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3) Development mining beneath the Big Bottom alluvial valley floor of <br />the Yampa River will not impact [he alluvial valley floor. <br />9) During mining of the 5 and 6 Mines, underground mine discharge from <br />those mines, combined with spoil spring discharge from the Strip Pit, <br />will increase dissolved solids concentration in the Williruns Fork River. <br />During low-flow of the river and assuming worst-case rnine <br />discharges, the concentration will increase by 224 mg/l, from the <br />historical mean of 332 mgJl to 576 mg/1. SAR will increase from the <br />historical mean of 0.44 to 5.05. The Williams Fork River would have a <br />medium salinity hazazd during low flow, and would have a low to <br />moderate sodium hazard at other times. The worst-case: discharges are <br />not expected to be reached based on past discharges; therefore, the <br />medium salinity hazard is not expected to be achieved. Impacts from <br />dissolved solids loading of the Williams Fork during irrigation season <br />would be minimal due to dilution resulting from high river stage. <br />10) After mining ceases, the 5 and 6 Mines will refill with water at about <br />one-half the rate of the inflow rate during mining, and il: may take on <br />the order of 16 years for the mines to completely fill. <br />11) After the 5 and 6 Mines refill, water may seep from the coal subcrop <br />into the Williams Fork alluvium. The seepage would b~~ driven by a <br />maximum pressure developing in the subcrop equal to:t head of 100 ft. <br />above the ground surface. This head would cause maximum seepage of <br />20 gpm. In a worst-case scenazio, the 20 gpm discharge would raise <br />SAR in the Williams Fork River alluvial water from the historical mean <br />of 3.6 to 9.4. The alluvial water is naturally higher in dissolved solids <br />and metals than the mine discharge water; therefore, increased SAR is <br />the only expected impact. <br />l2) The No. 9 Mine will not refill to the surface, but will refill to an <br />equilibrium level between the Twentymile Sandstone and the overlying <br />White Sandstone. <br />13) The No. 9 Mine portal backfill area is too small to generate enough <br />leachate to have a measurable effect on neazby aquifers. <br />Observed Hydroloeic Im~cts <br />The monitoring plan contained in the permit application has been designed <br />to verify the permittee's projected hydrologic impacts of mining. Section <br />2.05 of the permit application includes a description of th~~ observed <br />hydrologic impacts caused by mining at the Eagle Mines.:Each year REC <br />assesses the on-going impacts to the hydrologic system in its annual <br />hydrologic report. Observed impacts are summazized below. <br />23 <br />