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R 0 <br />December 2013 Mine Groundwater Quality Assessment ♦ 2 <br />Beginning in the third quarter 2012, MNGW -1 has been sampled triannually per <br />Henderson's GWMP. As documented in the 2012 Annual Water Monitoring Report <br />(Henderson, 2013a), a pH deviation measuring 6.4 standard units (s.u.) was observed (NPL <br />of 6.5 - 8.5) at MNGW -1 during the December 2012 monitoring event. This appeared to be <br />attributable to normal seasonal fluctuations /low flow winter conditions and consistent with <br />historical data that demonstrated a relative increase in metals /nutrients and decrease in pH <br />during low flow conditions and a reverse trend during spring and summer high flow <br />conditions. As stated in the Annual Monitoring Report, this seasonality would continue to be <br />assessed as future monitoring data becomes available. <br />On June 4 and 20, 2013, DRMS was verbally notified that an NPL exceedances for pH had <br />again occurred. A sample collected on May 28, 2013, during the spring runoff period, had a <br />measured pH value of 6.3 s.u. and a follow -up sample on June 19, 2013 also reported a pH <br />of 6.3 s.u. Based on these deviations, Henderson Mine provided written notification (dated <br />July, 11, 2013; Henderson 2013b) to DRMS per Section 5.3 of the GWMP. <br />Henderson also performed a trend analysis on the pH data and the results did not appear to <br />be anomalous, or obvious outliers. Per the GWMP, the monitoring frequency at MNGW -1 <br />for pH was assessed and increased from triannually to monthly in August 2013 to better <br />evaluate pH trends and the underlying water chemistry that may be causing low pH values at <br />this location. <br />1.2 OBJECTIVES <br />This Technical Memorandum fulfills the commitment of providing the results of the <br />additional sampling at MNGW -1 and summarizes both these recent and historical water <br />quality data in order to evaluate the potential causes of reduced pH which can then be <br />utilized to guide further monitoring and /or develop a mitigation plan, as appropriate. <br />AJAX Ltd ♦ 28828 Cedar Circle ♦ Evergreen, CO 80439 ♦ (303) 241 -2057 <br />Clear Creek Associates* 6155 E. Indian School Rd. ♦ Scottsdale, AZ 85251 ♦ (480) 659 -7131 <br />