with the bulk-GCL void ratio. The bulk-GCL void ratio was defined by Petrov et al. (1997) as:

eb ¼

HGCL  Hs Hs

(8)

Where HGCL is the GCL height; and Hs is the height of solids in the GCL. The height Hs is defined by

Hs ¼

Mbent

rs ð1 þ w0 Þ

þ

Mgeo

rsg

(9)

where Mbent is the mass of bentonite per unit area in the GCL, Mgeo is the mass of geosynthetics per unit area in the GCL, rs is the density of bentonite solids, rsg is the density of polypropylene geotextile solids; and w0 is the initial water content of the bentonite. The diffusion coefficients of sodium and chloride inferred from GCL diffusion measurements done with 3e5 g/L solutions decrease linearly with decreasing final bulk-GCL void ratio. The diffusion coefficient was shown to depend on the source solution and, upon significantly increasing the NaCl concentration, the diffusion coefficient inferred also increased. The diffusion coefficients were estimated to range from 1  1010 to 2  1010 m2/s. Lange et al. (2009) studied the diffusion of various metals for the following four cases where a GCL might serve as an effective barrier against metals and metalloids: acidic rock drainage, gold-mine tailings, lime-treated mine effluent, and municipal solid waste. The averaged diffusion coefficients for Cu, Cd, Zn, Fe, and Ni covered a narrow range from 6.7  1011 to 8.9  1011 m2/s. The diffusion coefficients for As, Al, Mg, Mn, and Sr range from 8.0  1011 to 1.6  1010 m2/s. The diffusion coefficients of the individual metals did not change significantly upon changing the composition of the solution, which suggests that, although the composition of the solution has some effect on the diffusion coefficient of the metal, sorption onto the GCL is the dominant factor controlling the metal mobility.

11

5.4.2. Diffusion of organic species through geosynthetic clay liners The diffusion mechanisms of VOCs were also quantified for virgin GCLs (Lake and Rowe, 2004; Rowe et al., 2005; Ganne et al., 2008) containing sodium bentonite. Lake and Rowe (2004) observed no significant increase in the diffusion coefficient for bulk-GCL void ratios ranging from 4.1 to 4.8. Their results indicate that bulk-GCL void ratios ranging from 4.1 to 4.8 correspond to low normal stress and that the corresponding diffusion coefficients represent an upper bound for diffusion coefficients. In further studies, Rosin-Paumier et al. (2011) reported that the composition of the bentonite (natural sodium versus calcium-activated) has no significant impact on the diffusion of VOCs. At 23
C, the diffusion coefficients for VOCs in bentonite range from 1 to 3  1010 m2/s for bulk-GCL void ratios ranging from 3.7 to 4.8. Mendes et al. (2013) quantified the diffusion of seven phenolic compounds through GCLs and reported diffusion coefficients for virgin GCLs that range from 5  1011 to 1.3  1010 m2/s for methylphenols and from 5  1011 to 6.3  1011 m2/s for chlorophenols. These values fall in the lower range of diffusion coefficients reported in the literature for the diffusion through virgin GCLs. 5.4.3. Aged GCLs after cation exchange Rosin-Paumier et al. (2011) subjected a GCL containing natural sodium bentonite to cation exchange by permeating the GCL with a synthetic leachate containing a mixture of monovalent and divalent cations in proportions representative of what is found in actual landfill leachate. They reported that cation exchange leads to an increase in the hydraulic conductivity of the GCL by a factor 8.5. They then studied diffusion in three specimens cut from the sample and subjected to cation exchange and reported an increase in the diffusion coefficient of VOCs compared with that of virgin GCL specimens for a bulk-GCL void ratio of 3.9. Trichloroethylene exhibits the largest increase in diffusion coefficient of the VOCs studied: from 1.0  1010 to 2.6  1010. The diffusion coefficients for dichloromethane (DCM) and dichloroethane (DCA) increase by a factor about 1.4 (from 2.3  1010 to 3.1  1010 m2/s and from 1.9  1010 to 2.6  1010 m2/s, respectively). This ratio is not as large as the analogous ratio for the increase in the hydraulic conductivity of the GCL. In addition, a decrease in the bulk-GCL void ratio from 3.9 to 3 would negate the effect of cation exchange so that the diffusion coefficient would not increase for DCM and DCA. Such a decrease in the bulk-GCL void ratio could occur over the lifetime of the landfill as the height of the waste increases. Thus, a detrimental effect of cation exchange on the hydraulic conductivity of a GCL is not necessarily indicative of a detrimental effect on the diffusion coefficient of VOCs through GCLs. This result remains to be confirmed for other GCLs and other pollutants. For phenolic compounds in aged GCL specimens, diffusion coefficients range from 2  1010 to 6.4  1010 m2/s for methylphenols and from 8.1  1011 to 2.5  1010 m2/s for chlorophenols. Due to the high sorption of methylphenols observed in the control cell, only the order of magnitude of these diffusion coefficients is considered significant (Mendes et al., 2013). Mendes et al. (2014a) also quantified the diffusion of bisphenol A after cation exchange for the same GCL specimen. They obtained a diffusion coefficient of 2.2  1010 m2/s. 5.5. Diffusive transfer through geomembranes 5.5.1. Diffusion of inorganic species Rowe (2005) presented the results of a measurement of the diffusion of chloride through a geomembrane that, at the time of publication, had run for 12 years. The receptor concentration in this measurement remained below about 0.02% of the source

