Relative Abundance of Monovalent and Divalent Cations and the ...

Relative Abundance of Monovalent and Divalent Cations and the Impact of Desiccation on Geosynthetic Clay Liners Craig H. Benson1 and Stephen R. Meer2 Abstract: Laboratory experiments were conducted on a geosynthetic clay liner 共GCL兲 containing Na–bentonite to determine how the swell index and hydraulic conductivity of GCLs are affected by wet-dry cycling with solutions having different relative abundance of monovalent and multivalent cations. Relative abundance of monovalent and multivalent cations was characterized by the RMD of the test solution, which is defined as the ratio of the total molarity of monovalent cations to the square root of the total molarity of multivalent cations at a given ionic strength. RMD was found to control the final swell index, relative abundance of monovalent and divalent cations in the final exchange complex, and the final hydraulic conductivity of bentonite exposed to wet-dry cycling. Ionic strength affects the number of wet-dry cycles required for a change in hydraulic conductivity to occur and the rate of change in swell index. Large increases in hydraulic conductivity and loss of swelling capacity occurred for solutions having RMD艋 0.07 M1/2. Modest or small changes in hydraulic conductivity and swell index were obtained when the RMD was 艌0.14 M1/2. These findings suggest that chemical analysis of the pore water in cover soils may prove useful in evaluating the compatibility of GCLs and cover soils used in applications where wet-dry cycling may occur. DOI: 10.1061/共ASCE兲1090-0241共2009兲135:3共349兲 CE Database subject headings: Clay liners; Landfill; Geosynthetics; Hydraulic conductivity.

Introduction Geosynthetic clay liners 共GCLs兲 are factory-made clay liners that consist of a layer of bentonite 共3.2– 6.0 kg/ m2兲 bonded to a geosynthetic material. Most GCLs contain bentonite sandwiched between two geotextiles that are bonded using needle punching, stitching, or adhesives. In some cases, the bentonite is bonded directly to a geomembrane or a geomembrane is laminated to one of the geotextiles. For GCLs that do not include a geomembrane, the effectiveness as a hydraulic barrier is controlled by the hydraulic conductivity of the bentonite. The hydraulic conductivity of the sodium bentonite used in most GCLs is on the order of 10−9 cm/ s when permeated with deionized 共DI兲 water at stresses typical of cover applications 共Shan and Daniel 1991; Petrov and Rowe 1997; Shackelford et al. 2000; Jo et al. 2001兲. GCLs present an attractive alternative to compacted clay liners as the hydraulic barrier layer in landfill cover systems because of their low hydraulic conductivity 共⬇10−9 cm/ s to DI water兲, ease of installation, thinness, and perceived resistance to environmental stresses 共e.g., freeze-thaw and wet-dry cycling兲共Bouazza 2002兲. However, the bentonite in GCLs is sensitive to chemical interactions with the hydrating liquid, and ion exchange that occurs in bentonite can alter its physical properties. In particular, 1 Wisconsin Distinguished Professor and Chairman, Dept. of Geological Engineering, Univ. of Wisconsin-Madison, Madison, WI 53706 共corresponding author兲. E-mail: [email protected] 2 Engineer, Sigma Environmental Services, Inc., 1300 West Canal Str, Milwaukee, WI 53233. E-mail: [email protected] Note. Discussion open until August 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this paper was submitted for review and possible publication on October 29, 2007; approved on May 13, 2008. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 135, No. 3, March 1, 2009. ©ASCE, ISSN 1090-0241/2009/3-349–358/$25.00.

exchange of multivalent cations for the native Na results in increased hydraulic conductivity and decreased swell potential 共Shan and Daniel 1991; Gleason et al. 1997; James et al. 1997; Ruhl and Daniel 1997; Petrov and Rowe 1997; Shackelford et al. 2000; Egloffstein 2001; Jo et al. 2001, 2004, 2005; Kolstad et al. 2004, Guyonnet et al. 2005; Lee and Shackelford 2005; Lee et al. 2005; Katsumi et al. 2007兲. When cation exchange is concomitant with wet-dry cycling, increases in hydraulic conductivity can occur that may render a GCL ineffective as a hydraulic barrier 共Melchior 1997; Lin and Benson 2000; Egloffstein 2001; Benson et al. 2007; Meer and Benson 2007兲. These increases in hydraulic conductivity have been attributed to macroscopic features 共e.g., cracks, intergranule pores兲 formed during drying that do not swell shut during rehydration 共Lin and Benson 2000; Melchior 2002; Benson et al. 2007; Meer and Benson 2007兲. Recent field studies of covers with GCLs have confirmed that multivalent-for-monovalent cation exchange can occur relatively rapidly and that large increases in hydraulic conductivity of GCLs can occur under some circumstances 共Egloffstein 2001; Melchior 2002; Benson et al. 2004, 2007; Meer and Benson 2007兲. Water percolating downward from the overlying cover soils is believed to be the source of the multivalent cations 共Melchior 2002; Benson et al. 2007; Meer and Benson 2007兲. For example, Meer and Benson 共2007兲 measured the hydraulic conductivity, swell index, and cation exchange complex of GCLs exhumed from four landfill covers that had been in service between 4.1 and 11 years. They found that divalent cations 共primarily Ca兲 had replaced most of the native Na in the exchange complex of the bentonite. Hydraulic conductivities of the exhumed GCLs fell in a broad range 共5.2⫻ 10−9 – 1.6⫻ 10−4 cm/ s兲, but most were greater than 10−6 cm/ s. The swell index of bentonite from the exhumed GCLs was typical of Ca–bentonite, and analysis of the exchange complex showed that nearly all of the native Na had been replaced by Ca, and to some extent Mg. Meer and Benson 共2007兲 also reviewed field data for 15 GCLs

