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III III I II I II IIII III Doc Date:1211112001 <br />~I~ <br />' (~ 2.05.6.3 PROBABLE HYDROLOGIC CONSEQUENCES ~~ <br />2.05.6.3.1 GROUNDWATER <br />Information collected in the baseline hydrology characterization together with <br />information concerning the hydrologic impacts of coal mining within the Raton <br />Basin as well as some of the surface mining regions in the Western United <br />States was used to prepare an assessment of the potential impacts of the <br />proposed mining on both the quantity and quality of groundwater resources both <br />within and adjacent to the mine permit area. <br />2.05.6.3.1.1 Quantity <br />Proposed strip mining at the Rimrock Coal Mine Site will disturb the Vermejo <br />Formation down to the minable coal seam. Local groundwater flow should only <br />be minimally impacted by the presence of the mine due to the pumping and <br />discharge of mine inflows. Mine inflows are difficult to project precisely <br />without detailed mine plan information and measurements of aquifer <br />characteristics from site specific tests. However, a crude projection of the <br />maximum rate of mine inflow was calculated using the method of McWhorter <br />(1981), and assuming that the mine pit is approximately 100 feet wide, <br />advances at a rate of 4 feet per day and reaches a maximum pit length of 500 <br />feet. The affected aquifer was assumed to be 10 feet in thickness, have a <br />hydraulic .conductivity of 1.88 ft per day and be under unconfined conditions. <br />These are probably somewhat conservative assumptions because the hydraulic <br />conductivity was taken from the USGS regional study for producing wells in the <br />Raton/Vermejo/Trinidad aquifer and would be biased toward the higher yielding <br />• units within these formations. The resulting calculations, as provided <br />below, suggest a maximum inflow rate of about 1.6 gpm. <br />Qmax - 2*R ((Sya*T*B2)/12)1/2 * tl/2 at t - L/R <br />- 8 * ((.1*18.8*102)/12)1/2 * (125)1/2 <br />- 354 ft3/day <br />where: Qmax - the maximum inflow rate in ft3/day <br />R - 4 ft/day, the rate of pit advance <br />Sya - 0.1, the apparent specific yield <br />T - 18.8 ft2/day, the transmissivity of the affected <br />aquifer calculated by multiplying the hydraulic <br />conductivity of 1.88 ft/day by 'the aquifer <br />thickness, b. <br />b - 10 ft., the aquifer thickness <br />L - 500 ft., the maximum pit length <br />This estimate should be viewed as an order of magnitude value because aquifer <br />,~ tests have not been performed on the Vermejo aquifer unit that would be <br />-1- <br />