Waste-Soil Interactions that Alter Hydraulic Conductivity

Abstract

Liquid-soil compatibility of soil hydraulic conductivity is reviewed with respect to the materials and methods used in evaluating waste-soil interactions, factors influencing interpretation of test results, and the interactions that are thought to alter significantly the hydraulic conductivity of clay soils. Significant increases in hydraulic conductivity may result from flocculation of clay particles due to interactions with electrolyte solutions, shrinkage of the soil matrix in the presence of concentrated organic solvents, and acid-base dissolution of the soil. Observed effects typically are greater in rigid-wall permeameters than flexible-wall permeameters. Considerable evidence supports the use of the Gouy-Chapman theory for describing the influence of aqueous solutions on the fabric and, therefore, the hydraulic conductivity of clay soils. However, swelling test results suggest that the Gouy-Chapman theory does not account properly for shrinkage effects which have been observed to result in large increases in hydraulic conductivity upon permeation with concentrated organic solvents. Three mechanisms may contribute to an increase in the hydraulic conductivity of clay soils upon permeation with acid permeants: (1) flocculation of the clay, (2) dissolution of the clay minerals (aluminosilicates), and (3) dissolution of other minerals (e.g., CaCO3) in the clay soil. Dissolution and piping of clay minerals leads to increases in hydraulic conductivity. Dissolution of carbonates initially leads to buffering, re-precipitation, pore clogging, and a decrease in hydraulic conductivity. Depletion of the buffering capacity leads to a decrease in pH, dissolution of constituents, and a possible increase in hydraulic conductivity. The measured hydraulic conductivity of a compacted sand-bentonite mixture is shown to be significantly affected by the sequence of permeation to a saturated calcium solution. The effect, termed “first exposure”, has important implications with respect to the application of laboratory test results for evaluation of the suitability of a material for a waste containment barrier.