Hydraulic Conductivity of Geosynthetic Clay Liners with Sodium Bentonite to Coal Combustion Product Leachates

Chen, Jiannan N.; Benson, Craig H.; Edil, Tuncer B.

doi:10.1061/(ASCE)GT.1943-5606.0001844

Open access

Technical Papers

Jan 11, 2018

Hydraulic Conductivity of Geosynthetic Clay Liners with Sodium Bentonite to Coal Combustion Product Leachates

Authors: Jiannan N. Chen, A.M.ASCE [email protected], Craig H. Benson, F.ASCE [email protected], and Tuncer B. Edil, Dist.M.ASCE [email protected]Author Affiliations

Publication: Journal of Geotechnical and Geoenvironmental Engineering

Volume 144, Issue 3

https://doi.org/10.1061/(ASCE)GT.1943-5606.0001844

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Abstract

Experiments were conducted to evaluate the hydraulic conductivity of geosynthetic clay liners (GCLs) containing granular sodium bentonite that were permeated with coal combustion product (CCP) leachates. Chemical properties of the CCP leachates were selected from a nationwide survey of CCP disposal facilities. Five synthetic leachates were selected from this database to represent a range of CCP disposal facilities: typical CCP leachate (geometric mean of CCP chemistry), strongly divalent cation leachate (aka low-RMD leachate), flue gas desulfurization (FGD) residual leachate, high ionic strength ash leachate, and trona ash leachate. Typical GCLs from two U.S. manufacturers were used. Hydraulic conductivity tests were conducted on non-prehydrated and subgrade hydrated (by compacted soil for 60 days) GCL specimens at effective stresses ranging from 20 to 450 kPa. At 20 kPa, GCLs permeated directly had high hydraulic conductivity (

> 10^{- 6} m / s

) to trona leachate and moderate to high hydraulic conductivity (

10^{- 10}

to

10^{- 7} m / s

) to the other CCP leachates. Hydraulic conductivity was strongly related to the ionic strength of the leachate and inversely related to the swell index of the bentonite when hydrated in leachate, as demonstrated in past studies on leachates from other waste streams. Increasing the effective stress from 20 to 450 kPa caused the hydraulic conductivity to decrease up to three orders of magnitude. Hydration on a subgrade prior to permeation has only modest impact on the hydraulic conductivity to CCP leachate. Hydration by permeation with deionized (DI) water prior to permeation with trona leachate resulted in hydraulic conductivity up to three orders of magnitude lower than obtained by direct permeation, suggesting that deliberate prehydration strategies may provide chemical resistance to CCP leachates.

Introduction

Coal combustion products (CCPs) are residuals from coal-fired power plants that are disposed in waste-containment facilities when they cannot be used beneficially in other applications (Edil and Benson 2002, 2006; Benson et al. 2014). When in contact with water, CCPs can release a variety of cations and anions into solution, which have potential to contaminate groundwater. Consequently, new regulations in the United States require that CCP disposal facilities incorporate a composite liner consisting of a geomembrane overlying a soil liner at least 0.6 m thick and having a hydraulic conductivity no more than

1 \times 10^{- 9} m / s

(Federal Register 2015). The regulations include a provision permitting alternatives materials to be used in lieu of the soil liner, including geosynthetic clay liners (GCLs).

GCLs consist of a thin layer of bentonite clay (5–10 mm) sandwiched between two geotextiles bonded by stitching or needlepunching. In some cases, a geomembrane is laminated to the GCL (Benson et al. 2007). The effectiveness of a GCL as a barrier to fluid flow is controlled by the hydraulic conductivity of the bentonite, which is composed primarily of the clay mineral montmorillonite (Mesri and Olson 1971; Shackelford et al. 2000; Jo et al. 2001; Kolstad et al. 2004a). Species and valence of cations in the exchange complex associated with the montmorillonite surface strongly affect the hydraulic conductivity of GCLs. For most new GCLs, sodium (

{Na}^{+}

) is the primary bound cation (Shackelford et al. 2000; Jo et al. 2005), and the bentonite is referred to as Na-bentonite.

Na-bentonites have very low hydraulic conductivity to deionized (DI) water (

\sim 10^{- 11} m / s

) because osmotic swelling in the presence of the monovalent

{Na}^{+}

reduces the size and increases the tortuosity of the pore space containing mobile water (Norrish and Quirk 1954; van Olphen 1977; McBride 1994; Prost et al. 1998; Jo et al. 2001; Kolstad et al. 2004a). However, replacement of monovalent cations on the surface by divalent or polyvalent cations, or increasing the ionic strength (I) of the surrounding pore water, suppresses osmotic swelling, resulting in higher hydraulic conductivity (Egloffstein 1997; Shackelford et al. 2000; Jo et al. 2001; Kolstad et al. 2004a). Replacement of

{Na}^{+}

by cations with higher valence is thermodynamically favorable (Sposito 1981, 1989; McBride 1994) and occurs when Na-bentonite is in environments where divalent or polyvalent cations are present. CCP leachates can release an abundance of

{Ca}^{2 +}

and

{Mg}^{2 +}

cations, which can replace

{Na}^{+}

, suppress osmotic swell, and alter the hydraulic conductivity (Meer and Benson 2007; Benson and Meer 2009; Scalia and Benson 2010).

This study was conducted to evaluate how CCP leachates affect the hydraulic conductivity of GCLs containing granular Na-bentonite. Hydraulic conductivity tests were conducted on specimens of typical GCLs from two manufacturers in the United States using five CCP leachates and DI water. GCL specimens were permeated directly with leachate or hydrated on a subgrade prior to permeation to simulate field conditions more realistically. Tests were conducted at effective stresses ranging from 20 to 450 kPa to evaluate how the hydraulic conductivity varies as a CCP disposal facility is filled.

Background

Ionic strength and relative abundance of monovalent and polyvalent cations in the pore water are master variables affecting swelling and hydraulic conductivity of bentonite for pH between 2 and 13 (Jo et al. 2001; Kolstad et al. 2004a). Ionic strength is defined by

I = \frac{1}{2} \sum_{i = 1}^{n} c_{i} z_{i}^{2}

(1)

where

c_{i}

= molar concentration of the

i

th ion in solution; and

z_{i}

= valence of the

i

th ion. Relative abundance of monovalent and polyvalent cations is described using the parameter RMD (Kolstad et al. 2004a)

RMD = \frac{M_{M}}{\sqrt{M_{D}}}

(2)

where

M_{M}

= total molarity of the monovalent cations; and

M_{D}

= total molarity of the polyvalent cations. Solutions with higher ionic strength or lower RMD (greater abundance of polyvalent cations) tend to suppress osmotic swelling of bentonite, resulting in GCLs with higher hydraulic conductivity (Jo et al. 2001; Kolstad et al. 2004a; Scalia et al. 2014). Kolstad et al. (2004a) indicated that RMD has a significant effect on swelling and hydraulic conductivity at low to moderate ionic strength (

< 50 mM

), whereas ionic strength is more important at high ionic strength (

> 50 mM

).

The hydraulic conductivity of GCLs to actual leachates has been studied by Ruhl and Daniel (1997), Petrov and Rowe (1997), Shan and Lai (2002), Katsumi and Fukagawa (2005), Katsumi et al. (2007), Bradshaw and Benson (2014), and Bradshaw et al. (2015). These studies have focused on the hydraulic conductivity of GCLs to municipal solid waste (MSW) leachates, which tend to be sodic and ammonia-rich with high levels of dissolved and suspended organic matter and trace concentrations of anthropogenic organic compounds (Bradshaw and Benson 2014; Bradshaw et al. 2015). In contrast, the influence of CCP leachates on hydraulic conductivity has received only modest attention (Ashmawy et al. 2002; Benson et al. 2014; Chen et al. 2015). CCP leachates are inorganic, have little to no dissolved or suspended organic matter, and can have higher ionic strength and a greater preponderance of polyvalent cations than MSW leachates (Benson et al. 2014; Salihoglu et al. 2016).

