The overall objective of this project was to demonstrate in situ biogeochemical transformation of chlorinated ethenes in a subsurface bioreactor, and to validate engineering guidance for the process. The importance of this technology to the Department of Defense (DoD) is its potential to overcome limitations of purely biological or chemical remediation systems. It also has the potential to address issues with contaminant rebound via generation of long-lasting reactive minerals in aquifers. Often, the synergistic effects are not recognized and remediation strategies focus solely on either the biological or the chemical mechanisms, often to the detriment of the other mechanism. This project was initiated to understand the potential benefits of a biogeochemical treatment system and the key factors controlling the process.

Biogeochemical transformation is defined as "processes where contaminants are degraded by abiotic reactions with naturally occurring and biogenically formed minerals in the subsurface." These biogenically formed minerals are created as a result of biogeochemical reactions that occur typically under anaerobic iron- and sulfate-reducing conditions. Biogenically formed minerals are particularly important because they are continuously replenished in the subsurface under these redox conditions and have high surface area and reactivity. In situ biogeochemical treatment systems are defined as systems that capitalize on or enhance such natural processes via engineered reaction zones in the subsurface. Examples of this application include permeable reactive barriers (e.g., biowalls), in situ bioreactors, and injection of organic carbon (e.g., soluble electron donors or vegetable oil emulsions) into a contaminated aquifer with the specific purpose of creating biogenically formed minerals. In situ biogeochemical transformation involves biological formation of reactive minerals that can destroy chlorinated solvents such as trichloroethene (TCE) without accumulation of toxic intermediates such as vinyl chloride. Iron sulfides are one class of minerals that have been identified as being reactive with chlorinated solvents.

This project was unable to demonstrate continued biogeochemical treatment to harmless products, and both the biotic and biogeochemical reactors stalled at cis-DCE. Bacteria capable of partial dechlorination outcompeted the reactive minerals in the first section of the reactor, and sulfate was fully reduced to sulfides and precipitated within this first section as well (presumably as the iron sulfides mackinawite and pyrite). However, the results did produce several lessons valuable for future applications of this technology. Specifically, the results and subsequent analysis showed that it is critical to balance the iron and sulfate fluxes, to ensure sulfate reducing conditions throughout the reactor, and to perform site-specific testing. Results also suggested that hematite should be used as the iron source for this process. Finally, spreadsheets were developed that are useful for designing and evaluating future applications of this technology.

Although this demonstration was not successful, the results and analysis indicate that properly designed in situ biogeochemical treatment is applicable for both source areas and plumes, and it can be implemented in a variety of configurations including in situ bioreactors, biobarriers, injection systems, and recirculation systems, and it is potentially applicable to chlorinated ethenes as well as other contaminants of concern. (Project Completion - 2015)