REDOX ZONATION AND THE REMEDIATION OF CONTAMINATED GROUND-WATER SYSTEMS

The observed distribution of redox processes, and the hydrologic factors that control redox processes, are important factors in the design of remedial strategies for contaminated ground-water systems. In current engineering practice, natural attenuation mechanisms such as contaminant oxidation/reduction are commonly used in conjunction with contaminant removal/destruction technologies to achieve overall site remediation. Design of such systems requires (1) an accurate delineation of ambient redox processes, and (2) evaluation of how redox processes are affected by contaminant removal/destruction technologies. Field methods for assessing redox processes in ground-water systems, therefore, are a crucial component of modern remedial investigation/feasibility studies (RI/FS). An example of the interplay between naturally occurring redox processes and engineered remedial actions is the Naval Submarine Base Kings Bay site in southern Georgia. The site is an abandoned landfill where chlorinated ethenes were disposed in the 1970s. Initial investigations (1997) showed that sulfate-reducing conditions predominated near the landfill and that relatively high concentrations (5,000 ug/L) of perchloroethene (PCE) rapidly transformed to trichloroethene (TCE), cis-dichloroethene (cis-DCE), and vinyl chloride (VC). However, because the contaminant plume extended to a nearby housing development, further remedial action was required. In 1998, in-situ oxidation by Fenton’s reagent injection was used to remove PCE from the contaminant source area. This treatment lowered PCE concentrations below 50 ug/L, but shifted the redox conditions from sulfate-reducing to Fe(III)-reducing. Because highly chlorinated ethenes such as PCE and TCE are less efficiently degraded under Fe(III)-reducing conditions, the natural attenuation capacity for PCE at this site has been lowered. Conversely, because(VC) is more efficiently degraded under Fe(III)-reducing conditions, the natural attenuation capacity for VC has been enhanced. In both cases, an accurate description of how redox conditions have changed in space and time is an integral part of assessing overall site remediation.