Many studies have been conducted with GCL specimens permeated directly by leachate and without opportunity for the bentonite to hydrate in water or a more dilute solution, known as direct permeation (e.g., Jo et al. 2001; Kolstad et al. 2004a; Lee and Shackelford 2005; Scalia et al. 2014). Prehydration can promote osmotic swelling prior to permeation, providing resistance to alterations in hydraulic conductivity under some circumstances (Petrov and Rowe 1997; Petrov et al. 1997; Ruhl and Daniel 1997; Stern and Shackelford 1998; Lin and Benson 2000; Vasko et al. 2001; Ashmawy et al. 2002; Shan and Lai 2002; Kolstad et al. 2004b; Lee and Shackelford 2005; Bradshaw et al. 2013; Bradshaw and Benson 2014). Moreover, in field applications, GCLs hydrate prior to contact with leachate as water in the soil subgrade migrates upward into the GCL before waste is placed or before leachate reaches the GCL (Daniel et al. 1993; Vasko et al. 2001; Bradshaw et al. 2013).

The effect of prehydration depends on the ionic strength of the permeant solution, with modest effects for more dilute solutions (

I < 50 mM

) and a more significant effect for more concentrated solutions (

I > 100 mM

, Lee and Shackelford 2005). For water contents characteristic of hydration on subgrade (water

content < 100 %

), prehydration can have an adverse impact. Bradshaw et al. (2013) reported that GCLs prehydrated on a subgrade had 1.3–4.1 times higher hydraulic conductivity to MSW leachate than GCLs permeated directly using a broad range of subgrade soils, ranging from sand to highly plastic clay. They attributed the higher hydraulic conductivity to cation exchange during subgrade hydration. All of these studies have focused on simple solutions or leachates with a narrow range of chemical composition. None have addressed how subgrade prehydration affects the hydraulic conductivity of GCLs exposed to the broad range of chemistries associated with CCP leachates, which range from dilute to concentrated and sodic to calcic (Benson et al. 2014).

Materials

Geosynthetic Clay Liners

Two commercially available Na-bentonite GCLs that are commonly used in the United States were used in the study (and are referred to herein as CS and GS). The GCLs have similar properties, but are manufactured by different companies. The bentonite is sourced from different mines and is processed using different techniques. Both GCLs contain granular Na-bentonite encapsulated between a woven and a nonwoven geotextile and bonded together by needlepunching. The average dry bentonite mass per unit area [ASTM D5993 (ASTM 2014)] was

3.72 \pm 0.11 kg / m^{2}

(variation is standard deviation) for the CS GCL and

3.76 \pm 0.12 kg / m^{2}

for the GS GCL. The average initial moisture content was

10.1 \pm 0.7 %

(CS) and

7.8 % \pm 0.5 %

(GS), and the initial thickness (measured to the nearest 0.1 mm with a digital caliper based on six measurements) was 7.7–9.8 mm (CS) and 7.2–8.2 mm (GS). Both GCLs contain sand-size bentonite granules, with

D_{50} = 1.0 mm

(CS) or 0.3 mm (GS) (Fig. 1). Bentonite in the GS GCL has finer granules than the CS GCL.

Fig. 1. Granule-size distributions of bentonites in GCLs in this study

Quantitative X-ray diffraction (XRD) showed that bentonite in the CS GCL consisted of 84% montmorillonite, 9% quartz, 3% feldspar, 2% clinoptilolite, 1% calcite, and

\leq

1% mica. Bentonite in the GS GCL had 86% montmorillonite, 5% quartz, 3% feldspar, 3% cristobalite, 2% augite, and 1% clinoptilolite. Cation exchange capacity (CEC) and concentrations of initial bound cations (

{Ca}^{2 +}

,

{Na}^{+}

,

{Mg}^{2 +}

, and

K^{+}

) were determined using the ammonium acetate extraction in ASTM D7503 (ASTM 2010) (five replicate tests). The CEC was

73.8 \pm 3.0

(CS) and

72.8 \pm 3.4

(GS)

{cmol}^{+} / kg

. The exchange complex of the CS GCL includes

{Na}^{+} - 33.3 {cmol}^{+} / kg

,

K^{+} - 2.7 {cmol}^{+} / kg

,

{Ca}^{2 +} - 21.4 {cmol}^{+} / kg

, and

{Mg}^{2 +} - 9.0 {cmol}^{+} / kg

. For the GS GCL, the exchange complex includes

{Na}^{+} - 29.7 {cmol}^{+} / kg

,

K^{+} - 2.70 {cmol}^{+} / kg

,

{Ca}^{2 +} - 21.9 {cmol}^{+} / kg

,

{Mg}^{2 +} - 7.3 {cmol}^{+} / kg

. Both bentonites are predominantly Na-montmorillonite, but have appreciable

{Ca}^{2 +}

in the exchange complex, which is common in U.S. bentonites (Lee and Shackelford 2005; Meer and Benson 2007; Bradshaw et al. 2013; Scalia and Benson 2010; Scalia et al. 2014).

CCP Leachates

The CCP leachates used in the testing program were selected by Benson et al. (2014) after analyzing the composition of leachates from CCP disposal facilities throughout the United States, as reported in the Electric Power Research Institute’s (EPRI) CCP leachate database (EPRI 2006, 2009). A detailed description of the database and the analysis to select the synthetic leachates has been provided by Benson et al. (2014).

{Na}^{+}

,

K^{+}

,

{Ca}^{2 +}

,

{Mg}^{2 +}

,

{Cl}^{-}

, and

{SO}_{4}^{2 -}

are the predominant ions in all leachates. The relationship between RMD and ionic strength for the CCP leachates in the EPRI database is shown in Fig. 2.

Fig. 2. Relationship between *RMD* and I for CCP leachates in EPRI database

Each of the five CCP leachates in Benson et al. (2014) was selected for testing to represent different conditions encountered in CCP disposal facilities (Fig. 2):

•

Typical CCP leachate (

I = 39.5 mM

and

RMD = 0.16 M^{1 / 2}

) representing the geometric mean ionic strength and RMD of all of the CCP leachates;

•

Strongly divalent cation CCP (low RMD) leachate (

I = 48.0 mM

and

RMD = 0.014 M^{1 / 2}

);

•

Flue gas desulfurization residue (FGD) leachate (

I = 96.8 mM

and

RMD = 0.39 M^{1 / 2}

);

•

High ionic strength CCP (high strength) leachate (

I = 177 mM

and

RMD = 1.0 M^{1 / 2}

); and

•

Trona ash (trona) leachate (

I = 755 mM

and

RMD = 4.5 M^{1 / 2}

).

Synthetic leachates representing these conditions were created by dissolving reagent-grade

Ca {SO}_{4}

,

{Na}_{2} {SO}_{4}

,

Mg {SO}_{4}

,

K_{2} {SO}_{4}

, NaCl, and

{CaCl}_{2}

in DI water. Other chemical properties of the CCP leachates are summarized in Table 1. Concentrations in the leachates were checked by geochemical equilibrium analysis using MINTEQA2, as described by Benson et al. (2014).

