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Freezout Lake Wildlife Management Area (WMA) receives drainage from adjacent irrigated land. Results of a 1986 reconnaissance study and a 1990-92 detailed study indicated that some selenium concentrations in water, bottom sediment, and biota in Freezout Lake WMA wetlands were higher than established criteria and standards, raising concerns about potential toxicity to aquatic organisms and water birds. In 199495, a second detailed study was conducted to determine the distribution, mobilization, and accumulation of selenium associated with irrigation drainage from land underlain by glacial-lake deposits in the southern part of the Freezout Lake WMA. Interpretations presented in this report are based on data obtained in 1986-95 from 25 soil or drill-core sites, 8 ground-water sites, 5 soil-moisture sites, 30 surface-water sites, 35 bottomsediment sites, and 21 biological sites in a 20 squaremile area in and near the southern part of the WMA. Freezout Lake WMA is bordered on the south and east by seleniferous glacial-lake deposits. Precipitation and selenium-free irrigation water infiltrate the deposits and dissolve and mobilize selenium. Selenium-rich ground water then discharges into open irrigation drains. The irrigation drains discharge into wetlands of Freezeout Lake and Pond 5 in Freezout Lake WMA. In the wetlands, selenium is removed from water and accumulates in bottom sediment and biota. Sparingly soluble selenium-enriched gypsum that formed prior to irrigation probably constitutes a slowly diminishing reservoir of selenium in the glaciallake deposits. As selenium-free irrigation water percolates through soil, it dissolves this naturally occurring selenium. Selenium concentrations in soil moisture and shallow ground water are further increased by evapoconcentration. Maximum concentrations were 1,100 micrograms per liter (ug/L) in soil moisture and 190 |ng/L in ground water. Selenium in irrigation drainage is derived primarily from ground-water discharge. The acute-toxicity criterion for aquatic life of 20 jug/L was exceeded in water from all four sampled drains that convey water primarily from glacial-lake deposits; the highest measured concentration was 180 iiig/L. During the irrigation season, selenium concentrations in irrigation drainage were lower than during the non-irrigation season because of dilution from increased flow caused by irrigation. Selenium loads in irrigation drains were highest during the beginning of the irrigation season. Irrigation drainage discharges into wetlands in Freezout Lake WMA. The average annual load of selenium from irrigated glacial-lake deposits to Freezout Lake WMA wetlands is about 200 pounds, or about 0.06-0.08 pound per acre of irrigated land. Selenium that discharges from irrigation drains to Freezout Lake WMA wetlands is converted rapidly by biogeochemical processes from soluble, oxidized selenate to insoluble, reduced selenium species, which accumulate in the reduced, organic-rich bottom sediment. Consequently, selenium concentrations are low in lake water (less than 3 fig/L), except in small areas near the mouths of drains. Selenium concentrations are high in bottom sediment (maximum of 13 micrograms per gram) and decrease with distance from the mouths of drains. Biota samples typically had higher selenium concentrations than national average background concen-

water-resources investigations report

Detailed study of selenium in glacial-lake deposits, wetlands, and biota associated with irrigation drainage in the southern Freezeout Lake area, west-central Montana, 1994-95