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it ~ L0 INTRODUCTION: PASSIVE WATER TREATMENT APPROACHES <br />This work plan is the result of lab-scale testing of microbial water treatment performed by <br />' Shepherd Miller, Ina (SMI). These tests were run as part of a lazger site investigation of the <br />hydrology and geochemistry of the West Pit at the San Luis Mine. The pilot testing outlined in <br />this work plan will develop the information necessary to implement a passive water treatment <br />system, and to generate the data necessary to demonstrate the effectiveness of the system to <br />regulatory agencies and other interested parties. During the pilot testing SMI will also optimize <br />the engineering aspects of the overall treatment design. <br />Two water treatment approaches were tested in the SMI laboratory study. The first approach <br />involved the use of sulfate-reducing bacteria (SRB) in an anaerobic environment, The second <br />approach utilized microbial biofilms on limestone pebbles in an aerobic environment to remove <br />manganese and residual dissolved sulfide. <br />The proposed on-site pilot-scale testing would test one water treatment approach. This approach <br />would utilize SRB to treat the manganese, sulfate, and TDS within the pit - in situ. To <br />stimulate the activities of naturally occurring SRB the water in the backfill, the West Pit would <br />receive additions of organic carbon through injection wells. The iron-bearing rocks that <br />comprise the backfill in the pit would be expected to react with the dissolved sulfide to <br />precipitate solid metal-bearing compounds. It is also possible that after reducing conditions have <br />been formed and maintained for a long period of time (several months to a few years) that <br />manganese concentrations will decrease. <br />It is possible that within a few years of initial in situ treatment in the West Pit the water quality <br />will have improved sufficiently to allow release of excess pit water by seepage to the Rito Seco, <br />without additional treatment. However, the pilot study must be conducted for several months to <br />evaluate this possibility. <br />• <br />Baarle Mountain Resources Shepherd Miller, /nc <br />llgraniielp-0rive170026TreportldmgloBachmeMb.doc 1 lurte 8, /999 <br />