Table 1. Chemical Properties of CCP Leachates

Property	Leachate
Property	Typical CCP	Low RMD	FGD	High strength	Trona
Na (mM)	11.1	1.3	42.1	109	645
K (mM)	1.9	0.3	1.3	0.6	5.1
Ca (mM)	7.8	15.4	13.6	12.8	12.5
Mg (mM)	1.2	1.0	3.05	5.8	6.3
${Cl}^{-}$ (mM)	2.0	0.7	22.1	48.3	—
${SO}_{4}^{2 -}$ (mM)	14.5	16.8	27.2	49.2	343
Ionic strength (mM)	39.5	48.0	96.8	178.0	755.0
RMD ( $M^{1 / 2}$ )	0.16	0.014	0.39	1.0	4.4
pH	8.5	8.5	8.5	8.5	11.0
$EC at 20 ° C$ ( $S / m$ )	0.2	0.2	0.6	1.0	4.1

Permeant solutions were prepared by dissolving reagent-grade

{Cl}^{-}

and

{SO}_{4}^{2 -}

salts in Type II DI water [ASTM D1193 (ASTM 2011a)]. The solutions were stored in collapsible carboys with no head space to limit interaction with atmospheric carbon dioxide (

{CO}_{2}

). Composition of each solution was checked periodically, and the solution replaced if a change in chemistry occurred.

Subgrade Soil

A silty clay soil was selected for subgrade prehydration of GCLs. The subgrade soil was obtained from a landfill in the Midwestern United States and was intended to represent typical or average conditions chemically. Pore water in the subgrade has similar ionic strength, but slightly lower RMD (

I = 3.8 mM

and

RMD = 0.021 M^{1 / 2}

), than the average subgrade pore water defined by Scalia and Benson (2010) and ASTM D5084 (ASTM 2016a, Section 6.1.2, Option ii). Scalia and Benson (2010) defined average pore water based on a review of the pore-water chemistry of surficial soils at waste-containment facilities. Physical and chemical properties of the subgrade soil are summarized in Table 2. Major cations in the pore water were determined by batch tests following the procedure described by Meer and Benson (2007). The major cation concentrations were

{Na}^{+} = 0.70 mM

,

K^{+} = 0.17 mM

,

{Ca}^{2 +} = 0.99 mM

, and

{Mg}^{2 +} = 0.68 mM

.

Table 2. Properties of Subgrade Soil

Parameter	Value
Physical properties
Specific gravity of solids ( $G_{s}$ )	2.61
Plasticity index (PI)	27
Liquid limit (LL)	40
Gravel (%)	0.0
Sand (%)	1.3
Fines (%)	98.7
Optimum water content (%)^a	18.8
Max dry unit weight ( $kN / m^{3}$ )^a	16.5
Pore-water chemistry
${Na}^{+}$ (mM)	0.7
$K^{+}$ (mM)	0.2
${Ca}^{2 +}$ (mM)	1.0
${Mg}^{2 +}$ (mM)	0.7
pH	7.5
EC ( $S / m$ )	0.05
Ionic Strength (mM)	3.8
RMD ( $M^{1 / 2}$ )	0.02

a

Standard Proctor compaction as per ASTM D698 (ASTM 2012b).

Methods

Hydraulic Conductivity

Hydraulic conductivity tests were conducted on 100-mm-diameter GCL specimens using flexible-wall permeameters following the methods in ASTM D5084 (ASTM 2016b) and ASTM D6766 (ASTM 2012a). The falling headwater–constant tailwater method was used. GCL specimens were trimmed from rolls supplied by the manufacturers following the method described by Jo et al. (2001). Excess geotextile fibers were carefully removed from the edge of each GCL specimen, and bentonite paste prepared with the permeant liquid was applied to the periphery of the specimen as described by Jo et al. (2001). Disks of nonwoven geotextile (mass per

area = 240 g / m^{2}

and

thickness \approx 1.8 - 2.4 mm

depending on stress) were placed on both sides of the GCL to evenly distribute flow. Geotextile disks were used in lieu of porous stones, which have propensity to clog. New geotextile disks were used for each specimen. The disks were submerged in the permeant liquid for saturation and squeezed to remove air bubbles. An effective stress of 20 kPa was applied initially with an average hydraulic gradient of 190.

GCL specimens permeated directly with leachate were hydrated in the permeameter with leachate for 48 h with the headwater applied and the effluent valve closed. Permeation followed immediately afterwards and was conducted until the hydraulic conductivity was steady, the ratio of incremental outflow to inflow was within

1 \pm 0.25

, and the pH, major cation concentrations (

{Na}^{+}

,

K^{+}

,

{Ca}^{2 +}

, and

{Mg}^{2 +}

), and electrical conductivity of the effluent were within 10% of those of the influent. Reaching these hydraulic and chemical equilibrium criteria required days (very permeable specimens) to up to 2 years (less permeable specimens). When relatively high hydraulic conductivities (

10^{- 8} m / s

or higher) were obtained, the influent was spiked with rhodamine water tracing (WT) dye to stain flow paths through the GCL to ensure that sidewall leakage was not occurring. A duplicate test was also conducted to ensure the result was reproducible.

After the hydraulic and chemical equilibrium criteria were reached at 20 kPa, the effective stress was increased incrementally to 100, 250, and 450 kPa, with the hydraulic conductivity measured at each stress until the equilibrium criteria in D5084 were satisfied. Tests were conducted over this range of stress to simulate the effective stresses that would be encountered during waste filling, representing waste depths up to approximately 30 m. The hydraulic conductivity of specimens directly consolidated under 100 or 450 kPa effective stresses was also tested to investigate the case of rapid filling of waste in a CCP disposal facility.

Prehydration

Effects of prehydration on a subgrade were evaluated by hydrating GCL specimens on the silty clay subgrade soil following the methods described by Bradshaw et al. (2013). Specimens of subgrade soil were compacted to the maximum dry unit weight at optimum water content per standard Proctor in 150-mm-diameter and 116-mm-tall compaction molds. Subgrade specimens of this height are sufficient to ensure that the thickness of the subgrade layer does not affect hydration of the GCL (USEPA 1996a). After compaction, each subgrade specimen was extruded and placed on a 150-mm-diameter PVC disk. A 150-mm-diameter GCL specimen was placed on top of the subgrade specimen (woven side of the GCL contacting the subgrade), and a 1.5-mm-thick geomembrane disk, a nonwoven geotextile, and a PVC top plate were placed on top of the GCL specimen. A latex membrane was placed around the entire setup and two O-rings were used on each end to seal the latex membrane to the PVC top and bottom plates. The entire assembly was placed into a water-filled chamber maintained at

20 \pm 2 ° C

, with 10 kPa applied to simulate the stress applied by a leachate collection system.

GCL specimens used to evaluate subgrade hydration were hydrated on the subgrade for

60 \pm 2 days

to simulate the time period between completion of liner installation and initial placement of CCPs. Bradshaw et al. (2013) reported that the water content of GCLs increases rapidly during the first 30 days of hydration on a subgrade, but increases slowly thereafter and is relatively constant after approximately 60 days. After the hydration period, the 150-mm GCL specimen was trimmed to a diameter of 100 mm for hydraulic conductivity testing. The remainder of the specimen was used for water content, exchange complex, CEC, swell index (SI), and fluid loss (FL) testing.

Water content of the CS GCL increased from the 10.1% prior to hydration to 68.3% after subgrade hydration, on average. For the GS GCL, the water content increased from 7.8 to 70.4%, on average. The water contents after subgrade hydration are consistent with GCL water contents (65.0–70.0%) measured by Bradshaw et al. (2013) near equilibrium and reflect water migrating into the GCL via capillary conduction and vapor phase transport.

