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<br /> <br />1 <br /> <br /> <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />1 <br />7.0 LONG-TERM RESPONSE INVESTIGATION <br />Concurrent with the hydrologic investigations, BMRI will be evaluating potential long-term <br />response actions related to the elevated TDS and sulfate concentrations within the West Pit <br />backfill. The hydrologic investigations may provide additional information for the long-term <br />response action or identify new alternatives, but nonetheless BMRI proposes to proceed with <br />this work concurrently to minimize the time required to develop along-term option. <br />Based upon the analysis presented in Section 3.3 and Appendix B discussing the geochemistry <br />and associated reactions, it appears that the source of the elevated TDS and sulfate <br />concentrations contained within the West Pit is associated with the oxidation of the trace <br />sulfides contained within the backfilled material. Since the backfilled materials have been <br />inundated by the recovering water table, this oxidation and secondary mineralization process <br />has ceased. Therefore, the secondary mineralizatian is expected to flush from the material <br />afrer a number of pore volumes of groundwater have passed through the backfill. <br />A temporary system could be developed to pump water from the backfill material, treat the <br />water to reduce sulfate levels and then re-inject the water back into the backfill. Such a <br />system may even expedite the movement of pore volumes through the system and the <br />associated reduction in the elevated constituents. <br />The final system cannot be developed until the hydrologic characterization work described in <br />Section 5 is completed, but evaluation of several alternatives can begin at this time. BMRI <br />has identified several passive and active treatment technologies which exist to remove sulfate. <br />These technologies use either membrane separation or biological reduction. Water extracted <br />from the pit would be passed through the appropriate active technology with TDS and sulfate <br />concentrations reduced to appropriate levels prior to re-injection. The systems BMRI has <br />currently identified include microfiltration, nanofiltration, and anaerobic biological treatment. <br />BMRI will initiate literature and communication searches of these technologies to identify <br />previous or current applications with similar conditions. Once the workable treatment systems <br />have been identified, further evaluations could include bench-scale testing and on-site pilot <br />testing using actual site ground waters, as appropriate. <br />22 <br />