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Page 1 COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale -Page 15 Permlt No. CO-0045675 2. Descriotion of Discharge Point 002 and Required Practices: (Con Y.) Because there is no discrete discharge point, the definition of 002 includes the area immediately upstream of the historic seeps (which are no longer in existence), and the downgradienl groundwater zone in the Rito Seco alluvium (which is subject to moniodng and remediaflon through a groundwater capture well system (see figure 2of the permit). The permit includes peAormance standards in the form of BMPs, monitoring and reporting requirements and compliance schedules, which, when all of the requirements are taken as a whole, form a reasoned approach to ensuring that the water quality in the Rito Seco will not be adversely affected by any flow from discharge point 002. Precflcally, a discharge to surface water from 002 no longer occurs, due to the implementaton of the water management plan proposed in TR-026. Monftodng to ensure the continued efficacy o(the wafer management plan and compliance schedules which will be required throughout the period o(Mis permit fo be implemented in the case ofunexpected flow from the West Pit or other manttestabons of flow from the zone of affected ground water. The monitorng and compliance schedules set lorth (or discharge point 002 in Part I.A.S describe moniorng and reporting requirements and prescribe appropriate compliance schedules as applied to a system that is hydrogeologicallycompticated but that is understood well enough to measure compliance with the peAormance standards. The changes in this permit relative fo discharge point 002 are made to clarify those actions that BMRI is already obligated to take to protect the water quality in the Rfto Seco. The additional details provided in the compliance schedules are intended to define the time frames within which those actions and responses must occur. The flow of groundwater from the backfilled West Pit will continue to be prevented through the implementation of a conditon requiring that the average groundwater table elevation for wells BF-4 and BF-5 be maintained at or below 8582 ft. ams/. A BMP comprised of establishing a hydraulic gradient reversal between the West Pit and the adjacent Rifo Seco alluvial aquifer as measured by wells BF-4, BF-5, M-16 and M-20, and a compliance schedule for 002 in the event of a cessation o(the hydraulic gradient reversal currenfly in effect, is included in Part I.A.5. o(the permit. A detailed hydrologic characterization of groundwater Bows in the vicinity of the West Pit was conducted as part of the response actions implemented by BMRI/BMG under the OMG permit. Results obtained from the hydrologic characterizabon supported an interpretation that the Bow of groundwater from the West Pit was restricted to the area of the window. Thus, sufficient lowering o(the groundwater level in the West Pit results in a condition where the groundwater level elevation in the West Pit would be lower than the elevation o(suAace water in the Rtto Seco adjacent to the window. Therefore, the Bow ofgroundwater in the immediate vicinity o(fhe West Pit would flow toward and into the pit. Currently, the elevation o(the groundwaterlevel in the backfilled West Pit has been lowered suthciently to reverse the direction ofgroundwater flow in the vicinity of the window. The decrease in the elevation o(the groundwater level will be maintained as a compliance condition throughout the time period of the CDPS permit by pumping and treating the West Pit backflll groundwater. The hydraulic gradient reversal causes the formation of a groundwater divide in the Rito Seco alluvial aquifer downgradienl from the window. The groundwater divide elfecbvely severs the groundwater flow connection between the West Pit and the Rifo Seco alluvial aquifer. Groundwater flow upgredient of the groundwater divide is toward the West Pit and groundwater flow downgradienl of the groundwater divide is along the natural westeriy hydrologic gradient. Should a manifestation o(flow from the West Pit occur, further contamination of the Rito Seco alluvial aquifer will be controlled through the combination o(water level elevation maintenance and a compliance schedule (for Bow from the West Pit that could emanate as suAace seeps) or a BMP (for downgradienl migration o(the plume of affected groundwater in the Rito Seco alluvial aquifer). For the purposes ofregulating the flow and quality of water in that portion of the affected groundwater [hat historically contributed to the formation o/surface seeps, the monthly average elevabon o(the groundwater table, as measured on a weekly basis in West Pit 6ackfill extraction wells BF-4 and BF-5, will be used to evaluate compliance with the following condition: The groundwater table elevation, based on the average of all measured values for each calendar month in the West Pit backfill groundwater moniorng wells BF-4 and BF-5, must be equal to or lower than an elevation of 8582 feet above mean sea level (ft. emsQ.