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TAB 17 <br />• PROBABLE HYDROLOGIC CONSEQUENCES <br />r.,.....a w,«e- <br />Introduction. This Tab contains a discussion of the probable hydrologic consequences of <br />the Nucla mining plan on the quality and quantity of surface and ground water for the <br />proposed permit and adjacent areas. The significance of each impact or potential impact <br />was determined. The determination of significance has bean made considering the impact of <br />a probable hydrologic consequence on the quality of the human environment,. existing water <br />uses, and the intended postmining land use of the area. <br />Water Inflows to Pits and Dramsowns. <br />Introduction. This section describes the methods and results used to determine the <br />probable hydrologic consequences of mining in the Nucla East mining area to ground water <br />quantity. In order to make the impact assessments, two different analysis techniques were <br />• employed. First, pit inflow volumes were determined on an annual basis using an <br />analytical approach developed by McWhorter, 1982. The second analysis involved the <br />determination of annual pit inflow rates and annual drawdowns in the adjacent overburden <br />and coal aquifers as a result of the pit inflows. This analysis employed the use of the <br />USCS 3-dimensional finite-difference ground water flow model MODFLOW. <br />The two analysis techniques show a range in year 5 pit inflow volumes of from 20.5 to 44 <br />acre feet. This is largely due to the fact that the McWhorter analysis assumes no <br />recharge. It was felt that one analysis technique should incorporate the effects of <br />recharge and thus it was incorporated in the MODFLOW model runs. All drawdown projections <br />were calculated using the higher pit inflow rates that include the effects of recharge. <br />McWhorter Analysis for Calculating Pit Inflows. The following is a discussion of the <br />assumptions and input data used in performing the McWhorter pit inflow analysis. The <br />saturated thickness for the overburden was determined by using average yearly water level <br />r-1 <br />L J <br />values from adjacent monitoring wells and a contour map of the top of the Dakota coal. <br />The Dakota coal's saturated thickness was calculated by using the average coal seam <br />thickness per year (Tables 17-1 and 17-2) <br />Values used for hydraulic conductivity, <br />17-1 Revised 04/11/88 <br />