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Mr Kemt Gorham September23, 2002 Page 2 The following is in response to the additional issues outlined in your September 5, 2002, letter. Recently, a storm event filled the sediment pond at the mine rite. Thu event k'kely oceurnd the afternoon of Juy 29, 2002. Some 68 hours later, an August 1, 2002, the pond war observed during an inspection. The water level had receded to the level of the top of the 4-inch riser and the high water mark war estimated at approximately 5 feet above the rarer dunng the peak of the storm inflow. Review of the approved permit application page 958Rk indicates that 5 feet of head above the rarer would take approximately 60-70 hourr to discharge, which is in lone wfth the observation ofactualperfonnance. TJn~r page alto refers to rediment estimates and required storage capacity for the l0year storm event, calculated to be 4.91 ac ftwater and rediment combined). To the best of my knowledge, this recent storm event war not sampled by EFMC, nor have any other storm-related events been sampled Thir it likely a function of the lack ofpersonnel on-rite dunng there events. The CDPS Industrial General Permit requiter monthly visits to check far flow or discharge from the pond outfall and sampling of the discharge if it it acctrning. The Division recognises that the monthly visits are unkke/y to coinczde withpand discharge, especially at a rite where discharge fr infizguent. However, in the absence of actual discharge sampler, the Dlvirion relies on pond performance estimates to ensure that water quality is maintained. Thu information it l~pically Included in the permit application and it developed using accepted hydrologic and engineering prindp/er. The Division war unable to latate any information in the permit application that estimates rediment effluent concentration resulting from a storm event. Quantitative data available in the pernzit application ondygives an estimate of total sediment delawered to the pond and total storage (acTive and passive) volume available in the pond. Given the difficulty in collecting actual discharge samples, the Dlvirion requires that calculations estimatingpand performance during and after a rtomr event are Included in the Ratan Creek permit appb~catian. Given the as-built condition, the Raton Creek sediment pond will discharge during and after a storm event. Using the as-built pond configuration, please submit the necessary calculations estimating the went concentration for settleable solids from the >0 year storm event. Tbir permit material should be submitted under the pending permit renewal no. 4 (RN-04) and in the form ofpermit text, SEDCAD or other documentation, and maps ar required Response: M~8/H has completed the required calculations to demonstrate that the discharge from the sediment pond during a 10-year, 24hour event will meet effluent limitations. The calculations were conducted utilizing the SEDCAD 4 computer model. Attached are the results of the demonstration and revised pages for insertion into the permit. 2. fllro, regarding the rediment pond at the site, the permit application states on page 950 that `modification to the pond will be made to meet the applicable regulations for permanent retention. " The regulatory requirement Ir Rule 4.05.17 Portmining and Rehabilitation of Sedimentation Ponds, Diversions, Impoundments, and Treatment Facilities. Please review this Rule when addressing the issue of pond performance and rediment storage and cleanout.