Compatibility of geosynthetic clay liners with leachate from CCP management facilities

Experiments were conducted to evaluate whether coal combustion product (CCP) leachates adversely affect the hydraulic conductivity (K) of geosynthetic clay liners (GCLs) as an alternative material in lieu of a compacted clay liner (CCL). Chemical properties of CCP leachates were compiled based on a nationwide survey of CCP disposal facilities. Five synthetic leachates were developed to represent a range of conditions encountered in CCP disposal facilities: typical CCP leachate, strongly divalent cation fly ash leachate, flue gas desulfurization (FGD) residual leachate, high ionic strength ash leachate, and trona ash leachate. Nine GCLs were tested: two Na-bentonite (NaB) GCLs, six bentonite-polymer (P-B) GCLs, and one bentonite polymer composite (BPC). Hydraulic conductivity tests on GCLs were conducted on prehydrated (60-d on subgrade soil or DI water) and non-prehydrated GCL specimens. Tests were run to hydraulic and chemical equilibrium at effective stresses of 20 kPa, and then higher effective stresses were applied up to 450 kPa. At 20 kPa, the GCLs with NaB had high hydraulic conductivity to CCP leachates (10-10 to 10-6 m/s). Hydraulic conductivity was directly proportional to the ionic strength of the leachate and inversely proportional to the swell index of the bentonite when hydrated in leachates. The hydraulic conductivity of B-P GCLs with low polymer loading (? 1.2%) behaved similarly to NaB GCLs. However, the hydraulic conductivity of GCLs with polymer loading ? 1.9 % is controlled by the clogging behavior of polymer hydrogel. Polymer elution was observed during the hydraulic conductivity tests, and increasing hydraulic conductivity was associated with cumulative polymer elution. Increasing the effective stress from 20 to 450 kPa caused the hydraulic conductivity to decrease by up to three orders of magnitude. At 450 kPa effective stress, the hydraulic conductivities of all tests were lower than 10-9 m/s except for one test with Trona leachate. This observation indicates that high effective stress can minimize the effect of CCP chemistry on the hydraulic behavior of GCLs. Subgrade hydration had no beneficial effect on hydraulic conductivity; however, prehydration with DI water can effectively protect the GCLs due to a large amount of immobile water retained in the bentonite.