CHROMIUM-CONTAMINATED SOIL TREATMENT BY IN-SITU STABILIZATION

There is the potential for indigenous bacteria at contaminated sites to transform toxic hexavalent chromium (Cr(VI)) to less toxic trivalent chromium (Cr(III)) and/or precipitate the chromium such that it sorbs to the aquifer solids in situ. In laboratory column experiments with contaminated site soil, the addition of molasses stimulated these processes by indigenous bacteria, such that effluent water from the columns contained less chromium than control columns and the chromium dropped below the drinking water limit of 0.1 mg/L within 4 to 19 days. The presence of various bacterial electron acceptors impacted the removal of chromium, with nitrate resulting in the greatest chromium removal. After selected periods of flushing amended ground water through the columns, removing the amendments from the flushing water did not result in effluent chromium concentrations increasing above the drinking water limit. In addition, batch equilibrium tests with the post treated soil indicated that most of the chromium removal that occurred was irreversibly associated with the soil such that even acid and oxidizing conditions did not extract the majority of the chromium. The results indicate that in situ stabilization by enhanced bioactivity may be a viable remediation method and that risks associated with later re-release of the chromium into the ground water may be minimal.