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• Section 4.3.3). An aerobic treatment step may also help reduce residual organics and total <br />dissolved solids (TDS) in the effluent. <br />4.3.2 In-Situ Biological Treatment <br />In an in situ treatment, liquid organic cazbon would be added through injection wells into the <br />backfilled material in the pit. This organic carbon would be oxidized by naturally occurring <br />microorganisms using a range of electron acceptors (e.g., oxygen, nitrate, femc iron, sulfate). <br />The electron acceptor is chemically reduced as the organic cazbon is oxidized. Electron <br />acceptors that should be found in the West Pit include oxygen (dissolved and gas phase), <br />manganese III and N, nitrate, ferric iron, and sulfate. These electron acceptors will be used in a <br />predictable order, with oxygen consumed first, and sulfate last (Zehnder and Stumm, 1993). <br />An amount of organic cazbon sufficient to reduce all of the oxygen, manganese, nitrate, iron, and <br />the majority of the sulfate would be added to the backfill. The highly pem~eable backfill <br />material should provide a reasonable amount of mixing of the injected organic cazbon during <br />transport away from the injection wells, allowing effective coverage in the backfill with a <br />minimum number of wells. As the chemical conditions in the pit backfill become more reducing, <br />manganese and iron may become more soluble through reductive dissolution reactions. When <br />oxidized iron and manganese have been consumed, sulfate reduction will become the dominant <br />redox process. Metal sulfides (e.g., FeS,) and cazbonates (e.g., MnCO3) will form in conjunction <br />with sulfate reduction. Precipitation of sulfides and carbonates will reduce the concentration of <br />metals and sulfate in the pit backfill. At chemical steady-state, the reduction of sulfate will <br />produce concentrations of sulfate that aze generally less than 100 mg/L. <br />Manganese concentrations, under chemical conditions buffered by sulfate reduction, will <br />probably be controlled by adsorption on FeS (Huerta-Diaz et al., 1998), which is formed by the <br />reaction of sulfide with iron, formation of manganese carbonate, or manganese sulfide. <br />Manganese removal by precipitation of manganese sulfide, under reducing conditions, is <br />observed only after most other metal sulfides have precipitated (Cheong et al., 1998). <br />• <br />Ba~(le M11oun(ain Resources, /rtc. <br />pa/002671reporltlmnrchrpllvd6wVmrtg(march.doc 45 Mnrch 1?. 1999 <br />