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concentration, lying within the range of analytical uncertainty for the chemical analysis. Rowe (2005) also cites a study by August and Tatsky (1984) that concludes that negligible diffusion of heavymetal salts from a 0.5 M acid solution occurs through a HDPE GM over a four year measurement period. Based on these results, Rowe (2012) concluded that an intact GM is an excellent barrier against advective and diffusive migration of inorganic contaminants from a leachate. 5.5.2. Diffusion of organic species August and Tatsky (1984) found that the permeation rates in a HDPE GM of strongly polar penetrant molecules range from 7  107 m3 m2 d1 for methanol to 9.4  106 m3 m2 d1 for trichloroethylene. A number of studies on the use of GMs have focused on the diffusion of VOCs through virgin HDPE GMs (Park and Nibras, 1993; Prasad et al., 1994; Müller et al., 1998; Sangam and Rowe, 2001; Touze-Foltz et al., 2011; Park et al., 2012) virgin PVC, linear lowdensity polyethylene (LLDPE) GMs with and without a coextruded ethylene vinyl-alcohol (EVOH) inner core (McWatters and Rowe, 2008, 2010; Eun et al., 2014), fluorinated HDPE GMs (Sangam and Rowe, 2005), and aged HDPE GMs (Rowe et al., 2003; Islam and Rowe, 2009). The diffusion coefficients of VOCs in virgin HDPE GMs are 0.37  1013 and 22.8  1013 m2/s for benzene and DCM, respectively, with partition coefficients ranging from 1.8 to 189. The resulting permeation coefficient lies between 1 and 70  1012 m2/s. Sangam and Rowe (2005) examined how surface fluorination of a HDPE GM affects the diffusion of VOCs. In this study, surface fluorination consisted of applying elemental fluorine, which exchanges with hydrogen along polymer chains at the surface of a polyolefin substrate. The partition coefficient remains essentially the same after surface fluorination; however, surface fluorination reduces both the diffusion and the permeation coefficients by factors between 1.5 and 4.5, depending on the hydrocarbon examined. McWaters and Rowe (2010) studied diffusive migration of the aqueous and vapor phases of benzene, toluene, ethylbenzene, and xylenes through a 0.76-mm-thick PVC-P GM and a 0.76-mm-thick LLDPE GM. For the PVC GM, diffusion coefficients ranged from 5 to 10  1013 m2/s for diffusion from both the aqueous and vapor states. For the LLDPE GM, diffusion coefficients ranged from 2.5 to 5  1013 m2/s. The partition coefficients for PVC ranged from 100 to 1075 with respect to aqueous-phase concentrations. The partition coefficients for vapor-phase concentrations ranged from 22 to 290. For the LLDPE GM, partition coefficients ranged from 200 to 475 for aqueous-phase concentrations and from 44 to 123 for vapor-phase concentrations. The resulting permeation coefficients thus range from 130 to 750  1012 m2/s for PVC GMs and from 60 to 110  1012 m2/s for LLDPE GMs. McWaters and Rowe (2010) also studied the diffusive properties of two coextruded GMs, one with a polyamide inner core and the other with an EVOH inner core, and a standard 0.53-mm-thick LLDPE GM. The results indicate a significant reduction in mass flux through the coextruded GMs compared with the conventional LLDPE GM. The EVOH coextruded GM has the lowest permeation coefficients of about 8  1015 m2/s for diffusion from the aqueous phase. These permeation coefficients for the EVOH coextruded GM are upper bounds; actual values may be even lower. The polyamide coextruded GM has higher permeation coefficients than the EVOH coextruded GM, ranging from 5 to 8  1014 m2/s from the aqueous phase. The standard LLDPE GM has the highest diffusion coefficients, which range from 2 to 4  1013 m2/s. The partition coefficients for the EVOH coextruded GM range from 160 to 700 for aqueous-phase concentrations. For the polyamide coextruded GM,

