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Evaluation of Potential Impacts <br />From The Proposed Nix Property Santl and Gravel Operation <br />On Shallow Groundwater Levels, Water Wells and Wetlands <br />3.4 Proposed Mining and Reclamation Plans <br />The mine and reclamation plans were reviewed to assess potential effects from the proposed <br />mining, and post-mining conditions. Several pit configurations have been proposed for the Nix <br />property. The current mine plan is shown on Sheet 2 with the reclamation plan shown on Sheet <br />3. <br />Consideration of potential impacts from wet mining must take into account the leveling of the <br />groundwater table that results from creation of ponds by sand and gravel extraction below the <br />groundwater surface. Lake leveling occurs when, upon excavation, the sloping groundwater <br />table seeks an equilibrium elevation across the exposed pit (see Figure 3). The resulting <br />groundwater surface on the upgradient side of the pit is lowered relative to the pre-mining <br />groundwater elevation and comparably raised on the downgradient side of the pit. The <br />groundwater level in the exposed pit will equilibrate at an elevation representing the point at <br />which the inflow at the upgradient end of the excavation is equivalent to the outflow at the <br />downgradient end. In general, this is the pre-mining groundwater level elevation at the•midpoint <br />of the exposed pit. The steeper the slope of the groundwater table, the Beater the difference <br />between the post-mining ("leveled") and the pre-mining groundwater level at the upgradient and <br />downgradient ends of the lake. This generally benefits groundwater users downgradient of the <br />excavated pit by increasing the saturated thickness of the aquifer. Alternatively, the upgradient <br />groundwater users may be impacted by a reduction in groundwater level (saturated thickness) in <br />close proximity to the mined pit. <br />As a result of lake leveling, WWE recommended modification of the mine pit configurations to <br />minimize the historic groundwater elevation difference across the pit in closest proximity to the <br />upgradient water supply wells. This new pit configuration is shown on Sheet 2. )n addition to <br />the reconfiguration of the pit sizes and orientation, backfilling portions of some mining areas was <br />discussed. Based on spoil material calculations, Owens Bros. revised their reclamation plan to <br />show considerable backfilling of Mine Areas 2 and 3 immediately north of Weld County Road <br />28. This backfilling greatly increases the distance between the post-mining water surface and the <br />011-074.D00 Wright Water Engineers, Inc. Page 7 <br />April 2002 <br />