2018-01-31_PERMIT FILE - C2009087A (5)

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and quality of any groundwater which may discharge to the surface water system. The following sections characterize groundwater occurrence and conditions as a basis for evaluation of these • potential impacts. Mine Area Hydrogeology, Groundwater Occurrence, and Relationship to Mining Disturbance - The regional groundwater conditions were described in Section 2.04.5. Within the Mine Permit and adjacent areas, groundwater conditions have been studied using data obtained from a series of monitoring wells completed in five water - bearing hydrostratigraphic zones: the underburden beneath the Wadge Coal, the Wadge Coal, the overburden above the Wadge Coal, alluvium along the Little Grassy Creek and Grassy Creek drainages, and mine spoils. The wells were selected after consultation with the CDRMS and further evaluation of existing wells constructed at the Seneca II Mine to monitor groundwater levels and groundwater chemical quality, and for testing to determine the hydraulic properties of the various zones. The locations of the wells are shown on Map 2.04.7 - Ml, Hydrology. A summary of the groundwater monitoring intervals when data was collected and the wells were sampled is presented in Table 2.04.7 -T1, Summary of Groundwater Monitoring. As discussed on a regional basis in Section 2.04.5, groundwater occurrence and movement in the Mine Permit area and adjacent area are generally controlled by the climate, stratigraphy, geologic structure, surface drainage patterns and, to a lesser degree, the area topography and adjacent previous surface coal mining. These conditions are generally illustrated by Figure 2.04.6 -F1 and the geologic cross - sections presented in Exhibit 2.04.6 -E1, Geologic Information — Geologic Cross Sections (confidential). Water well lithologic and completion logs are presented in Exhibit 2.04.7 - E1, Hydrologic Information. • Potentially Affected Groundwater Units and Groundwater Aquifer Characteristics — Groundwater potentially related to the proposed mining activities occurs in the five hydrostratigraphic units within and adjacent to the PSCM permit area. The two shallow aquifers include the mine spoils aquifer at the adjacent Seneca II Mine and the alluvial aquifer along Little Grassy Creek and Grassy Creek. The three bedrock units include the Wadge coal and the underburden and overburden below and above the coal. The bedrock units form part of the basal Williams Fork aquifer described by Robson and Stewart (1990). Those and other authors of previous studies, as well as monitoring at the nearby Twentymile Mine, indicate that the major regional sandstone aquifers ( Twentymile and Trout Creek aquifers) are hydrologically isolated from the units to be disturbed by mining at the PSCM and are unlikely to be affected by the proposed mining. The isolation is created by the very low hydraulic conductivities (0.00081 ft/d horizontal and 0.00044 ft/d vertical [Robson and Stewart, 1990]) of the intervening shale sequences above and below the Wadge coal and its overburden and underburden. Additional details on the stratigraphy and hydrogeologic characteristics are provided in Section 2.04.5. Recharge to the groundwater system occurs primarily as infiltration of precipitation and snowmelt in the up -dip, upland areas near and beyond the boundaries of the Permit Area (Map 2.04.5 -M1, Regional Geology and Hydrology), as described in Section 2.04.5. Robson and Stewart (1990) reported that recharge varies as a function of precipitation, increasing with increasing precipitation, and that the average recharge rate to bedrock aquifers in the area is 0.31 inch per year. Recharge to the potentially affected units occurs where those units are exposed in the southwest and southeast parts of the permit area. The proposed PSCM is down -dip of and adjacent to backfilled spoils from • the Seneca II Mine. The spoils receive approximately three inches per year of recharge from infiltrating precipitation and snowmelt (Williams and Clark, 1994). PSCM Permit App. 2.04 -35 Revision 12/17/09