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where the in-service hydraulic conductivity could be determined. Of these 15 GCLs, 11 had hydraulic conductivities ⬎10−6 cm/ s and nine had hydraulic conductivities ⬎10−5 cm/ s. Only one GCL had a hydraulic conductivity typical of a new GCL with sodium bentonite 共2 ⫻ 10−9 cm/ s兲. GCLs that had undergone increases in hydraulic conductivity also exhibited near complete cation exchange 共Ca or Mg for Na兲. In contrast, the GCL that had a hydraulic conductivity 2 ⫻ 10−9 cm/ s did not exhibit cation exchange, despite being buried for 5 years. The cover soil overlying this GCL was sodic 共Na rich兲, and therefore was not a source of multivalent cations. The findings in Meer and Benson 共2007兲 suggest that large increases in hydraulic conductivity can occur in response to wetdry cycling when water percolating through the cover profile is dominated by multivalent cations. In contrast, when the percolating water is dominated by monovalent cations, wet-dry cycling should have much less effect 共and potentially no effect兲 on the hydraulic conductivity of GCLs. This hypothesis was evaluated in this study by subjecting bentonite from a GCL as well as GCL specimens to wet-dry cycling using solutions having different relative abundance of monovalent and divalent cations 共Na and Ca兲. After each wet-dry cycle, the hydraulic conductivity of the GCL and the swell index and cation exchange complex of the bentonite were determined. Changes in the hydraulic conductivity, swell index, and the cation exchange complex were related to the relative abundance of monovalent and multivalent cations in the hydrating solution.

Table 1. Properties of Cover Soils for Elution Tests

ID

Soil type

Percent fines 共%兲

Liquid limit

Plasticity index

Paste pH

CaCO3 content 共%兲

74 20 17 24 31 45 43 99 82 7 31

26 36 39 24 32 49 43 42 24 —a 28

11 9 11 12 8 18 13 25 2 —a 13

7.4 7.2 5.6 5.1 5.8 5.2 5.0 7.9 7.8 8.0 7.2

2.6 2.2 1.0 1.0 1.0 1.4 1.3 0.9 1.9 1.9 0.5

1 Sandy clay 2 Silty sand 3 Silty sand 4 Clayey sand 5 Silty sand 6 Silty sand 7 Silty sand 8 Lean clay 9 Sandy silt 10 Silty sand 11 Clayey sand a Not measured.

Hydraulic conductivity of the as-received GCL was determined in a flexible-wall permeameter following the methods described in ASTM D 5084 共ASTM 2004兲. DI water was used as the permeant liquid, the effective confining stress was 20 kPa, and the hydraulic gradient was 75. Two specimens were tested, and had hydraulic conductivities of 2.1⫻ 10−9 and 1.0⫻ 10−9 cm/ s. Swell Index Tests

Materials and Methods Geosynthetic Clay Liner A roll of GCL provided by a manufacturer was used as the source of the GCL specimens and bentonite used in the study. The GCL contained granular sodium bentonite encased by two geotextiles 共a slit-film woven geotextile and a nonwoven geotextile兲 bonded by needle punching. The mass per unit area of air-dry bentonite in the GCL was 4.3 kg/ m2, the initial air-dry thickness ranged from 5.8 to 7.0 mm, and the average initial air-dry water content of the bentonite was 7.0%. The bentonite consisted of sand-size granules 共0.075– 2.0 mm兲 composed primarily of clay-size particles 共87% finer than 2 ␮m兲 and had a liquid limit of 504 and a plasticity index of 465. X-Ray diffraction showed that the bentonite contained 80% montmorillonite, 7% plagioclase, 6% cristobalite, and trace levels 共艋2 % 兲 of illite, mica, heulandite, gypsum, and quartz. The cation exchange complex of the bentonite was determined by extraction using the ammonium acetate method 共Thomas 1982兲 on 10 g of dry bentonite crushed to pass a No. 20 United States standard sieve. Soluble salts were extracted beforehand using the saturation extraction procedure described in Rhoades 共1982兲 with a solid-DI water ratio of 1:5. The ammonium acetate extraction was conducted with a 1 M ammonium acetate solution at a solid-liquid ratio of 1:5. The mixture was shaken for 24 h, after which the solid and liquid were separated by vacuum filtration using Whatman No. 42 filter paper. Concentrations of the exchangeable cations Na, K, Ca, and Mg in the extract were measured using atomic absorption spectroscopy 共AAS兲 following USEPA Method 200.7. The exchange complex of bentonite in the GCL contained Na 共73% mole fraction兲, Ca 共22% mole fraction兲, Mg 共3% mole fraction兲, and K 共2% mole fraction兲 in the initial condition.

