Natural and anthropogenic loading of dissolved selenium in Colorado River Basin

摘要

Selenium (Se) is an essential micronutrient, which becomes an environmental toxicant at higher concentrations. Large areas of farmland in Colorado River Basin (CRB) generate salinized drainage with Se concentrations >5 μg/L, the U.S. Environmental Protection Agency chronic water-quality criterion for the protection of aquatic life. Almost all of the dissolved Se in the Colorado River and its tributaries is originally 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 (SeO_4~(-2)) in soils and agricultural drainage water of dry climates by evaporation. Minor (0-5%) amounts of Se are present as the selenite (HSeO_3~- & SeO_3~(-2)), but these species and the more reduced Se have high sorptive affinity towards organic matter and iron oxyhydroxides. The concentration of Se in the Lower CRB water is constant (?2.5 μg/L), but extreme evapotranspiration rates in the Imperial Valley, California, increase Se values in irrigation drainage to up to ～300 μg/L. Anthropogenic activities have greatly increased the mobilization and cycling of Se in this Basin. Presently only ～20% of dissolved Se load and 47% of total salinity are attributable to natural sources. The bulk of Se load (～70%) results from irrigation, where Se concentrations in drainage water are greatly enhanced by high nitrates from fertilizers. The city of Vernal, Utah sewer lagoons and two coal-fired power plants contribute ～10% of total Se.