Monday, November 13, 2006

EC and SAR Effects on the Hydraulic Conductivity of Santa Clara County Soils.

Jocelyn Evans, UC Davis, One Shields Ave, Davis, CA 95616

Several sites in Santa Clara County, Ca, are, or will be irrigated with tertiary treated wastewater with a sodium adsorption ratio (SAR) of 4.  Treated wastewater is known to cause soil swelling and dispersion due to high sodium contents and low electrical conductivities. Research has shown that decreases in hydraulic conductivity (HC) occur as solution electrical conductivity (EC) decreases for a given SAR, and that a soil's saturated HC is most affected by solutions of a higher SAR.  Water quality thresholds and soil hydraulic conductivities were determined for 30 soils from Santa Clara County, Ca.  Solutions with an SAR of 3, 6 or 12, and with total salt concentrations of 500, 100, 50, 10, 5, 1, and 0 mmolc/L were passed through soil columns using a constant head. Flux rates were measured to determine saturated hydraulic conductivity. Soil properties including texture by pipette, COLE, soil cation exchange capacity (CEC), mineralogy and organic and inorganic carbon content were also determined to understand the hydrologic behavior of these soils. The greatest decline in hydraulic conductivity occurred with SAR 12 solutions that were at or below a total salt concentration of 5 mmolc/L. Saturated hydraulic conductivities were most affected at clay contents of 40 percent, by the presence of high activity clays, and larger exchange capacities. In contrast, only small decreases in saturated HC were observed for samples that were largely composed of coarser fractions.