Sustainability of reductive dechlorination at chlorinated solvent contaminated sites: Methods to evaluate biodegradable natural organic carbon


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Virginia Tech


Reductive dechlorination is a significant natural attenuation process in chloroethene-contaminated aquifers where organic carbon combined with reducing redox conditions support active dechlorinating microorganisms. At sites where natural organic carbon (NOC) associated with the aquifer matrix provides fermentable organics, the ability to measure the NOC is needed to assess the potential for the long-term sustainability of reductive dechlorination. This study focused on developing a method to measure the potentially bioavailable organic carbon (PBOC) associated with aquifer sediment.

To measure NOC and evaluate its biodegradability, liquid extraction techniques on aquifer sediment were investigated. Single extractions with different extracting solutions showed that extractable organic carbon associated with the sediment ranged from 1-38% of the total organic carbon content (TOCs). Bioassay experiments demonstrated that 30-60% of the extractable organic carbon can be utilized by a microbial consortium. Alternating between 0.1% pyrophosphate and base solutions over multiple extractions increased the rate of removal efficiency and targeted two organic carbon pools. The result of the investigation was a laboratory method to quantify organic carbon from the aquifer matrix in terms of the PBOC. In the second part, the extractable PBOC was shown to biodegrade under anaerobic conditions, to produce H2 at levels necessary to maintain reductive dechlorination, and to support reductive dechlorination in enrichment cultures. For the third part of the research, the difference in extractable organic carbon inside and outside of a chloroethene-contaminated plume was examined through the combination of PBOC laboratory data and field parameters. Supported by ground-water constituent data, the PBOC extraction and bioassay studies showed that less extractable organic carbon was present inside than outside of the chloroethene plume. The final part of the research investigated the distribution of PBOC extractions across six contaminated sites. PBOC extractions were directly correlated to the TOCs, soft carbon content, and level of reductive dechlorination activity at the sites. Based on these correlations, a range for organic carbon potentially available to subsurface microorganisms was proposed where the upper bound consisted of the soft carbon and the lower bound consisted of the PBOC.



Natural Organic Carbon, Reductive Dechlorination, Monitored Natural Attenuation, Potentially Bioavailable Organic Carbon