Swell Index and Fluid Loss Tests

Hydration characteristics of the bentonite were evaluated using swell index tests [ASTM D5890 (ASTM 2011b)] and fluid loss tests [ASTM D5891 (ASTM 2016a)] using CCP leachate as the hydrating liquid. Control tests were conducted with DI water. The fluid loss test was conducted with Whatman No. 42 (Pittsburgh, Pennsylvania) filter paper using slurry with 6% solids and a pressure of 689 kPa.

Chemical Analysis

pH and electrical conductivity (EC) of influent and effluent samples from the hydraulic conductivity tests were recorded immediately after sampling using Accumet XL50 benchtop meters (Fisher Scientific, Hanover Park, Illinois). Influent and effluent samples were filtered using 0.22-μm filter paper, preserved with trace-grade nitric acid, and stored at 4°C. Major cation concentrations were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Vista-MPX ICP-OES (Varian, Santa Clara, California) following USEPA Method 6010B (USEPA 1996b).

Results and Discussion

Hydraulic conductivities for both GCLs permeated directly with the CCP leachates at 20 kPa are shown in Fig. 3 and summarized in Table 3. Final thickness of the GCL specimen and SI of the bentonite from the GCL in the permeant liquid are also in Table 3. The SI in Table 3 were measured after prehydration. For specimens directly permeated with CCP leachate, the SI in Table 3 correspond to bentonite contacted with CCP leachate without any prehydration.

Fig. 3. Hydraulic conductivity of GCLs directly permeated with CCP leachate at 20 kPa as a function of ionic strength

Table 3. Summary of Hydraulic Conductivity Testing Conditions and Outcomes

GCL	Prehydration method	Permeation liquid	Final GCL thickness (mm)	Swell index ( $mL / 2 g$ )	Hydraulic conductivity ( $m / s$ ) at specified effective stress
GCL	Prehydration method	Permeation liquid	Final GCL thickness (mm)	Swell index ( $mL / 2 g$ )	20 kPa	100 kPa	250 kPa	450 kPa
CS	None	DI water	10.8	36.0	$2.6 \times 10^{- 11}$	—	—	—
		Typical CCP	5.4	29.5	$5.7 \times 10^{- 10}$	$1.3 \times 10^{- 10}$	$3.3 \times 10^{- 11}$	$1.5 \times 10^{- 11}$
		Low RMD	5.1	24.0	$2.0 \times 10^{- 9}$	$1.3 \times 10^{- 10}$	$2.3 \times 10^{- 11}$	$1.1 \times 10^{- 11}$
		FGD	5.2	18.5	$8.8 \times 10^{- 9}$	$1.3 \times 10^{- 10}$	$3.3 \times 10^{- 11}$	$1.7 \times 10^{- 11}$
		High strength	5.2	15.0	$1.7 \times 10^{- 7}$	$4.6 \times 10^{- 8}$	$1.2 \times 10^{- 8}$	$9.8 \times 10^{- 10}$
		High strength	5.8^a	15.0	$1.7 \times 10^{- 7}$	$4.5 \times 10^{- 9}$ ^a	$1.2 \times 10^{- 8}$	$9.8 \times 10^{- 10}$
		Trona	5.1	8.0	$1.2 \times 10^{- 6}$	$1.2 \times 10^{- 7}$	$7.4 \times 10^{- 9}$	$1.1 \times 10^{- 9} 3.2 \times 10^{- 12}$ ^b
			5.1	8.0		$1.8 \times 10^{- 7}$ ^a
			7.5^a, 4.9^b	8.0, 8.0		$1.8 \times 10^{- 7}$ ^a
	Subgrade	Typical CCP	5.4	25.0	$7.6 \times 10^{- 11}$	$1.1 \times 10^{- 11}$	$6.5 \times 10^{- 12}$	$2.4 \times 10^{- 12}$
		Low RMD	5.2	22.0	$1.0 \times 10^{- 9}$	$4.5 \times 10^{- 10}$	$3.9 \times 10^{- 11}$	$2.2 \times 10^{- 11}$
		FGD	5.4	18.0	$8.0 \times 10^{- 8}$	$2.3 \times 10^{- 11}$	$5.1 \times 10^{- 12}$	$1.9 \times 10^{- 12}$
		High strength	5.3	20.0	$8.7 \times 10^{- 7}$	$1.2 \times 10^{- 8}$	$3.2 \times 10^{- 11}$	$1.6 \times 10^{- 11}$
		Trona	5.1	21.5	$2.6 \times 10^{- 6}$	$1.0 \times 10^{- 7}$	$2.1 \times 10^{- 9}$	$5.0 \times 10^{- 10}$
	DI water	Trona	9.2	34.0	$7.0 \times 10^{- 10}$	—	—	—
GS	None	DI water	10.3	32.5	$3.6 \times 10^{- 11}$	—	—	—
		Typical CCP	5.5	24.5	$3.2 \times 10^{- 10}$	$5.7 \times 10^{- 11}$	$1.3 \times 10^{- 11}$	$5.8 \times 10^{- 12}$
		Low RMD	5.2	21.5	$9.6 \times 10^{- 9}$	$7.7 \times 10^{- 9}$	$5.1 \times 10^{- 9}$	$1.1 \times 10^{- 9}$
		FGD	5.2	19.0	$1.8 \times 10^{- 8}$	$1.5 \times 10^{- 8}$	$3.3 \times 10^{- 9}$	$3.2 \times 10^{- 10}$
		High strength	5.1	13.0	$1.5 \times 10^{- 8}$	$9.9 \times 10^{- 9}$	$4.3 \times 10^{- 11}$	$1.7 \times 10^{- 11}$
		High strength	5.8^a	13.0	$1.5 \times 10^{- 8}$	$6.5 \times 10^{- 10}$ ^a	$4.3 \times 10^{- 11}$	$1.7 \times 10^{- 11}$
		Trona	5.2	8.0	$5.4 \times 10^{- 8}$	$8.1 \times 10^{- 9}$	$1.4 \times 10^{- 9}$	$5.3 \times 10^{- 11} 3.0 \times 10^{- 12}$ ^b
		Trona	7.0^a, 5.0^b	8.0, 8.0	$5.4 \times 10^{- 8}$	$1.3 \times 10^{- 10}$ ^a	$1.4 \times 10^{- 9}$	$5.3 \times 10^{- 11} 3.0 \times 10^{- 12}$ ^b
	Subgrade	Typical CCP	5.5	27.5	$5.9 \times 10^{- 11}$	$4.7 \times 10^{- 11}$	$3.9 \times 10^{- 11}$	$3.5 \times 10^{- 12}$
		Low RMD	5.3	30.0	$1.3 \times 10^{- 9}$	$4.5 \times 10^{- 10}$	$3.9 \times 10^{- 11}$	$2.2 \times 10^{- 11}$
		FGD	5.3	22.0	$1.8 \times 10^{- 8}$	$1.5 \times 10^{- 8}$	$5.0 \times 10^{- 9}$	$2.9 \times 10^{- 10}$
		High strength	5.2	24.0	$1.3 \times 10^{- 8}$	$7.2 \times 10^{- 9}$	$1.7 \times 10^{- 10}$	$1.8 \times 10^{- 11}$
		Trona	5.1	24.0	$1.5 \times 10^{- 8}$	$3.7 \times 10^{- 9}$	$4.2 \times 10^{- 10}$	$1.8 \times 10^{- 11}$
	DI water	Trona	9.3	30.0	$6.4 \times 10^{- 11}$	—	—	—

Note: Thickness measured at end of test at 450 kPa unless specified otherwise; swell index measured with permeant solution after hydration step.

a

Corresponds to test on specimen consolidated directly to 100 kPa prior to permeation.

b

Corresponds to test on specimen consolidated directly to 450 kPa prior to permeation.

