Denice Nelson

Enhanced Reductive Dechlorination - A Broader Perspective

Enhanced reductive dechlorination of chlorinated compounds has been applied via in situ engineered anaerobic systems in increasing numbers over the past decade. Knowledge gained from these field applications, combined with academic literature, allows us to have a broader understanding of the technology. Microbial communities can be engineered through manipulation of aquifer inputs, creating in-situ reactive zones. Over a short segment of the groundwater flow path, the microbial biomass is shifted to a succession of facultative degraders and fermenters, sulfate reducers, acetogens and methanogens. At low - to-moderate electron donor loading rates, the aquifer microbial continuum is dominated by species that produce C-1 compounds and acetate as the fermentation products. When electron donor loading increases to high levels a significant portion of the electron donor flows into pathways that generate fatty acids. The network of anaerobic metabolic processes is introduced as an electron donor cascade. Bacteria occupying several points in the electron donor metabolic cascade have been shown to reductively dechlorinate halogenated compounds. Biostimulation using late -cascade compounds such as lactate and met hanol bypass much of the productive dechlorinating communities. These observations were developed from the authors’ research and published reports of numerous other field applications and lab research.

Tracers as Tools for Design and Evaluation of Injection-Based In Situ Groundwater Remediation Systems

In situ treatment of contaminants by the direct injection of a reagent solution into the subsurface is an emerging remedial strategy. Three case studies are presented where tracers were used to evaluate various design and operational components at sites treating contaminants in situ. Tracers were used to determine site-specific design parameters including groundwater velocity, effective radius of injected fluids, and required frequency of injections. Additionally, tracers were used to verify hydraulic connectivity between the injection field and down-gradient observation wells, provide information on hydrogeology and solute-transport behavior (i.e., permeability differences, preferential channeling, and fractured flow) within the targeted treatment areas, and in two instances, used in conjunction with reagent delivery for determination of decay half-life coefficients. These case studies demonstrate that tracers can be used as practical tools to aid in designing full-scale systems and evaluating operation and long-term performance of in situ treatment strategies.