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<br />out its area of outcrop on the crest and north slope of the Williams Fork <br />Mountains. The Twentymile Sandstone also receives recharge at its outcrop, <br />but because the outcrop width is so narrow and the slopes are so steep <br />where the outcrops occur on the south face of the mountains, this recharge <br />contribution is probably very small. Most of the recharge to the Twenty- <br />mile Sandstone near the mine probably occurs in the upper elevations of <br />the mountains by downward leakage from the Upper Williams Fork. The Lewis <br />Shale is also recharged by infiltration at the surface, but because of <br />its low permeability, its rate of recharge is very slow. Recharge to the <br />Lewis Shale also occurs where permeable units in the Upper Williams Fork <br />are in contact with thin sandstone units in the Lewis Shale. <br />• Infiltration of surface waters and subsequent ground-water recharge is <br />especially significant to this study. The climate of the study area, <br />with low precipitation and high evaporation, suggests minimal ground-water <br />recharge. <br />In order to quantify this hydrologic variable at the Trapper mine, a <br />ground water recharge study was done. Monthly data on precipitation, <br />infiltration, surface runoff, evapotranspiration and soil moisture <br />storage were compiled and evaluated to estimate recharge. Mass balance <br />calculations were used to estimate monthly ground-water recharge rates. <br />The results of the study as presented in Table 2.7-23 showed that no <br />infiltration will be generated during a normal year at the Trapper Mine. <br />2-529 <br />