The overarching goal of this project was to develop a more complete understanding of the impacts of in situ remediation technologies on groundwater quality and relevant subsurface processes. This information is needed to realize the full potential of combined remedies, to more effectively treat difficult hazardous waste sites, and to develop ecologically sound long-term site management strategies. The specific objectives of the project were to: (a) identify secondary impacts of two prominent in situ remediation technologies, thermal treatment and anaerobic bioremediation, on long-term groundwater quality; (b) establish methods to predict the extent of both positive and negative post-remediation impacts on groundwater quality; and (c) develop strategies to overcome, or take advantage, of secondary impacts (e.g., pH reduction, release of electron donor) to achieve both immediate and long-term remedial objectives.

The research program was structured around four main tasks that were designed to elucidate the impacts of; (1) thermal remediation on groundwater quality and subsequent biological treatment, (2) electron donor delivery on metal sulfide formation and associated reductions in aquifer permeability, (3) pH reductions on groundwater geochemistry, microbial ecology and activity, (4) project reporting and data transfer. The work was based upon the expertise of the investigators in the areas of thermal treatment (Pennell) and bioremediation (Löffler and Cápiro), and a strong track record of collaborative research to examine complex interactions between biological, physical, and chemical processes, all of which will impact post-remediation groundwater quality. Site specific materials used in the experimental program were selected in consultation with remediation project managers (RPMs) and SERDP & ESTCP personnel.

Task 1: Impacts of Thermal Treatment on Groundwater Geochemistry and Combined Remedies.

Results from this work demonstrated that thermal treatment of soils resulted in electron donors and fermentable substrates (formate, acetate, propionate and butyrate) that were able to support microbial reductive dechlorination of tetrachloroethene (PCE) to ethene. Additionally, in separate column experiments, low temperature heating (35ºC) improved microbial reductive dichlorination performance, resulting in up to 3.5 times greater dechlorination of PCE to ethene compared to a typical groundwater temperature (15ºC). These observed improvements in microbial reductive dechlorination of PCE during low temperature heating of soil columns were greater than previously reported in batch enrichment culture and microcosm studies.

Task 2: Impacts of Metal Sulfide Formation and Dissolution on Aquifer Permeability.

Substantial reductions in permeability were observed due to the formation of iron (II) sulfide (FeS) precipitates that restricted or blocked pore throats. Column studies demonstrated that FeS precipitation can cause permeability reductions of up to 80%, and that this impact is most apparent in soils with high organic carbon content (e.g., 2.4% w/w). In aquifer cell studies, FeS precipitation caused local reductions in permeability, which led to preferential flow. Following re-establishment of oxic conditions, iron oxide formation resulted in further reductions in permeability and changes in preferential flow paths.

Task 3: Impacts of Remedial Strategies on pH and Microbial Reductive Dechlorination.

Dechlorination of PCE to ethene occurred at pH 5.5 in some microcosms, but was not observed in sediment-free enrichment cultures. The findings suggest that certain soils have pH buffering capacity and generate micro-environments within aggregates or near the solid-water interface where microbes may experience different pH than in the bulk liquid phase. Further, the PCE-to-ethene dechlorinating consortium Bio-Dechlor Inoculum™ (BDI) failed to dechlorinate at pH 5.5, but recovered dechlorination activity following pH adjustment to pH 7. The length of low pH exposure (8 to 40 days) affected recovery of dechlorination activity; however, vinyl chloride (VC), rather than ethene was the dechlorination end product even after short-term pH 5.5 exposure, suggesting that VC-dechlorinating Dehalococcoides mccartyi strains are particularly susceptible to low pH inhibition.

The research provides site managers, regulatory officials, consultants, and researchers with relevant new information about the impacts of thermal treatment and biostimulation on groundwater biogeochemistry and quality, and demonstrate the benefits of combined remedies. The project served to identify: (a) key secondary impacts of thermal treatment and biostimulation on groundwater quality, (b) strategies to overcome negative impacts (e.g., pH reduction) and to take advantage of positive impacts (e.g., electron donor release), and (c) guidelines for RPMs to effectively implement remedies to achieve ecologically sound and lasting solutions. A number of important conclusions and recommendations can be made from this work, including:

• Thermally-released substrates should be considered as part of a treatment train with enhanced bioremediation, particularly in low-permeability zones where substrate delivery is challenging. 
• Low-temperature heating may offer a more sustainable remediation approach over traditional energy-intensive thermal treatment by enhancing microbial activity leading to more complete dechlorination to ethane. 
• Local changes in soil permeability and preferential flow could contribute to slower contaminant mass transfer (e.g., dissolution and desorption), which could decrease mass flux and increase source zone longevity. Alternatively, such permeability reductions may increase local residence times, which could facilitate more complete reaction processes, including dechlorination of PCE to ethane. 
• The presence of certain soils may provide protection from low pH conditions (< pH 6) potentially due to soil cation exchange capacity (CEC), and rebound from low pH exposure is possible, although complete recovery of VC to ethene reductive dechlorination may be irreversibly impacted. 
• Members of the Sulfurospirillum genus contribute to PCE dechlorination under low pH condition.