Oxyanion concentrations in eastern Sierra Nevada rivers .1. Selenium

Selenium, a naturally occurring trace element, is a potential water quality problem in the western U.S.A. due to widespread areas of Se-rich source rock, arid climate, and consequent potential for evapoconcentration. Selenium concentrations in the Truckee, Walker, and Carson River systems which drain the eastern slope of the low Se content Sierra Nevada, California and Nevada, were low, ranging from less than 0.3 nmol/L (nM) to about 16 nM. Selenium was concentrated by evaporation in the flow systems but did not behave conservatively with respect to Cl-. Selenium concentrations were lower in the saline-alkaline terminal lakes of the Truckee and Walker Rivers than in the downstream reaches of the rivers. Potential Se ''sinks'' in the systems include bacterial reduction of Se in anoxic water and sediments and biomethylation of Se and subsequent degassing of the volatile Se species to the atmosphere.