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<br />There were larger concentrations of most trace elements including <br />arsenic, copper, lead, and zinc in bottom sediment collected from the <br />Uncompahgre River at site 4 (Colona) than in bottom sediment collected from <br />site 9 (Delta) (table 18). The lead concentrations in the sample collected at <br />site 4 exceeded the upper baseline (table 12) for Western United States soils <br />and exceeded the maximum lead concentration reported for the nine initial <br />reconnaissance investigations. Also, the zinc concentration for site 4 <br />exceeded the upper baseline for soils, but was within the range of zinc con- <br />centrations for the nine reconnaissance investigations. The sample collected <br />at site 9 had the smallest concentrations of several trace elements including <br />chromium, nickel, vanadium, and zinc reported for the six bottom-sediment <br />samples collected for this study. Selenium concentrations in bottom-sediment <br />samples from the Uncompahgre River were slightly greater than the upper base- <br />line for Western United States soils and were within the range of selenium <br />concentrations reported for the nine initial reconnaissance investigations <br />(table 12). The selenium concentrations in bottom sediment from the <br />Uncompahgre and Gunnison Rivers were approximately equal. <br /> <br />Selenium concentrations in both bottom-sediment samples collected from <br />Sweitzer Lake (table 18) were substantially greater than the upper baseline <br />for soils in the Western United States. However, the selenium concentrations <br />are less than the largest selenium concentration in bottom sediments reported <br />for the nine initial reconnaissance investigations (85 ~g/g). There were sub- <br />stantial differences in concentrations of barium, lead, selenium, and uranium <br />between the two bottom-sediment samples collected at Sweitzer Lake. The <br />bottom-sediment sample collected near the dam in January probably was more <br />representative of bottom sediment in Sweitzer Lake than the sample collected <br />near the south shore in November. The sample collected near the dam had <br />greater selenium and uranium concentrations (table 18) than the sample col- <br />lected near the south shore. The uranium concentrations for the sample <br />collected near the dam exceeded the upper baseline for Western United States <br />soils (table 12). The larger lead concentrations in the sample collected near <br />the dam may be attributed to the proximity of a boat dock, where gasoline may <br />be a source of lead. The bottom-sediment samples from Sweitzer Lake generally <br />had greater concentrations of chromium, nickel, selenium, and uranium and <br />smaller manganese concentrations than the bottom-sediment samples from the <br />rivers. Small concentrations of DDD, DDE, and dieldrin were detected in the <br />bottom-sediment samples collected from Sweitzer Lake; the maximum concentra- <br />tion was 0.8 ~g/kg of DDE (table 19). <br /> <br />Differences in trace-element concentrations between size fractions for <br />the six bottom-sediment samples collected for this study generally were not <br />large (table 18), but there were exceptions. Selenium concentrations were <br />greater in the less than 2-mm size fraction than in the less than 0.0625-mm <br />size fraction in five of the six samples, and the magnitude of the differences <br />was variable. The largest difference in selenium concentrations between size <br />fractions was in the sample collected in November from Sweitzer Lake near the <br />south shore. The selenium concentration in the less than 2-mm size fraction <br />was 18 ~g/g compared to 8.6 ~g/g in the less than 0.0625-mm size fraction. <br />Concentrations of manganese in the less than 2-mm size fraction for the <br />bottom-sediment samples collected from the Uncompahgre River were about <br />25 percent greater than in the less than 0.0625-mm size fraction. <br /> <br />48 <br />