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.
Both GCLs have very low hydraulic conductivity to DI water (
) and higher hydraulic conductivity to each of the CCP leachates (
). 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 (
to
). The effect of RMD of the leachate is evident at lower ionic strengths (
) (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
to
, whereas the hydraulic conductivity to low-RMD leachate was approximately one order of magnitude higher (
to
). Although some of this difference in hydraulic conductivity is attributable to ionic strength (
based on the trend in Fig.
3), the majority is likely caused by suppression of osmotic swell by
and
.
At lower ionic strengths (
; FGD leachate, typical CCP leachate, and DI water), the CS and GS GCLs had essentially the same hydraulic conductivity [Fig.
3, within
, 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
), and lower SI in DI water after permeation with low-RMD leachate (
for GS versus
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
(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.
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 (
) 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.
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
to approximately
, and the hydraulic conductivity increases from approximately
to
as the ionic strength of the permeant liquid increases (DI water to trona ash leachate) (Table
3).
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
where
= hydraulic conductivity to CCP leachate; and
= hydraulic conductivity to DI water. Eq. (
3) was obtained by nonlinear least-squares regression (
) 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
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 (
) 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:
Eq. (
4) (
) 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
,
,
, and
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.
Permeation by the four Ca-rich leachates (typical CCP, low-RMD, and FGD leachates with
) replaced most of the
present in the original exchange complex with
from the leachate (Fig.
7). The greatest replacement by
occured with the low-RMD and typical CCP leachates, which had the greatest preponderance of divalent cations (lowest RMD of leachates used).
was also sorbed preferentially to
, 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
concentration of all leachates (trona leachate slightly higher).
For both bentonites, the SI in DI water after permeation with these leachates is
(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
during permeation, primarly because of exchange of
for
(Fig.
7 and Table
4). The mole fraction of
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 (
) because the high ionic strength of the leachate suppressed osmotic swell. However, SI of the bentonite in DI water was
after permeation with trona leachate, which is indicative of a
-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
for the CS GCL and
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
lower at 100–250 kPa and
lower at 450 kPa.
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 (
and
for
and
) 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
compared with
for the original GCL, and the exchange complex was enriched in both
and
(Tables
3 and
4). Similarly, for the GS GCL, the SI to DI water after subgrade hydration was
, relative to
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
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
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– for CS and 32.0– 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
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
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.
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
, on average, as the effective stress increased from 20 to 100 kPa and
, on average, as the effective stress increased from 20 to 250 kPa. Hydraulic conductivity of the more permeable GCLs decreased approximately
, on average, as the effective stress increased from 20 to 100 kPa, and
, 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
at 450 kPa, and some had hydraulic conductivity exceeding
at 450 kPa, which is too permeable to be equivalent to a 0.6-m-thick compacted soil liner having hydraulic
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
, 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.
Hydraulic conductivities 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 , 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 () are more than an order of magnitude higher than normally anticipated for a GCL (). 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.