Swell index tests were conducted according to methods described in ASTM D 5890 共ASTM 2004兲. Air-dried bentonite was ground using a mortar and pestle until 100% passed a No. 200 United States sieve. Approximately 90 mL of DI water was poured into a clean 100 mL graduated cylinder. Two grams of dry bentonite was then placed in the graduated cylinder in 0.1 g increments. Additional test solution was used to rinse any particles adhering to the sides of the cylinder and to fill the cylinder to the 100 mL mark. After 24 h of exposure, the swell index was recorded. Solutions Column elution tests were conducted on 11 soils sampled from the surface layer of landfills throughout the United States to obtain an estimate of the ionic strength and relative abundance of cations in water contacting GCLs in the field. A sample of each cover soil was collected in a 20 L bucket, which was sealed and shipped to the laboratory for testing. Index properties of the soils are summarized in Table 1. The column elution tests were conducted in rigid-wall permeameters similar to those described in ASTM D 5856 共ASTM 2004兲. Specimens were prepared by compaction in a stainless steel compaction mold 共diameter= 105 mm, height= 75 mm兲 to a dry unit weight corresponding to 85% relative compaction per standard Proctor to simulate the low compactive effort normally applied to surface layer soils. Both ends of the specimen were covered with disks of nonwoven geotextile, and a porous stone was placed on top of the upper geotextile to distribute the influent water. Effluent from the column was analyzed for concentrations of Ca, Mg, Na, and K by AAS, as described previously. A solution representing synthetic rainwater was allowed to slowly drip 共2 mL/ h兲 onto the porous stone to simulate the slow unsaturated infiltration that might occur in the field. Ionic composition of the synthetic rainwater was based on an analysis of rainwater chemistry from 18 locations in North America, Europe, and

350 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / MARCH 2009


Table 2. Ionic Strength, RMD, pH, and Electrical Conductivity of Test Solutions Used in Laboratory Tests

Fig. 1. Ionic strength 共a兲; RMD 共b兲 from column tests on cover soils as function of eluted volume

Asia 共Meer and Benson 2004兲. The synthetic rainwater was prepared by dissolving NaCl and CaCl2 salts in DI water to create a solution having an ionic strength of 0.0008 M, RMD of 0.02 M1/2, and pH 7.1. RMD is defined as M m / M 1/2 d , where M m⫽total molarity of monovalent cations and M d⫽total molarity of multivalent cations in the solution, and represents the relative abundance of monovalent and multivalent cations at a given ionic strength. For inorganic aqueous permeant solutions, ionic strength and RMD are master variables controlling the hydraulic conductivity of GCLs for pH between 2 and 12 共Jo et al. 2001, 2004; Kolstad et al. 2004兲. The denominator of RMD includes divalent and other multivalent cations because Jo et al. 共2001兲 show that multivalent cations have a similar impact on hydraulic conductivity at the same molarity, regardless of valence. However, in most pore waters in cover soils including those evaluated in this study, divalent cations dominate the multivalent cations. Thus, the multivalent nomenclature is retained herein for consistency with past studies, but the solutions that were used in this study only contained monovalent and divalent cations. Ionic strength and RMD of the effluent from the cover soil elution tests is shown in Fig. 1 as a function of eluted volume. Both ionic strength and RMD typically decreased with eluted volume and then leveled off, although the magnitude of the drop varied considerably. Between cover soils, the steady-state ionic strength varied by approximately a factor of ten and the RMD varied by approximately a factor of 20. Effluent from the more clayey soils typically had lower ionic strength than effluent from the other soils, which may reflect the greater affinity of clays for

