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RULE 2 - PERMITS <br />area. Thus, it can be estimated that the recharge to the bedrock groundwater system is <br />approximately 0.89 acre -foot per year. The spoil recharge characteristics are expected to <br />be similar to the bedrock units. The spoil discharge is dependent on the hydraulic <br />properties of the downgradient bedrock units and the rate of flow that is much higher than <br />in the bedrock units. The spoil storage is dependent on the clast size and configuration of <br />replaced Seneca II spoil, which is variable. <br />The discharge of groundwater from bedrock units into the alluvium along Grassy and <br />Little Grassy Creeks can be estimated using the equation Q = K• I •A, as described above <br />(Freeze and Cherry, 1979). The hydraulic conductivity of the overburden adjacent to the <br />alluvium is estimated to be 2.5 ft/d„ the average of 0.03 and 5 ft/d shown for interburden <br />in Williams and Clark (1994, Table 4), which is the overburden for the proposed PSCM. <br />The hydraulic gradient in the overburden near the Grassy and Little Grassy Creek valleys <br />averages approximately 0.02, based on the potentiometric surface map shown on Figure <br />2.4.7 -175. The potentiometric surface map shows groundwatwer flow toward both valleys <br />occurring primarily from the east - southeast. The alluvial aquifer cross - sectional area <br />along the length of the valleys is estimated to be approximately 240,000 ft ( 12,000 linear <br />feet of aquifer averaging 20 feet in thickness). Thus, the calculated average groundwater <br />discharge from bedrock to alluvium is: <br />Q =K•I•A <br />= (2.5 ft/d) - (0.02) • (240,000 ft 2 ) <br />= 12,000 ft /d <br />= 62 gpm <br />= 0.14 cubic foot per second <br />The groundwater discharge can be expected to vary seasonally in response to varying rates <br />of precipitation recharge to the shallow bedrock, seasonal changes in groundwater levels <br />in the alluvium and surface water elevations. <br />Modeled water - budget information presented by Robson and Stewart (1990, Table 9) <br />indicates that for the Grassy Creek drainage (including Little Grassy Creek), groundwater <br />discharge to streamflow averages 0.5 cubic foot per second (cfs). Considering the above <br />calculations of the groundwater flow rate in the alluvium of the Grassy and Little Grassy <br />Creek valleys (75 ft /d) and the groundwater discharge rate from bedrock to the alluvium <br />in those valleys (12,000 ft /d), it would appear that most of the groundwater discharge <br />from bedrock to the alluvium is ultimately carried down - valley as surface water <br />streamflow because of the limiting factor of the relatively small hydraulic conductivity of <br />the alluvium. <br />Water quality for the bedrock units, alluvium, and spoils aquifers was determined by <br />collection and analysis of numerous groundwater samples from monitoring wells in the <br />permit area and just to the north (alluvial well SGAL70). A summary of the results is <br />presented in Table 2.04.7 -T3, and a complete listing of the data is presented in Exhibit <br />2.04.7 -E1, Hydrologic Information. Trilinear diagrams illustrating the major -ion <br />components in the groundwater are presented in Figures 2.04.7 48, 2.04.7 49, 2.04.7 410 <br />2.04.7 411 and 2.04.7 412 for the overburden, Wadge Coal, underburden, alluvium, and <br />spoils units, respectively. <br />PSCM Permit App. 2.04 -52 Revision 03/05/10 <br />