the partition coefficients range from 120 to 430 and, for the LLDPE GM, the partition coefficients range from 180 to 450 for aqueousphase concentrations. In summary, the permeation coefficients range from 2 to 6  1012 m2/s for the EVOH coextruded GM, from 7 to 22  1012 m2/s for the polyamide coextruded GM, and from 60 to 200  1012 m2/s for the LLDPE GM. The EVOH coextruded GM thus offers a five-to twelve-fold decrease in the permeation coefficient compared with a 2.0-mmthick HDPE GM. The question is of the prediction of the evolution of the diffusion coefficient, the partition coefficient, and the permeation coefficient as it may not be possible in the future to keep on performing measurements for the variety of contaminants that can be encountered in environmental applications. Theory to predict the evolution of the diffusion coefficient, the partition coefficient, and the permeation coefficient as functions of solubility, octanolewater partition coefficient, and molecular diameter of various VOCs is given in the literature for the diffusion of VOCs through virgin HDPE GMs (Sangam and Rowe, 2001; Joo et al., 2004; Park et al., 2012). The diffusion of polybriominated diphenyl ethers (PBDE) (Saheli et al., 2011) is currently under study. Touze-Foltz et al. (2012b) studied the diffusion of phenol, ocresol, p-cresol, 2,4-xylenol, 3,4-xylenol, 2-chlorophenol, 4chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,6tetrachlorophenol, 2,3,5,6-tetrachlorophenol, pentachlorophenol, and BPA through a virgin HDPE GM. The results show that the partition coefficient is linked to the aqueous solubility and the noctanol-water partition coefficient of the contaminant. This latest result is logical because less polar contaminants are less soluble in water. No clear link is found with the molecular diameter when all phenolic compounds are taken into account (especially BPA). However, for chlorophenols a strong correlation appears between molecular weight and partition coefficient, which may be related to polarity. An analysis of the sole chlorophenols (2-CP, 4-CP, 2,4-DCP, 2,3,6-TCP, 2,3,4,6-TeCP, 2,3,5,6-TeCP, and PCP) suggests that the partition coefficient is closely linked with the degree of substitution of chlorine atoms on the phenolic nucleus, a phenomenon that may also be attributed to a difference in polarity of the various chlorophenols studied. The partition coefficients of phenolic compounds are small compared with those of VOCs with similar chemical structure (e.g., benzene compared with phenol and toluene compared with ocresol and p-cresol), which suggests that the partition coefficient decreases as the hydroxyl group solubilizes the molecule. The diffusion coefficient is well correlated with the aqueous solubility and the n-octanol-water partition coefficient of the phenolic compounds (see Fig. 9). A good correlation with the molecular diameter is also obtained when one disregards the results obtained for BPA. These trends are consistent with those previously obtained for VOCs. However, the range of obtained for the parameters differs significantly from that obtained for the VOCs, which means that the empirical equations available in the literature that were derived for VOCs are not valid for predicting the evolution of the permeation coefficient (see Fig. 10) of phenolic compounds, except for predicting its evolution with molecular diameter (excluding the parameters obtained for BPA). These trends require further confirmation for other chemical families before attempting to predict the diffusion coefficient based only on the molecular diameter (for molecular diameters close to 0.5 nm). 5.5.3. Effect of geomembrane ageing on diffusion Islam and Rowe (2009) examined the effects of aging of HDPE GMs on the diffusion and the partition of benzene, toluene,