Both GCLs have very low hydraulic conductivity to DI water (

2.6 - 3.6 \times 10^{- 11} m / s

) and higher hydraulic conductivity to each of the CCP leachates (

> 10^{- 10} m / s

). Hydraulic conductivity is strongly related to the ionic strength of the leachate; as the ionic strength increases from 40 to 755 mM, the hydraulic conductivity increases up to four orders of magnitude (

10^{- 10}

to

10^{- 6} m / s

). The effect of RMD of the leachate is evident at lower ionic strengths (

< 50 mM

) (Fig. 3). The typical CCP and low-RMD leachates have similar ionic strength (39.5 versus 48.0 mM), but the low-RMD leachate has a much greater preponderance of divalent cations. Hydraulic conductivity of both GCLs permeated with typical CCP leachate ranged from

3.2 \times 10^{- 10}

to

5.7 \times 10^{- 10} m / s

, whereas the hydraulic conductivity to low-RMD leachate was approximately one order of magnitude higher (

2.0 \times 10^{- 9}

to

9.6 \times 10^{- 9} m / s

). Although some of this difference in hydraulic conductivity is attributable to ionic strength (

1.75 \times

based on the trend in Fig. 3), the majority is likely caused by suppression of osmotic swell by

{Ca}^{2 +}

and

{Mg}^{2 +}

.

At lower ionic strengths (

< 50 mM

; FGD leachate, typical CCP leachate, and DI water), the CS and GS GCLs had essentially the same hydraulic conductivity [Fig. 3, within

2 \times

, the repeatability of hydraulic conductivity tests on GCLs (Daniel et al. 1997)]. The exception is the hydraulic conductivity to low-RMD leachate, for which the GS GCL was more permeable than the CS GCL. Bentonite from the GS GCL had lower SI relative to bentonite from the CS GCL in low-RMD leachate (21.5 versus

24.0 mL / 2 g

), and lower SI in DI water after permeation with low-RMD leachate (

9.5 mL / 2 g

for GS versus

15.0 mL / 2 g

for CS). In contrast, hydraulic conductivity of the CS GCL was more sensitive to ionic strength when the ionic strength was above 50 mM (Fig. 3). For ionic strengths

\geq 178 mM

(high-strength and trona leachates), the CS GCL had hydraulic conductivity more than one order of magnitude higher than the GS GCL (Fig. 4), all other factors being equal.

Fig. 4. Comparison of hydraulic conductivity of GS and CS GCLs permeated directly with CCP leachates at 20 kPa effective stress

The coarser bentonite granules in the CS GCL may have contributed to greater sensitivity to ionic strength and higher hydraulic conductivity than the GS GCL at the highest ionic strengths. Katsumi et al. (2002) report that Na-bentonite GCLs containing bentonite with smaller granules are less permeable to more concentrated salt solutions (

> 200 mM

) than GCLs with larger granules. The effect of granule size becomes more important as the ionic strength increases because swelling of the bentonite diminishes with increasing ionic strength, precluding a transition from a dry granular material prior to permeation to a gel after permeation. Thus, the hydraulic conductivity of GCLs with finer granule sizes may be less sensitive to CCP leachates. Fig. 5 illustrates remnant granules that did not swell in the CS GCL after permeation with trona leachate, which has the highest ionic strength of the CCP leachates.

Fig. 5. Image of bentonite in CS GCL after permeation with trona leachate at 20 kPa showing remnant granules; examples of remnant granules identified with arrows

Free Swell, Fluid Loss, and Hydraulic Conductivity

Hydraulic conductivity of the GCLs as a function of SI in the leachate is shown in Fig. 6(a) (average swell index from two replicate tests reported). Hydraulic conductivity is strongly and inversely related to swell index, as previously reported by Jo et al. (2001) and Kolstad et al. (2004a), indicating that bentonite swelling is the primary mechanism controlling the pore space and hydraulic conductivity of Na-bentonite GCLs permeated with CCP leachates. For these GCLs, SI of the bentonite decreases from approximately

36.0 mL / 2 g

to approximately

8.0 mL / 2 g

, and the hydraulic conductivity increases from approximately

2 \times 10^{- 11}

to

3 \times 10^{- 7} m / s

as the ionic strength of the permeant liquid increases (DI water to trona ash leachate) (Table 3).

Fig. 6. Hydraulic conductivity to CCP leachate ( $K_{CCP}$ ) at 20 kPa by direct permeation as a function of (a) SI or (b) FL using CCP leachate as hydrating liquid; $K_{DI}$ = hydraulic conductivity to DI water at 20 kPa

The relationship between hydraulic conductivity and SI is approximately log-linear for the CS and GS GCLs with the CCP leachates [Fig. 6(a)]. This contrasts the nonlinear relationships between the logarithm of hydraulic conductivity and swell index reported by Jo et al. (2001) and Kolstad et al. (2004a). The reason for this difference in trends is not evident because the granule-size distributions for the CS and GS GCLs are similar to those for the GCLs evaluated by Jo et al. (2001) and Kolstad et al. (2004a), the solutions used by Kolstad et al. (2004a) were prepared with blends of reagent-grade salts like those used in the current study, and the mineralogy of the bentonites for both GCLs were similar to the GCLs used by Jo et al. (2001) and Kolstad et al. (2004a).

The good correspondence between hydraulic conductivity and SI shown in Fig. 6(a) indicates that SI is a useful indicator of the hydraulic conductivity of Na-bentonite GCLs with granular bentonite permeated with CCP leachates, as previously shown for other solutions and leachates by Jo et al. (2001) and Kolstad et al. (2004a). This relationship can be defined by

\log \frac{K_{CCP}}{K_{DI}} = 5.3 - 0.2 SI

(3)

where

K_{CCP}

= hydraulic conductivity to CCP leachate; and

K_{DI}

= hydraulic conductivity to DI water. Eq. (3) was obtained by nonlinear least-squares regression (

R^{2} = 0.84

) on pooled data from the CS and GS GCLs measured at 20 kPa after normalizing the hydraulic conductivities to CCP leachate by the hydraulic conductivity to DI water (reported in Table 3). The trend line in Fig. 3 corresponds to

K_{DI} = 2.6 \times 10^{- 11} m / s

for the CS GCL.

Fluid loss measured using ASTM D5891 is also used as an indicator of the hydraulic conductivity of GCLs to leachates (Liu et al. 2014). Bentonites that undergo osmotic swelling and have lower hydraulic conductivity when hydrated with leachate also deposit a less-permeable filter cake in the fluid loss test, resulting in lower fluid loss. Normalized hydraulic conductivity (

K_{CCP} / K_{DI}

) of the GCLs is shown in Fig. 6(b) as a function of fluid loss for bentonite from the CS and GS GCLs hydrated in the CCP leachates (average fluid loss from two replicate tests reported). Like with SI, the relationship between hydraulic conductivity and FL is approximately log-linear and can be represented by the following relationship:

\log \frac{K_{CCP}}{K_{DI}} = 0.11 FL - 0.92

(4)

Eq. (4) (

R^{2} = 0.74

) was obtained using data pooled from the CS and GS GCLs with the hydraulic conductivity measured at 20 kPa using the same procedures used to develop Eq. (3).

