POTENTIAL INFLUENCE OF CLAY SLICKENSIDES ON SUBSURFACE CONTAMINANT TRANSPORT

Slickensided features within subsurface clays may function as preferential pathways for contaminants, even though such clays appear impermeable. Geotechnical investigations have historically focused on the suitability of in situ clays as liners for waste disposal and storage sites. However, the investigations did not quantify the role slickensides might have in chemical migration. In this study, twenty “undisturbed” cores containing slickensides were collected from a subsurface (6.0 m depth) clay stratum at a once-proposed municipal landfill. The cores were permeated with water and three selected organic liquids in fixed-wall constant head permeameters. Water (baseline) hydraulic conductivities were determined for all cores, followed by a randomly assigned water or organic liquid treatment, with five cores in each treatment. The last pore volume through each core included a dye (i.e., Brilliant Blue FCF) designed to visually detect macropore flow. The organic liquids included xylene, methanol and acetone at differing concentrations. Although the clay stratum previously was considered an adequate confining layer (< 1.0 x 10^-7 cm s^-1), slickensided clays had baseline conductivities to water that ranged from 4.4 x 10^-8 to 8.8 x 10^-4 cm s^-1. Slickensided clays treated with organic solvents showed increased conductivities up to 1.4 orders of magnitude larger than their respective baselines. Cores with initially low (< 1.0 x 10^-7 cm s^-1) baseline conductivities had the largest increases in conductivity, while those with higher baseline conductivities had modest conductivity increases, which were more reflective of the density to viscosity ratio of the permeants. Clays treated with pure xylene had the largest increases in conductivity, up to 1.4 orders of magnitude. Clays treated with increasing concentrations of methanol and acetone did not show significant conductivity increases until a 90% solution was permeated. Visual observations of dyed cores revealed distinct vertical and linear preferential flow along slickenside planes. These results have significant implications regarding the disposal and storage of surface waste and suggest that undisturbed (in situ) slickensided subsurface clays are more conductive of fluids than previously assessed.