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Stewart Lake, located adjacent to the Green River near Jensen, Utah, is administered by the State of Utah as a Waterfowl Management Area. Waterfowl use is heavy during periods of spring and fall migration and during the breeding season. Although it lies beside the Green River, Stewart Lake only receives water from the Green River during flooding periods. Almost all the inflow to Stewart Lake comes from agricultural drains under irrigated fields near Jensen. Several of the drains have selenium concentrations in the range of 50 to 104 parts per billion, which exceed the aquatic standard of 5 parts per billion and the drinking water standard of 10 parts per billion. Water from several seeps is also high in selenium. Irrigation is probably the major cause of the problem. Stewart Lake acts as a sink and accumulates about 75 percent of the inflowing selenium in the sediment and in biological tissue of waterfowl and fish. The soil of these fields is high in selenium content as well as salt, since the soil is derived from the Mancos Shale, which is prevalent in the Jensen area. Drainage water seeping through the soil picks up selenium compounds and carries these to Stewart Lake, where they accumulate in the sediment and in the bodies of fish and birds. Ashley Creek flows from the south slope of the Uintah Mountains. Most of the water in Ashley Creek is diverted in the Steinaker Reservoir, where it is stored for irrigation and municipal uses. Some of the return flow from fields near Vernal is drained and goes into Ashley Creek. This return flow contains a moderate amount of selenium. A major source of selenium seems to be seepage from the Vernal Sewage Lagoons and nearby Winter Storage Pond, which are underlain by fractured shale, thus releasing much water as seepage. The Mancos Shale formation is known be to a significant source of selenium. There is little or no selenium in the water in the lagoons. However, the standing water from these lagoons seeps into the shale, picks up selenium compounds, and carries it down to Ashley Creek and the Winter Storage Pond. Seeps below this area have extremely high selenium levels, some as high as 16,000 parts per billion. The detailed study determined that Ashley Creek downstream from the Sewage Lagoons has the largest concentration of selenium of any body of water in the middle Green River basin. Selenium concentrations in lower Ashley Creek average about 70 to 90 parts per billion. There are also elevated selenium levels, 300 to 400 parts per billion, in the Mantle Gulch area, which drains into lower Ashley Creek. Some of the irrigation drains in the Vernal area contain slightly elevated selenium levels, but these need to be studied further.

Send questions or comments about this report to the author, David L. Naftz (dlnaftz@usgs.gov) 801.908.5000.

For more information about USGS activities in Utah, visit the USGS Utah District home page.