Ionic strength 共M兲

RMD 共M0.5兲

pH

Electrical conductivity 共S/m兲

0 0.005 0.005 0.005 0.011 0.011 0.011 0.025 0.025 0.025 0.025 0.025

— 0.007 0.07 0.7 0.007 0.07 0.7 0.007 0.07 0.10 0.14 0.7

7.0 6.5 6.7 6.6 6.2 6.0 6.0 6.5 6.2 6.2 6.3 6.0

0.05 9.9 7.7 6.0 22.0 18.0 14.9 44.2 36.2 36.0 35.6 32.0

cations. A similar dependence on soil type is not evident in the RMD of the effluent, with effluent from the clayey and nonclayey soils having a wide range of RMD. Eleven solutions were used in the study. Ionic strength and RMD of the solutions are summarized in Table 2. The RMD of these solutions spans the range of RMD in the effluent from the column elution tests 关Fig. 1共b兲兴. Three ionic strengths were used 共0.005, 0.011, and 0.025 M兲. The upper bound on steady concentrations observed in the elution tests was represented by an ionic strength of 0.005 M, and the upper bound from the entire set of elution data was represented by an ionic strength of 0.025 M 关Fig. 1共a兲兴. The intermediate ionic strength 共0.011 M兲 was used to represent an intermediate condition. Type II DI water 共ASTM D 1193, ASTM 2004兲 was used as a control. Jo et al. 共2001, 2004兲 and Kolstad et al. 共2004兲 show that cation valence is the most significant factor affecting swell of bentonite and hydraulic conductivity of GCLs at a given molarity, whereas cation species for a given valence has no measurable impact. Thus, all solutions were prepared with anhydrous NaCl and CaCl2 dissociated in Type II DI water 共i.e., Na was the sole monovalent cation and Ca was the sole divalent cation兲. Batch Tests Batch tests were conducted using bentonite from the GCL to evaluate how the combined effects of desiccation and cation exchange affect swelling of the bentonite and composition of the exchange complex. For each batch test series, 80 g of bentonite from the GCL was ground using a mortar and pestle until it passed the No. 20 United States sieve. A 2 g subsample was set aside for swell index testing, and the remainder was mixed in 2 L glass jars with one of the salt solutions summarized in Table 2. The jars were capped and tumbled end-over-end at 30 rpm. After tumbling, the slurry was emptied into polypropylene pans for air drying under a vacuum hood. The mass of each pan was weighed daily. Once the mass was stable, the bentonite was reground until it passed the No. 20 United States standard sieve. A 10 g portion of the reground bentonite was set aside to measure the cation composition of the exchange complex and 2 g was set aside for a swell index test. The bentonite was then mixed again with an identical salt solution at the same solid-to-liquid ratio.



Fig. 2. Ca, Mg, Na, and K concentrations in batch-test solution as function of tumbling time

This process was repeated until the 80 g of bentonite was exhausted, or the swell index and exchange complex data indicated that exchange of Ca for Na was complete. The tumbling time required to achieve equilibrium between the bentonite and the solution was determined by collecting aliquots of the solution over time while tumbling the mixtures for 48 h. Each aliquot was vacuum filtered using a Buchner funnel and Whatman No. 42 filter paper. Cation concentrations in the filtrate were determined by AAS as cited previously. Results of the analysis showed that the concentrations of Na, K, Ca, and Mg did not change significantly after approximately 14 h 共Fig. 2兲. Therefore, all batch tests were conducted with a convenient 24 h tumbling time. Hydraulic Conductivity Tests Falling-head hydraulic conductivity tests were conducted on the GCLs in flexible-wall permeameters according to methods described in ASTM D 5084. An average effective stress of 20 kPa was applied to simulate the effective stress applied to the GCL in a final cover with approximately 1 m of cover soil. No backpressure was applied so that samples for pH and electrical conductivity measurements could be collected easily. The hydraulic gradient that was applied varied depending on the hydraulic conductivity of the specimen, with a maximum hydraulic gradient of 75 being applied to specimens with lower hydraulic conductivity. This gradient is higher than the field gradient. However, Shackelford et al. 共2000兲 indicate that the hydraulic conductivity of GCLs is relatively insensitive to the hydraulic gradient. Thus, the effect of the elevated gradient is believed to be negligible. Lower gradients were used for the more permeable specimens to permit convenient collection of effluent. GCLs specimens were prepared for hydraulic conductivity testing using the method described in Jo et al. 共2001兲. A razor knife was used to cut the GCL along the outer circumference of a stainless steel cutting ring 共152 mm diameter兲. A small volume of permeant liquid was applied to the GCL along the inner circumference of the cutting ring during trimming to induce local hydration and prevent loss of bentonite. After cutting, the specimen was removed from the cutting ring and excess geotextile fibers along the edge of the GCL were removed. Bentonite paste, prepared with bentonite from the GCL and the permeant liquid, was applied around the perimeter of the GCL to reduce the potential for sidewall leakage.

The GCLs were placed in the permeameter and allowed to hydrate in the permeant liquid for 48 h under no hydraulic gradient as recommended in Jo et al. 共2001兲. The solutions shown in Table 2 were used as permeant liquids. After hydration, a hydraulic conductivity test was conducted following the methods in ASTM D 5084. Each specimen was permeated for 30 days to simulate percolation during wet spring conditions, as suggested by Lin and Benson 共2000兲. During this period, the data were inspected to determine if the hydraulic conductivity was steady 共⫾25% from the mean and no visible trend兲 and the ratio of incremental inflow to outflow was between 0.75 and 1.25. In all cases, both of these criteria were satisfied during the 30 day permeation period, and generally in ⬍10 days. Because the intent was to simulate cation exchange that occurs during a wet spring condition, establishing chemical equilibrium between the bentonite and the permeation liquid was not a termination criterion. Wet-dry cycling was conducted using the method in Lin and Benson 共2000兲. After permeation, GCL specimens were air dried on a bench until the weight of the specimen did not change. Overburden pressure was not applied during drying. Meer and Benson 共2007兲 compared hydraulic conductivities of specimens dried with and without overburden pressure, and indicate that overburden pressure during drying had no noticeable effect on the hydraulic conductivity. Typically 7 – 10 days were required to complete the drying cycle, after which the air-dry water content of the bentonite was on the order of 20–30%. Specimens were subjected to 5–9 wet-dry cycles using this procedure. After the final wetting, the thickness of the GCL was measured, the water content and swell index of the bentonite were determined, and the composition of the exchange complex was measured. The wet-dry procedure that was used is severe, as the GCL is dried to a water content at the low end of water contents observed in the field 共Meer and Benson 2007; Benson et al. 2007兲. However, as described subsequently, the GCLs that experienced changes in hydraulic conductivity due to wet-dry cycling following the aforementioned procedure had hydraulic conductivities at the end of testing comparable to those measured on specimens exhumed from field sites by Melchior 共2002兲, Meer and Benson 共2007兲, and Benson et al. 共2007兲.

