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Based on the results of the laboratory tests, the specific objectives of the proposed pilot-scale <br />field test aze: <br />1. Demonstrate that sulfate reduction will occur in situ as a result of injection of organic <br />cazbon at a single well within the backfilled pit. <br />2. Demonstrate that sulfate reduction will occur on a time scale that is similar to that seen in <br />laboratory columns (a time frame of a few months). <br />3. Demonstrate that after in situ reduction of sulfate in the backfilled pit, the manganese <br />concentrations can be reduced to acceptable limits. <br />4. Ascertain that there is no permanent degradation of water quality as a consequence of in <br />situ treatment. <br />1.1 Summary of Laboratory Columu Results <br />Three laboratory columns were constructed and operated to remove sulfate by sulfate reduction <br />• and to determine whether or not manganese could be removed. Water from BF-2 was used as the <br />feed solution; this water contained approximately 710 mg(L sulfate and 5.3 mgfL manganese. <br />These columns differed by their material composition or flow chazacteristics. The first column <br />contained gravel and alluvial soils and had I pore volume exchanged every 3 days. The second <br />column contained alluvial soils, and a third column contained waste rock from the West Pit <br />backfill. The second and third columns both had an exchange rate equivalent to 1 pore volume <br />per every 6 months; water samples that were removed from the second and third columns for <br />testing were replaced with fresh BF-2 water. <br />All of the laboratory columns received sufficient organic carbon to reduce 800 mg/L sulfate in <br />the feed solution, or approximately a 1.1 times excess the actual concentration of dissolved <br />sulfate. The experiment was not designed to have an excess of organic cazbon; rather the system <br />was designed to deliver the exact theoretical amount of organic carbon required to reduce the <br />sulfate concentration to 0 mg/L. The excess organic carbon resulted from calculations that were <br />based on earlier analyses of the water, but the actual water used as a feed solution had a lower <br />n <br />U <br />BaBle Mountain Rtsourca Shepherd Miller, Inc <br />Ilgranilclpdrivell002671nportrldmglaCachmenl6.doc 2 June 8, 7999 <br />