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Page 1 <br />COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division <br />Rationale -Page 16 Permit No. CO-0045675 <br />The historical seeps were caused by the Row ofgroundwater from the West Pit and may, in the future, a~'so tie <br />caused by natural variation in the Row of groundwater in the vicinity of the historical seeps. The area of <br />historical seepage will be mondored on a monthly basis for surface seeps and a compliance schedule will be <br />followed in the event that Row of the seeps occurs. <br />2. Description of Discharge Point 002 and Required Practices: (Cont.) <br />The compliance schedule for the historical seeps is warranted because there is a low, but non-zero, <br />probability that there might exist natural hydrologic Rows that result in surface seepage at the historical seeps <br />that would be unrelated to groundwater Bow from the West Pit (as measured by the groundwater level <br />elevation and groundwater potentiomenMc suAace map). <br />The comp/lance schedule is implemented under any of four conditions. The first is when or R fhe average <br />groundwater table elevation of all measured values for a calendar month in wells BF-4 and BF-5 is higher than <br />the measured RRo Seco sfreambed bottom elevation of 8582 ft. amsl. The second condition is when orrf <br />suAace flow is observed in fhe vicinity o(the historic seeps. The third condition is if the average groundwater <br />table elevation of all measured values (or a calendar month measured of M-32 and M-33 is higher than tfie <br />elevation of the measured streambed elevation at that location, which has been defined as 8540 R. amsl. The <br />fourth condition is based on a comparison o(water qualify standards and surface wafer quality results <br />obtained from samples collected from the RRo Seco surface water sampling station RS-2. Each condRion as <br />describe herein has associated wdh it a compliance schedule that initiates evaluation o(other conditions <br />associated with the monitoring system. As such, the monitoring and compliance schedules consfdute a <br />program ensuring that all possible manifestations of discharge associated with discharge point 002 are <br />property described and incorporated in the permit. The specific components of the compliance schedule <br />require a logical progression of actions consistent with the hydrogeology of the site, and respond to the issues <br />of concern for this permR, specificafly the water quality in the RRo Seco. The acb'ons inGude accelerated <br />monitoring, immediate reporting and update, increased visual observation o/fhe north arroyo wall and historic <br />seepage area, and increased pumping and increased water treatment prior to discharge through discharge <br />poinl001. <br />The BMP /or groundwater Bow downgradient from fhe groundwater divide that has established in the Rito <br />Seco alluvial aquifer, consists o(a groundwater capture system. The water management plan consists of <br />pumping two groundwater capture wells (M-32 and M-33) located downgradient of the Rito Seco alluvial <br />groundwater plume. The groundwater table elevation in the groundwater capture wells will be maintained at <br />an elevation of 8540 ft. amsl to prevent the migration of affected groundwater in the RRo Seco alluvial aqui/er. <br />Maintaining a groundwater table elevabion of 8540 ft amsl in the groundwater capture wells M-32 and M-33 will <br />ensure that a groundwater divide will form and be maintained downgradient of the capture wells, which will <br />prevent further downgradient migration of Rile Seco alluvial groundwater from upgradient of the capture wells. <br />The operation o(the groundwater capture wells will allow natural Rushing of the upgradient Rito Seco alluvial <br />aquifer to occur. <br />The constraints on the groundwater pumping rate in the Rito Seco alluvial aquifer are different than fhe <br />constraints on fhe groundwater pumping rate in the backfilled West PR. The hydraulic properties of the RRo <br />Seco alluvium impose an upper limit on the rate o(groundwafer pumping, which under conditions of <br />groundwater pumping, is not the case for fhe backfilled Wes[ Pit. Because otthe natural hydraulic properties <br />o/the RRo Seco alluvial aquifer, the groundwater pumping rate can only be increased to a value that is <br />reflective of the hydraulic properties o/the alluvium. Thus, the nature o/the compliance actions to be taken, <br />and the GmR o1 the groundwater pumping rate, under conditions where the average groundwater table <br />elevation at M-32 and M-33 is above an elevation of 8540 R ams/, are ditTerent than the action that would be <br />taken i( the average groundwater table elevation in the West Pit is above 8582 ft amsl. <br />Based on the hydrologic characterization conducted by the permRee, there is a minimal, but non-zero, <br />possibility that a portion o(fhe groundwater Rushed from the RRo Seco alluvial aquifer might Bow into the Rito <br />Seco. This possibility results tram the sinuous nature o(the incised aroyo channel olthe Rifo Seco. The <br />potential influence ofgroundwater flow on the water quality of the RRo Seco will be assessed using wafer <br />quality monrtodng data collected at surface water monitoring station RS-2. <br />Ouarterty reports of fhe data collected to assess fhe implementa8on of the required best management <br />prec8ces ('BMP reports? will tie submitted to the WQCD as part o(this permR. The quarterty BMP repo~YS /or <br />