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Selenium, when bioavailable, is toxic in small <br />concentrations to fishes. A number of water samples collected in <br />the study area contained detectable concentrations of selenium <br />and allow us to analyze its spatial distribution and possible <br />sources. The four southern ,samples, the irrigation return flow <br />at 23, irrigation return flow at station 18 flowing ,into <br />Roubideau ponds, one Roubideau pond, and the pond at station 20 <br />all contained 3 ug L'1 of Se. We assume these are from similar <br />water sources. The Gunnison River at station 22 contained 4 µg <br />L'1 of Se. The similarity of these values indicates that much of <br />this water is derived from the Gunnison River and its <br />tributaries, is used for irrigation, and returns to the river. <br />ESCALANTE STATE WILDLIFE AREA ON THE GUNNISON RIVER <br />N- <br />old rives channel <br />t. oL oo rat <br />Selenium concentrations ug/L <br />in surface and ground water <br />irrigation return <br />river ctwmel / backwater <br />6 ~ <br />t ~ <br />nd 3 <br />~' irrigation return <br />ircigotion return <br />Gunnison River <br />3 <br />nd <br />3 <br />backwater nd O ~ Q <br />nd l/ Ponce <br />escarpment <br />~- Roubideau Creek <br />l <br />Figure 18. Selenium concentrations (µg L'1) in study area <br />waters. <br />25 <br />