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Response to DRMS Adequacy Review (4) — Cotter LP -21 Mine Reclamation Plan Amendment <br />Cotter Corporation (N.S.L.) (Cotter) submits this response to the April 9, 2013 letter from Tim <br />Cazier, Division of Reclamation, Mining and Safety ( "DBMS "), to Glen Williams, Cotter. The <br />DRMS' comments are in italics and Cotter's responses are in bold. <br />LP -21 Mine, File No. M- 1977 -305, Amendment (AM4) Application Adeguacy <br />Review <br />Page ESWMP -4, Section 7.1 - Engineering Approach. The last sentence implies the <br />improvements discussed in the drainage plan are recommended The Operator needs to <br />commit to implementing the improvements in the drainage plan and as modified pursuant <br />to the comments by the DRMS in this and subsequent adequacy reviews. <br />Cotter commits to implement the improvements described in the drainage <br />plan and as revised by subsequent Adequacy Reviews. <br />2. Page ESWMP -S, section7.2. The NOAA Atlas 2, Volume III charts provided in the <br />attachments are illegible due to the small scale. Please state the specific design <br />storm depths used for runoff analyses for both the 10 year and 100 year, 24 -hour design <br />events. <br />The design storm precipitation depths used in the analysis for this site are 1.9 <br />inches and 3.0 inches, respectively, for the 10 -year and 100 -year, 24 -hour <br />events. This information was taken from the NOAA ATLAS 2, Volume III <br />isopluvial charts. The use of these values can be seen in the "Master Network <br />Summary" worksheet enclosed in Attachment #1. <br />3. Page ESWMP -S, section 7.3. <br />a. The second paragraph states the surface soils at the site are considered Hydrologic <br />Soils Group (HSG) B. However, the second paragraph on page ES9MP -3 states <br />the soils trend to a clay loam. Furthermore, satellite images suggest subbasins <br />OFF 10 and OFF 20 contain significant rock outcrops, which is additionally <br />supported by the soil survey map on Figure U3 indicating most of subbasin OFF <br />20 consists of SMU 88 - "Rock Outcrop - Orthents Complex". This evidence <br />suggests HSG "D " might be more representative for runoff analyses for OFF 10 <br />and OFF 20. Please revise the selected curve numbers (CN) to reflect HSG D <br />(CN 89 -poor), or provide documentation to substantiate the claim of HSG R <br />While much of the area above the site, making up the offsite basins, is <br />characterized as SMU 88 (Rock outcrop - Orthents complex), these areas are <br />not made -up simply of exposed rock. In fact, the exposed rock portions of <br />the site are typically in very steep faces which comprise only a small <br />percentage of the overall site acreage. The outcrops themselves are obviously <br />very impermeable regarding stormwater, but the surrounding soil, by USDA <br />definition, consists of gravelly, stony loams in the typical profile (fine sandy <br />