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Previous investigations at Travis AFB concluded that groundwater in portions of the Base is contaminated with chlorinated hydrocarbons and petroleum hydrocarbons. The nature and extent of the contamination are described in Remedial Investigation (RI) reports prepared for four operable units (OUs) at Travis AFB. Each OU covers a portion of the Base and encompasses multiple sites. Natural attenuation of contaminated groundwater is a potential remedy considered for the NEWIOU. In the remedy selection process, the Natural Attenuation alternative (Alternative 2) was compared with the No Action alternative (Alternative 1) and the Extraction, Treatment, and Discharge alternative (Alternative 3). In general, sites considered favorable for natural attenuation are those for which "active" remediation (i.e., extraction, treatment, and discharge) may not be cost-effective because of the relatively low concentrations of contaminants, strong sorption of pollutants on soils, and the potential for natural processes to remediate the contaminant plume in a reasonable amount of time while being fully protective of human health and the environment. The NEWIOU Groundwater WOD selected natural attenuation as the interim remedy for one site (LF006) and identified two sites and a portion of five other sites where the selection of the interim remedy has been deferred, but where natural attenuation will be assessed further. More details regarding the relationship of these sites to the Natural Attenuation alternative are provided in the FS, Proposed Plan, and Groundwater IROD. While the nature and extent of contamination at these sites are characterized in the RI reports, data are lacking with respect to natural attenuation. The understanding of natural attenuation has only recently developed to the point where a systematic assessment of the complex processes involved is possible.
This work explores two innovative technologies for the remediation of chlorinated ethene solvents contaminating groundwater: (1) groundwater circulation wells (GCWs) with downwell zero-valent metal reductive dechlorination reactors, and (2) constructed vertical subsurface flow wetlands. Both the natural dechlorination in wetland sediments, and the engineered dechlorination in a well using zero-valent metals have major implications for the treatment of Air Force pollutants, with the potential to save millions of dollars annually in long term remediation at hundreds of sites across the Air Force. Complementary modeling and column studies examined the potential for controlling and treating groundwater contamination using groundwater circulation wells (GCWs) with downwell zero-valent metal reductive dechlorination reactors. The construction of the field scale wetland research facility includes two complete wetland cells (140 x 60 feet each), fully contained. Chemical analysis of samples drawn from the various depths of wetland sediment suggests a very heterogeneous development of microbial activity relevant to reductive dechlorination over the course of one year of operation. Concentration contours of PCE, TCE, and nitrate suggest that reductive dechlorination is taking place when more readily reducible electron acceptors (like nitrate) are depleted. PCE is reduced ten-fold from inflow to outflow, even with significant short-circuiting of flow from the bottom sediments to the outflow. Data suggests 100-fold treatment is possible.