2007-11-01_PERMIT FILE - C1982056A (3)

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NW Mains Ventilation Shaft and Thickener Undertlow Project (TROT-391 -Drainage and sediment control for the NW Mains Ventilation Shaft site is provided by upgradient diversion and access road ditches, perimeter topsoil berms, a rock containment berm filter along the entire southeast side of the pad, pad topsoil stockpile collection ditches, gravel surfacing, downgradient containment berm, and the combination of sump and rock check-dam at the low point of the berm, and the natural vegetative filter existing between the shaft site and Fish Creek. Under elevated flow conditions, discharges may pass through the sump and rock filter berm and flow into adjacent low- Iying areas. Under these conditions, effective control and treatment will still occur, since the sump and berm will act as sediment traps, the flow path will be extended, and the low-lying areas and vegetative filter will serve to control flows and remove settleable solids. As a relatively small area (approximately 1.8 acres), and given the effectiveness of the drainage and sediment control measures, the additional disturbance that would be involved in constructing a sediment pond is not warranted and off-site sediment contributions can effectively be controlled by these measures under a Small Area Exemption, consistent with applicable provisions of Rule 4.05.2(3)(6). In mid- 2005, TCC experienced an unanticipated discharge of production mine water due to the failure of both primary and back-up secondary pressure-relief valves on the underground system. While the flows associated with this discharge significantly exceeded anticipated runoff flows for the design storm, the drainage control system of the sump, rock check dam, and downgradient vegetative filter functioned together to effectively control and treat discharge flows. The effectiveness of the existing control and treatment measures was demonstrated by the fact that no flow ever reached Fish Creek, and sampling results for the ponded discharge water in the low-lying areas associated with the vegetated filter indicated that the water would meet applicable effluent discharge standards. Under normal conditions, there will be no discharge or mixing of mine water with surface drainage from the site. One of the cased holes, completed in the pad area, is used as a pressure relief/accumulator for the production mine water system. System pressure relief and back-up were designed to prevent any surface dischazge of production mine water, however, a catastrophic failure of both primary and back-up pressure relief valves resulted in a discharge in mid-2005. A manual ball valve was installed at that time to seal the casing and prevent any future discharge. If suitable system pressure relief controls can be installed to assure that no future discharge will occur, the manual valve may be removed or replaced with afail-safe valve that would allow for free movement of air in the pressure relief/accumulator pipe but prevent fluid discharge, at some point in the future. So long as no discharge is anticipated or occurs, the provisions of Rule 4.05.2 are not applicable. Given the limited site area, a "D" soil type (Binco silty clay loam), the drainage and sediment control measures, and a distance of approximately 500 feet between the shaft site and Fish Creek, calculated peak flow and runoff from the site for the 10-yeaz, 24-hour storm event would be approximately 1.65 cfs and 0.13 acre-feet, respectively. Gravel and vegetative filter efficiency was modeled using the SEDCAD4 check dam and grass filter utility. The SEDCAD4 demonstrations are presented in Exhibit 8S. The estimated settleable solids concentration under peak discharge conditions would be approximately 0.42 ml/I. This settleable solids level is consistent with natural sediment levels for runoff from undisturbed lands, given similaz conditions, and would be significantly reduced as the runoff passes through the vegetated area between the shaft site and Fish Creek. 18-Kish[ Ventilation Shaft (TR0444) -Water & Earth Technologies, Inc. was contracted to conduct SEDCAD modeling and prepare design reports for the Bleeder Shaft Pad Small Area Exemption and access road drainage ditch and culvert design. As noted in their report (Exhibit 8T, April 2005), drainage and sediment control for the bleeder shaft pad site is provided by: • Two upgradient diversion ditches (to divert undisturbed runoff around the facility) • Two downgradient drainage ditches (to collect runoff from the disturbed area) • Gravel surfacing of the pad • The use of 20-inch diameter sediment logs (wattles) or silt fencing around the lower portion of the topsoil stockpiles • The use of Curlex single net (Curlex [) erosion control blankets over seeded cut and fill slopes • The combination of a sump, rock filter (2-foot rock berm), and grass filter The combined runoff from the disturbed area will be collected in the downgradient drainage ditches and conveyed first through the sump and graded rock filter, and then through the grass filter prior to discharging through undisturbed natural vegetation between the pad and Fish Creek. As a relatively small area (approximately 4.25 acres), and given the effectiveness of the drainage and sediment control measures, the additional disturbance that would be involved in TR07-60 2.05-84 06/04/07