Details of the Proposed Action

Transcription

1 Details of the Proposed Action Proposed Action This alternative is based on the proposed Plan of Operations submitted to the Forest Service on June 4, 2010 by Shoshone Silver/Gold Mining Company for the Rescue Mine. The proposed plan would allow Shoshone Silver/Gold Mining Company to conduct mining and milling operations at the Rescue Mine. Figure 1 The Rescue Mine is an underground gold mine located approximately one half mile southeast of Warren, Idaho in Section 11, T22N R6E, Boise Meridian (Figure 1). The project area encompasses approximately 170 acres and is located adjacent to Warren Creek which eventually drains into the Salmon River approximately 15 miles Page 1

2 downstream. The project area includes the Rescue Mine Millsite, waste rock and tailings disposal areas, and temporary access and exploration roads. The mine site is accessed from Forest Road #50340 (Figure 2). Figure 2 The mine site currently has an access road, exploration roads, various ditches and pipelines, three drill pads, a main adit, three abandoned adits, one new adit/portal under construction, a mill building, a makeup water tank, a generator/shop building, various ancillary buildings, a primary settling pond, a secondary overflow pond, a tailings pond, and a stormwater/streamflow infiltration pond (Figure 3). Page 2

3 Figure 3 In addition to reauthorizing continuing use of the existing infrastructure along with the personnel and equipment needed to maintain and operate the existing mine, the proposed action would: Increase the capacity of the existing tailings pond in two phases. Construct a new waste rock dump. Clear a flat pad on which two makeup water tanks would be installed. Fix a leak on the existing primary process water infiltration pond by installing a bentonite seal. Authorize the construction of three new water infiltration basins. Create a process water management system for ore milling. Create a stormwater management system that utilizes overland flow to infiltrate water and/or infiltrates water using a stormwater infiltration pond. Authorize temporary road construction and maintenance necessary to facilitate the proposed activities. Conduct exploratory core drilling and/or trenching. The layout of the mine site under the proposed expansion is shown in Figure 4 and a detailed description of new facilities is provided below. Page 3

4 Figure 4 Tailings Pond The existing tailings pond is full and of insufficient volume to handle the amount of tailings expected to be generated as the mine returns to production over the next few years. Two new tailings ponds would be constructed in phases. Both would be located where the existing tailings pond is located with each phase excavating a consecutively larger footprint. The phase one tailings pond would be constructed by clearing the trees and topsoil for future reclamation and excavating material from the existing pond to create the pond s containment berm. Figure 5 shows an approximation of the phase one configuration. A temporary access road (# ) would be constructed to encircle the pond by creating approximately a ten foot wide crest on the pond berm. The first phase pond would have a capacity of approximately 6,000 cubic yards, it would be lined with 40 mil EPDM liner (or equivalent), and have a lined, armored overflow channel on the northwest end leading to the adjacent stormwater pond. A decant tower and perforated pipe would be installed to capture drainage water from the tailings. This would act as a sump from which to pump recycled tailings water to the makeup water tanks or pond. Page 4

5 Figure 5 The construction of the second phase tailings pond is optional and implementation would depend upon whether sustained future production at the mine requires more tailings storage volume. Construction would consist of building a second pond on top of the first phase pond after it is filled with tailings. Figures 6 and 7 display an approximation of the phase two construction. The phase two pond would cover approximately the same area as the first and extend southwestward from the phase one development. The temporary road around the first phase pond would be abandoned and relocated around the second phase pond. The same type of liner and decant system as described above for phase one would be used in the phase two pond. Page 5

6 Figure 6 Figure 7 Page 6

7 Following the construction of each phase, the outside slopes of the pond berms would be revegetated with native forbs and grasses, a six-foot high cyclone fence would be erected around the perimeter of the tailings pond berm to restrict wildlife access, and the southeast end of the pond would be gently sloped. Final reclamation of the tailings ponds would consist of dewatering, stabilizing with cement (if needed), placing a two-foot thick lift of native fill material over the surface of the tailings, recontouring the outer berm to achieve 2:1 (H:V) slopes, and applying revegetation treatments (seed, fertilizer, mulch, and slash). All tailings would be transported as a slurry from the mill building to the tailings pond via a plastic pipeline. After the solid tailings are deposited in the pond the residual water would be pumped via another pipeline up to the mill makeup water tanks or pond (Figure 4). An alternative tailings disposal method that may be employed would be a sand or paste backfill system that involves partially dewatering the tailings slurry, possibly adding binders such as cement, and pumping it back into the mine to fill voids left by ore and waste rock removal. Waste Rock Dump The existing waste rock dump is expected to be filled when construction of the new decline (a downward sloping mine tunnel) is completed. Further mine development would require additional space to dispose of approximately 4,600 cubic yards of waste rock. The new waste rock dump would be constructed in two phases. Figures 8 through 10 display an approximation of the waste rock dump. The first phase of the dump would consist of two separate lifts of waste rock, each being approximately fifteen feet high. Phase 2 would have roughly three fifteenfoot lifts. Trees, slash, root wads, and topsoil would be retained for future reclamation. Less than 0.1 miles of new temporary road construction (# and # ) would provide access across the bottom and top of the dump. All temporary roads would be insloped with a ditch discharging to a buffered outlet on undisturbed ground on either side of the dump. Dump reclamation would involve recontouring the dump so that all the slopes were at an approximately 2:1 (H:V) angle. Stockpiled trees, slash, root wads, and topsoil would be redistributed on the area. The top of the dump would be backsloped and crowned to direct stormwater away from the regraded dump face and onto undisturbed ground on either side of the dump. All disturbed ground would be revegetated. The operator would reclaim each lift prior to construction of the next. Makeup Water Tanks Two plastic water tanks of up to 5,000 gallons capacity each would be placed in a gently sloping area just off the temporary road above the millsite (Figure 4). A 1,000 square foot area would be bladed flat to accommodate the tanks. The tanks would provide water to the mill via a plastic pipeline. The tanks would receive water pumped up from an underground sump in the mine and/or recycled decant water from the tailings pond or the makeup pond. Page 7

