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