Results and Discussion Swell Index Swell index of the bentonite subjected to wet-dry cycling using the batch procedure is shown in Fig. 3共a兲. A decrease in swell index following the first wet-dry cycle was observed for all solutions. For some solutions 共DI water and salt solutions having RMD= 0.7 M0.5兲, the decrease in swell index was followed by an increase in swell index with additional wet-dry cycles 共bentonites that “retained swell”兲, whereas the swell index changed negligibly or decreased modestly 共bentonites that “lost swell”兲 in the other solutions. For most solutions, the swell index ceased to change significantly after 3 – 4 cycles 共DI water is an exception兲. The bentonites that retained swell 共solid symbols兲 retain a swell index of at least 23 mL/ 2 g after repeated wet-dry cycling, whereas the bentonites that lost swell 共open symbols兲 have a swell index 艋15 mL/ 2 g. These changes in swell behavior reflect the relative abundance of monovalent and divalent cations in the hydrating solution, and their impact on the primary cations in the exchange complex. This correspondence is illustrated in Fig. 4; swell index increases monotonically as the mole fraction of Na in the ex-



Fig. 3. Swell index 共a兲; mole fraction of Na on exchange complex of the bentonite 共b兲 as function of number of wet-dry cycles

change complex increases. Except for one case, the bentonites that have lost swell have a mole fraction of Na less than 0.4. The bentonites that retained swell were hydrated with solutions having the greatest relative abundance of monovalent cations: DI water or solutions having an RMD= 0.7 M0.5. DI water is included as a hydration solution that contains predominantly monovalent cations, because the cations present in the batch liquid prepared with DI water are derived primarily from soluble

Fig. 4. Relationship between swell index of bentonites in batch tests and mole fraction of Na in exchange complex

Na–salts in the Na–bentonite. In contrast, the bentonites that lost swell were hydrated with solutions having a greater abundance of divalent cations 共RMD艋 0.07 M0.5兲, which resulted in the replacement of Na by Ca as the number of wet-dry cycles increased. This exchange effect is illustrated in Fig. 5 using data from the batch tests conducted with an ionic strength of 0.005 M and RMD= 0.007 M0.5. The rate at which this effect occurs is

Fig. 5. Mole fraction of Na and Ca as function of number of wet-dry cycles for bentonite in batch tests. Solution had ionic strength of 0.005 M and RMD= 0.007 M0.5.



Fig. 7. Hydraulic conductivity of GCLs permeated with solutions having ionic strength= 0.005 and 0.025 M and RMD ranging from 0.007 to 0.7 M0.5 as function of number of wet-dry cycles

Fig. 6. Swell index of bentonite from final cycle of batch tests as function of ionic strength 共a兲; RMD 共b兲 of solution

illustrated in Fig. 3共b兲. More rapid replacement of Na by Ca occurs with wet-dry cycling when the ionic strength is higher or the RMD is lower. Replicate tests were conducted with two solutions that produced bentonites that retained swell 共DI water and a solution with I = 0.005 M and RMD= 0.7 M0.5兲 to determine if the decrease in swell index observed for these solutions during the first cycle was anomalous. Nearly identical swell indices were obtained from these replicate tests, even for DI water, confirming that the decrease observed in the first cycle in all tests was a real phenomenon 关Fig. 3共a兲兴. However, the mechanism responsible for this decrease in swell index during the first cycle remains unknown. The final swell indices 共i.e., after the last wet-dry cycle兲 of the bentonites that retained swell vary with the ionic strength of the hydrating solutions. The largest final swell index 共40 mL/ 2 g兲 corresponds to hydration with DI water and the lowest to a solution having an ionic strength of 0.025 M 共25 mL/ 2 g兲. The solutions with low and intermediate ionic strength 共0.005 and 0.011 M兲 yielded an intermediate 共and the same兲 swell index 共35 mL/ 2 g兲. All of these solutions also have RMD of at least 0.7 M0.5. This variation in swell index with ionic strength reflects the well-known sensitivity of osmotic interlayer swell to concentration in bentonites where the exchange complex is composed predominantly of monovalent cations 关Fig. 3共b兲兴. In contrast, the final swell indices of the bentonites that lost swell are essentially independent of ionic strength, which reflects the insensitivity of crystalline interlayer swelling to concentration in bentonites

