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Kathy Welt and Christine Johnston Mazch 7, 1997 (DRAFT) Page 6 for MCC to either utilize alternative storage locations or to pump the water to the surface and discharge it to the North Fork in accordance with the current NPDES permit, or to Lone Pine Gulch (to be proposed as a new NPDES outfall). V. BASIS FOR LARGE CAPACITY SUMP USE AND SUMP OPERATIONS Motivating Factors for Sump Use This section explains the many reasons which compelled MCC to utilize the NW Panel sealed sump and to plan on using the NE Panel sealed sump when feasible. Historically, F- and B-Seam combined inflows have been approximately 12 gpm on an average annual basis. On the few occasions that large inflows were experienced, they generally ranged from 50 to 100 gpm and dropped off to low levels (<5 gpm) within a matter of days. In 1996, by contrast, due primarily [o inflows through the B East Mains fault, the average annual groundwater inflow increased to nearly 200 gpm, with a peak day inflow rate of 2,500 gpm and a peak monthly inflow rate of approximately 650 gpm. Initially, it was infeasible to pump the inflows to the surface as quickly as water was entering the mine. Therefore, another sump (NE Tailgate sump) was constructed north of the B East Mains fault to provide temporary storage while sufficient pumping and pipeline capacity was constructed to remove the water from the mine. Even after MCC developed the capability [o pump the unexpected groundwater inflows to the surface, these flows overwhelmed the hydraulic capacity of the treatment processes in the sedimentation ponds. With the unprecedented groundwater inflows in 1996, MCC experienced considerable difficulty complying with the daily effluent limit of 70 mg/L for total suspended solids (TSS). In November 1996, MCC received a letter from the CWQCD which summarized the perniit exceedances in 1996 and which specified that there must be no more permit exceedances attributable to inflows from the fault if MCC was to avoid violation fees of up to $10,000 per day. The only practical way to guarantee that the hydraulic capacity of the system would not be exceeded was to create storage capacity. Because surface storage sites with sufficient volume were not available at the mine, the below-ground use of previously-mined azeas became mandatory. An important factor in the decision to use the mined panels for underground storage is the requirement that MCC must operate the sedimentation ponds to maintain storage capacity for the full runoff volume from the 10-year, 24-hour storm event. Thus, the operational capacity of the ponds to receive mine discharges is less than the physical capacity of the ponds. Compounding the problem of increased intows to the treatment facility was the presence of a different kind of colloid which had not been previously encountered. This colloid did not lend itself to sedimentation utilizing the flocculents/polymers that had historically been effective. Although MCC is now coping with this colloid by applying heavy doses of alum, this is not a practicable long- term solution due to the large volumes of floc produced. Mine water treatment feasibility studies are currently focused on how to best address this particulaz colloid, along with the other range of constituents which comprise TSS in outflows from the mine. This colloid appears to have been picked up in transit to, or while stored in, the NE Tailgate or other sumps. Water quality analysis on samples taken directly from the fault shows the water to be of sufficient quality to meet NPDES discharge standards for the North Fork.