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3) If a leacha[e occurred, is i[ likely to affect down-gradient users of <br />ground water? <br />4) What criteria should be used [o minimize [he potential for ground water <br />contamination? <br />Potential for a Leacha[e to Occur <br />The recharge to aquifers at the Trapper Mine site is by direct infiltration <br />through the thin soils and by flow losses from the intermittent streams when they <br />are flowing. Most of the infiltrating recharge is from snowmelt, and [he rate of <br />infiltration is probably very slow (Golder Associates, 1976). <br />The upper unit of the Williams Fork Formation receives recharge throughout its <br />area of outcrop on the crest and north slope of the Williams Fork Mountains. The <br />Twentymile Sandstone also receives recharge at its outcrop, but because the out- <br />crop width is so narrow and the slopes are so steep where the outcrops occur on <br />[he south face of the mountains, this recharge contribution is probably very <br />small. Most of the mine-area recharge [o [he Twen[ymile Sandstone probably <br />occurs in the upper elevations of the mountains by downward leakage from the <br />Upper Williams Fork. However, most water within [he Twentymile Sandstone in the <br />area of the Big Bo[[om synclines probably moves into [he area from the east, <br />rather than being derived Erom mine-area recharge. The Lewis Shale is also <br />recharged by infiltration a[ [he surface, but because of its low permeability, <br />its rate of recharge is very slow. <br />Infiltration of surface waters and subsequent groundwater recharge is especially <br />significant to this study. Even if liquids are present within the disposed <br />wastes, infiltrating water is [he means by which any contaminants could be dis- <br />placed or leached. The contaminants might then migrate downward from the waste <br />body, through the unsaturated zone, and into Che groundwater system. The climate <br />of the study area, with low precipitation and high evaporation, suggests minimal <br />groundwater recharge. Wastes disposed of above the groundwater table may effect- <br />ively be hydrologically isolated. <br />4-45 <br />