Selenium contamination in the Colorado River Basin, western USA： Remediation by leaching and nanof＇dtration membranes

Selenium （Se） is an essential micronutrient to biota, but can become a potent toxicant at elevated concentrations. The natural sources and chemical properties of Se species make the boundary between deficiency and toxicity narrow for some biota, with both phenomena common around the globe. Large areas of farmland in the Colorado River Basin （CRB） generate salinized drainage water with Se concentrations much higher than 5 μg/L, the U.S. Environmental Protection Agency chronic water-quality criterion for the protection of aquatic life. We have carried out detailed field and laboratory studies to investigate Se geochemistry and remediation in two of these areas： the Middle Green River Basin, Utah and the Salton Sea Basin, California, located respectively in the Upper and Lower CRB. Results from these and other studies show that approximately 90% of the dissolved Se in the Colorado River and its tributaries originally is derived from the Upper Cretaceous Mancos Shale and equivalent pyritic marine units that outcrop in the Upper CRB. Selenium is mobilized commonly by biogeochemical oxidation of this pyritic shale and is concentrated mainly as selenate （SeO4^2-） in soils and agricultural drainage water of dry climates by evaporation. Minor （0%-5%） amounts of Se are present as the selenite species （HSeO3^-） and （SEO3^2-）, but these species and the more reduced species, elemental Se （SeO） and selenide （Se^2-）, have much lower solubility and/or have high sorptive affinity towards organic matter, clay minerals and iron oxyhydroxides. The concentration of dissolved Se （-2.5 μg/L） and salinity in the Lower Colorado River water are among the highest of the world major rivers. Because of low precipitation （7 cm/a） and extreme evapotranspiration （-1.8 m/a） rates in the Salton Sea Basin, California, Se values in irrigation water imported from the Colorado River increase to 〉300 μg/L in drainage wastewater. Removal of Se from contaminated wastewater by nanofiltration membranes was demonstrated in laboratory and pilot-scale field experiments.