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Chlorinated solvent contamination of groundwater is a widespread problem at many military and civilian facilities. Traditional cleanup technologies such as air sparging, pump-and-treat methods, and natural attenuation have well-documented limitations for the remediation of chlorinated aliphatic hydrocarbons (CAH). The objectives of this project were to demonstrate the ability of the In-Situ Reactive Zone (IRZ) technology to enhance the rates of remediation of CAHs and to gather information under varying site conditions that could be used to evaluate long-term treatment effectiveness.
Enhanced reductive dechlorination (ERD), an IRZ technology, was used to biologically degrade CAHs under anaerobic conditions. Through subsurface reagent injection, existing aerobic or mildly anoxic aquifers were altered to form highly anaerobic reactive zones using molasses as the source of organic carbon. Molasses is a commonplace reagent, and its injection equipment is simple and economical from an engineering perspective. During this project, the IRZ technology was demonstrated at two Air Force bases (AFB). Based on these results, as well as other field applications by ARCADIS and other practitioners, a joint Environmental Security Technology Certification Program/Air Force Center for Environmental Excellence technical protocol was written.
At the end of the demonstration, CAH concentrations remained above maximum contaminant levels (MCL) at both AFB sites. The initial performance objectives of 80% concentration reductions within 1 year also were not met. However, after 2 years of injections at the Hanscom AFB site in Massachusetts, over 99% removal of trichloroethene (TCE), cis-1,2-dichloroethene (DCE), and vinyl chloride (VC) was observed where the most substantial, consistent doses of substrate were achieved. Substantial degradation of cis-1,2-DCE lagged approximately a year or more behind TCE degradation. Substantially increased levels of methane appeared to predict continuing degradation of CAHs. Perhaps most significantly, concentrations remained at or even below the final treatment levels for at least a year following any injections.
A 27-month treatment program at Vandenberg AFB in California was initially hampered by the low buffering capacity of the aquifer, which caused the pH to drop below desired levels at injection points. This issue was overcome in the last 5 months of treatment with the addition of a sodium bicarbonate buffer to the injectate. The average reduction within the IRZ was on the order of 65% (from approximately 1,000 parts per billion [ppb] to approximately 350 ppb), although up to 85% reductions in TCE were measured in some wells. Similar concentration reductions were being observed nearly a year after the last injection.
In situ treatment offers many advantages over traditional ex situ (i.e., pump-and-treat) systems, including significant savings on capital expenses and remediation time, destruction rather than containment of contaminants, and elimination of the need for groundwater withdrawal, treatment, and discharge. In this project, effective treatment of TCE was demonstrated at sites with differing geologic and biogeochemical conditions. The results also demonstrated that IRZ systems can effectively treat the areas of highest contaminant concentrations, thereby accelerating site closure.
However, this project also demonstrated some potential limitations of IRZ treatment that remedial project managers should be aware of, including (1) long lag times (greater than 1 year) may be encountered before complete dechlorination is measured; (2) substantial time (more than 1 year) may be required for optimizing the system, incurring significant operations and maintenance costs; (3) groundwater chemistry impacts may extend significant distances downgradient of the injection points; and (4) flexible management is needed to fine-tune treatment at a given site. Water "pushes" to expand the radius of influence following injections were found to be helpful in some cases. The results also demonstrated the importance of controlling aquifer pH during reagent injections and the need for careful monitoring to avoid problems and adjust injection times and conditions. Heterogeneous flow and variable flow directions were found to prolong lag times to complete dechlorination. However, lag times at the demonstration sites ultimately were overcome with sufficient reagent loading. (Project Completed - 2006)