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ethylbenzene, and xylenes. For this study, two different 1.5-mmthick HDPE GMs were aged in the laboratory at 85
C by immersion in a synthetic leachate for up to 32 months. Partition and diffusion measurements were made at room temperature on both unaged and aged GMs by using a dilute aqueous solution. The diffusion and partition coefficients decrease as aging progresses. After aging the GM for 10e32 months, the inferred permeation coefficients decreased by 36%e62%. This decrease in the diffusion, partition, and permeation coefficients is related to the increase during aging in GM crystallinity. These results for aging are consistent with previous results from Rowe et al. (2003), who showed that the permeation coefficient for a 14-year-old HDPE GM sampled from a leachate lagoon is between four and five times less than that obtained for unaged HDPE GMs typical of what is produced today (i.e., no virgin specimen was available). Measurements were recently performed to elucidate the effect of GM aging on the diffusion of phenolic compounds through GMs. The diffusion of 2,4,6-trichlorophenol, 2,3,5,6-tetrachlorophenol, and pentachlorophenol through two

17-year-old high-density polyethylene geomembranes (HDPE GMs) was studied (Touze-Foltz et al., 2016). The first was stored indoors away from direct sunlight whereas the second was exposed for the entire time in a south-facing corner of a pond. Both unexposed and exposed specimens, although under different exposure conditions, experienced ageing. Partition coefficients ranged from 30 to 190, increasing with the number of chlorine atoms in the contaminant. Diffusion coefficients for the unexposed geomembrane ranged from 9  1014 to 2.4  1013 m2/s and from 9  1014 to 2.8  1013 m2/s for the exposed geomembrane. These values were compared with those reported in the literature for diffusion of the same phenolic compounds through a virgin HDPE geomembrane from a different manufacturer: the exposure did not noticeably affect the partition coefficients or diffusion coefficients. This result was linked to the temporal evolution of the unexposed geomembrane. Furthermore, in spite of their 17-year ageing, both geomembrane specimens still had diffusive characteristics that were consistent with a virgin geomembrane obtained from a different manufacturer.

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Mendes et al. (2014b) made preliminary measurements of the diffusion of chlorophenols (4-chlorophenol, 2,4-dichlorophenol, 2,4,6-tricholophenol, 2,3,5,6-tetrachlorophenol, and pentachlorophenol) through PE films produced from the same base resin as a HDPE GM that is commercially available in Europe. The molar mass in number (Mn) and the molecular weight (Mw) of the resin, as determined by “ultrahigh-temperature” gas-phase chromatography, are Mn ¼ 44.1 kg/mol and Mw ¼ 136.7 kg/mol. The PE density is 0.911 kg/m3 and the crystallinity fraction is 51%. The films were fabricated from pellets. Some of the virgin PE film specimens were aged by thermo-oxidation at 105
C, which leads to the formation of carbonyl compounds (alcohols, carboxylic acids, etc.) and to an expected increase in crystallinity. The final carbonyl concentrations in the oxidized samples were between 0.1 and 0.4 mol/kg, which was obtained after 300 h of thermo-oxidation (Pons, 2012). For such carbonyl concentrations, the crystallinity fraction ranged from 54% to 66% according to the calibration curve proposed by Pons (2012) for the same polymer as used in this study. These

