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16-SEP-2005 17:07 FROM-Colorado Division of Minerals & Geology +13038328106 T-644 P103/003 F-292 <br />concentration in the West Pit baekfill is statistically indistinguishable f[rom] pre- <br />mining condition!?' <br />The parameters that raise regulatory questions are TDS, fluoride, sulfate, manganese and iron. In the box <br />and whisker plots (figures 7, 8 and 9), it is visually evident that current water quality as it pertains to iron, <br />tanganese, sulfate and TDS, are better today than in the pre?minin0 samples. <br />The plot for fluoride does not provide the evidence, visually, to support the Operator's conclusions. <br />,Rather, the Division, and the operator, resorted to standard statistical techniques to evaluate the <br />distribution of fluoride in the pre- and post-mining periods. The following table (in mg/L) compares the <br />results: <br />Fluoride Mean Std Dev Mean - I std dev Mean + 1 std dev <br />PE -mmin 3.717 0.873 2.844 4.590 <br />Current 4.140 1.067 3.070 5.207 <br />This verifies that the pre-mining and post-mining conditions for fluoride are statistically indistinguishable, <br />based on an assessment of the distribution, and assuming the data are normally distributed. Given the <br />tight range of the data in both the pre- and post-mining data sets, and the strong overlap at the one <br />standard deviation level, there is little statistical indication for a shift in the distribution of fluoride <br />concentrations. <br />Based on the groundwater regulations, and the conclusion that ambient groundwater conditions have been <br />met, the Division has no basis to require mitigation of groundwater quality. <br />When the West Pit was backfilled, acid-generating backfiII was placed in the pit first then covered with a <br />variable thickness of non-acid generating sediments from the Rito Seco alluvium and/or Santa Fe <br />Formation. Soluble acid salts that formed on the waste rock when exposed above the water table went <br />back into solution upon saturation. In time, provided this rock remains saturated, those salts were <br />removed by incoming groundwater, and such groundwater was treated. Most of the acid-generating <br />backfill was saturated by incoming groundwater and therefore has been rinsed, treated, and discharged. <br />Similar backfill above the maintained water level has not been rinsed as it has not been re-saturated. This <br />rock will continue to form acid salts at a rate commensurate with its exposure to air and water. <br />To avoid seepage from the West Pit to the Rito Seco, the Operator has maintained a water level in the pit <br />that is at or below the Rito Seco level. It is presumed, based on an assessment of the backfill mineralogy <br />submitted in earlier reports, that backfill for some several feet above the water level that has not been <br />rinsed, will need to remain unsaturated, if current West Pit water quality is to abide. For this reason, it is <br />prudent to develop a system to maintain the current West Pit water level for the far future to preclude <br />seepage to the Rito Seco and to maintain current West Pit water quality. The proposed transmission of <br />West Pit water to the farm west of the mine provides a reasonable solution. <br />Cc: Allen Sorenson <br />Jim Finley, Telesto <br />Bill Lyle, Phelps Dodge <br />Cheryl Linden, AGO