Operation of In Situ Biological System for Treatment of Hexavalent Chromium at the Selma Pressure Treating Superfund Site

摘要

Groundwater at Selma Pressure Treating Superfund Site is impacted by discharges of hexavalent chromium. The extended plume was treated by the full-scale in situ bioremediation (ISB) recirculation system to convert soluble hexavalent chromium to trivalent. The treatment area is 450 thousands square feet and the impacted zone extended vertically from the top of the groundwater at 45 feet to 120 feet below ground surface (bgs). ISB was accomplished by adding a biodegradable substrate (molasses) to the injected groundwater. Injection well biofouling was counteracted by delivering the substrate in pulses, cleaning injection wells with a hydrogen peroxide solution and compressed air, and by daily injections of disinfectant solution containing chlorine dioxide (biocide solution). In 21 months of continuous around-the-clock operation, 155 million gallons of groundwater was recirculated and an approximately 250 pounds of hexavalent chromium converted. The concentration of hexavalent chromium has decreased from as much as 3,000 micrograms per liter (μg/L) to a less than 50 μg/L (MCL level).