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4.4.3.4 In-situ Biological Bench-Scale Study <br />• <br />A 4-foot column of pit backfill material comprised of backfill removed during installation of <br />well BF-6 was constructed and flooded with water extracted from well BF-2. Organic cazbon <br />was added in a quantity sufficient to reduce all of the oxygen, manganese, nitrate, iron, and <br />sulfate in the pit material and water. Each week, 0.5 liter of water was removed from the column <br />for monitoring purposes, and replaced with water from BF-2 containing the same ratio of organic <br />cazbon used in the column startup. To facilitate the creation of anaerobic conditions, a small <br />quantity of inocula (50 g) was added from an anaerobic digester. <br />After five weeks of operation, a sample from this column was tested for a full suite of water <br />quality parameters. Table 4-4 shows the quality of the interstitial water in this column as <br />compazed to water used as feed from well BF-2. Sulfate reduction has occurred in this column <br />neazly to completion. However, iron and manganese concentrations remain elevated, indicating <br />that reductive dissolution of these constituents is still occurring in the column. The concentration <br />• of these elements would be expected to decrease as metal sulfides form and the soluble forms of <br />iron and manganese become exhausted. <br />4.4.3.5 Passive Biological Beuc6-Scale Study <br />Two columns were constructed to simulate treatment with an anaerobic upflow cell followed by <br />an aeration cell. A 4-foot column, 6 inches in diameter, was used for construction of the <br />anaerobic cell. The physical matrix in this column consisted of alluvial soils that were collected <br />from the Rito Seco stream bank. An inoculum of sludge from an anaerobic digester was added at <br />the start of the anaerobic column operation. The organic cazbon requirement for the anaerobic <br />column was calculated for complete removal of oxygen, manganese, nitrate, iron, and sulfate <br />based on theoretical reaction stoichiometry (Zehnder and Stumm, 1993), and the chemical <br />composition of groundwater from BF-2. The organic cazbon was added to the columns daily. <br />Water from BF-2 was added to the anaerobic column at a flow rate of 2 mL per minute. Effluent <br />from the anaerobic column overflow was added to the top of the aerobic column. This aerobic <br />column had the same dimensions as the anaerobic column, and the physical matrix was <br />• comprised of pebble limestone and quartz. These materials were inoculated with approximately <br />Barrie Mounmin Resources, /nc. <br />p:1100267IreponslmnrchrptlbZ6wrrmngrmarch.doc 56 Marrfi 22, 1999 <br />