where the exchange complex consists primarily of divalent cations 关Fig. 3共b兲兴共Norrish and Quirk 1954; McBride 1994; Kolstad et al. 2004兲. The sensitivity of the final swell index to ionic strength and RMD of the test solution is shown in Fig. 6. RMD has a greater effect on the swell index of bentonite than ionic strength after repeated cycles of wetting and drying. Swell index is modestly affected by ionic strength, varying by at most 10 mL/ 2 g over the range of ionic strengths that were used, and is more strongly affected by RMD, decreasing by 19– 25 mL/ 2 g as the RMD decreases. Hydraulic Conductivity Hydraulic conductivities of GCL specimens subjected to repeated wet-dry cycling via permeation followed by air drying are shown in Fig. 7. Increases in hydraulic conductivity of 4–5 orders of magnitude occurred in the GCL specimens permeated with test solutions having RMD= 0.007 M0.5, whereas no discernable change in hydraulic conductivity occurred when the GCLs were permeated with solutions having RMD= 0.7 M0.5, regardless of whether the ionic strength was high or low. This distinct difference in behavior is consistent with the changes in swell index shown in Fig. 3, and can be attributed to the amount of Ca-for-Na exchange that occurred during wet-dry cycling. Final mole fractions of Na and Ca on the exchange complex of bentonite from the GCLs permeated with solutions having RMD= 0.7 M0.5, 共XNa = 0.65 and 0.66, XCa = 0.28 and 0.26兲 were found to be similar to those for bentonite from a new GCL 共XNa = 0.73, XCa = 0.22兲共Table 3兲. In contrast, the mole fractions of Na and Ca for GCLs permeated with test solutions having RMD= 0.007 M0.5 共XNa = 0.03 and 0.02, XCa = 0.84 and 0.94兲 show that nearly complete replacement of Na by Ca occurred during wet-dry cycling. Behavior between these two extremes was observed for the GCLs permeated with solutions having ionic strength= 0.025 M and intermediate RMD 共0.07– 0.14 M0.5兲. In particular, the final hydraulic conductivity decreased and mole fraction of Na in the exchange complex increased as RMD increased 共Table 3兲. The large increases in hydraulic conductivity evident in Fig. 7



Table 3. Hydraulic Conductivity of GCL and Mole Fraction of Na, Ca, Mg, and K in Exchange Complex of Bentonite after Final Cycle of Wetting and Drying Ionic strength 共M兲 — 0.005 0.005 0.025 0.025 0.025 0.025 0.025

RMD 共M0.5兲

Total number of cycles

Hydraulic conductivity 共cm/s兲

—

0

0.007 0.7 0.007 0.07 0.10 0.14 0.7

9 9 5 5 5 5 9

1.0⫻ 10−9 2.1⫻ 10−9 2.5⫻ 10−4 1.6⫻ 10−9 2.4⫻ 10−5 7.1⫻ 10−6 4.9⫻ 10−7 1.3⫻ 10−8 2.9⫻ 10−9

for lower RMD are caused by desiccation cracks in the bentonite that do not swell shut during rehydration. Cracks formed in the bentonite of all GCLs during drying, regardless of the test solution that was used. Examples of cracks formed during desiccation are shown in Fig. 8 for the tests conducted with I = 0.005 M and RMD= 0.007 M0.5 关Fig. 8共a兲兴 or 0.7 M0.5 关Fig. 8共c兲兴共white arrows on the photographs in Fig. 8 indicate locations of cracks兲. These

Mole fraction in exchange complex Na

Ca

Mg

K

0.73

0.22

0.03

0.02

0.03 0.66 0.02 0.14 0.24 0.25 0.65

0.84 0.26 0.94 0.81 0.70 0.68 0.28

0.08 0.05 0.01 0.03 0.04 0.05 0.05

0.05 0.02 0.03 0.02 0.02 0.02 0.02

cracks swelled shut during rehydration with solutions having RMD= 0.7 M0.5 关Fig. 8共d兲兴, but did not completely close in GCLs permeated with test solutions having RMD= 0.007 M0.5 关Fig. 8共b兲兴. The effect of Ca-for-Na exchange on swelling of the bentonite is responsible for these differences in self-healing capacity. GCLs exposed to solutions that induce Ca-for-Na exchange 共lower RMD兲 lose their swelling capacity 关Fig. 3共a兲兴 and ability to

Fig. 8. Bentonite in GCL specimen permeated with I = 0.005 M and RMD= 0.007 M0.5 solution after air drying 共a兲; following final wetting cycle 共b兲; exposed bentonite of GCL specimen permeated with I = 0.005 M and RMD= 0.7 M0.5 solution following after air drying 共c兲; and following final wetting cycle 共d兲. Examples of cracks illustrated with white arrows. JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / MARCH 2009 / 355


Fig. 9. Hydraulic conductivity of GCL at end of final wet-dry cycle versus RMD of permeant solution

self-heal, as reported previously by Lin and Benson 共2000兲, Meer and Benson 共2007兲, and Benson et al. 共2007兲. In contrast, GCLs exposed to solutions that do not induce Ca-for-Na exchange 共higher RMD兲 retain their swelling capacity 关Fig. 3共a兲兴 and ability to self-heal during rehydration following desiccation.

