Hydraulic Conductivity Reductions Resulting from Clay Dispersion within Alluvial Sediments Impacted by Sodium-rich Water

By Lance J. King and Harold W. Olsen, Colorado School of Mines, Golden, Colorado, and George N. Breit, U.S. Geological Survey, Denver, Colorado.

ABSTRACT

Substantial reductions in hydraulic conductivity (HC) of soils and sediment can result from dispersion of clay into flowing ground water. Smectitic clays with high amounts of exchangeable sodium are particularly likely to disperse into ground water if the ionic strength of the pore water substantially decreases. Our experimental study evaluated the HC decrease possible due to clay dispersion resulting from a sodium-rich leachate plume moving through smectite-containing alluvium near the Norman Landfill, Oklahoma. Core samples of alluvium were placed in a triaxial cell and flushed with background water, NaCl solutions and distilled water. These tests detected substantial dispersion of clay that was associated with decreases in HC of more than two orders of magnitude from 4.38 x 10^-3 cm/sec to 2.05 x 10^-5 cm/sec (centimeters per second) in sediments containing less than 5 percent clay-sized particles. The change in HC attributable to clay dispersion can change the distribution of hydraulic conductivity in an aquifer. Such changes need to be considered in forecasting plume migration and developing remediation strategies.