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

Abstract

Experiments were conducted to evaluate the hydraulic conductivity of geosynthetic clay liners (GCLs) containing granular sodium bentonite that were permeated with coal combustion product (CCP) leachates. Chemical properties of the CCP leachates were selected from a nationwide survey of CCP disposal facilities. Five synthetic leachates were selected from this database to represent a range of CCP disposal facilities: typical CCP leachate (geometric mean of CCP chemistry), strongly divalent cation leachate (aka low-RMD leachate), flue gas desulfurization (FGD) residual leachate, high ionic strength ash leachate, and trona ash leachate. Typical GCLs from two U.S. manufacturers were used. Hydraulic conductivity tests were conducted on non-prehydrated and subgrade hydrated (by compacted soil for 6 days) GCL specimens at effective stresses ranging from 2 to 45 kPa. At 2 kPa, GCLs permeated directly had high hydraulic conductivity (>1−6 m/s) to trona leachate and moderate to high hydraulic conductivity (1−1 to 1−7 m/s) to the other CCP leachates. Hydraulic conductivity was strongly related to the ionic strength of the leachate and inversely related to the swell index of the bentonite when hydrated in leachate, as demonstrated in past studies on leachates from other waste streams. Increasing the effective stress from 2 to 45 kPa caused the hydraulic conductivity to decrease up to three orders of magnitude. Hydration on a subgrade prior to permeation has only modest impact on the hydraulic conductivity to CCP leachate. Hydration by permeation with deionized (DI) water prior to permeation with trona leachate resulted in hydraulic conductivity up to three orders of magnitude lower than obtained by direct permeation, suggesting that deliberate prehydration strategies may provide chemical resistance to CCP leachates.