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22 MAYLAND ET AL. Rumble (1985) measured water quality parameters, including Se, in surface-coal and bentonite-mine impoundments and livestock ponds in the Northern High Plains. He reported mean Se values of 2.2, 10.6, and 1.1 µg Se/L in the three potential livestock-water sources, respectively. H.F. May- land (1986, unpublished data) also sampled livestock-water impoundments throughout central Montana in known seleniferous areas and found 0.4 to 0.7 µg Se/L in most of these waters. A few samples contained as high as 270 µg Se/L, many times greater than the USEPA (1477) allowable value of 10µg Se/L in drinking water but is near the 100 µg/L level of Se in blood considered adequate for animal requirements. In a report on water-quality trends in the nation's rivers, Smith et al. (]987) noted that, over an 8-yr period, 7Salo of the 211 stations reported values at or below the 1µg Se/L laboratory detection limit. Four sites had higher concentrations in 1981 than in 1974, whereas 23 had lower concentrations. Rivers draining some of the seleniferous regions contained relatively high Se concentrations. Selenium mean values and trends for specific stations (R.A. Smith, 1987, personal communication, Table 2-2) show a decline in flow- corrected values of Se in three rivers flowing out of seleniferous areas. These decreases in Se outflow were also accompanied with decreases in total dis- solved salts (TDS) from the two Colorado sites. The TDS information was not available for the Montana site. Improvements in irrigation water manage- ment in western Colorado and modifications of the crop-fallow system in north-central Montana have occurred during this same period. These prac- tices would account for decreases in river-Se levels if also accompanied by decreases in overall salinity. Smith et al. (1987) reported an increasing trend in arsenic (As) and cad- mium (Cd) concentrations in many of the rivers. Fossil-fuel combustion, the largest source of both elements, introduces these elements into the aquatic environment through (i) atmospheric deposition of combustion products and (ii) runoff from fly ash storage areas near power plants and other nonfer- rous smelters. Fossil-fuel combustion also releases Se to the environment Table 2-2. Mean Se values and concentration trends over the 1974 through 1981 sampling period for river systems in seleniferous areas (R.A. Smith, 1987, personal communi- cation). River station Location Dissolved Se Conc. Trend' Total Se Conc. Trend' µgl L µg/L Gunnison River Near Grand 8.0 Down 10.0 Down Junction, CO Colorado River Colorado-Utah 5.5 Down 8.5 Down boundary Marias River Chester, MT 1.0 Down 1.0 Down Cheyenne River Cherry Creek, SD 3.0 NS 4.0 NS Little Missouri Waterford City, ND 1.0 NS 1.0 NS Missouri River Fort Randall, SD 2.0 NS 2.0 NS 1' Trends are significant at P < 0.10. NS =not significant.