CASE STUDY OF SELENIUM MOBILIZATION FROM MANCOS SHALE: RELATIONSHIP TO NITRATE CONTAMINATION

Investigations in Western Colorado have established relationships for groundwater and surface water between elevated selenium and dissolved nitrogen concentrations. It has been hypothesized that selenium is oxidized and thus mobilized by nitrate in irrigated shale terrains in Western Colorado. This evaluation provides a case study for a site in the Gunnison River Basin that supports and extends this relationship. Investigation of hydrogeochemical conditions in the Devil's Thumb area of Delta County found nitrate concentrations in excess of 1,000 mg/L with corresponding elevated selenium concentrations of up to 26,000 mg/L. Elevated nitrate concentrations and changes in hydrological conditions are attributed to the construction and attendant irrigation of a new golf course, as based on a comprehensive hydrogeologic and geochemical study that included groundwater and surface water chemistry and isotopic analyses. Preliminary analysis shows that trends in selenium concentrations match those of nitrate. This study revisits the effects of nitrate on selenium mobilization as previously observed by other researchers in the context of a well characterized case study. These results have implications for management of nitrogen fertilizers in settings susceptible to selenium mobilization.