Water Quality and Aquitard Permeability

Flow test on artificially sedimented illite-silt mixtures designed to simulate aquitard soils show that permeability changes accompanying changes in pore water dissolved salt concentration depend on the exchangable sodium percentage and the structural state of the clay. The dissolved salt concentration at which clay swelling causes permeability reductions and the concentration that causes clay dispersion both vary directly with exchangeable sodium percentage. Comparisons suggest that at equal exchangeable sodium percentages, flocculated-disaggregated illite particles swell more and disperse at slightly higher dissolved salt concentrations than when the particles have been previously aggregated into domains by drying. Permeability increases accompanying increases in dissolved salt concentrations are small. When the clay is not dispersed, permeability increases are proportional to the degree of clay swelling existing prior to the increase. Permeability increases in completely dispersed clays are inversely related to exchangeable sodium percentage. Subsequent replacement of seawater by freshwater recharge could result in more than an order of magnitude decrease in the permeability of aquitards.