Sorption experiments and a numerical model were used to assess the effects of irrigation and drainage on the depth distribution of selenite and selenate in soils from the western San Joaquin Valley, California. The sorption studies show that selenate (15 to 12,400 micrograms per liter of selenium) is not adsorbed to soil, whereas selenite (10 to 5,000 micrograms per liter of selenium) is rapidly adsorbed. The time lag between adsorption and desorption is considerable, indicating nonreversible sorption, and an equation describing the desorption reaction is determined by the concentration of adsorbed selenite prior to desorption. A numerical model was used to qualitatively evaluate the nonreversible sorption reaction and its affect on selenite mobility during leaching by irrigation water. Model results show that selenite is resistant to leaching and therefore can represent a potential long-term source of selenium to ground water. In contrast, selenate behaves as a conservative constituent under alkaline and oxidized conditions and is easily leached from soil.
Chemical analyses of soils from irrigated and non-irrigated sites confirm the experimental and modeling results. Increasing salinity and soluble concentrations of selenate at a site irrigated for more than 40 years indicated leaching and downward displacement by irrigation water. Decreasing salinity and soluble concentrations of selenate at a nonirrigated site probably resulted from evaporation from the shallow water table. Adsorbed selenium in the irrigated and nonirrigated soils is mostly selenite and has been minimally affected by irrigation and drainage.
