2010-04-20_REVISION - M1977300 (9)

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Environmental Protection Plan, Schwartzwalder Mine 15-1 • 15. MITIGATION OPTIONS AND CONSTRUCTION SCHEDULE Rule 6.4.20(15) requires that the EPP provide a detailed construction schedule for (1) all facilities designed to contain or transport toxic materials, acid-forming materials, or designated chemicals used in the extractive metallurgical process and (2) all facilities proposed to contain, hold, or for disposal of material that has the potential to cause acid mine drainage. The Schwartzwalder Mine has been reclaimed, and no new facilities are planned for mining activities, mine operations, or extractive metallurgical processes. However, facilities will be constructed for the mitigation of groundwater in the alluvium and fill. Mitigation strategies for the alluvium and fill were proposed by DRMS in a letter dated October 21, 2009. These options and others were considered in a preliminary screening of mitigation alternatives. The mitigation options proposed and considered included the following: (a) convey, transport or divert surface water around or away from toxic-forming material; (b) capture and/or retain surface water run-off; (c) remove the alluvium/fill and replace with inert fill; (d) pump and treat the pore water in the alluvium/fill; (e) isolate the creek from the alluvium/fill; (f) capture and treat the pore water in the alluvium/fill using a funnel-and-gate style permeable reactive barrier (PRB) system; (g) capture and treat the pore water in the alluvium/fill using an engineered wetland; • (h) set up institutional controls to limit human activities (and exposure) at the site. These strategies and others were evaluated in light of EPA's nine clean-up criteria (40 CFR 300.400): (a) Overall protection of human health and the environment, (b) Compliance with applicable or relevant and appropriate requirements, (c) Long-term effectiveness and permanence, (d) Reduction of toxicity, mobility or volume through treatment, (e) Short-term effectiveness, (f) Implementability, (g) Cost, (h) State acceptance, and, (i) Community acceptance. Sustainability and effectiveness were identified as particularly important criteria, based on the remoteness of the site, closure and reclamation status, longevity of constituents, and the potential post-closure uses of the property for recreation, wildlife, and open space. Two mitigation options were selected to be carried forward to engineering studies, feasibility evaluations, and potentially design and construction: an engineered wetland and a permeable reactive barrier (funnel and gate) system. Preliminary details for an engineered wetland and a permeable reactive barrier (funnel • and gate) system are described in the following sections, along with an approximate timeline for engineering studies, evaluation, DRMS review, and construction. Limited source removal will also occur in 2010, as part of the RML termination. The limited source removal is described in Section 15(b)(iii). 4109C.100419 Whetstone Associates