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Water Quality Update
South Fork of Powder River
The Wyoming Department of Environmental Quality (WDEQ) standard for selenium was exceeded 90% of the time during the monitoring season. No associations between field measurements and selenium were apparent. A comparison to previous monitoring results indicated that selenium concentrations have not changed significantly.
Murphy Creek
The WDEQ standard for selenium was exceeded 30% of the time during the monitoring season. Selenium in Murphy Creek may be influenced by seasonal variability. Elevated concentrations of selenium are prevalent during the spring run-off months.
Willow Creek
The WDEQ standard for selenium was exceeded 27% of the time during the monitoring season. Elevated selenium concentrations corresponded with increased discharge rated and lower stream temperatures. The selenium present during the winter months may indicated a change in the redox state of the water. The PRCD’s water quality consultant WWC Engineering is currently working on a Use Attainability Analysis to petition the WDEQ for a site specific selenium standard on Murphy Creek. The petition demonstrates that selenium within the watershed is naturally occurring at concentrations exceeding the WDEQ standard for aquatic life.
In general, selenium and general water chemistry results were similar to the monitoring completed on the streams between 2008 and 2009 and previous monitoring completed between 2003 and 2006. Elevated selenium concentrations in Willow Creek (sampling site is upstream of irrigation) and Murphy Creek, which are not effected by irrigation, suggest that the selenium is naturally occurring. Additionally, elevated selenium concentrations in the spring and winter months may indicate that the redox state (oxidizing or reducing) of the water affects selenium concentrations. Based on the selenium concentrations observed in Willow Creek and Murphy Creek, the elevated selenium in South Fork of Powder River may be attributed to natural sources although anthropogenic sources may also contribute.
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