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Page 10 of 27 <br /> Ground Water <br /> The Trout Creek Sandstone,the Twentymile Sandstone, and the White Sandstone are bedrock aquifers <br /> currently being used for ground water supplies in the general vicinity of the Williams Fork Mines. The <br /> Middle Sandstone is not currently used as a ground water supply in the vicinity. The alluvial bodies <br /> associated with the Yampa River and Williams Fork River contain limited ground water and are not <br /> considered major aquifers in the general area. Alluvial bodies along the Yampa River up and downstream <br /> of the general area,however, are significant sources of ground water. High yield irrigation and municipal <br /> water supply wells are completed in the Yampa River alluvium in those areas. Also,the Yampa River <br /> alluvium outside the vicinity is widely used as a source of domestic and livestock watering. Within the <br /> vicinity, alluvial ground water is not a significant source of water put to beneficial use. <br /> The sandstone aquifers in the Big Bottom Synclinal Basin are recharged at their subcrops beneath the <br /> stream/alluvial systems of the Yampa and Williams Fork Rivers, and at their outcrops in upland areas. <br /> From a recharge area, flow would be generally northward, downdip toward the axis of the Big Bottom <br /> syncline. Faults may provide conduits of flow for ground water wherever a fault is not sealed with fine- <br /> grained gouge material. <br /> The sandstone aquifers are under atmospheric pressure (water table conditions)near their recharge areas <br /> and under hydrostatic pressure (artesian conditions)within the structural basins or at discharge points. <br /> Hydrostatic pressures in the sandstone aquifers increase with depth and are at a maximum in the axial <br /> areas of the Big and Round Bottom Synclines. Artesian conditions are developed in aquifers which are <br /> confined by overlying and underlying strata with low permeabilities. Several of the wells drilled into the <br /> bedrock aquifers within the area exhibit artesian flows at the surface. <br /> Ground water in the Trout Creek Sandstone and Williams Fork Formation is predominantly calcium and <br /> sodium bicarbonate types. Water in contact with coals is a calcium sulfate type and can contain fluoride, <br /> iron,manganese, selenium,and sulfate concentrations in excess of U.S. Public Health Service drinking <br /> water standards,with the water contained in the coals and thin discontinuous sandstones generally being <br /> of poorer quality than that from the massive regional sandstone aquifers. <br /> The Yampa River and the Williams Fork River alluviums contain alluvial ground water. These alluvial <br /> water bearing units may store, and release water used by the overlying vegetation, and may sustain a <br /> component of baseflow to the associated river systems. These alluvial units may provide recharge to rock <br /> aquifers and are recharged by rock aquifers within the ground water study area. <br /> Alluvial water quality is variable, depending on the underlying rock and source of alluvial material. <br /> Ground water from the Yampa River alluvium is primarily sodium sulfate type. Dissolved solids average <br /> 4,586 mg/1 with a maximum measure of 8,810 mg/l. Groundwater for the Williams Fork alluvium is <br /> primarily of the sodium bicarbonate type. Total dissolved solids average 1,009 milligrams per liter(mg/1) <br /> with a maximum measured value of 1,510 mg/1. Maximum primary and secondary drinking water <br /> standards are exceeded in both aquifers for many parameters including barium, cadmium, chloride, <br /> chromium,pH, sulfate,and selenium. In addition, average concentration values for chloride,total <br /> dissolved solids, iron, lead,manganese, and sulfate exceed EPA primary and secondary standards. <br /> Williams Fork Mines Prepared by: R ReiRey M.S. GISP <br /> C1981044 December 2023 <br />