The North Platte River Watershed near Casper, Wyoming, has several streams and waterbodies listed as impaired for selenium on the state of Wyoming's 303(d) list, including the North Platte River. The source of selenium in the North Platte River Watershed near Casper is the local geology that consists of cretaceous tertiary shale formations with high levels of selenium. Past assessments of selenium in the watershed have focused on the influence irrigation practices on these cretaceous shales have on selenium concentrations in the river system. The Casper Canal delivers irrigation water from Alcova Reservoir, located approximately 25 miles southwest of Casper, to approximately 24,000 acres of irrigated lands in the CasperAlcova Irrigation District. This project, initiated in 2009 by the Wyoming Department of Environmental Quality (WDEQ), was a Total Maximum Daily Load (TMDL) project for selenium in the North Platte River and four of its major tributaries.To explain variability in the selenium data, a spatial regression approach was used to correlate watershed characteristics to selenium concentrations at several locations in the North Platte River Watershed near Casper. Possible sources were identified as the watershed characteristics with the highest correlation to selenium concentrations. This analysis expanded on an earlier U.S. Geological Survey study that applied a similar methodology of regressing watershed characteristics to predict in-stream selenium concentration. This TMDL project incorporated the regression analysis to aid in allocating the loading to various sources within the watershed and to help guide implementation in the watershed for future projects.It was found that most of the variability in selenium can be explained by the percent of irrigated land, the length of irrigation canals, and the percent of cretaceous shale in each subwatershed. Background levels of selenium from sources other than irrigation practices were also evaluated to determine the ability of the tributary streams to meet the current water-quality criteria. 445

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