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January 9. 1996 -2- 933 2713 nn5 <br /> Passive treatment of acid drainage developed through contact with mine waste and natural <br /> mineralization is an emerging technology. The concept of passive treatment involves <br /> routing acid drainage to engineered treatment cells where natural biological processes result <br /> in the buffering of pH, sulfate reduction, and metals precipitation. Passive treatment <br /> systems are rarely maintenance free, and generally require monitoring to assure effectiveness <br /> as well as periodic replacement of organic substrate. A general discussion of the specific <br /> application to the Longfellow Koehler mine complex followed by a description of the <br /> technology is presented in the sections below. <br /> Application of Passive Treatment at the Longfellow Koehler Mine Complex <br /> Passive treatment of acid drainage can only be successfully implemented in conjunction with <br /> an effective restoration program to minimize quantity of flow. Due to the relatively long <br /> residence times (5 to 20 days) required to achieve sulfate reduction and metals precipitation, <br /> it is imperative that the flow rates be minimized. In-situ physical and chemical stabilization <br /> of mine wastes through surface reclamation, storm water management, and groundwater <br /> cutoff structures, can greatly reduce infiltration and flow through the waste materials. The <br /> remaining residual flow through the mine wastes can be collected and routed to engineered <br /> cells for treatment. <br /> Passive biological treatment of acid drainage as well as neutral metal bearing mine drainage <br /> has been evaluated using bench scale and pilot scale systems at a variety of sites (Mudder et <br /> al., 1995; Bender and Phillips, 1995). Passive treatment systems if utilized at the <br /> Longfellow Koehler Mine Complex would require the following conditions: <br /> ► Anoxic conditions; <br /> ► A source of sulfate (common in acid waters at the site); <br /> ► A source of readily metabolized carbon (provided by the substrate); <br /> ► An effective flow collection system; and, <br /> ► A design which maximizes flow through and contact time with the <br /> substrate. <br /> Engineered passive biological treatment cells would be used at the Longfellow Koehler Mine <br /> Complex as part of final closure to treat low flow seeps after reclamation and installation of <br /> surface and ground water controls. Seeps would be sampled prior to any construction <br /> activities to verify they are amenable to passive treatment. If amenable, a passive treatment <br /> cell(s) would be excavated below the site of the seep, most likely at the present pond <br /> location. The bottom and sides of the cells would be lined with geomembrane. A gravel <br /> Golder Associates <br />