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<br />ground surface at the Piceance Site (based on a ground surface elevation of about 6,200 <br />feet). <br />'The Lower Aquifer is composed of fractured oil shale and marlstone of the Parachute <br />Creek Member of the Green River Formation. The Lower Aquifer extends from the base <br />of the Mahogany Zone to the top of the Saline Zone. The Lower Aquifer is also referred <br />to as the Leached Zone because of the dissolution and removal of minerals, mainly <br />nahcolite, by percolating groundwater. This has resulted in enhanced porosity and <br />permeability of the Leached Zone. The boundary between the Leached Zone and the <br />underlying Saline Zone, below which minerals are not being dissolved, is called the <br />Dissolution Surface. The Lower Aquifer is located from approximately 1,050 feet to <br />1,400 feet below ground surface in the vicinity of the Piceance Site (based on a ground <br />surface elevation of about 6,200 feet). <br />'The following discussion presents data describing existing groundwater quality, both <br />on a regional and asite-specific basis. The chemical quality of groundwater in the <br />Piceance Creek Basin varies both within and among the aquifers. <br />The water in the alluvium is classified as a sodium bicarbonate type. Concentrations of <br />major ions indicate that the alluvial groundwater is similar in chemistry to that of the <br />Upper Aquifer. Concentrations of total dissolved solids (TDS) range from 470 to 6,720 <br />milligrams per liter (mg/1) and average 1,750 mg/1 over the entire basin (Weeks et al. <br />1974). Generally, concentrations of dissolved solids increase in the downstream <br />direction, with increases in sodium and bicarbonate from recharge areas to discharge <br />areas. These increases are believed to reflect influences from irrigation-return flows, <br />contributions from bedrock groundwater discharging to the alluvium, and <br />concentration by evapotranspiration. <br />Water in the Upper Aquifer can be classified as a sodium bicarbonate type with a TDS <br />concentration ranging from less than 400 to 2,000 mg/1 (Weeks et al. 1974). Calcium, <br />magnesium, and sulfate concentrations in the Upper Aquifer are greater than in the <br />Lower Aquifer. The Lower Aquifer can be classified as a sodium bicarbonate-chloride <br />type with TDS concentrations ranging from less than 500 to greater than 60,000 mg/I <br />and averaging over 13,000 mg/1. Most of the highest TDS concentrations are located <br />just above the top of the Saline Zone near the Dissolution Surface (Coffin et al. 1971). <br />Weeks and others report that concentrations of some trace elements in the Lower <br />Aquifer are great enough to be of environmental concern (Weeks et al. 1974). Barium, <br />boron, and lithium are consistently found in high concentrations in the northern part of <br />the basin, and concentrations of barium occasionally exceed drinking water standards. <br />Other constituents in the groundwater that have elevated values and, on average, <br />exceed drinking water standards include arsenic, cadmium, chloride, iron, mercury, <br />manganese, lead, and selenium. Elevated levels of barium, boron, lithium, copper, <br />chromium, strontium, and fluoride are also common in this geohydrologic unit. <br />G-3 <br />