Field Cell Verification of the Effects of Concentrated Organic Solvents on the Conductivity of Compacted Soils

Abstract

Controversy has developed over the use of elevated hydraulic gradients to test the laboratory conductivity of compacted soils to liquid organic chemicals. Questions also remain as to how representative the laboratory measurements are of field conductivities. This study was undertaken to clarify these questions. Fixed wall laboratory permeameters and concrete field test cells were used to compare the conductivity of compacted soils of three different mineralogies to acetone and xylene. Laboratory testing was done at hydraulic gradients of 31, 91, and 181 at two initial moisture contents. The major clay mineral in each of the soils was kaolinite, mica, and montmorillonite. Since the montmorillonite was derived from bentonitic rock, it will hereafter be referred to as bentonite. The hydraulic gradients tested showed no consistently significant effect on the conductivity of the soils to water (0.01N CaSO4), acetone or xylene. Laboratory conductivities to water ranged from 3.94 x 10-9 to 1.96 x 10-8 cm s-1 for all soils. Conductivities to concentrated acetone and xylene were typically 1 to 3 orders of magnitude greater than when water was the permeant. In the field cells, waste xylene penetrated the 15 cm thick compacted soil liners in 2 to 70 days. Acetone required 19 to 704 days to produce leachate. Field conductivities to acetone and xylene increased to 1 and 2 orders of magnitude greater, respectively, than the laboratory values with water. Chemical concentrations in soil samples and visual observations of dye patterns in the soil indicated that the chemicals moved in cracks and large pores between soil structural units. Conductivity changes measured in the field test cells showed similar trends and compared favorably with the laboratory tests using fixed wall permeameters. Conductivity increases in the field averaged 1 order of magnitude less than in the laboratory, presumably due to differences in the purity and water content of the waste under field conditions.