Exchange Complex

The exchange complex of the bentonites before and after direct permeation is shown in Fig. 7 and summarized in Table 4 in terms of mole fractions computed as the quotient of the total charge associated with primary cations and the cation exchange capacity. When the mole fractions sum to less than 1.0, cations other than the primary cations

{Na}^{+}

,

K^{+}

,

{Ca}^{2 +}

, and

{Mg}^{2 +}

that were considered in the analysis occupy some of the exchange sites contributing to the CEC. Similar changes in the exchange complex are evident for the CS and GS GCLs. For both GCLs, the primary cations comprised a larger fraction of the CEC after permeation with CCP lecahate, indicating that other cations in the original exchange complex were replaced by cations in the leachate.

Fig. 7. Mole fractions of primary cations in the exchange complex in bentonite initially, after subgrade hydration, and after meeting chemical equilibrium criteria with direct permeation: (a) CS; (b) GSGCLs; each bar in graph is stacked (top to bottom) as ${Na}^{+}$ , $K^{+}$ , ${Ca}^{2 +}$ , and ${Mg}^{2 +}$

Table 4. Exchange Complex, Cation Exchange Capacity, and Swell Index to DI Water

GCL	Prehydration method	Permeant liquid	Mole fraction of bound cations				CEC ( $cmol + / kg$ )	Swell index to DI water ( $mL / 2 g$ )
GCL	Prehydration method	Permeant liquid	${Na}^{+}$	$K^{+}$	${Ca}^{2 +}$	${Mg}^{2 +}$	CEC ( $cmol + / kg$ )	Swell index to DI water ( $mL / 2 g$ )
CS	None	Not permeated^a	0.45	0.04	0.29	0.12	73.2	36.0
	Subgrade	Not permeated^a	0.40	0.02	0.38	0.17	76.2	22.0
	None	Typical CCP	$< 0.01$	0.02	0.83	0.11	72.8	12.0^b
		Low RMD	0.01	0.02	0.90	0.06	75.1	15.0
		FGD	0.04	0.03	0.77	0.13	74.4	13.0
		High strength	0.21	0.01	0.57	0.10	74.1	14.5
		High strength (100 kPa)	0.22	0.01	0.55	0.12	70.1	—
		Trona	0.52	0.03	0.27	0.07	78.2	33.0
		Trona (100 kPa)	0.49	0.03	0.33	0.07	74.8	—
		Trona (450 kPa)	0.53	0.03	0.33	0.08	80.0	—
	Subgrade	Typical CCP	0.01	0.02	0.82	0.15	70.7	—
		Low RMD	0.01	0.02	0.79	0.15	69.0	—
		FGD	0.09	0.02	0.66	0.18	72.3	—
		High strength	0.20	0.02	0.49	0.18	71.1	—
		Trona	0.57	0.03	0.34	0.06	75.1	—
GS	None	Not permeated^a	0.42	0.03	0.31	0.10	71.4	32.0
	Subgrade	Not permeated^a	0.35	0.02	0.36	0.12	74.3	27.0
	None	Typical CCP	0.01	0.02	0.88	0.09	73.0	15.0
		Low RMD	0.00	0.02	0.90	0.07	69.0	9.5
		FGD	0.02	0.02	0.80	0.10	74.0	11.0
		High strength	0.16	0.02	0.56	0.19	73.8	18.0
		High strength (100 kPa)	0.16	0.02	0.60	0.19	74.0	—
		Trona	0.56	0.03	0.28	0.06	80.6	26.0
		Trona (100 kPa)	0.52	0.02	0.33	0.06	66.3	—
		Trona (450 kPa)	0.54	0.03	0.35	0.06	77.2	—
	Subgrade	Typical CCP	0.01	0.02	0.87	0.10	70.4	—
		Low RMD	0.01	0.02	0.84	0.13	72.1	—
		FGD	0.05	0.03	0.78	0.14	71.8	—
		High strength	0.18	0.01	0.47	0.18	69.6	—
		Trona	0.62	0.02	0.30	0.01	74.6	—

Note: Hyphen indicates data not available; data correspond to bentonite after completing hydraulic conductivity test at 450 kPa unless indicated otherwise.

a

Analysis of bentonite prior to permeation and after applied prehydration condition.

b

SI of oven-dried bentonite after reaching chemical equilibrium criteria in hydraulic conductivity test.

Permeation by the four Ca-rich leachates (typical CCP, low-RMD, and FGD leachates with

RMD < 1

) replaced most of the

{Na}^{+}

present in the original exchange complex with

{Ca}^{2 +}

from the leachate (Fig. 7). The greatest replacement by

{Ca}^{2 +}

occured with the low-RMD and typical CCP leachates, which had the greatest preponderance of divalent cations (lowest RMD of leachates used).

{Ca}^{2 +}

was also sorbed preferentially to

{Mg}^{2 +}

, which is consistent with the lyotropic series (McBride 1994). The ony exception is the GS GCL permeated with the high-strength leachate, which had the second highest

{Mg}^{2 +}

concentration of all leachates (trona leachate slightly higher).

For both bentonites, the SI in DI water after permeation with these leachates is

9.5 - 13.0 mL / 2 g

(Table 4), indicating the Na-bentonite has transformed to a Ca-bentonite and no longer undergoes osmotic swell when hydrated in DI water, as observed in previous studies conducted by Jo et al. (2001, 2004), Meer and Benson (2007), and Scalia and Benson (2010).

GCL specimens permeated with Na-rich leachate trona leachate were enriched in

{Na}^{+}

during permeation, primarly because of exchange of

{Na}^{+}

for

{Mg}^{2 +}

(Fig. 7 and Table 4). The mole fraction of

{Na}^{+}

increased from 0.45 to 0.54 for the CS GCL and from 0.42 to 0.56 for the GS GCL. Bentonite from both GCLs had low SI in the trona leachate (

8.0 mL / 2 g

) because the high ionic strength of the leachate suppressed osmotic swell. However, SI of the bentonite in DI water was

26.0 - 33.0 mL / 2 g

after permeation with trona leachate, which is indicative of a

{Na}^{+}

-rich exchange complex and is comparable to SI for the original Na-bentonite.

Influence of Prehydration on Hydraulic Conductivity

The influence of prehydration of GCLs prior to permeation with leachate was evaluated using two approaches. The first approach consisted of hydrating the GCL specimens on a subgrade for 60 days prior to permeation with the CCP leachate to simulate hydration of the GCL after installation and prior to waste placement (described in the “Methods” section). The second approach consisted of permeating the GCL with DI water for 60 days prior to permeation with CCP leachate. The more realistic effect of subgrade hydration was evaluated with both GCLs and each of the CCP leachates. Hydration by permeation with DI water, which would require special provisions in practice, was evaluated with both GCLs and the trona leachate, which had the highest ionic strength and largest impact on hydraulic conductivity for both GCLs with direct permeation.

Hydraulic conductivity of GCL specimens hydrated on a subgrade and then permeated with CCP leachates is shown in Fig. 8 relative to hydraulic conductivity to the same leachates via direct permeation. When the effective stress was low (20 kPa), hydration on the subgrade had a beneficial effect on hydraulic conductivity for the more dilute typical CCP and low-RMD leachates, with a reduction in hydraulic conductivity attributable to subgrade hydration of

2 - 7 \times

for the CS GCL and

5 - 7 \times

for the GS GCL. For the more concentrated FGD, high strength, and trona leachates, the GS GCL had approximately the same hydraulic conductivity with or without subgrade hydration. At higher effective stress, subgrade hydration had a more consistent and significant effect on hydraulic conductivity to CCP leachates. For the CS GCL, the hydraulic conductivity to CCP leachate for both GCLs typically is at least

2 \times

lower at 100–250 kPa and

3 - 5 \times

lower at 450 kPa.

