2014-10-31_PERMIT FILE - C1981044A

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Effects of Mine Discharge on Surface Water Quality Water quality data, including physical properties, major and minor ions, etc., has been collected on a quarterly basis since 1981. Monthly data is available since 1981 for pH, conductivity, and temperature. This data was collected upstream and downstream of the mine site on the Williams Fork River. To characterize the effects of mine discharge on the Williams Fork River, a mass balance analysis for mean monthly dissolved solids and SAR was completed as summarized in Exhibit 34, Summary of Dissolved Solids and SAR Data for the Williams Fork River. The following formula was used to calculate the flow - weighted average dissolved solids and SAR values. Ct = (Q1C1 + Q2C2)Qt Q 1 = discharge of Williams Fork at the Upstream Q2 = No. 5 + No. 6 mine discharge Q3 = No. 1 Strip Pit discharge Qt = Total flow C 1 = Mean dissolved solids, calcium, magnesium, or sodium concentration of the Williams Fork River C2 = Weighted concentration of dissolved solids, calcium, magnesium, or sodium concentration of the No. 5 and 7 North Angle discharges, which are a combination of discharges from the Nos. 5 and 6 Mines. C3 = Mean dissolved solids, calcium, magnesium, or sodium concentration of the No. 1 Strip Pit Ct = Flow- weighted dissolved solids, calcium, magnesium, or sodium concentration downstream of the mine site in the Williams Fork River The calculated effect of the mine discharge and Strip Pit discharge is to increase dissolved solids concentrations during a low -flow event from 332 mg/L to 576 mg/L and to increase the SAR from 0.44 to 5.05. Given these results, the addition of these discharges to the Williams Fork River water would result in a low to moderate sodium hazard and maintain a medium salinity hazard during low flow conditions. Since actual discharges have always been less than the maximum predicted discharges, the likelihood of there being any significant impact to the Williams Fork River is minimal. Also, impacts to agriculture would be minimal because the river flow over the entire irrigation season would be higher than the 7 -day, 10 -year low flow. Quality Effects of Postmining Seepage on Williams Fork Alluvial Water After the mines refill, water may move out of the coal sub -crop into the William Fork Alluvium. While there is no indication that the flooded mine workings will be under high heads in the sub -crop areas, significant heads might occur. Recent excavation of the E Seam area near the 5A portal indicates that this area of the mine is not naturally under any hydrostatic head. Therefore, it is expected that the maximum possible head on the No. 5 and No. 6 Mines in the sub -crop areas will be approximately 100 feet above ground surface. Possible seepage rates from the seams into the sub -crop area were calculated assuming that the water would move up- dip from the nearest flooded mine workings in each mine to the associated sub -crop area. The nearest workings for the No. 5 Mine area are 800 feet from the sub -crop, and for the No. 6 Mine area 1,200 feet away. Using Darcy's Law, a permeability of 2.5 feet per day, aquifer thickness of 12 feet, and sub -crop width of 1,000 feet, the potential seepage rates for the E and F Seam sub -crops are estimated to be approximately 19.5 gpm (0.043 cfs) and 13.0 gpm (0.044 cfs), respectively. The potential seepage from the coal seams could affect water quality in the alluvium downstream of the sub -crop areas. There is approximately 5.5 million square feet of alluvium in these areas. Using a recharge rate of three inches per year, approximately 0.044 cfs of fresh water (assumed to be the same quality as the existing alluvial water) is added on an annual basis. This value is based on a lysimeter study performed at the Seneca Mine (Bob Williams, USGS, personal communication). In addition, the William Fork River water recharges the alluvium in the spring. There is an average water level rise in the alluvium each year of approximately two feet. Assuming a specific yield of 10 percent, this converts to approximately 0.035 cfs of water on an annualized basis. TR14 -36 2.05 -45 Revised 03/20/14