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continues, the backfill water will approach the irrigation water quality. Time periods for this to <br />occur are given later in this section. <br />Spoil Water Infiltration Into Backfill Figure 2.05.6(3)-1 shows how infiltrating water may <br />build up in the spoil north of the Meehan Draw drainage protection pillar (Meehan Draw will not <br />be disturbed by mining) and begin to seep into the backfill. It should be noted that the mining <br />area north of Meehan Draw will be reclaimed as Dryland Pasture and will not be irrigated during <br />reclamation (see Map 2.05.5-1, Post Mining Land Use). Any accumulated spoil water may enter <br />one or more of the minor sandstone beds of the overburden in the backfill. This annual <br />infiltration is calculated below assuming a 10' average backfill height, a hydraulic conductivity <br />of 2.1'/day (from GW -N9) and a hydraulic gradient of 0.053 (based on the water level gradients <br />along the old Peabody Highwall). <br />Seepage into the backfill is equal to: (10' average backfill height) x (1,900' wide seepage area) x <br />(hydraulic conductivity of 2.1'/day) x (hydraulic gradient of 0.053) x (1/43,560 cu. ft. per ac- ft.) <br />x (365 days per year) is equal to 17.7 ac -ft per year, or an average of about 11 gpm. It is strongly <br />believed that spoil water seepage into the backfill will not have any significant impact on water <br />quality, flow rates, well usage, etc. due to four reasons: <br />1) As described in this section, the water quality of the spoil water will be at a maximum 6% <br />to 10% higher in TDS than the existing overburden water quality, which is relatively <br />poor. Regardless of the seepage rates into the sandstone zones, the water quality will be <br />very similar and consistent with the overburden, with a TDS from 2,800 to 3,500 ppm. <br />2) Due to the low seepage rates, the movement is very slow. <br />3) The sandstones above the Dakota coals have historically been too poor in quality and too <br />low in flow rates to provide for useful wells. For this reason, no known wells in the <br />vicinity of the mine area have been completed in the Dakota Formation. It is very <br />unlikely that any new wells will be completed in the Dakota Formation in the vicinity <br />since the Brushy Basin Member of the Morrison Formation provides significantly better <br />water quality and flow rate and is located only 160 feet deeper than the Dakota LDx <br />Seam at NHN permit area. <br />4) The local wells are completed in the Brushy Basin Member of the Morrison Formation, <br />which is below the Dakota coals. Although this zone has significantly better quality <br />water, these sandstones are separated from the spoil by shale layers with very low <br />permeability; therefore, these aquifers cannot be affected by the spoil water. <br />6) Impact of Backfill Water Ouafity on Surface Water Ouality. <br />Spoil Water Quality The chemistry of the water interacting with the spoil is described in the <br />previous section. In order to determine the impact of the spoil water quality on the surface water, <br />it is first necessary to predict the expected quality of the spoil leachate for the NHN Mine. The <br />Section 2.05.6(3) Page 23 April 2016 (PR -01) <br />