The Effectiveness of Woodchip Bioreactors to Remove Selenium from Tile Drain Water in Eastern South Dakota.

Many South Dakota soils are composed of sedimentary marine shales containing high levels of selenium.  Selenium may occur in the form of selenite (SeO₃) and selenate (SeO₄), which are water soluble and can be leached from the root zone via deep percolation or exported through agricultural subsurface drains.  The delivery of selenium to natural aquatic ecosystems through the tile drainage system may cause adverse impacts on fish and wildlife, as well as livestock and humans. Because selenium is a micronutrient, very small amounts of selenium is beneficial, but too high can be toxic (the Safe Drinking Water Act threshold level is 0.05 ppm). Denitrifying woodchip bioreactors is a practice that typically is installed to remove nitrate from the subsurface drainage water. In this study we explore if the reduction and immobilization of selenium as a result of the biological process within a bioreactor would be the secondary benefit of the bioreactors. Selenium ions can be reduced to insoluble elemental selenium and deposited within the reactor under anaerobic condition, carried out microbes after the reduction of nitrate-nitrogen is complete. We are utilizing three bioreactors field-scale bioreactors installed near Baltic, Montrose and Arlington in eastern South Dakota. We have collected water samples upstream and downstream of these bioreactors and analyzed the water samples for selenium. Preliminary results showed selenium concentrations up to 170 ug/l in the drainage water and that bioreactors may immobilize up to 98% of the selenium present in the tile drain water. The removal rates may change as the bioreactors mature or flow rates through the reactors changes.