SELENIUM LOSS IN THE ARKANSAS RIVER ACROSS THE HIGH PLAINS, USA

Concentrations of most dissolved constituents are high in the Arkansas River in southeast Colorado and southwest Kansas. Dissolved stream loads calculated from U.S. Geological Survey chemical and flow data and Colorado and Kansas agency chemical data for 1979-2008 for the river and major tributaries indicate a gain in sulfate (SO₄) and loss of selenium (Se) during this period in the river from Pueblo, Colorado, to the Colorado-Kansas line. The expected major sources of SO₄ and Se are weathering of marine Cretaceous shales. Although the sources are primarily natural, the concentrations of most constituents dissolved in the Arkansas River increase substantially across Colorado primarily due to evapotranspiration associated with extensive irrigated agriculture and shallow reservoirs. Regional calculations for SO₄ indicate the dissolved load in the river at the Colorado-Kansas line is about three times that of the Arkansas River at Pueblo below Fountain Creek. In contrast, the dissolved Se load in the river at the Colorado-Kansas line is approximately one-half that in the river at Pueblo below Fountain Creek. Some Se is released from weathering of rocks within the watershed between Pueblo and the state line and added to the river water as indicated by Se concentrations in tributaries. Explanations for the net loss of dissolved Se include removal by sedimentation in surface water, chemical reduction in soils and sediments, plant uptake, and volatilization. Removal might be important in soils and sediments underlying irrigated fields where dissolved selenite and selenate could be reduced to elemental Se. Volatilization of Se can arise from biomethylation in surface water, soils, and plants. Diaz, Johnson, and Naftz (2009) report that volatilization appears to be the dominant mechanism of Se removal from the Great Salt Lake. The Se concentration in the Arkansas River in the High Plains is typically an order of magnitude or more than in the Great Salt Lake. Volatilization may be an important mechanism to consider in the mass balance of Se in the Arkansas River watershed as well as other river systems with high Se concentration.