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' , ~ • 1 <br />Mr. Bruce Humphries <br />January 3, 2000 <br />Page 5 <br />Secondly, in response to DMG adequacy letter Comment L, in [he letter of adequacy letter from <br />James Dillie (DMG) to Bill Lyle (BMRI/BMG) dated May 26, 1999, BMRI agreed to return the <br />quality of water at M-11 R to satisfy the established permit condition values. Maimtaining M-11 R <br />as [he point of compliance for the Rito Seco alluvial aquifer is consistent with minimizing the <br />potential effect of historical mining activities on groundwater resources in the vicinity of the <br />West Pit. <br />3.2 Performance Monitoring <br />Performance monitoring of the downgradient hydraulic barrier system will be accomplished <br />within the monitoring plan proposed in TR-026, and in subsequent amendments to tlme <br />performance monitoring plan as outlined in DMG letters of adequacy dated May 26, 1999 and <br />September 15, 1999. The alluvial aquifer performance monitoring wells M-Zl, M-22, M-23, M- <br />24, M-26, and M-31 are all located within or near the affected portion of [he Rito Seco alluvium <br />and the Shewart-CUSUM control charts, as outlined in TR-026, will serve to track the <br />improvement in water quality of the alluvial groundwater system. <br />According to the hydraulic barrier design outlined in Section 2.1, the hydraulic barrier will be <br />established upgradient from the performance monitoring well M-26, which Imas remained <br />unaffected by historic flows from [he West Pit backfill. Thus, there will be a regularly monitored <br />and evaluated sanmpling location downgradient of the Iydraulic barrier tlmat will be established in <br />the Rito Seco alluvium. The water quality monitoring data gathered a[ M-26 will provide an <br />adequate measure of the effectiveness of the hydraulic barrier in eliminating the downgradient <br />migration of Rito Seco alluvial groundwater that has been affected by historic tows from the <br />West Pit backfill. <br />