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the results of that model and the flow and transport model of Robson and <br />Saulnier (1981) both indicated that the potentiometric surface shows flow from <br />both alluvial and bedrock aquifers toward Piceance Creek. The groundwater <br />quality in the Piceance Creek alluvium varies from relatively fresh water in the <br />headwaters of Piceance Creek to mildly brackish near the mouth of Piceance <br />Creek, before its confluence with White River. The dissolved solids <br />concentration in Piceance Creek alluvium increases from upstream to <br />downstream and the average dissolved-solids concentration is 1750 mg/L <br />(Robson and Saulnier, 1981). The quality of alluvial groundwater is affected by <br />the following processes. <br />• ANuvial inflow occurs from springs which discharge near the mouth of several <br />tributary streams such as Willow Creek, Hunter Creek, Stewart Gulch, and <br />Ryan Gulch. The specific conductance of spring discharge from these <br />springs increases from 500 to 800 µS/cm to 1200 to 1500 µS/cm, from south <br />to north along Piceance Creek valley. <br />Water-sample analyses from alluvial wells in the Piceance Creek alluvium <br />indicate that groundwater quality can be high in dissolved sulfate which <br />appears to result from a combination of groundwater discharge from the <br />Uinta Formation and slow groundwater velocities. Well P-73-4 near the <br />mouth of Horse Draw has alluvial groundwater with adissolved-solids <br />concentration of 6710 mg/L (Saulnier, 1978). However, this well is near the <br />site of a former Shell Oil Co. facility and may have been contaminated by <br />experimental mining operations in the early 1970s. <br />Natural groundwater discharge along bedrock fractures can contribute saline <br />groundwater to the alluvium, especially in the lower reaches of Piceance <br />Creek in the area near Alkali Flat and Stinking Spring in lower Yellow Creek <br />where specific conductivities of greater than 10,000 µS/cm may be observed <br />in natural spring discharge (Saulnier, 1978). <br />Manmade disturbances have caused contamination of the alluvial <br />groundwater in areas where poorly completed exploration wells have flowed <br />saline groundwater at or near ground surface as observed in Alkali Flat on <br />Lower Piceance Creek (Saulnier, 1978). <br />Alluvial boreholes drilled by the USGS indicate that locally, the Piceance <br />Creek alluvium contain dense organic-rich clay (Welder, 1987). These clay <br />beds may have developed upstream of natural dams developed along <br />Piceance Creek due to the periodic flash floods and debris flows which occur <br />in Piceance Creek Basin (Saulnier, 1978). Stagnant groundwater occurring <br />in this clay-rich alluvium may have higher concentrations of dissolved solids <br />derived from these organic clays. <br />Sentem6er 7. 1999 2 <br />