results for crystallinity fraction should be compared with 51% crystallinity fraction for virgin PE. For PE films, the partition coefficients increase with increasing number of chlorine atoms for chlorophenols, whereas the diffusion coefficients generally decrease with increasing number of chlorine atoms. In addition, the diffusion coefficients decrease with the increase in crystallinity that occurs upon aging. The permeation coefficients, however, increase; contrary to what was reported by Rowe et al. (2003) when they compared the diffusion of chlorinated and aromatic hydrocarbons through a new modern HDPE GM with that through a 14-year-old HDPE GM. These results show that the partition coefficient is linked to the aqueous solubility and the n-octanol-water partition coefficient of the contaminant. In view of the results reported by TouzeFoltz et al. (2012a,b), this latest result seems logical because less polar contaminants are less soluble in water. Although no clear evidence supports a link with the molecular diameter, for chlorophenols the molecular weight correlates well with the partition coefficient; a result that may be related to polarity.

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An analysis of the chlorophenols under study (4-CP, 2,4-DCP, 2,4,6-TCP, 2,3,5,6-TeCP, and PCP) implies that the partition coefficient is closely linked with the degree of substitution of chlorine atoms on the phenolic nucleusda phenomenon that may also be attributed to a difference in polarity between the various chlorophenols studied, as noticed by Touze-Foltz et al. (2012b). The diffusion coefficient and permeation coefficient correlate well with the aqueous solubility and the n-octanol-water partition coefficient of the chlorophenols, and they also correlate well with the molecular diameter. These trends are consistent with previous trends obtained for the diffusion of VOCs and phenolic compounds through GMs. However, the ranges obtained for the parameters differ significantly, so the empirical equations published for VOCs cannot be used to predict the evolution of the partition coefficient of the chlorophenols studied herein or of the phenolic compounds studied by Touze-Foltz et al. (2012b). However, equations previously used in the field of PE films for food packaging and that link the diffusion coefficient to the molecular mass (Brandsch et al., 1999; Helmroth et al., 2002) prove to apply to the data discussed herein for GMs (see Fig. 10).

6. Conclusion The first objective of this review paper is to present the state-ofthe-art measurement techniques for evaluating properties of transfer of chemicals in lining materials for landfills. The second objective is to summarize the current knowledge regarding advective and diffusive transfer parameters for GMs and GCLs, included multicomponent GCLs. The retention curves of GCLs are discussed first because they are crucial for predicting transfer through barriers, including GCLs that contain bentonite with a low water content and that may be installed in environmental applications and therefore subject to desiccation. Thus, knowledge of the hydration path is important to adequately predict advective transfer in GCLs or composite liners containing GCLs. Recently obtained results provide evidence that load has a significant effect on the retention curves of GCLs. Section 3 discusses the quantification of advective flow rates in GMs and multicomponent GCLs. Although no dedicated standard exists for GCLs, a procedure was recently developed to quantify advective flow in these materials. The existing methodologies allow us to study the potential evolution of the flow rate in GMs during their lifetime at a site and to detect a decrease in performance, which occurs, for example, to oxidized bituminous GMs when left uncovered. For multicomponent GCLs, the procedure developed, which will become a French standard, allows us to differentiate between the hydraulic performance of the various multicomponent GCLs currently on the market. The evidence shows that, when the film or coating is sufficiently thick, it is this component rather than the bentonite in the GCL that maintains the lining. This brings into question the definition of multicomponent GCLs that cannot be categorized as caly geosynthetic barriers. Section 4 discusses the advective flow rate due to holes either in GMs used in association with GCLs or in the film or coating of multicomponent GCLs. The thickness of the film or coating was found to have no detectable influence on the meter scale. In addition, results obtained in the steady state are not significantly impacted by the composition of the GM, its texture, the composition of the bentonite, or bentonite ageing as regards cation exchange and cation exchange combined with hydrationedesiccation cycles. Based on the data presented herein, the question of GCL ageing does not appear relevant if the GCL is used in association with a GM in a composite liner.

