2009-03-19_PERMIT FILE - C1982057

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• The Trout Creek Sandstone of the Illes Formation lies approximately 300 to 400 feet below the Wadge Coal seam and approximately 60 to 100 feet below the Wolf Creek Coal seam. The Trout Creek sandstone is essentially under confined conditions and is separated from the surface mining by finer grained lithologies of the Williams Fork Formation, which are predominantly gray to black siltstones, limey shales, and silty, fine-grained sandstones. Because of the thickness and low permeability of the strata separating the Trout Creek Sandstone from the mining and the upward hydraulic head below the mined strata, it is unlikely that the mining could have any effect on the water quality of this unit. For these reasons, the current coal mining operations at Seneca II-W are unlikely to significantly impact bedrock groundwater offsite, and that no point of compliance is warranted for bedrock groundwater. Points of Compliance - Alluvium In the southern section, the mine is adjacent to Hubberson Gulch along the southwest (Figure 1). Hubberson Gulch has confluence with Dry Creek, which flows along the western edge of the mine. In the northern section of Seneca II-W, Dry Creek lies to the west and the 009 tributary to Sage Creek lies to the east. It is possible that these valleys receive direct runoff or groundwater inflow from the mine operation. The 009 and 015 tributaries are both in the Sage Creek basin. In both cases, mining has occurred in only a small portion of the ridgelines. In both basins, the bedrock dips to the west, while these drainages drain to the east. As a result, any spoils water generated at the ridge tops would drain along the bedrock dip, to the west and away from these tributaries. Seneca's NPDES permit agrees with this position, as these two basins are designated `stormwater only', and do not contain any spoil water. As a result, these alluvial materials do not justify groundwater compliance wells. The largest influence to alluvial water quality was predicted in Dry Creek. Even here, the downgradient extent of influence is expected to be limited due to the low hydraulic conductivity of the alluvium and additional downgradient dilution and geochemical attenuation (discussed previously). Nonetheless, the alluvial material has the highest hydraulic conductivity of the materials downgradient of the mine site and would be most indicative of any offsite impacts. Therefore, monitoring well DCAL-02 will be designated a compliance well to monitor for any impact to the Dry Creek alluvium. Historical and current water quality of monitoring well DCAL-02 can be seen in Table 1. The well is currently showing exceedances of the dissolved manganese and dissolved iron agricultural and domestic use standards. Although there was no background (i.e., premining) or ambient (i.e., before 1/31/94) sampling conducted directly on the Dry Creek alluvium, there was sampling conducted on Hubberson Gulch, which is located adjacent to Seneca II-W upstream of Dry Creek and has been affected by mining prior to 1/31/94; and Watering Trough Gulch, which is upstream of Dry Creek but has not been affected by mining throughout the sampling record. These Hubberson and Watering Trough Gulch wells show ambient concentrations higher than the Regulation 41 Domestic Use and Agricultural Use Standards for the certain constituents described below. Table 2 provides a statistical summary of ambient water quality data for all these alluvial wells. • The ambient cutoff date chosen was 1/31/94, as specified in Reg. 41. The maximum observed values on TR-63 7-3b-2 Revised 3/09