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r CHAPTERTHREE Atfecte.d Environment , from Uinta wells (20-5 and 20-8) and the A-Crroove portion of the solution mining well (20-3) indicate that only the Uinta Formation portion of the Upper Aquifer would be considered a USDW. During drilling and subsequent sampling of monitoring well 20-5 (Uinta Formation), concentrations of fluoride (5 to 8.3 mg/1), chloride (769 mg/1), sulfate (290 to 390 mg/I), nitrite (2.5 mg/1), chromium (0.066 mg/1), iron (0.5 to 13.3 mg/I), manganese (0.051 to 0.405 mg/l), zinc (13.3 mg/1), and pH (8.8) exceeded Colorado groundwater standazds. Mahogany Zone The Mahogany Zone is a kerogen-rich oil shale zone that separates the Upper Ayiuifer from the Lower Aquifer spatially, but not entirely hydrologically. The Mahogany Zone is considered a semi-confining layer and it is approximately 180 feet thick at American Soda well 20-1. The vertical hydraulic conductivity, which controls the vertical exchange of water between the Upper Aquifer and the Lower Aquifer, varies throughout the rock formation. There are insufficient data available at the Piceance Site to evaluate vertical gradients. However, published data indicate that downward vertical gradients probably exist in the upland azeas of the project and upward gradients may occur near Piceance Creek (Robson and Saulnier 1980). Lower Aquifer The Lower Aquifer consists of the Leached Zone of the Parachute Creek Member. The Leached Zone is generally located below the Mahogany Zone and above the Dissolution Surface. Based on geologic information from American Soda boreholes, the thickness of tl~e Lower Aquifer ranges from 342 feet bgs in well 29-2 to the south to 440 feet bgs in well 19-2 in the northwestern part of the Project Area. In the wells in section 20 at the project site, the thickness ranges from approximately 367 to 405 feet bgs. The Dissolution Surface (i.e., ba:;e of the Lower Aquifer) is located at an approximate elevation of 4,700 to 4,800 feet msl within the project boundary. Based on static water elevations measured on September 21, 1998, in well 20-1 (6,087.69 feet msl), we1129-2 (6,119.84 feet msl), we1120-4 (6,138.25 feet msl), and well 20-4 (5,108.63 feet msl), groundwater flow in the Lower Aquifer below the Piceance Site is generally to the east- southeast at 100 degrees azimuth, and the horizontal hydraulic gradient is approximately 0.014 feet per foot. Ina 7-day pump test of the Lower Aquifer conducted at the adjacent lease propem~ to the west of the Piceance Site, transmissivity varied from 280 to 350 square feet per day in four observation wells. However, transmissivity has not been determined for American Soda wells completed in the Lower Aquifer. Storage coefficient data from a 7-day pump test conducted on the adjacent lease property to the west of the Piceance Site ranged from 2x105 to 7x105 (Cole et al. 1995). Lower Aquifer water is generally classified as a sodium bicazbonate-chloride type. TDS concentrations of the Lower Aquifer range from about 500 mg/1 to greater than 60,000 mg/1 and are dependent on depth and location. Data indicate that the salinity of the Lower P,quifer 3-20 Groundwater