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TECHNICAL MEMORANDUM <br />To: Bill Lyle <br />Date: January 3, 2004 <br />Page 8 <br />feet above mean sea level (amsl). Part of the objective of backfilling the West Pit was to <br />' place sulfide-bearing waste rock material below the ground water table to limit oxidation, <br />which has been successfully accomplished as indicated by performance monitoring of the <br />West Pit ground water chemistry. <br />Mining that formed the West Pit resulted in a lowering of the landscape and essentially <br />translocated the stratigraphic relationships to lower elevations south of pre-mining <br />locations. That is, the fault gouge, clay layer still exists on the south side of the West Pit <br />and continues to act as an aquitazd. The Santa Fe Formation still lies above the fault <br />gouge, clay layer, and the Rito Seco alluvium still overlies the Santa Fe Formation material <br />adjacent to the current stream location. Thus, the biggest effect of mining, from the <br />standpoint of geologic relationships among the local rock types, was to provide a direct <br />contact between backfill material and the Rito Seco alluvium. Because of the relative <br />magnitudes of hydraulic conductivity among the current materials at the West Pit, ground <br />water flow from the bedrock aquifer is directed preferentially to the southeastern corner of <br />the West Pit where backfill material is in direct contact with the alluvial material. <br />On a local basis, the hydraulic properties of the West Pit backfill material result in a <br />focusing of bedrock ground water flow into the West Pit. Given the total depth of the <br />' West Pit (approximately 140 feet) the current geologic and hydrologic post-mining system <br />is analogous to the pre-mining system in that the majority of ground water flow from the <br />West Pit occurs at shallow depths. Thus, the current West Pit ground water chemistry is <br />used in comparing current and pre-mining conditions. <br />Water Chemistry Comparison <br />Comparisons between the pre-mining and current chemical concentrations for total <br />dissolved solids (TDS), fluoride, sulfate, manganese, and iron are shown in Figures 7, S, <br />and 9. Copper concentrations were also evaluated. These constituents were identified <br />during development of TR-26 performance monitoring program as most characteristic of <br />the geochemical conditions in the West Pit ground water.. During development, review <br />' and final approval of TR-26, a close examination of the West Pit water chemistry was <br />performed to determine the post-mine, backfllled West Pit geochemistry and patential <br />applicable methods which to manage the water. Subsequently, the TR-26 review <br />examined a wide array of potential naturally occurring and mine induced constituents and <br />identified a specific list of constituents which representatively chazacterized the post-mine <br />West Pit geochemistry. These performance constituents include manganese, sulfate, <br />fluoride, copper, iron, total dissolved solids, calcium, temperature, and pH. <br />Figures 7 through 9 aze box and whisker plots of the available water chemistry data. The <br />' box and whisker plot allows a visual representation of the water chemistry data <br />distribution, In the figures shown: (1) the solid dots represent outliers, (2) the whiskers <br />correspond to the 5`" and 95`h percentile, (3) the right and left edge of the "box" correspond <br />I <br />P:Y,rojene~2/2-nexmonHswluisJuanluis4r2ftvrrylryy,remine vnrpis.Aoc <br />I <br />