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We suspect that the reason that Se is not detected in the <br />anaerobic groundwater is because it occurs in the insoluble <br />chemical species selenide (SSSA 1989). However, when soil or <br />possible groundwater containing selenide enters the Gunnison <br />River it is oxidized to selenate which is soluble., bioavailable <br />and well-dispersed through the water column, as it is at station <br />21. <br />Surprisingly, and unfortunately, the highest Se <br />concentrations occurred in the very productive backwater at <br />station 21. Thus, it appears that backwaters will all have some <br />Se concentration due to the Gunnison River. Tn addition to this, <br />if groundwater rich in sulfate salts is discharged from <br />floodplains to the backwaters, higher salt and Se concentrations <br />occur. Thus, backwaters may produce chemical problems for larval <br />fishes. <br />On the north, the irrigation return flow contains 5 ug L-1, <br />and the small pond at number 4, which we suspect is also fed by <br />irrigation return flow, contained 6 ug L-1 of Se. The ground <br />water at stations 19, 1, and 3 did not contain detectable <br />concentrations of Se. <br />Water from the Gunnison River system was analyzed by Butler <br />et al. (1991) to determine Se concentrations. They found the <br />Uncompahgre .River at Delta to contain the highest Se <br />concentrations. Concentrations in the Uncompahgre River were <br />highest in the winter samples. <br />26 <br />