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1999) which are completed to the lower Uinta Formation in the vicinity of <br />well 20-1. Well 20-8 is a water supply well which provides process water for the <br />American Soda Pilot Plant. The initial sample analyses from these wells <br />indicated dissolved solids concentrations in the 1500 to 2500 mg/L range and <br />+/-300 mg/L dissolved-sulfate concentrations typical of groundwater from the <br />Uinta Formation. However, since mid-1998, sample analyses from these wells <br />indicate increases in dissolved-solids and sodium-bicarbonate concentrations. <br />This trend coincides with initiation and increased usage of supply water pumped <br />from well 20-8. The drawdown coincident with the increased production from <br />well 20-8 appears to have resulted in bringing Upper Aquifer water from the <br />Parachute Creek Member contaminated by well 20-1 into the groundwater <br />produced from well 20-8 during operations, and well 20-5 during sampling. The <br />water produced from these wells thus has the higher sodium-bicarbonate <br />signature characteristic of the Parachute Creek Member, the sulfate signature <br />characteristic of the Uinta Formation, and the overall higher dissolved-solids <br />concentration due to the plume of saline water created due to the presence of <br />well 20-1 (Figure 2). <br />Lower Aquifer <br />The Lower Aquifer is represented by the stratigraphic interval between the <br />Mahogany zone and the base of the Parachute Creek Member, at the basin <br />margins, or the upper surface of the intact, non-dissolved soluble saline minerals <br />in this unit in the central portion of the basin. The upper part of the Lower Aquifer <br />is the lean oil-shale zone immediately below the Mahogany Zone, commonly <br />referred to as the "B-Groove" aquifer. In much of the basin the lower part of the <br />Lower Aquifer is represented by the dissolution zone at the top of intact soluble <br />mineral. This part of the Lower Aquifer is also referred to as the "Leached Zone" <br />because it represents that part of the formation where leaching of soluble <br />minerals, primarily Nahcolite, has created zone of secondary porosity and <br />permeability which can be extensive and include the B Groove. The removal of <br />the minerals has led to local brecciated zones and fracturing which increase the <br />permeability and groundwater content of the oil-shale horizons in the lower <br />Parachute Creek Member. <br />The water quality of the groundwater in the Lower Aquifer generally follows the <br />pattern displayed by the Upper Aquifer. That is, generally fresher water at the <br />basin margins and the headwaters of Piceance Creek, with a gradual increase in <br />dissolved solids moving from recharge areas to discharge areas. However, there <br />is more variability in Lower Aquifer water quality due to the presence of the <br />soluble saline minerals. In addition, the Lower Aquifer groundwater has a <br />different trace-element distribution, probably due to accessory minerals <br />associated with the saline mineral suite. The Lower Aquifer tends to contain very <br />high concentrations of dissolved fluoride (greater than 10 mg/L), higher levels of <br />barium, boron, lithium, and, locally, arsenic, than found in the Upper Aquifer <br />(Weeks et al. 1974; Robson and Saulnier, 1981; Saulnier, 1978). <br />Seniamher 7. 1999 6 <br />