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Summary of Water Sources <br />The Gunnison River valley in the study area has surprisingly <br />complex hydrologic patterns. We had expected that the dynamic <br />nature of the river would have created a river system that was <br />completely connected to the ground water system throughout the <br />study area. Instead, we found little groundwater connection. <br />The base level (low flow) of the Gunnison River apparently <br />creates the base level below which ground water can not drop. <br />However, this base level control only affects minimum water <br />levels. It does not greatly influence the height or chemical <br />characteristics of this water. <br />Several different water sources influence the study area. <br />Surprisingly, much of the groundwater in the study area has very <br />different chemical characteristics from the Gunnison River. <br />Adjacent groundwater elevations are typically higher than the <br />Gunnison River and apparently are recharged by irrigation return <br />flows back toward the river. This water flows not only through <br />ditches to the river, but much ditch water is lost to the ground <br />en route. This groundwater is later discharged back to the <br />Gunnison River, as can be seen in the backwater at station 21. <br />This returning water has higher salt concentrations than the <br />Gunnison River,, and also is. highly reduced. Iron is soluble in <br />highly reducing conditions and where groundwater seeps from the <br />banks and oxidizes iron precipitates are abundant. <br />The Gunnison River in this area is a gaining river, much as <br />would be found in a high mountain valley. Water is returning <br />from numerous irrigation ditches, recharging slopes and <br />floodplain water tables creating higher water levels in <br />floodplains than in the river. In addition, floodplain ground <br />waters are chemically distinct from the river, being more saline. <br />Surface waters in the study area have detectable concentrations <br />of Se, while the groundwaters had less or no Se. This most <br />likely is due to the chemical speciation of Se in aerobic surface <br />waters where it is soluble vs. anaerobic groundwaters where it is <br />insoluble, and not in the water column. Thus, floodplains may be <br />performing an important Se removal function. <br />27 <br />