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Finally, the diffusion of organic and inorganic species through GCLs and GM is discussed. Until recently very few species had been investigated. Data now exist on various types of geomembranes, especially as regards the diffusion of VOCs. A recent study has shown that the empirical laws used for predicting the diffusion of VOCs in GMs cannot be extended to other chemical families, such phenolic compounds. This situation emphasizes the need for further research in this field. References Abuel-Naga, H.M., Bouazza, A., 2010. A novel laboratory technique to determine the water retention curve of geosynthetic clay liners. Geosynth. Int. 17 (5), 313e322. omembranes e dictionnaire des termes relatifs aux AFNOR, 2013. NF P84-500. Ge geomembranes. AFNOR, 2008. NF P84-705. Geosynthetic Barriers e Determination of the Swelling, Flow and Permeability Characteristics of Geosynthetic Clay Liners (GCL) Using an Oedopermeameter d Characterisation Test and Performance Test (English version). AFNOR, 2006a. EN ISO 10318. Geosynthetics d Terms and Definitions. AFNOR, 2006b. EN 14150. Geosynthetic Barriers d Determination of Permeability to Liquids. Ahari, M., Touze-Foltz, N., Mazeas, L., Guenne, A., 2011. Quantification of the adsorption of phenolic compounds on the geotextile and bentonite components of four geosynthetic clay liners. Geosynth. Int. 18 (5), 322e331. Ahel, M., Tepic, N., 2000. Distribution of polycyclic aromatic hydrocarbons in a municipal solid waste landfill and underlying soil. Bull. Environ. Contam. Toxicol. 65 (2), 236e243. Ali, M.A., Singh, R.M., Bouazza, A., Gates, W.P., Rowe, R.K., 2014. Effect of vertical stress on GCL thermal conductivity. In: Proceedings 7 International Congress on Environmental Geotechnics, 10e14 November, Melbourne, Australia, 8p. ASTM 2010. Standard D5084. Standard Test Methods for Measurement of Hydraulic Conductivity Of Saturated Porous Materials using a Flexible Wall Permeameter, ASTM International, West Conshohocken, PA, 2010, DOI: 10.1520/D5084-10, www.astm.org. August, H., Tatsky, R., 1984. Permeabilities of commercially available polymeric liners for hazardous landfill leachate organic constituents. In: Proceedings International Conference on Geomembranes, Denver, USA, pp. 163e168. Azad, F.M., 2011. Investigation of the Behaviour of Clay Liners at the Base of Municipal Solid Waste Landfills. PhD Thesis. University of Sydney, NSW, Australia, p. 203. Banat, F., Al-Bashir, B., Al-Asheh, S., Hayajneh, O., 2000. Adsorption of phenol by bentonite. Environ. Pollut. 107 (3), 391e398. Bannour, H., Stoltz, G., Delage, P., Touze-Foltz, N., 2014. Effect of stress on water retention of geosynthetic clay liners. Geotext. Geomembr. 42, 629e640. Bannour, H., Barral, C., Touze-Foltz, N., 2015. Altered geosynthetic clay liners: effect on the hydraulic performance of composite liners. Eur. J. Environ. Civ. Eng. 19 (9), 1155e1176. http://dx.doi.org/10.1080/19648189.2015.1005161. Bannour, H., Touze-Foltz, N., 2015. Flow-rate measurements in meter-size multicomponent geosynthetic clay liners. Geosynth. Int. 22 (1), 70e77. Bannour, H., Barral, C., Touze-Foltz, N., 2013a. Flow rate in composite liners including GCLs and bituminous geomembranes. In: International Conference on Geotechnical Engineering 2013, ICGE'13, Hammamet, Tunisia, February 21e23, pp. 809e819. , A., von Maubeuge, K.P., 2013b. Interface Bannour, H., Touze-Foltz, N., Courte transmissivity measurement in multicomponent geosynthetic clay liners. In: von Maubeuge, Kent P., Kline, J.P. (Eds.), Current and Future Practices for the Testing of Multi-component Geosynthetic Clay Liners, ASTM STP 1562, pp. 47e61. Barral, C., Touze-Foltz, N., Croissant, D., 2014. Flow rate quantification in multicomponent geosynthetic clay liners with the oedopermeameter method. In: Proceedings 10 ICG, September 23e26, Berlin, p. 8. Barral, C., Touze-Foltz, N., 2012. Flow rate measurement in undamaged multicomponent geosynthetic clay liners. Geosynth. Int. 19 (6), 491e496. Barral, C., Touze-Foltz, N., Loheas, E., Croissant, D., 2012. Comparative study of hydraulic behaviour of geosynthetic clay liners exhumed from a landfill cover and from a dam after several years in service. In: Proceedings of the 5th European Geosynthetics Congress, Valencia, p. 8. Barroso, M., 2005. Fluid Migration through Geomembrane Seams and through the Interface between Geomembrane and Geosynthetic Clay Liner. Ph.D. Thesis. University Joseph Fourier of Grenoble (France) and University of Coimbra (Portugal), p. 215. Barroso, M.C.P., Lopes, M.D.G.A., Bergamini, G., 2010. Effect of the waste pressure on fluid migration through geomembrane defects. In: Proceedings 9 ICG, Guaruja, Brazil, pp. 959e962. Barroso, M., Touze-Foltz, N., von Maubeuge, K., 2008. Influence of the textured structure of geomembranes on the flow rate through geomembrane-GCL composite liners. In: Proceedings Eurogeo4, Edinburgh, Scotland, p. 8. Barroso, M., Touze-Foltz, N., Saidi, F., 2006a. Validation of the use of filter paper suction measurements for the determination of GCLs water retention curves. In: Proceedings 8ICG, Geosynthetics, Yokohama, Japan, pp. 171e174.

