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• Est-mine Groundwater Quantity <br />The following discussions address the post-mine recharge characteristics of affected aquifers, aquifer <br />restoration, and impacts to flows from springs. <br />Post-mine Recharge: <br />Radian (1981) conducted an infiltration investigation of the spoils at Trapper and predicted that on balance <br />no percolation or resaturation of the spoil aquifers would occur under normal precipitation conditions. This <br />analysis indicated that recharge to the spoil aquifers will occur only with above normal precipitation <br />conditions. This conclusion was based upon an idealized representation of average monthly precipitation <br />amounts as compared to average potential monthly evapotranspiration. Experience at Trapper has shown <br />that some surface water infiltration and resaturation of the spoil aquifers does in fact occur as evidenced <br />by the documented increasing water levels in the backfill monitoring wells established at Trapper. A <br />discussion of Radian's (1981) water balance calculations is presented in Section 2.7.6. The entire Radian <br />Report (1981), evaluating hydrologic impacts associated with the management of coal combustion <br />byproducts from Craig Station, is available at the Trapper Mine offices. Relevant findings from the report <br />• are further discussed in Section 4.3.4.6. <br />Pre-mine recharge of the groundwater aquifers at Trapper occurs mainly in the outcrop areas of the <br />aquifer units, while recharge to the spoil aquifers is believed to occur over most of the backfilled areas of <br />the various pits. Radian (1981) expected permeabiliGes in the range of 1 x 10'3 to 1 x 10'' cm/sec for the <br />overburden spoils at Trapper. The permeabilities of the backfill aquifers from the four spoil wells <br />established at Trapper to date (wells GD-3, GF-5, GF-7, and GF-11) vary from 1.7 x 10'3 to 1.6 x 10~ <br />cm/sec. Table 2.7-21 b presents the calculated backfill permeabilities. These permeability values are <br />similar to values derived from the Upper Williams Fork aquifers in this area. Based upon these findings, <br />the capacity of the spoil aquifers to recharge from infiltration at the surface should at least equal the <br />combined recharge capacities of the aquifers disturbed by mining. <br />The geologic structures of Johnson, No-Name, and Flume drainages are similar to the Pyeatt structures <br />with beds dipping generally to the north at approximately 9 degrees (see Map M34). Groundwater flow in <br />the Upper Williams Fork aquifers has been shown in Section 2.7 to be controlled by geologic structure. <br />The aquifer units contributing water to the shallow alluvial fill systems north of the Trapper project area do <br />not extend very far south of the shallow alluvial fills before they crop out. Recharge to the Upper Williams <br />fork aquifers is mainly in their outcrop areas. The outcrop areas occur throughout the Trapper permit area <br />• with shallower units tending to outcrop along the northern portions of the permit area and deeper units <br />surfacing as one moves to the south. The northern edge of the mined area in each drainage is typically <br />several thousand feet south of the shallow alluvial aquifer systems identified to the north of the mine <br />permit area. Geologic structure generally controls the groundwater movement in the other drains~it ~~, d~~ <br />Approved:~2~~~ <br />4-236 <br />