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Evaluation of Potential Well Impacts From <br />The Proposed Nix Property Sand and Gravel Operation <br />• Mine Area 5 - 3 feet <br />Evaluation of upgradient impacts to wells resulting from the above elevation differences requires <br />an understanding of the shape of the post-mining groundwater surface upgradient of the pits. An <br />initial assessment of potential impacts is to assume that the maximum change in groundwater <br />levels upgradient of the pit is equivalent to the maximum elevation difference at the upgradient <br />end of the pit. However, the lake leveling effect does not remove groundwater from the alluvial <br />system and, as such, does not lower the regional groundwater surface. Lake leveling only <br />imparts a localized change in the groundwater table contours. <br />The resulting upgradient groundwater surface varies between the groundwater surface of the pit <br />and the pre-mining groundwater surface (see Figure 4). This groundwater surface is concave <br />upwazd, with its steepest gradient neazest the pit and shallowest gradient at its point of tangency <br />with the pre-mining groundwater surface. Its exact configuration is a function of the pre-mining <br />slope of the groundwater surface and the hydraulic properties of the aquifer. <br />WWE considered numerous methods for assessing the configuration of this curvilinear <br />groundwater surface and the likely distance to an area where there would be no effects on the <br />groundwater table. Many methods, including those for determining agricultural drain spacing, <br />ignore aquifer properties such as transmissivity and specific yield. As a result, W WE considered <br />the groundwater surface profile upgradient of the pit to be best represented by the Theis non- <br />equilibrium equation'. -Input values used for determining the drawdown (s) at a given distance (r) <br />from a pumping well (i.e., the gravel pit) include a pumping rate (Q), transmissivity (T), time (t) <br />and a specific yield (Sy). Aquifer properties of 200,000 gpd/ft and 0.2 were used for <br />transmissivity and specific yield, respectively. <br />z <br />'s= Q W(u)whereu=r S <br />4rrT 4Tt <br />011-074.000 Wright Water Engineers, Inc. Page 7 <br />November 2001 <br />