8 Figure 8 Figure 9 Page 8

9 Figure 10 Process Water Infiltration Areas The mill process water would be disposed of by infiltration into the ground. Several locations have been identified on the mine site where process water would be discharged (Figure 4). The existing primary infiltration pond would serve as both the primary mill makeup water pond and the primary infiltration pond. This pond is presently leaking water from a small seep on the outside of the berm lying between the pond and Warren Creek. This leak would be repaired by temporarily dewatering the pond and constructing an impermeable barrier along the streamside perimeter by excavating a slit trench and backfilling it with bentonite. Once the pond has been dewatered, the fine clay material that has built up on the bottom of the pond would be removed and placed on the waste rock dump in order to increase the vertical infiltration rate in the pond. Three smaller infiltration basins ranging from 1,000 to 3,000 square feet in surface area would be constructed to dispose of waste process water (Figure 4). The basins would each be excavated approximately five feet deep with the excavated material bermed up around the perimeter to increase their capacity. Water levels in the three basins would only be allowed to rise to within two feet of the top of the berm. Page 9

10 Process Water Management The primary source of mill water would be the 50 gallons/minute of water that is produced by the underground workings of the mine. The operator currently holds a water right on this source. There would be no discharge of process water to Warren Creek. Approximately 120 gallons/minute of water is required to operate all of the gold recovery equipment in the mill. When milling processes are occuring additional water from makeup water tanks or the makeup water/infiltration pond would be used to make up the approximate 70 gallon/minute shortfall. Stormwater Management There are three primary sources of stormwater runoff at the Rescue mine; the temporary roads, the mill site work pads and waste rock dumps, and flow in an intermittent stream that is diverted around the mine site. Drainage from these sources would either be dispersed overland or directed into infiltration basins. Stormwater that falls on the mill site work pads and existing waste rock dump would run overland to the primary infiltration pond. Temporary roads would have water either directed to the stormwater infiltration basin via ditches or have waterbars that direct flows to fill side of the road. The intermittent stream directly south of the main mine site runs for a couple of months during spring snowmelt and into early summer. It flows directly into the main mine site where it is captured in a catchment basin and piped and/or ditched to the stormwater infiltration basin. After the new tailings pond is constructed, the reconstructed stormwater infiltration pond would have a storage capacity of approximately 90,000 gallons. This may not be sufficient to contain the total streamflow during snowmelt. As such, an approximately 200 foot existing overflow pipe would be capped and perforated or slotted in order to distribute the flow over a larger section of ground for infiltration rather than have a single concentrated discharge at the end of the pipe. Additionally, structures to divert all or part of the stream across the minesite to the makeup water / infiltration pond would be installed. If water from the perforated overflow pipe consolidates into an overland flow, the stream would be diverted to the degree needed to avert overland flow into Warren Creek. Temporary Roads Approximately 0.3 miles of temporary road would be newly constructed. New temporary road construction would be needed in conjunction with the new waste rock dump, both phases of the tailings pond construction, and the exploration drilling (Figure 11). Road maintenance would occur throughout the project area on existing temporary roads. With the exception of temporary road # , maintenance activities would include; but is not limited to, routine blading, the removal of slides/slumps, maintenance/installation of safety berms, maintenance/installation of waterbars, cleaning of culverts, ditch cleaning, and clearing of brush/trees encroaching the roadway. Temporary road # would include the installation of a culvert in addition to other activities described above. Table 1 describes the temporary roads associated with the proposed action. Page 10

11 Figure 11 Page 11

12 Table 1. Proposed Temporary Road Construction and Maintenance Road # Activity Approximate Mileage Maintenance Maintenance Maintenance Maintenance New Construction and Maintenance Maintenance and Culvert Installation New Construction and Maintenance < New Construction and Maintenance < New Construction and Maintenance 0.1 Exploration Projects Five drill pads would be constructed in the project area (Figure 12). Four drill pads would be constructed in or adjacent to temporary roads already present on the landscape. One drill pad would require the construction of approximately 0.1 miles of temporary road (# ) to access the pad. Drill pads would be constructed to an approximate dimension of 15 feet by 30 feet to accommodate the drill rig and associated equipment. Following drilling, erosion control measures (drainage and revegetation) would be implemented as temporary reclamation. When there is no further use planned for a pad in the foreseeable future, it would be reclaimed by restoring the pad and the access road to the original hillslope contour. Trenching could occur anywhere within the cross-hatched area indicated in Figure 12. No new roads would be constructed; a tracked excavator would travel overland to access trenching sites. Trenches would be no more than 12 feet wide, 5 feet deep, and 20 feet long. Rock samples and photographs would be taken of any vein exposures. Trenches would be backfilled and reclaimed (seed, fertilizer, mulch, and slash) in the same operating season as they were opened. Any trees and slash cut to provide overland access for machinery would be scattered back over these routes. Final Reclamation The existing reclamation plan for the Rescue mine would be updated to reflect the additional reclamation tasks associated with the proposed expansion. Removal and disposal of all structures, equipment, hazardous materials and other debris would be required. Earthwork would be done to recontour the tailings ponds and waste rock dumps to reduce slope angles in order to reduce erosion and promote revegetation. All roads would be fully recontoured. All disturbed ground would be revegetated. Final approval and implementation of the Plan of Operations would not occur until a bond has been posted to cover the costs of reclamation. Page 12

13 Figure 12 Page 13