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Reconnaissance Investigation of Water Quality, Bottom Sediment, and Biota Associated with Irrigation Drainage in the Dolores Project Area, Southwestern Colorado and Southeastern Utah, 1990-91 ByDavid L. Butler, Richard P. Krueger. Barbara Campbell Osmundson, and Errol G. Jensen Abstract The Department of the Interior started a pro~ gram in October 1985 to identify the nature and extent of irrigation-induced water-quality prob- lems that might exist in the Western United States. Water. bottom-sediment, and biota samples were collected and analyzed for a reconnaissance inves- tigation during 1990-91 to identify potential water-quality problems associated with irrigation drainage in the Dolores Project area in southwest- ern Colorado and southeastern Utah. Concentrations of dissolved solids and sul- fate exceeded secondary maximum contaminant levels for drinking water in many water samples from irrigated and nonirrigated areas. Mc Elmo Creek and the Mancos River contribute substantial dissolved-solids loads to the San Juan River. Cadmium was detected in 19 water samples from 16 sites. Criterion to protect aquatic life from chronic exposure to cadmium was exceeded in two samples, however, these samples were collected from Summit Reservoir and Puett Reservoir. which are located outside the irrigated area served by the Dolores Project. Mercury was detected in II water samples at concentrations ranging from 0.1 to 1.2 micrograms per liter, and 6 of those sam- ples were collected at sites outside of the irrigated area served by the Dolores Project. Selenium concentrations exceeded the chronic aquatic-life criterion for selenium of 5 micrograms per liter in most water samples from Mc Elmo Creek, Navajo Wash, from newly irri- gated areas, and from the Mancos River. Irrigation drainage may be the primary source of selenium to Mc Elmo Creek. The maximum selenium concen- tration in water was 88 micrograms per liter in Navajo Wash, which drains irrigated land on Mancos Shale in the southern end of the Monte- zuma Valley. Only 1 of 15 water samples collected from streams that drain the Montezuma Valley north of Mc Elmo Creek had a selenium concen- tration greater than 1 microgram per liter. Samples of irrigation drainwater from newly (since 1987) irrigated land in the Yellow Jacket and Cahone areas had selenium concentrations ranging from 3 to 12 micrograms per liter. Selenium concentra- tions in the San Juan River were 2 micrograms per liter, and selenium was not detected in water sam- ples collected in nonirrigated areas. Concentra- tions of pesticides in water were less than levels harmful to aquatic life. Except for selenium concentrations in bottom-sediment samples from four sites, trace- element concentrations in bottom sediment in the Dolores Project area were not elevated when com- pared to soils in the western United States. The maximum concentration of an organochlorine pesticide in bottom sediment was 5.5 micrograms per kilogram of DDD in a sample from Summit Reservoir. Generally, selenium concentrations in biota in the Dolores Project area were greatest in sam- ples collected from Navajo Wash, in newly irri- gated areas in the Yellow Jacket and Cahone areas, and from the Mancos River basin. Selenium con- centrations in aquatic plants and aquatic inverte- brates were larger in samples collected in the newly irrigated areas than in the long-term irri- gated areas in the Montezuma Valley. Selenium concentrations in 10 of II aquatic-invertebrate samples from the newly irrigated areas exceeded a Abstract 1