Fig. 8. Hydraulic conductivity of GCLs to CCP leachates after prehydration on subgrade or by permeation with DI water versus hydraulic conductivity after direction permeation

The greater impact of subgrade hydration at low stress (20 kPa) for the GS GCL may indicate that the smaller granules in the GS GCL hydrate more thoroughly than the those in the CS GCL, promoting osmotic swelling prior to contact with leachate and lowering the hydraulic conductivity. Osmotic swelling from subgrade hydration, with a modest amount of water available, should have greater influence on hydraulic conductivity for CCP leachates having lower ionic strength.

The modest amount of cation exchange (

{Ca}^{2 +}

and

{Mg}^{2 +}

for

{Na}^{+}

and

K^{+}

) that occurred during subgrade hydration (Fig. 7) apparently had no consistent impact on hydraulic conductivity to the CCP leachates. However, the posthydration swell index to DI water of the CS GCL was

22 mL / 2 g

compared with

36 mL / 2 g

for the original GCL, and the exchange complex was enriched in both

{Ca}^{2 +}

and

{Mg}^{2 +}

(Tables 3 and 4). Similarly, for the GS GCL, the SI to DI water after subgrade hydration was

27 mL / 2 g

, relative to

32 mL / 2 g

for the original GCL. Thus, cation exchange during subgrade hydration did occur and affect swelling of the bentonite, but not sufficiently to consistently alter hydraulic conductivity to the CCP leachates.

The CS GCL specimen consolidated to 20 kPa and permeated with FGD leachate is an exception, with the specimen prehydrated on the subgrade

9 \times

more permeable than the specimen permeated directly with leachate. The reason for this large difference relative to the other leachates is not known. However, Bradshaw et al. (2013) and Bradshaw and Benson (2014) reported similar findings for some leachates and attributed the higher hydraulic conductivity to replacement of

{Na}^{+}

by divalent cations during prehydration, thereby exacerbating the impacts of exchange during permeation.

Prehydration with DI water for 60 days had a much more significant effect on hydraulic conductivity, as shown for trona leachate in Fig. 8. The CS and GS GCLs were approximately three orders of magnitude less permeable when prehydrated by permeation with DI water. Thus, prehydration strategies, like those used to create the dense prehydrated (DPH) GCLs described by Kolstad et al. (2004b) and Mazzieri and Di Emidio (2015), could be used to create GCLs with Na-bentonite that are more resistant to CCP leachates. However, even with prehydration by permeation with DI water, the hydraulic conductivity to trona leachate was higher than the hydraulic conductivity to DI water. Thus, prehydration did not provide complete protection against alterations in hydraulic conductivity. Permanence of the prehydration effect afforded by DI water over very long-term permeation with aggressive leachates has not yet been investigated.

Swell index tests were conducted to investigate how prehydration with DI water provides resistance to higher hydraulic conductivities. Bentonite was hydrated for 24 h in a conventional SI test. The supernatant above the swollen bentonite was then removed carefully from the graduated cylinder with a syringe to avoid loss of bentonite, and replaced with trona leachate. The graduated cylinder was then tumbled by hand for 10 min, and the SI measurement take after settlement of the bentonite was complete. SI of bentonites from both GCLs decreased modestly within 1 day after switching to trona leachate (38.0–

35.0 mL / 2 g

for CS and 32.0–

30.0 mL / 2 g

for GS), but then stabilized and remained essentially constant for an additional 20 days when the test was terminated.

These data indicate that water associated with osmotic swell is retained in the interlayer of montmorillonite after hydration and subsequent contact with trona leachate and provides for the lower hydraulic conductivity after prehydration with DI water. Lee and Shackelford (2005) and Jo et al. (2004) reported retention of immobile water from osmotic swell in prehydrated bentonite after switching permeant liquids from DI water to inorganic salt solutions, and Jo et al. (2004) reported that immobile water from osmotic swell reduces cation exchange by inhibiting diffusion of divalent cations into the interlayer of montmorillonite.

Influence of Effective Stress

Increasing the effective stress from 20 to 450 kPa reduced the hydraulic conductivity by up to three orders of magnitude for the GCLs directly permeated with leachates (Fig. 9). The data fall primarily into two groups: more permeable and less permeable GCLs at low effective stress. The more permeable GCLs typically had hydraulic conductivity exceeding

10^{- 8} m / s

at 20 kPa, including both GCLs permeated with trona leachate, the CS GCL permeated with the high strength leachate, and the GS GCL permeated with the FGD leachate. The less permeable GCLs typically had hydraulic conductivity no greater than

10^{- 8} m / s

at 20 kPa, and included both GCLs permeated with typical CCP leachate, the CS GCL permeated with low-RMD and FGD leachate, and the GS GCL permeated with high-strength leachate. No consistent distinction exists between the two groups by type of GCL or CCP leachate.

Fig. 9. Hydraulic conductivity of CS (closed) and GS (open) GCLs to CCP leachates as a function of effective stress

For both groups, the hydraulic conductivity decreased approximately two orders of magnitude when the stress was increased from 20 to 450 kPa. For the less permeable GCLs, the rate of decrease in hydraulic conductivity with increasing stress was greater initially, with the hydraulic conductivity dropping

10 \times

, on average, as the effective stress increased from 20 to 100 kPa and

50 \times

, on average, as the effective stress increased from 20 to 250 kPa. Hydraulic conductivity of the more permeable GCLs decreased approximately

5 \times

, on average, as the effective stress increased from 20 to 100 kPa, and

10 \times

, on average, as the effective stress increased from 20 to 250 kPa. The less permeable GCLs had greater SI (Table 3), and therefore greater osmotic swell and more bound water relative to the more permeable GCLs. Consequently, the pore space in the less permeable GCLs probably was more compressible as the effective stress increased, causing a greater reduction in hydraulic conductivity.

Although increasing the effective stress from 20 to 450 kPa caused a comparable reduction in hydraulic conductivity for both groups of GCLs and leachates, only the less permeable GCLs likely would meet the criteria for an alternative liner in the recent U.S. regulations for CCP disposal at higher effective stress. The more permeable GCLs had a hydraulic conductivity exceeding

10^{- 10} m / s

at 450 kPa, and some had hydraulic conductivity exceeding

10^{- 9} m / s

at 450 kPa, which is too permeable to be equivalent to a 0.6-m-thick compacted soil liner having hydraulic

conductivity \leq 10^{- 9} m / s

per the criteria in the Federal Register (2015). Meeting the equivalency requirement in the Federal Register (2015) requires that the GCL have a hydraulic conductivity no greater than

2.5 - 5.0 \times 10^{- 11} m / s

, depending on the thickness of the GCL.

Hydraulic conductivity tests were also conducted with high strength and trona leachate on GCLs consolidated directly to a higher effective stress (100 or 450 kPa) prior to permeation to simulate a scenario where a facility is filled rapidly relative to leachate generation. For these tests, the GCL specimen was initially consolidated to 100 or 450 kPa, and then the hydraulic gradient was applied. Permeation was continued until the aforementioned hydraulic and chemical equilibrium criteria were met. In contrast, all other specimens were initially consolidated to 20 kPa, permeated until the equilibrium criteria were met, and then incrementally consolidated to the next higher effective stress and repermeated. Hydraulic conductivities obtained from these tests are shown in Fig. 10 along with hydraulic conductivities measured at the same effective stress, but with the initial consolidation to 20 kPa followed by permeation to hydraulic and chemical equilibrium.

