2022-07-18_PERMIT FILE - M2022018

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South Hinsdale Response to Objections 12 July 2022 It is our practice, and we believe to be best engineering practice, to ALWAYS have an armored outfall on detention (and retention) basins used for sedimentation and collecting water from any significant area (more than about 50 x 50 feet, if not smaller), in case of significant, low-probability events. In the same way, we encourage our clients to divert uphill water as much as possible so that there is no contact of water with exposed earth or other sources and establish zones or cells within a drainage basin to control surface water as close to contact with "industrial activities"or construction as possible and thereby reduce the size of detention and outfall structures (as well as mud), and maximize evaporation and infiltration. For an activity like this, normally an armored outfall would be based on a three-inch depth at an elevation of 1 foot above the design of the basin (top of freeboard), or more if the 100-year probability event of 24-hour rainfall/snowmelt is greater than 3 inches. The basin of the detention basin itself of course is key to design, as well as the drainage basin area and characteristics. For mining projects, those are not of course fixed values but change as mining proceeds. Such an outfall for this type work would start with compacted earth (subsoil, not topsoil, which must be reserved for reclamation) or(in case of outcrops or cobbly subsoil) CDOT Class V1 material. A liner (PVC or HDPE, 10-30 mil), anchored above the estimated flowline, extending if possible to the floor of the basin, and if possible to the bottom of the bar ditch on the road or just above the OHWM of a perennial stream or observed streambed of an ephemeral stream (not intruding on a wetland). (In this case, the lower edge of the armor would be limited by the property line and NOT extend to the river's edge. We would also have to coordinate with USFS because of the ROW. In addition to staples or other ground anchors, large cobbles would be used to anchor the liner edges and along the edges of the normal flow line. Then additional Class VI or clean 3-inch- minus material would be placed above the liner and shaped to channel flow. if the liner cannot be extended to a streambed or OHWM or bottom of a ditch or swale, additional larger(6-inch+) cobbles would be placed along the lower edge. To protect against suspended sediment discharge (that is, capture sediment), in addition to the detention time of the basin, several methods are available. One or several may be used based on materials and other factors. Straw bales, straw or coin logs, silt fencing, fabric or excelsior blankets (generally placed as L shapes), gravel (3/4-inch minus) rock check dams, and other methods can be used. Final selection is done in the field based on good engineering judgment. County Specific Comment 14): water consumption and use—estimate for water consumption too low. Response: The calculations are not complete, perhaps because we did not specify details. First, although washing is specifically listed as a potential (emphasize potential) processing of pitrun materials, we do not expect much (if any) washing of materials on a regular basis, because the production of washed material is (a) not economical on a small operation like this given site conditions, and (b) not in much demand by the anticipated market in the South End and northern Archuleta County. The cost of renting and operating equipment for quite small quantities of washed material mean that it is unlikely that the break-even cost of such materials is going to be less than obtaining such materials from Arboles, Ignacio, or Durango, even with current fuel prices. And the primary market for washed materials is producing concrete and asphalt. The 10 gallon per ton is NOT the estimate for washed material, but for ALL materials produced at the site. This was calculated by estimating that less than 4% of production "at 5182-22-003 WAST£LIN£, INC. Page 58 of 107