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Recent analyses of samples from American Soda alluvial observation well 21-2 <br />show a slight increase in dissolved solids from 1998 to 1999, represented <br />primarily by an increase in the calculated concentration of sodium carbonate and <br />by increases in calcium and magnesium concentrations (Agapito, 1999). The <br />increase in the concentration of the dissolved solids in alluvial groundwater in this <br />ephemeral side gulch of Piceance Creek is likely due to an NPDES permitted <br />release of water from the American Soda retention ponds in the vicinity of the <br />waste-water retention ponds in the gulch below the American Soda Pilot Plant. <br />This release occurred immediately after discharge from a large storm event in the <br />creek bed above the retention ponds overwhelmed the capacity of the ponds. A <br />quantity of sodium-carbonate water was released from the ponds after the flood <br />event and continued until the pond levels stabilized. These releases as well as <br />infiltration from the flood event are the likely cause of the changes in water <br />quality observed in well 21-2. Alluvial groundwater in Piceance Creek was <br />observed to have a specific conductance of 2110 µS/cm in American Soda <br />observation well 21-5, downstream of the Yankee Gulch lease (Table 1). This <br />specific conductivity is within the range of alluvial groundwater reported in <br />Robson and Saulnier (1981) and Saulnier (1978). <br />Upper Aquifer <br />Upper Aquifer groundwater quality reflects contributions from both the Uinta <br />formation and the upper part of the Green River Formation between the <br />Mahogany Zone and the Uinta Formation (Weeks et al. 1974; Saulnier, 1978). In <br />some parts of the basin the lean oil shale zone immediately above the Mahogany <br />Zone forms a relatively prolific groundwater zone, the "A-Groove Aquifer". The <br />groundwater quality of both the Uinta and Green River Formation parts of the <br />Upper Aquifer vary from north to south and vertically in the basin in response to <br />the mineralogy of the formations, the speed of groundwater flow, and natural and <br />manmade disturbances to the flow system. <br />Groundwater quality in the Uinta Formation is generally higher in sulfate and <br />strontium concentration, lower in bicarbonate concentration, and slightly higher in <br />dissolved-solids concentration than water in the dolomitic marlstone of the <br />Parachute Creek Member of the Green River Formation part of the Upper <br />Aquifer. Because residence time in Piceance Creek Basin aquifers appears to <br />be directly correlated with dissolved-solids concentrations, Saulnier (1978) <br />attributed this water-quality difference in parts of the Upper Aquifer to the <br />relatively slower groundwater flow in the heterogeneous and cemented siltstones <br />of the Uinta Formation and the relatively faster flow in the fractured lean <br />marlstones of the Green River Formation. This difference is most striking when <br />comparing ranch wells completed in the Uinta Formation to springs issuing from <br />marlstones of the Parachute Creek Member which are interbedded with the Uinta <br />Formation (Saulnier, 1978). In part, the differences in the geochemical character <br />of these parts of the Upper Aquifer may be because the marlstones of the Green <br />River Formation contain more carbonate minerals. Also where the marlstones <br />September 7. 1999 <br />