Fig. 10. Hydraulic conductivity of GCLs to CCP leachates with 100 or 450 kPa applied prior to permeation relative to hydraulic conductivity at same stress but with chemical equilibrium criteria achieved at 20 kPa

Hydraulic conductivities

10 - 1,000 \times

lower were obtained when 100 or 450 kPa effective stress was applied prior to permeation. At 450 kPa, the hydraulic conductivity to trona leachate with the effective stress applied beforehand was

\leq 3.2 \times 10^{- 12} m / s

, or 1/340 that of the hydraulic conductivity obtained at 450 kPa when chemical equilibrium was established initially at 20 kPa. Despite being lower, the hydraulic conductivities at 100 kPa (

\geq 1.3 \times 10^{- 10} m / s

) are more than an order of magnitude higher than normally anticipated for a GCL (

\sim 1 \times 10^{- 11} m / s

). High effective stress needs to be achieved rapidly relative to permeation for a GCL to have very low hydraulic conductivity to aggressive CCP leachates.

The final thickness was lower (0.2 mm, Table 3) for both GCL specimens with 450 kPa applied before permeation, indicating that the bentonite in the GCL had lower void ratio when elevated stress was applied prior to permeation. In contrast, the final exchange complex of bentonite from GCLs with elevated stress applied before permeation was indistinguishable from the exchange complex of GCLs with the stress applied after permeation to reach the chemical equilibrium criteria (Fig. 11). In similar experiments, Petrov and Rowe (1997) reported that the void ratio and hydraulic conductivity were lower for the specimens consolidated prior to permeation. This suggests that sequencing of the stress application (before or after permeation with leachate) alters the hydraulic conductivity by a physical phenomenon rather than a chemical phenomenon, and that cation exchange occurs regardless of when the effective stress is applied.

Fig. 11. Mole fractions of major monovalent and divalent cations in exchange complex of bentonite in GCLs permeated with CCP leachates with 100 or 450 kPa applied prior to permeation relative to mole fractions at same stress but with chemical equilibrium criteria achieved at 20 kPa

Summary and Conclusions

Hydraulic conductivity of two geosynthetic clay liners to coal combustion product (CCP) leachates was evaluated in this study. Both GCLs contained granular sodium bentonite and were contained between needlepunched geotextiles. The GCLs were provided by two U.S. manufacturers and represent the two most commonly used GCLs in the United States. Five leachates were used in the experimental program representing leachate characteristic of CCPs at coal-fired power plants in the United States.

Hydraulic conductivity tests were conducted on the GCL specimens permeated directly with leachate or prehydrated on a subgrade or with DI water prior to permeation with CCP leachate. Effective stresses ranging from 20 to 450 kPa were used, and the sequence of application of the stress relative to permeation was evaluated. Tests with elevated stress applied prior to permeation with CCP leachate were conducted to simulate a CCP disposal facility that is filled rapidly relative to the rate of leachate generation. All hydraulic conductivity tests were conducted until the hydraulic and chemical equilibrium criteria in ASTM D5084, D6766, and D7100 were met. The exchange complex and swell index of the bentonite were evaluated prior to testing, after subgrade prehydration, and after permeation was completed.

In general, the findings of this study are consistent with the current body of knowledge regarding the impact of chemical solutions on the hydraulic conductivity of GCLs. The specific outcomes, however, are unique to CCP leachates. Based on the findings of this study, the following conclusions are drawn:

•

GCLs with granular Na-bentonite that are permeated directly or hydrated on a subgrade prior to permeation have higher hydraulic conductivity to CCP leachate than to DI water, as has been observed for leachates from other waste streams. Hydraulic conductivity is related strongly and directly to the ionic strength of the leachate. Relative abundance of polyvalent cations relative to monovalent cations in CCP leachates has a modest effect on hydraulic conductivity relative to ionic strength. Accordingly, chemistry of the CCP leachate has a strong influence on whether the hydraulic conductivity of a GCL will meet the equivalency requirements in the Federal Register (2015);

•

Hydraulic conductivity of GCLs with granular Na-bentonite permeated with CCP leachates is inversely related to swell index and directly related to the fluid loss of the bentonite when CCP leachate is used as the hydrating liquid. SI or FL can be used to screen the hydraulic conductivity of GCLs to CCP leachates. Equations were provided to estimate the hydraulic conductivity to CCP leachate based on the hydraulic conductivity to DI water and the SI or FL measured using CCP leachate as the hydrating liquid;

•

Hydration on a subgrade prior to permeation has only modest impact on the hydraulic conductivity to CCP leachate. At low effective stress (20 kPa), hydraulic conductivity of GCLs hydrated on subgrade prior to permeation have hydraulic conductivity comparable to the hydraulic conductivity obtained from direct permeation. At higher stresses (100–450 kPa), the hydraulic conductivity is

2 - 5 \times

lower when the GCL is hydrated on a subgrade prior to permeation with CCP leachate. Prehydration by permeation with DI water prior to CCP leachate provides much greater resistance to higher hydraulic conductivities (three orders of magnitude at high ionic strength);

•

Increasing the effective stress from 20 to 450 kPa causes a reduction in hydraulic conductivity to CCP leachates of approximately two orders of magnitude. The rate of reduction in hydraulic conductivity with increasing stress is higher at low effective stress for GCLs with lower hydraulic to CCP leachate at 20 kPa. The stress at which the GCL is evaluated has a strong influence on whether the hydraulic conductivity will meet the equivalency requirements in the Federal Register (2015). Any evaluation should be conducted at stresses reflecting field conditions; and

•

Lower hydraulic conductivities to CCP leachate can be achieved by applying high effective stress prior to permeation with leachate (e.g., rapid filling of a disposal facility) or by permeating the GCL with DI water prior to permeation (e.g., using a prehydrated GCL), especially for leachates with higher ionic strength. Permanence of the benefits achieved by prehydration with DI water over very long periods of time has not been evaluated.

Acknowledgments

The Electric Power Research Institute (EPRI) provided financial support for this study through a grant to the Office of Sustainability at the University of Wisconsin-Madison. EPRI also provided access to their CCP leachate database for use in this study. Fundamental Research Funds for the Central Universities (A0920502051619-89) provided support for Dr. Chen’s travel.

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Journal of Geotechnical and Geoenvironmental Engineering

Volume 144 • Issue 3 • March 2018

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This work is made available under the terms of the Creative Commons Attribution 4.0 International license, http://creativecommons.org/licenses/by/4.0/.

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Received: Jan 24, 2017

Accepted: Sep 1, 2017

Published online: Jan 11, 2018

Published in print: Mar 1, 2018

Discussion open until: Jun 11, 2018

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Jiannan N. Chen, A.M.ASCE [email protected]

Assistant Professor, School of Geoscience and Environmental Engineering, Southwest Jiaotong Univ., Chengdu 611756, China. E-mail: [email protected]

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Craig H. Benson, F.ASCE [email protected]

Dean, School of Engineering, Univ. of Virginia, Charlottesville, VA 22904 (corresponding author). E-mail: [email protected]

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Tuncer B. Edil, Dist.M.ASCE [email protected]

Professor Emeritus, Geological Engineering, Civil and Environmental Engineering, Univ. of Wisconsin-Madison, Madison, WI 53706. E-mail: [email protected]

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Abstract

Introduction

Background

Materials

Geosynthetic Clay Liners

CCP Leachates

Subgrade Soil

Methods

Hydraulic Conductivity

Prehydration

Swell Index and Fluid Loss Tests

Chemical Analysis

Results and Discussion

Free Swell, Fluid Loss, and Hydraulic Conductivity

Exchange Complex

Influence of Prehydration on Hydraulic Conductivity

Influence of Effective Stress

Summary and Conclusions

Acknowledgments

References

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