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Demonstration of Compliance Climax Molybdenum Company With Regulation 41 Permit M-1977-493 values, conditions are not allowed to deteriorate. The use of the entire data set from these wells considers the variability in observed concentrations over time in determining representative ambient quality. Furthermore, the available information (both water quality data and the operational and reclamation history) indicates that there have been no new or increased sources of groundwater contamination initiated subsequent to that date. In particular,the restart of Climax mining and milling activities in 2012 did not introduce a new or increased source of groundwater contamination in the Arkansas River Basin, as verified by the data trends in ARwell. 3.2.2 Ambient-Based Manganese and pH Values for ARwell Monitoring of ARwell began with four consecutive quarters of data from September 1993 through September 1994. Monitoring was suspended following the initial year of sampling, but resumed again in February 1997, and continued generally on a quarterly basis since then. All data were considered in establishing ambient quality. Compliance with Regulation 41 should therefore be based on consideration of all data collected over the period of record because the entire data set considers the full range of variability over the period of generally improving water quality in establishing an appropriately conservative estimation of ambient groundwater quality as of January 31, 1994. Based on the manganese and pH data available and the evidence that groundwater quality has improved over time, these data are appropriate for use in determining existing ambient quality for those parameters as of January 31, 1994, and are protective against further lowering of groundwater quality. Table 3-1, presents the summary statistics associated with all manganese and pH data from ARwell. For pH, the mean value represents the log transformed average of the hydrogen ion concentration (10-P"). The full data set for this well is included in Appendix B. Table 3.1: Ambient-Based Manganese and pH Calculations for ARwell Manganese, Field pH, Site Number: ARwell Dissolved Standard (mg/L) Units Summary Statistics—All data collected through 3rd Quarter - 2016 Number of Analyses 66 76 Number of Non-Detects 0 0 minimum 0.016 6.0 arithmetic mean 0.210 6.9 maximum 1.320 9.0 standard deviation(a) 0.201 0.48 Mean-/+2a 0.612 6.0 - 7.9 Monitoring data in Appendix B were compared against these values in accordance with Section 5.2.1 of the WQMP; maintaining manganese and pH at these levels would be in compliance with Regulation 41. December 2016 8