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Barroso, M., Touze-Foltz, N., von Maubeuge, K., Pierson, P., 2006b. Laboratory investigation of flow rate through composite liners consisting of a geomembrane, a GCL and a soil liner. Geotext. Geomembr. 24 (3), 139e155. Beddoe, R.A., Take, W.A., Rowe, R.K., 2011. Water-retention behavior of geosynthetic clay liners. J. Geotech. Geoenvironmental Eng. 1028e1038. November 2011. Beddoe, R.A., Take, W.A., Rowe, R.K., 2010. Development of suction measurement techniques to quantify the water retention behaviour of GCLs. Geosynth. Int. 17 (5), 301e312. Benson, C.H., 2013. Impact of subgrade water content on cation exchange and hydraulic conductivity of geosynthetic clay liners in composite barriers. Coupled Phenom. Environ. Geotech. 79e84. Benson, C.H., Thorstad, P.A., Jo, H.Y., Rock, S.A., 2007. Hydraulic performance of geosynthetic clay liners in a landfill final cover. J. Geotech. Geoenvironmental Eng. 133 (7), 814e827. Bouazza, A., Jefferis, S., Vangpaisal, T., 2007. Investigation of the effects and degree of calcium exchange on the Atterberg limits and swelling of geosynthetic clay liners when subjected to wet-dry cycles. Geotext. Geomembr. 25 (3), 170e185. Boutron, O., Gouy, V., Touze-Foltz, N., Benoit, P., Chovelon, J.-M., Margoum, C., 2009. Geotextile fibres retention properties to prevent surface water nonpoint contamination by pesticides in agricultural areas. Geotext. Geomembr. 27, 254e261. Brandsch, J., Mercea, P., Piringer, O., 1999. Modeling of additive diffusion coefficients in polyolefins. In: Risch, S.J. (Ed.), Food Packaging Testing Methods and Applications, ACS Symposium Series, 753, Washington DC, pp. 27e36. Brown, K.W., Thomas, J.C., Lytton, R.L., Jayawickrama, P., Bhart, S., 1987. Quantification of Leakage Rates through Holes in Landfill Liners. United States Environmental Protection Agency Report CR810940, Cincinnati, OH, p. 147. Cleary, B.A., Lake, C.B., 2011. 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