2011-12-05_REVISION - C1981012 (2)

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Zach — Rob INTEROFFICE MEMORANDUM TO ZACH TRUJILLO FROM: ROB ZUBER SUBJECT: NEW ELK MINE, C1981012, HYDROLOGY ADEQUACY FOR TR-61 DATE: 12/5/2011 CC: DAN HERNANDEZ Regarding the hydrology related to the SAE in the TR-61 application, I have the following adequacy concerns. 1. A larger scale map should be provided to better define drainage patterns within and adjacent to the proposed SAE area. In particular, it should be shown that the flow is not significantly concentrated upgradient of the silt fence. These sediment control structures are not designed to treat runoff that is concentrated. 2. At the south end of the proposed SAE, the applicant needs to explain the drainage pattern, especially in regard to the natural channel and the railroad bed. Are these features included in the proposed SAE? 3. It is unclear why a berm is required along the north side of the proposed SAE. There is already an embankment at this location for the existing pond, Pond 007A. Have the applicant explain why this berm is needed. 4. It is unclear what happens with drainage in the area west of the proposed SAE and east of ditch D24. Does this area drain to the ditch or should the SAE be extended to the west to include this area? 5. Regarding the SEDCAD input, the applicant should justify the use of the input value of 10 gpm per square foot. This value is at the high end of the range of values recommended in the SEDCAD4 Design Manual and User's Guide. 6. Regarding the SEDCAD output, the peak concentration of the treated runoff is reported as 1.00 ml/1. However, 0.5 mV1 is the daily maximum effluent limitation for a 10 -year, 24 -hour event in the CDPS permit for New Elk Mine. Have the applicant provide one of the following: • Explanation as to why this exceedance is acceptable, • a revised SEDCAD demonstration, or • a redesigned sediment control for the SAE. Finally, we should ask the applicant about the Sewage Package Treatment Plant that is shown within the proposed SAE. Is this a new facility? If so, it will likely require an additional Technical Revision. Let me know if you have any questions and/or comments. Thanks, Zach.