Laserfiche WebLink
<br />controlled primarily by groundwater discharge, which causes the water to be high in <br />dissolved solids and to have decreased sediment loads. <br />G.1.1.2 Parachute Creek Watershed <br />The Parachute Creek watershed covers approximately 200 square miles. Parachute <br />Creek flows in a southerly direction from its headwaters on the Roan Plateau in <br />Garfield County to the Colorado River south of the town of Parachute. <br />1n general, the water quality in the Parachute Creek drainage basin is typical of surface <br />waters from semi-arid areas of western Colorado. The water is slightly alkaline, with <br />mean pH values ranging from 8.0 to 8.5. The major anion in these waters is bicarbonate, <br />with some carbonate present when the pH exceeds 8.3. The concentrations of trace <br />metals in the waters are generally low and are well below established water quality <br />standards. Concentrations of chemical constituents are generally lowest in the <br />upstream reaches of the Parachute Creek drainage. Farther down the Parachute Creek <br />drainage, soluble salts accumulate from natural runoff and irrigation return flows (COE <br />1985) <br />G.1.2 Groundwater <br />The discussion of groundwater focuses primarily on the Piceance Site because no <br />mining will occur along the pipeline corridor or at the Parachute Site. <br />The principal water-bearing bedrock units within the Piceance Creek Basin include the <br />Uinta and Green River Formations. The underlying Wasatch Formation consists of low- <br />permeability materials. Alluvial sediments found within major drainages can also <br />provide locally important sources of groundwater. The groundwater system within the <br />basin is typically divided into three aquifers: (1) Alluvial Aquifer, (2) Upper Aquifer, (3) <br />Lower Aquifer (Weeks 1974). The Upper and Lower Aquifers are separated by the oil- <br />shale-rich, semi-confining Mahogany Zone. The general structure of the aquifer system <br />is presented in Figure G-1. <br />The Alluvial Aquifer occurs in valley bottoms along creeks. The saturated thickness <br />ranges from a few Feet up to 100 feet for a well location along the lower reaches of <br />Piceance Creek (Weeks 1974). Groundwater in the Alluvial Aquifer can occur under <br />unconfined or semiconfined conditions. <br />The Upper Aquifer consists of fractured, lean oil shale of the Parachute Creek Member <br />of the Green River Formation above the Mahogany Zone and sandstone, siltstone, and <br />fractured marlstone of the saturated portion of the overlying Uinta Formation. The <br />saturated thickness of the Upper Aquifer in the vicinity of the Piceance Site is <br />approximately 350 feet. The Upper Aquifer is located approximately 500 feet below <br />G-2 <br />