Laserfiche WebLink
TAB 17 <br /> PROBABLE HYDROLOGIC CONSEQUENCES <br /> Ground Water <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 been 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 /> Ground Water Inflows to Pits and Drawdowns. <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 /> USGS 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 /> 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). Values used for hydraulic conductivity, <br /> 17-1 Revised 04/11/88 <br />