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Field Screening of Water, Soil, Bottom Sediment, and Biota Associated with Irrigation Drainage in the Dolores Project and the Mancos River Basin, Southwestern Colorado, 1994 By David L. Butler, Barbara Campbell Osmundson, and Richard P. Krueger Abstract A reconnaissance investigation for the National Irrigation Water Quality Program in 1990 indicated elevated selenium concentrations in some water and biota samples collected in the Dolores Project in southwestern Colorado. High selenium concentrations also were indicated in bird samples collected in the Mancos Project in 1989. In 1994, field screenings were done in parts of the Dolores Project and Mancos River Basin to collect additional selenium data associ- ated with inigation in those areas. Selenium is mobilized from soils in newly inigated areas of the Dolores Project called the Dove Creek area, which includes newly (since 1987) inigated land north of Cortez and south of Dove Creek. Selenium was detected in 18 of 20 stream samples, and the maximum concentra- tion was 12 micrograms per liter. The Dove Creek area is unique compared to other study areas of the National Irrigation Water Quality Program because selenium concentrations probably are indicative of initial leaching conditions in a newly inigated area. Selenium concentrations in nine shallow soil samples from the Dove Creek area ranged from 0.13 to 0.20 microgram per gram. Selenium concentra- tions in bottom sediment from six ponds were less than the level of concern for fish and wildlife of 4 micrograms per gram. Many biota samples collected in the Dove Creek area had elevated selenium concen- trations when compared to various guidelines , and effect levels, although selenium concentra- tions in water, soil, and bottom sediment were relatively low. Selenium concentrations in 12 of 14 aquatic"invertebrate samples from ponds exceeded 3 micrograms per gram dry weight, a dietary guideline for protection of fish and wildlife. The mean selenium concentration of 10J micrograms per gram dry weight in aquatic- bird eggs exceeded the guideline for reduced hatchability of 8 micrograms per gram dry weight. Two ponds in the Dove Creek area had a high selenium-hazard rating based on a new protocol for assessing selenium hazard in the environment; however, waterfowl were reproducing at the two ponds. Three tributary streams of Mc Elmo Creek that drain inigated areas of the Montezuma Valley south of the creek were sampled in 1994. Mud Creek probably is the largest source of selenium to Mc Elmo Creek. Most biota samples from Mud Creek had elevated selenium concen- , trations when compared to guidelines for dietary items and freshwater fish. . Selenium concentrations in water samples collected in the Mancos River Basin upstream from Navajo Wash, which includes the Mancos Project, ranged from less than 1 to 10 micrograms A_I 1