2013-05-01_REVISION - M1978116 (9)

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Response to DRMS Adequacy Re,6ew (2) — Cotter SM -18 Mine Reclamation Plan Amendment Area, Colorado Parts of Dolores, Montrose, and San Miguel Counties lists the Bodot series runoff f'class as "very high ". Please revise the selected curve numbers (CN) to reflect HSG D (CN = 89,,80 - poor, fair), or provide documentation to substantiate the claim of HSG B. The mine sites evaluated by this office were visited on multiple occasions by experienced personnel. During these visits, part of the observations included evaluation of the vegetative cover and the general soils types for future quantification of runoff. These observations were not only performed for the specific mine site, but also for the probable watersheds thought to affect the site. Drainage evaluation and selection of runoff and roughness coefficients is a very subjective process. Our office selected values that tended to err on the conservatively high side relative to the overall runoff. We do not, however, wish to present numbers which, in our experience, produce results so conservative as to create unnecessary expenditures for the operator, or the public in general. Broad -based soil evaluations, such as the one included in the other documents, tend to cover very large areas of a regional analysis. While the Type 23, Bodot description includes cobbly clay loam, and cobbly silty clay, our observations of the specific sites did not find hydraulically "tight" soils. On the contrary, we found the soils to be on the sandy side, appearing to be well- drained. The NRCS description of these soils states that the Drainage Class of Type 23 Bodot soil is well - drained. Soils on the slopes are generally comprised of colluvium derived from sandstone and shale units for which weathering conditions of rainfall, sheet flow, and soil creep have preferentially removed finer -grain clays, leaving coarse, more permeable soils. We were onsite in a variety of conditions, including recent rainfall and snowfall events, and at no time did we observe soils throughout the sites which could be classified as Hydrologic Soils Group "D" (per the original SCS TR -55 classification — "High runoff potential. Soils having a very slow infiltration rate when thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a clay pan or clay layer at or near the surface, and shallow soils over nearly impervious material"). This would be an inaccurate description of the existing soils for the purpose of hydrologic evaluation. Our selection of CN and Manning's "N" coefficients involves looking at a variety and range of possible values found in several reliable and respected resources such as: 3