Dispersion and re-deposition of fine particles and their effects on saturated hydraulic conductivity

Abstract

Measurements of saturated hydraulic conductivity (K) have been used to study the rates of structural breakdown and pore clogging which occurred during leaching of 2 contrasting soils, Balkuling agricultural soil and a mining residue. Soil columns were leached with solutions of different electrolyte concentrations under saturated conditions using a Marriotte constant-head device. Measurements were made with either abrupt change of concentration from 500 to 1 mmol/L or gradual decreasing concentration from 500 through 100, 50, 10, to 1 mmol/L of NaCl followed by deionised water. The relative hydraulic conductivity (K/Ko) decreased substantially with time and with decrease in electrolyte concentrations for both soils. The decreases were attributed to various extents of internal swelling, dispersion, and decreases of pore radii as a result of detachment followed by re-deposition of the clay fraction during leaching. There was little difference in ultimate reductions in permeability between the abrupt and gradual changes of concentration from 500 to 1 mmol/L for both soils. The mining residue was clearly more prone to structural deterioration than the agricultural soil. Whereas the effluent particles from the Balkuling soil included domains or aggregates of clay crystals, those from the mining residue columns appeared to consist essentially of primary clay crystals indicating the less cohesive nature of this disturbed material. We also estimated the extent of permeability reductions using theoretical investigations based on particle size distribution. Predicted simulations agreed reasonably well with experimental data for both soils following column leaching with 1 mmol/L, with the best agreement observed for the mining residue.