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• Formation, as a whole, is an aquitard. However, there are coal seams within the <br />Upper Williams Fork which have permeabilities of 15 gallons/day/ft2. The over- <br />lying Lewis Shale has low porosity and permeability, acting more as an aquiclude <br />(confining bed) than as an aquifer or an aquitard. Its permeability is estimated <br />at 10-5 gallons/day/ft2. The alluvial aquifer associated with the Yampa <br />River is restricted in its occurrence to the flood plain of the river and is an <br />excellent aquifer with an estimated permeability of 500 to 5000 gallons/day/- <br />ft 2. <br />Peizometer studies conducted by Radian indicate that vertical cross bedding <br />groundwater flow occurs and that an up -section flow component develops to the <br />north and west of the mine. However, this up -section flow component does not <br />exist between the Yampa River alluvium and the Upper Williams Fork (Settergren, <br />1979 -Appendix N). <br />Depths to groundwater from the land surface are variable from about 120 to 250 <br />feet in the Upper Williams Fork which includes the coal seams mined at the Trap- <br />per Mine. Using data from wells constructed for the Radian studies and assuming a <br />permeability y of 50 gallons/day/ft` and a porosity of 0.2, estimated groundwater <br />velocity within the Twenty Mile Sandstone is about 0.6 ft/day. Actual velocities <br />in the Upper Williams Fork are much lower. <br />The general quality of groundwater in the Trapper Mine area appears to be good. <br />Total dissolved solids for all wells is less than 100 mg/1, which means that the <br />water is classed as fresh in the U.S. Geological Survey classification. The <br />water is relatively hard, containing significant quantities of calcium bicarbon- <br />ate and sulfate. Concentrations of minor and trace elements in the groundwater <br />are low. However, concentrations of lead, selenium, and arsenic are at or above <br />National Interim Primary Drinking Water Standards (NIPDSW) in some wells. For <br />the proposed National Secondary Drinking Water Regulations (NSDWR) it appears <br />that both iron and molybdenum exceed the limits. Concentrations of copper and <br />molybdenum exceed the NAS/NAE recommended limits for continuously -used irrigation <br />water. <br />4-92 <br />