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Situated about 130 feet above the "D" coal seam is the "E" coal seam aquifer. <br />This coal seam is currently being mined at the Grand Mesa operation. Like the <br />"D" seam, the aquifer subcrops below the glacial/alluvial material and <br />continues deep into the Piceance Basin at dip of 5-60. Recharge, flow, and <br />discharge conditions are also similar to those encountered in the "D" seam <br />aquifer. The transmissivity of the aquifer was determined to be 1.5 <br />ft2/day. The "E" seam aquifer is also confined and has a low permeability. <br />The operator currently pumps about 25 gallons per minute of water from the "E" <br />seam mine workings. The sources of these inflows are discussed in Volume 8 of <br />the application and shown on Map 6. Ground water in the aquifer is a sodium <br />bicarbonate type with TDS concentrations ranging from 1,242 mg/1 to 1,354 <br />mg/1. Because of the low permeabillties and poor water quality, large-scale <br />development of the aquifer has not occurred. <br />The glacial/alluvial aquifer is one of the most important water sources in the <br />area of the mine. The aquifer mantles about 2/3 of the surface in the mine <br />area to depths of over 200 feet. Recharge probably occurs through adjacent <br />bedrock aquifer discharges, influent leakage from stream flow, irrigation <br />return flows, and vertical infiltration of precipitation and snow melt. The <br />transmissivity of the aquifer was determined to be 1,885 ft2/day. The <br />storage coefficient is 7.5 x 10-2. Monitoring results indicate that the <br />aquifer is a calcium bicarbonate type with TDS concentrations ranging from 160 <br />mg/1 to 400 mg/1. Because of the moderate permeability and good water quality <br />characteristics, the aquifer is used near the mine for domestic, irrigation, <br />stock watering, and industriai purposes. <br />Ground water near the proposed mine has been deveioped for irrigation, stock <br />watering, and light industriai purposes. As discussed above, most of this <br />water comes from the glacial/alluvial aquifer. Ground water is not used <br />extensively for domestic purposes. Most domestic water comes from the town of <br />Colby's domestic water system. A listing of existing wells in the area is <br />presented in Table 2 of Volume 8. <br />Probable Hydrologic Consequences of Mining <br />During the proposed mining operations, piezometric surfaces in the "D" seam, <br />"E" seam, and the glacial/alluvial aquifers may be affected to some degree. <br />Several decades after cessation of mining, there is a potential that water <br />quality in both bedrock and surficial aquifers may also be affected. The <br />applicant's discussion of probable hydrologic consequences is contained in <br />Section IV of Volume 8 of the permit application. <br />The dewatering of the mine workings will cause piezometric surfaces to lower <br />in the vicinity of the mine. The aquifers that could potentially be affected <br />are the two coal seams and the glacial/alluvial aquifer. The applicant has <br />predicted future mine inflows and resultant water level drawdowns through <br />modeling techniques developed by McWhorter (1981). These predictions are <br />presented in Table 8 of Volume 8. The modeling predicts that the drawdown <br />effects resulting from dewatering unfractured coal seams will not extend to <br />the subcrop south (updip) of the mine workings. As a result, there is minimal <br />potential for lowered piezometric surfaces in the surficial aquifer through <br />unfractured coal seams. <br />-25- <br />