During the past several years, concern by the U.S. Department of the Interior has been increasing about the quality of irrigation drainage and its potential effects on the health of humans, fish, and wildlife. The Sun River area was selected for a reconnaissance investigation of irrigation drainage because sufficient information existed to indicate that potential problems of a toxic nature might exist. The purpose of the report is to describe concentrations of selected inorganic and organic constituents in water, bottom sediment, and biota and to compare the analytical results to various criteria and baseline information.The area of study included the Sun River Irrigation Project, Freezeout Lake Game Management Area, and Benton Lake National Wildlife Refuge. Water, bottom sediment, and biota were sampled at selected sites and analyzed for inorganic and organic constituents that could be toxic at large concentrations.Although selenium was of primary concern, other trace elements and selected pesticides also were analyzed. Some water-quality problems have been prevalent for many years in the Sun River Irrigation Project, including the Sun River and Muddy Creek. However, during this study, most sampling sites were free of concentrations of toxic constituents that are in excess of established criteria and standards. There was little change in arsenic, boron, mercury, and selenium concentrations in fish and invertebrates at Sun River sampling sites upstream and downstream from the irrigation project. Presently, the most serious threat within the irrigation project appears to be from nitrate in ground water. Water from some wells contains nitrate concentration in excess of drinking-water standards (10 milligrams per liter) established for the State of Montana.Freezeout Lake Game Management Area and Benton Lake National Wildlife Refuge had sampling sites where concentrations of toxic constituents in water, bottom sediment, and biota were moderately to considerably larger than established criteria and standards. The largest selenium concentrations in water and bottom sediment were from seeps that surround Benton Lake, with maximum concentrations of 580 micrograms per liter in water and 6.7 micrograms per gram in bottom sediment. Selenium was detected in most biological samples.Several eared-grebe livers from Freezeout Lake and several coot livers and eggs from Benton Lake had selenium concentrations indicative of contamination.Water from the irrigation project supplied to the Freezeout Lake and Benton Lake areas has dissolved-solids concentrations in the range of 500 to 700 milligrams per liter and trace elements only slightly larger than concentrations measured in the source water from the Sun River. No hydrologic connection is apparent between the water used for irrigation and the seeps sampled in this study. Irrigation drainage, natural basin runoff, and water from the seeps all contribute dissolved constituents to the lakes. To determine the relative proportions of constituents conveyed into the lakes by all sources,...
