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
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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.
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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-
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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.
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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
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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.
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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
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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
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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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