Creation of a subsurface permeable treatment zone for aqueous chromate contamination using in situ redox manipulation

An in situ redox manipulation (ISRM) method for creating a permeable treatm ent zone in the subsurface has been developed at the laboratory bench and i ntermediate scales and deployed at the field scale for reduction/immobiliza tion of chromate contamination. At other sites, the same redox technology i s currently being tested for dechlorination of TCE. The reduced zone is cre ated by injected reagents that reduce iron naturally present in the aquifer sediments from Fe(III) to surface-bound and structural Fe(II) species. Sta ndard ground water wells are used, allowing treatment of contaminants too d eep below the ground surface for conventional trench-and-fill technologies. A proof-of-principle field experiment was conducted in September 1995 at a chromate (hexavalent chromium) contaminated groundwater site an the Hanfor d Site in Washington. The test created a 15 m (similar to 50 feet) diameter cylindrical treatment zone. The three phases of the test consisted of (1) injection of 77,000 L (20,500 gallons) of buffered sodium dithionite soluti on in 17.1 hours, (2) reaction for 18.5 hours, and (3) withdrawal of 375,00 0 L (99,600 gallons) in 83 hours. The withdrawal phase recovered 87% to 90% of the reaction products. Analysis of post-experimental sediment cores ind icated that 60% to 100% of the available reactive iron in the treated zone was reduced. The longevity of the reduced zone is estimated between seven a nd 12 years based on the post-experiment core samples. Three and half years after the field test, the treatment zone remains anoxic, and hexavalent ch romium levels have been reduced from 0.060 mg/L to below detection limits ( 0.008 mg/L). Additionally, no significant permeability changes have been de tected during any phase of the experiment.