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I~ Since the pre-Cambrian monitoring wells were mostly completed within the pit area, water quality monitoring data were only available from these wells during the pre-mining period. ' however, during the mining phase, two dewatering wells were installed (DW-1 and DW-2), with mining water quality data being obtained from these wells. The water quality data from DW-2 indicates water quality in the pre-Cambrian rock that improves from the mining phase to the post-mining period. Average TDS and sulfate concentrations at DW-2 were 523mg/I and ' 177 mg/1, respectively for the mining phase and 289 mg/L and 93 mg/L, respectively, during the post-mining period (Table A-7 of Appendix A). Bicarbonate alkalinity averaged 280 mg/l, ' which has contributed to the slightly alkaline pH in the pre-Cambrian rock of 7.95 su. 2.5.2 Surface Water Quality BMRI has monitored surface water quality extensively through the life of the mining operation, from the pre-mining period, through the mining phase, to the post-mining period. A select number of parameters have been compiled in a series of tables presented in Appendix ' A of this report. The monitoring results are discussed briefly below. The discussion focuses on the parameters of interest due to the increasing or elevated levels including the major ' cations (calcium, magnesium, potassium, and sodium), anions (bicarbonate, alkalinity, chloride, and sulfate), manganese, and pH. ' Station RS-1 was used to establish background surface water quality conditions, as it is located above all mining-related activities. Total dissolved solids (TDS) concentrations were relatively low at RS-1 throughout the pre-mining and mining periods, averaging 80 mg/1(Table A-1 of Appendix A). The low TDS values at RS-1 are reflected in the low cation and anion ' concentrations, with calcium averaging 14 mg/I, while sulfate concentrations averaged 5.4 mg/I. Bicarbonate alkalinity values at RS-1 were relatively high (67 mgll}, which is reflected ' in the slightly alkaline average pH values at RS-1 of 7.5 standard units (su) (Table A-1 of Appendix A). As would be expected, this upgradient station has shown little dhange during the period of monitoring. The transition of Rito Seco from a mountain stream to an earthen channel, coupled with a reduced stream gradient that increases contact time, resulted in a more mineralized water at the downgradient stations (RS-2 and RS-5). Average pre-mining TDS concentrations at RS-2 ' and RS-5 were 98 mg/1 and 120 mg/1, respectively (Tables A-2 and A-3 of Appendix A). The ' 10 1