The enhanced anaerobic dechlorination (EAD) process is used for in-situ degradation of various chlorinated organic compounds. Electron donors must be delivered to the targeted treatment area and anaerobic subsurface conditions must be maintained for a period of time to degrade both the soluble and adsorbed contaminants. The most common EAD approaches use batch addition of either small volumes of high strength electron donors such as emulsified oil or solid phase hydrogen release compounds, or large volumes of diluted dissolved donors such as molasses or other carbohydrates. Both approaches typically rely on groundwater transport to carry the additives across the entire EAD targeted area. However, groundwater flow is generally laminar, predominantly horizontal, and soluble electron donors added in batch mode can only be adequately distributed in the subsurface with either high-density point installation or large volume addition, or some balanced combination of both. In addition, both batch approaches often require relatively high groundwater flow velocity to distribute the additives down gradient in reasonable time frames and before the electron donor is fully degraded. These difficult requirements for proper batch donor addition often cause dechlorination to stall midway through the process or have a limited treatment area due to a lack of donor distribution. Proper maintenance of neutral pH is a second important requirement for EAD, and is often not controlled adequately during batch addition approaches. Dehalococcoides, the organisms responsible for breakdown of cis-dichloroethene to vinyl chloride and ethene, are not active at a pH below 6.0-6.3. Batch addition methods provide little recourse to adjust pH without excessively raising pH in the area immediately surrounding the injection location. Continuous groundwater extraction and recirculation approaches to electron donor and pH buffer addition, however, address these issues and can provide faster and more thorough remediation than the batch processes. Groundwater recirculation provides greater donor distribution through increased injection volumes and hydraulic gradients. Alkalinity can be added as needed to counter decreases in pH and conducted in the form of a large scale titration. Bioaugmentation cultures, when needed, can also be added and quickly dispersed throughout the area. Groundwater recirculation systems for EAD typically divide the treatment area into sections and recirculate and amend groundwater as needed within each section depending on the size of the target area and the aquifer conditions. The proper design and implementation of groundwater recirculation for EAD will be presented and concepts reviewed.
Falatko, David M.; Fam, Sami A.; and Pon, George
"Applications and Benefits of Groundwater Recirculation for Electron Donor Delivery and pH-Adjustment During Enhanced Anaerobic Dechlorination,"
Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy: Vol. 16
, Article 8.
Available at: http://scholarworks.umass.edu/soilsproceedings/vol16/iss1/8