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~ Probgb(e Hydrologic Conreguenca - Chimrtcv Rack Mine <br />s <br />Prior to flow from the pit, resaturation of the materials in the pit must occur. The time necessary <br />~ for the resaturation of the backfilled pit can be estimated by utilizing the volume of the pit, the <br />]• infiltration rate, and the porosity of the materials within the pit. The volume of the pit that will <br />be resaturated can be calculated by multiplying the surface area (630,000 ft2) along with the <br />' estimated height that the groundwater surface would rise to afrer resaturation. Assuming that the <br />resaturation would raise the water level to 33 feet above the bottom of the pit (Piteau and <br />Associates, 1982), then the volume of materials (for both pits) that must be resaturated is <br />20,790,000 ft3. Prior to this resaturation occurring, a period of three years would be necessary <br />for significant vegetation growth that would allow the runoff curve to decrease to the point where <br />' significant infiltration will occur. Considering the assumed porosity of the backfill (15%), <br />3,118,500 ft' of water must infiltrate from the surface and from the formation to fill the pit to this <br />level. Inflows calculated in the equations presented above indicate an inflow rate of 14.92 gpm <br />from infiltration due to precipitation. Approximately 10 gpm is estimated to inflow to the pit from <br />underburden resulting in a total of 24.92 gpm of infiltration. Since the volume divided by the rate <br />should equal the time necessary for sufficient inflow, this would result in approximately 1.78 years <br />for pit resaturation. <br />This estimation of the time necessary for the resaturation of the pit appears realistic. Water levels <br />' in the pit Fluctuated from 1985 to 1991 (the last monitoring period) with no apparent trends in the <br />data. This fluctuation without upward trends indicates that ground water is apparently recharged <br />within the pit. <br />PICTURED CLIFFS SANDSTONE IMPACTS <br />A similar evaluation of the probable impacts to the Pictured Cliffs Sandstone cannot be performed. <br />The Pictured Cliffs is separated from coal seam "C" by approximately 35 feet. Between the fill <br />material and the sandstone is a shale, sandstone, and coal sequence. Shales are considered an <br />impermeable material and an associated vertical hydraulic conductivity is not available (Fetter, <br />1981). Impacts to this aquifer are therefore not anticipated. <br />FRUITLAND FORMATION IMPACTS <br />The Fruitland Formation is the coal-bearing unit in the area. This unit has small yields of water <br />from the coal seams. A former groundwater well W-3 screened in the "B" coal seam exhibited <br />poor water quality as indicated in the original permit document. This well had elevated levels of <br />sodium, calcium and sulfate. Comparison of these original results to the results from the well W- <br />16located in the spoils pile of the former east pit are mixed; however, great deviations in quality <br />are not observed. Water quality in the original coal seams was not potable. <br />Two other wells, W-9 and W-10 were also screened in the Fruitland Formation. The well W-9 <br />was considered the downgradient point-of-compliance well in this formation at the site. This well <br />Revised hydrologic-053\OS-23-0(g:00AM) 14 <br />