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(P332) Response of selenium-contaminated sediments to passive remediation efforts at Stewart Lake, Utah.
Yahnke, James*,1, Smith, Delbert1, 1 U.S. Bureau of Reclamation, Technical Service Center, PO Box 25007, Denver, Colorado
ABSTRACT- Since 1995, we have monitored total selenium in sediment at 20 sites in Stewart Lake, Utah. In 1997, natural flooding by the Green River, enhanced by a newly excavated inlet channel and supplemented by flows from Ashley Creek, reduced total selenium concentrations in sediment throughout Stewart Lake dramatically from a geometric mean of 15 ppm to 10 ppm. Since then, remediation efforts have focused on enhancing the effects of natural flooding from the Green River and the ability to better manage draining the lake. Inlet and outlet water control structures and drainage channels, have been constructed to facilitate draining of the lake to enhance the oxidation of selenium in sediments. Greater oxidation should result in a higher degree of dissolution of selenium during flooding. Agricultural drains, which were the primary source of selenium to the lake, were diverted to the Green River in the fall of 1997 and spring of 1998. Between the 1997 flood and the September 1998 sediment sampling, total selenium increased to a geometric mean of 16 ppm. Most of the increase occurred in the northern part of the lake nearest the drains. Seeps along the north end of the lake remain a source of selenium in an area isolated by the drainage channels. Total selenium concentrations in sediment have remained essentially unchanged since 1998. Selenium speciation samples indicate that oxidation has occurred when the lake has been drained. An attempt at enhancing oxidation in 2000 by tilling the sediments in several test plots had no evident effect. An attempt at enhancing oxidation in 2000 by tilling the sediments in several test plots had no measurable effect. An attempt to desorb selenite by liming several test plots in 2001 was only slightly successful; however, liming also caused significant chemical reduction in the already soluble selenate in the surface sediments. Overall the chemically oxidized environment created during the period that the lake is drained is replaced by reducing conditions during flooding. Attempts at flushing selenium by flooding have not been successful.
Key words: selenium, remediation, bottom sediment, wetland
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