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(1994) report that recharge to the spoils aquifer from precipitation falling on the aquifer may be as <br />high as 3.0 inches per year. <br />• Groundwater flow determined from evaluation of water level elevations in wells e s completed in the <br />basal Williams Fork aquifer was generally from recharge areas in the south and then toward the <br />north and northwest through the permit area. Potentiometric surface contours (Robson and Stewart, <br />1990, Figure 33) for the basal Williams Fork aquifer indicated that flow converges toward Grassy <br />Creek. A hydrologic divide that generally follows the crest of the Tow Creek anticline separates the <br />groundwater flow system in the permit area and the general area from the groundwater flow system <br />to the east, west, and south. The portion of the divide in the Grassy Gap area separates the <br />hydrologic system of the PSCM permit area and general area from the hydrologic system of <br />Twentymile Park to the southeast. Hydraulic heads in the basal Williams Fork aquifer are above the <br />land surface in much of the area. Insufficient data were available to determine the flow patterns in <br />the Twentymile and Trout Creek aquifers in the area. The Yampa River forms a regional hydrologic <br />boundary on the north. <br />Regional Groundwater Quality - Groundwater quality in the general area was summarized by <br />Robson and Stewart (1990) based on more than 1,000 water quality analyses from wells in the area. <br />Water in the Twentymile aquifer is generally a sodium bicarbonate type with dissolved solids <br />concentrations between 300 and 600 milligrams per liter (mg/L). Groundwater in the basal Williams <br />Fork aquifer is generally of sodium or calcium bicarbonate type, with dissolved solids <br />concentrations ranging from 300 to 1,400 mg/L. Calcium bicarbonate water more commonly <br />occurs near recharge areas, and sodium bicarbonate water becomes more common farther <br />downgradient along the groundwater flow path. The transition from one type to the other is a result <br />• of cation exchange, whereby calcium and magnesium in the water are exchanged for sodium and <br />potassium found in clay within the aquifer matrix. Sulfate concentrations in the groundwater <br />reportedly range from 50 to 400 mg/L in areas not disturbed by mining and from 50 to 1,500 mg/L <br />in disturbed areas, where dissolved solids concentrations also can be 3,000 mg/L or more. <br />• <br />PSCM Permit App. 2.04 -23 6/15/09 <br />