Practical Implications The hydraulic conductivities obtained after wet-dry cycling with the solutions having RMD艋 0.07 M0.5 are in the range of 7.1 ⫻ 10−6 – 2.5⫻ 10−4 cm/ s. These hydraulic conductivities are similar to those of the most permeable GCLs exhumed from final covers by Melchior 共2002兲, Benson et al. 共2007兲, and Meer and Benson 共2007兲, which ranged between 4.8⫻ 10−5 and 9.4 ⫻ 10−4 cm/ s. These hydraulic conductivities are also similar to the final hydraulic conductivities reported by Lin and Benson 共2000兲 for GCLs subjected to wet-dry cycling in the laboratory using 0.0125 M Ca solutions. In contrast, the GCLs subjected to wet-dry cycling with solutions having RMD of 0.7 M0.5 have a final hydraulic conductivity less than 2.9⫻ 10−9 cm/ s, which is similar to the average hydraulic conductivity of the exhumed GCLs described in Mansour 共2001兲共1.9⫻ 10−9 cm/ s兲. This correspondence between field and laboratory conditions for GCLs having both high and low hydraulic conductivities suggests that the testing method used in this study, while severe, provides a reasonable estimate of field conditions. Influence of the relative abundance of monovalent and multivalent cations on the hydraulic conductivity after wet-dry cycling is shown in Fig. 9 in terms of the hydraulic conductivity measured at the end of the last cycle versus the RMD of the permeant liquid. The hydraulic conductivity decreases rapidly as the RMD increases from 0.007 to 0.14 M0.5. Maximum hydraulic conductivities of the GCLs exhumed by Meer and Benson 共2007兲 that were not overlain by a geomembrane are also shown in Fig. 9 along with the RMD of the pore water in the cover soil. Good agreement exists between the relationship between hydraulic conductivities and RMD for the specimens tested in this laboratory study and for the GCLs exhumed by Meer and Benson 共2007兲. The exception is the GCL exhumed from Site D by Meer and Benson 共2007兲. This GCL was exhumed from a cover over a small landfill in a national forest in northern Wisconsin where wet conditions are common and tree cover is extensive. Thus, the GCL at Site D may have been exposed to fewer wet-dry cycles

than the other GCLs, and may not have reached its final hydraulic conductivity when sampled. Additionally, the ionic strength of the pore water from the cover soil at Site D was the lowest of those evaluated by Meer and Benson 共2007兲. This factor may also have contributed to a greater number of wet-dry cycles being required before a large increase in hydraulic conductivity occurred 共see subsequent discussion兲. No threshold in RMD is apparent in Fig. 9 above which the hydraulic conductivity of the GCL is unaffected by wet-dry cycling, although a threshold may have been identified if tests had been conducted for RMDs between 0.14 and 0.7 M0.5. However, the trend in Fig. 9 suggests that only modest changes in hydraulic conductivity should be expected when the permeant liquid has RMD⬎ 0.14 M0.5. Findings reported in Meer and Benson 共2007兲 suggest that RMD of pore waters can be estimated reliably using the batch water leach test defined in ASTM D 6141 共ASTM 2004兲. Thus, batch water leach testing may prove to be useful for evaluating the compatibility between cover soils and GCLs. Comparison of the hydraulic conductivities in Fig. 9 corresponding to RMD of 0.007 and 0.7 M0.5 and ionic strengths of 0.005 and 0.025 M suggests that RMD has a greater influence on final hydraulic conductivity than ionic strength. The greater importance of RMD is also evident in Fig. 7. For RMD of 0.7 M0.5, essentially the same hydraulic conductivity was obtained for both ionic strengths for all wet-dry cycles. Similarly, for RMD of 0.007 M0.5, comparable hydraulic conductivities were obtained for ionic strengths of 0.005 and 0.025 M after the hydraulic conductivity increased and leveled off 共although the final hydraulic conductivities are slightly higher for the tests conducted with the 0.005 M solution兲. However, when the ionic strength was higher, the hydraulic conductivity increased after fewer wet-dry cycles. These findings suggest that ionic strength controls the number of cycles required for an increase in hydraulic conductivity, and RMD controls the final hydraulic conductivity. Similar effects were observed for index swell of the bentonite exposed to wet-dry cycling using the batch procedure 共Fig. 3兲, and are apparent in the Na and Ca fractions in the exchange complex 共Table 3兲. As shown in Fig. 10, the mole fraction of Na and Ca at the end of wet-dry cycling is essentially a unique function of the RMD of the permeant solution, and is independent of the ionic strength. Given that GCLs in covers generally are anticipated to have a service life of decades during which they could be exposed to numerous wet-dry cycles, RMD of the pore water of the adjacent cover soil should be a primary factor considered when evaluating the compatibility of GCLs and cover soils. Moreover, the pore water in adjacent cover soils should be evaluated under realistic elution conditions, as the RMD of the pore water will vary as the water content of the cover soil changes during wetting and drying. Changes in water content alter cation concentrations and the ionic strength, and result in a nonlinear variation in RMD even when the population of cations in the pore water remains unchanged 共i.e., due to the square root term in the denominator of RMD兲. The final hydraulic conductivity of the GCLs evaluated in this study is shown in Fig. 11 as a function of the index swell of the bentonite after the final hydraulic conductivity test. The hydraulic conductivity diminishes as the index swell increases, which reflects the effect of cation exchange on the ability of the bentonite to swell during rehydration and close macroscopic features that formed during drying. Relatively low hydraulic conductivities 共艋10−8 cm/ s兲 are achieved when the index swell exceeds 15 mL/ 2 g, which is the same threshold observed for bentonites that lost swell in the batch tests 共Fig. 3兲. The trend between hydraulic conductivity and index swell also has a shape similar to



