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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale -Page 12 Permit No. CO-00.95675 Vl. A, 2. Description of Discharge Poin! 002: (Cons,) groundwater from the West Pit x~as restricted to the area of the window. Thus, sufficient lowering of the groundx~ater level in the West Pit results in a condition where the groundwater level elevation in the West Pit would be lower than the elevation of surface water in the Rito Seco adjacent to the window. Therefore, the flow of groundwater in the immediate vicinity of the West Pit would flow toward and into the pit. Currently, the elevation of the groundwater level in the backf~lled West Pit has been lowered sufficiently to reverse the direction ojgroundwater flow in Ure vicinity ojthe x3ndow. The decrease in the elevation ojtlre groundwater level xdll be maintained as a compliance condition throughout the tune period of the CDPS permit by pumping the West Pit backf~lt groundx~ater. The seepage front is subdivided according ro two components of groundx~ater flow that jonn as a consequence of lowering the groundwater level elevation in the West Pit: (1) the seepage from flow that resulted historically in the jonnatian ojsurface seeps along the arroyo sidew•all ojthe Ruo Seco directly adjaceru to the window and (2) dre seepage front flow within the Rito Seco alluvial aquifer that exists as a zone of affected groundx~ater that flows along the naturally occurring ltydroulic gradient parallel to sub-parallel to the Rito Seco. The hydraulic gradient reversal causes the jonnation of a groundwater divide in dre Rito Seco alluvial aquifer downgradient Jran the window. The groundwater divide effectively severs the groundwater flow connection between the West Pit and the Rito Seco alluvia! aquifer. Cround++~ater flow upgradient of the groandx~ater divide is toward the West Pit and groundwater flow downgradient of tare groundwater divide is along the natural westerly hydrologic gradient. Each manifestation of the seepage front will be controlled through the combirwtion oja BMP and compliance schedule (for seepage front flox• that could emanate as surface seeps) or a BMP (for downgradient migration of the seepage front in the Rito Seco alluvial aquifer). For the purposes of regulating the flow and quality of rater in that portion of the seepage front that historically contributed to the jonnation of surface seeps, the eln•ation of the groundx~ater table, as measured on a weekly basis in the West Pit backfill extraction x~ell BF-4, xill be used to evaluate compliance xdth the following condition: The historical seeps are parr of this component ojthe seepage jrontJlow and, due to natural variation in the flow of groundwater in the vicinity of the historical seeps, a compliance schedule will be followed in the event that flow at the seeps occurs. The compliance schedule for the historical seeps is warranted because there is a low, but non-zero, probability that there rnig/u exist hydrologic flows that result in active surface seepage at the historical seeps that x~ould be unrelated to groundx~ater flow from the Wesr Pit (as measured by the ground++•ater level elevation). 77re BMP for the second carnponent of groundwater flow, drat is, groundwater flow downgradient from the groundwater divide that has established in the Rito Seco alluvial aquifer, consists oja groundwa[er capture system. The water management plan consists of pumping two graurulx•ater capture wells (M-32 and M-33) located downgradient of the Rito Seco alluvia( groundwater plume. The groundwater level elevation in the groundwater capture wells will be maintained at an elevation of fl. anal !o prevent the migration ojaJfected groundwater in the Rita Seco alluvial aquifer. The operation of the groundwater capture wells xnll allow natural flushing aJthe upgradient Rito Seco alluvial aquifer to occur. Based on Ure hydrologic characterization conducted by the pernritee, there is a minimal, but non-zero, possibility that a portion of the groundwater flushed from the Rito Seco alluvial aquifer might flow into the Rim Seco. This possibility results front the sinuous nature of the incised arroyo channel of the Rito Seco. The potential influence of groundwater flow on the water quality of the Rito Seco will be assessed using rater quality monitoring data. Monthly monitoring data reports will be submitted to the WQCD as pan of this pennir. For purposes of the Colorado Discharge Permit System, the permittee will be required to submit documentation regarding om~ signiflcaru changes m the above described system, as x~ell as the monitoring data as described previously.