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Letter to J.J. Jones -2- August 27. 1996 baseline which would facilitate development of numeric protection levels at a future time when toxicity of molybdenum and sodium is better understood, or when the Water Quality Control Commission assigns standards for these elements. CMC has stated that inclusion of arsenic, barium, beryllium, chloride, mercury, nickel, antimony and selenium in the monitoring program is unnecessary because these constituents would not be expected in the precambrian bedrock in more than trace amounts. It is the Division's opinion that the potential presence of these constituents in the bedrock, even in trace amounts, is sufficient cause to screen the ground water for their presence. This is because constituents present in trace amounts in the rock can cause water quality problems if the constituents are dissolved, and dissolution of trace elements is more likely to occur in acidic environments caused by weathering of pyrite that is present in the mine, in the waste rock, and in the mill tailing. CMC has stated the opinion that analysis of raw mine water to screen for constituents to be included in the monitoring program is not appropriate because the ore body has a completely different mineralization than the development and cut materials that compose the mine fill. The Division offered the idea of screening mine water to develop a monitoring constituent list as a less expensive alternative to screening water collected from the monitoring wells over a period of several sampling episodes, however, either screening method is acceptable to the Division. The Division continues to believe that screening mine water to determine monitoring parameters is an appropriate approach because: a. The Henderson Mine fill is not the only potential source for ground water contamination at the Henderson Mine. Mine water is impounded, in various stages of treatment, in surface ponds at the mine. Leakage from the ponds is a potential source of ground water contamination. Sludges from water treatment that are stored at the mine are also a potential source of ground water contamination. b. Some percentage of the mine waste fill must certainly consist of development rock taken out of the alteration halo surrounding the ore body. This rock would be mineralized, and would constitute a potential source of ground water contamination. For these potential contaminant sources, a preliminary screening of mine water would be an appropriate method for development of an initial ground water monitoring parameter list. Screening of the mine water would be used only for the purpose of eliminating parameters from the list. Then if monitoring of the wells indicates that other parameters may be eliminated without jeopardizing the integrity of the monitoring program, such eliminations could be made. Also, it is the Division's view that since mill tailing is the primary potential source of ground water contamination at the mill site, screening of tailing seep water is an appropriate method of developing a parameter list for the mill site