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• Local water bearing units comprised of discontinuous sandstones and coal beds are present <br />throughout the general area, and the three coal beds (the Lennox, Wadge and Wolf Creek beds) in <br />the lower member of the Williams Fork Formation constitute locally important water bearing units. <br />Robson and Stewart (1990) state that the thin sandstone and coal water bearing units of the lower <br />member appear to function as a single hydrologic unit about 300 feet thick and containing about 50 <br />percent shale. It is confined above by the marine shale of the middle member, and the shale beds <br />within the unit function as leaky confining beds limiting its hydraulic connection to the underlying <br />Trout Creek aquifer. In addition to the basal Williams Fork water bearing unit, even more localized <br />water bearing units have formed in the re- saturated spoil of the reclaimed surface coal mine pits of <br />the Seneca Mine just east of the proposed portal facility area (Williams and Clark, 1994). <br />The Trout Creek aquifer is the lower of the two regional aquifers, generally occurring about 1,000 <br />to 1,100 feet below the top of the Twentymile Sandstone and about 250 feet below the Wadge coal. <br />The Trout Creek aquifer averages about 100 feet thick but can be up to 140 feet thick in the area. <br />Robson and Stewart (1990) indicate that the upper boundary of the aquifer is poorly defined by non - <br />marine mudstones, thin, poorly - developed coals, and silty sandstones, all of which can be classified <br />as confining beds. The confining beds vary in thickness and lateral continuity and thus form a leaky <br />confining layer. Beneath the Trout Creek aquifer is about 300 feet of marine shale that hydraulically <br />isolates it from the underlying formations. <br />Regional Aquifer Hydrologic Characteristics - Hydraulic characteristics of the above aquifers <br />were evaluated by Robson and Stewart (1990) on the basis of 126 data points from field and <br />laboratory tests. They indicate that the geometric mean hydraulic conductivity of the basal Williams <br />• Fork aquifer (0.11 feet per day [ft/d]) is about 10 times greater than that of the Twentymile aquifer <br />(0.014 ft/d) and about 20 times greater than that of the Trout Creek aquifer (0.0051 ft/d). The largest <br />hydraulic conductivity values reported (60 and 500 ft/d) were in the reclaimed spoil at the Seneca II <br />Mine (Williams and Clark, 1994). The smallest mean values of hydraulic conductivity reported by <br />Robson and Stewart (1990) were for the shale beds that form confining layers. The mean horizontal <br />hydraulic conductivity for the shales was 0.00081 ft/d. Williams and Clark (1994) reported data <br />showing vertical hydraulic conductivities less than 10 -6 ft/d for shales in the permit area. Based on <br />aquifer thicknesses, the mean hydraulic conductivity values translate to transmissivities of 10 to 25 <br />square feet per day (ft /d) for the basal Williams Fork aquifer, 0.5 to 0.8 ft /d for the Trout Creek <br />aquifer and, because of the wide variation in thickness of the Twentymile aquifer, a large range of <br />values that average 3.5 ft /d for that aquifer. Transmissivities were not reported for the reclaimed <br />spoil. Porosity values measured on rock samples from outcrops and drill cores (Robson and Stewart, <br />1990) showed averages of 14 percent for the basal Williams Fork aquifer, 12 percent for the <br />Twentymile aquifer and 15 percent for the Trout Creek aquifer. Storage coefficients of 0.0002 to <br />0.001 for the basal Williams Fork aquifer indicated that the aquifer was under confined conditions <br />where tested. Storage coefficients of about 0.1 would be expected for unconfined areas of the <br />aquifers. <br />Recharge to the regional groundwater system occurs mostly in the spring in the higher - altitude parts <br />of the area when snowmelt infiltrates into the aquifers. Recharge from summer thunderstorm <br />precipitation may occur, but most of that water would be lost to evapotranspiration. The area -wide <br />average recharge estimated by Robson and Stewart (1990) was 0.31 inch per year (in/yr). The rate <br />• of recharge to the spoils aquifer is greater than the area -wide average because of the near absence of <br />stratification in the spoils and their greater permeability than the other aquifers. Williams and Clark <br />PSCM Permit App. 2.04 -22 6/15/09 <br />