Fig. 12. Hydraulic conductivity versus time for GCL permeated with 0.0125 M CaCl2 solution for 1,600 days, dried to gravimetric water content of 50%, and permeated again with 0.0125 M CaCl2 solution 共adapted from Benson et al. 2007兲 Fig. 10. Mole fraction of Na or Ca in exchange complex of bentonite from GCL specimens subjected to wet-dry cycling as function of RMD of permeant solution. For specimens permeated with DI water, RMD was set at 1.5 M0.5.

the relationship between hydraulic conductivity and RMD, which suggests that RMD of the permeant liquid is the primary factor affecting the hydraulic conductivity of GCLs exposed to wet-dry cycling. The correspondence between hydraulic conductivity and index swell in Fig 11 suggests that index swell testing may prove to be a convenient means to assess the compatibility of GCLs with cover soils. However, the index swell data in Fig. 11 are from bentonites that have undergone repeated wet-dry cycling. A swell test conducted with pore water and fresh bentonite from a GCL would not necessarily show a similar diminished swell index, as reported by Meer and Benson 共2007兲. Rather, an index swell procedure with pore water and wet-dry cycling would need to be developed. An additional important consideration is that cation exchange alone will not result in large increases in hydraulic conductivity. Most pore waters tend to be dilute 共e.g., as shown in Fig. 1兲, and cation exchange with dilute solutions that contain predominantly

Fig. 11. Hydraulic conductivity of GCL at end of final wet-dry cycle versus swell index for final wet-dry cycle

divalent or multivalent cations is known to result in GCLs with hydraulic conductivities on the order of 10−8 cm/ s 共Egloffstein 2001; Jo et al. 2004, 2005; Benson et al. 2007兲. This effect is illustrated in Fig. 12, which shows hydraulic conductivity versus time for a GCL permeated with a 0.0125 M CaCl2 solution by Benson et al. 共2007兲. The GCL was permeated for 1,599 days with the CaCl2 solution, which resulted in a gradual increase in hydraulic conductivity to 2.3⫻ 10−8 cm/ s that was maintained for the duration of the test period. After 1,599 days, the specimen was removed and dried to a water content of 50%, and then permeated again with the CaCl2 solution for 30 days. A single cycle of desiccation caused the hydraulic conductivity of the GCL to increase to 4.7⫻ 10−6 cm/ s. Thus, large increases in hydraulic conductivity of GCLs that are initially hydrated require both cation exchange and desiccation. Protective methods that ensure initial hydration of the bentonite and prevent wet-dry cycling 共e.g., a GCL overlain by a geomembrane that is promptly covered with soil兲 are likely to preclude large increases in hydraulic conductivity even if cation exchange occurs.

Summary and Conclusions A series of batch tests and hydraulic conductivity tests was conducted on a GCL containing Na–bentonite to evaluate how swell index and hydraulic conductivity are affected by wet-dry cycling in solutions having different relative abundance of monovalent and multivalent cations. Relative abundance of monovalent and multivalent cations was characterized by the RMD of the test solution, which is defined as the ratio of the total molarity of monovalent cations to the square root of the total molarity of multivalent cations. Test solutions having a range of RMD and two ionic strengths were prepared using NaCl and CaCl2 dissolved in DI water. Results of the batch and hydraulic conductivity tests showed that RMD controls the final swell index, the relative abundance of monovalent and divalent cations in the final cation exchange complex, and the final hydraulic conductivity of bentonite exposed to wet-dry cycling. Ionic strength affects the number of wet-dry cycles required for a change in hydraulic conductivity to occur and the rate at which the cation exchange complex and swell



index change with wet-dry cycling. Lower swell index and higher hydraulic conductivity were obtained as the RMD decreased, which reflected greater replacement of the native Na in the bentonite exchange complex with Ca. Large increases in hydraulic conductivity and loss of swelling capacity occurred when the solution had an RMD艋 0.07 M0.5. Modest or small changes in hydraulic conductivity and swell index were obtained when the RMD was 艌0.14 M0.5. These findings suggest that chemical analysis of the pore water in cover soils may prove useful in evaluating the compatibility of GCLs and covers soils used in applications where wet-dry cycling may occur. The findings also illustrate that large increases in hydraulic conductivity of GCLs used in cover applications where the bentonite is initially hydrated occur when the native Na is replaced by multivalent cations and the bentonite is exposed to wet-dry cycling. Protective measures that ensure initial hydration and prevent wet-dry cycling or cation exchange will likely prevent large increases in hydraulic conductivity of GCLs used in cover applications.

Acknowledgments Financial support for this study was provided by the United States Environmental Protection Agency 共USEPA兲共Contract No. 2CR361-NAEX兲 and the United States National Science Foundation 共NSF兲 under Grant Nos. CMS-9900336 and CMMI-0625850. David Carson and Thabet Tolaymat were the project managers for the portion funded by USEPA. The findings in this paper are solely those of the writers. This paper has not been reviewed by USEPA or NSF. Endorsement by USEPA or NSF is not implied and should not be assumed. Tammy Rauen, Sabrina Bradshaw, and Joseph Scalia of the University of Wisconsin-Madison assisted with the testing program. James Olsta of CETCO provided valuable comments during preparation of the manuscript.

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