Schaefer, C., G. Lavorgna, M. Annable, and A. Haluska.
ESTCP Project ER-201428, 167 pp, 2018

In a study of long-term behavior in chlorinated ethene DNAPL source areas following in situ bioaugmentation in heterogeneous media, monitoring was performed up to 3.7 years following active TCE bioremediation using a high-density monitoring network. Soil sampling, passive flux meters, and push-pull tracer testing was performed. Results showed that biogeochemical conditions remained favorable for reductive dechlorination of chlorinated ethenes despite the absence of lactate, lactate fermentation transformation products, or hydrogen. While ethene levels suggested relatively low dechlorination of the parent TCE and daughter products, CSIA showed that the extent of complete dechlorination was much greater than indicated by ethene generation. Results of push-pull tracer testing confirmed that DNAPL remained in a portion of the source area, consistent with soil and groundwater data. Overall study results suggest biological processes have the potential to persist to treat TCE years after cessation of active bioremediation, thereby serving as an important component of remedial design and long-term attenuation. Reliance on ethene generation alone as an indicator of complete dechlorination significantly underestimated the extent of complete dechlorination, as CSIA analysis provided a more reliable estimate, thus highlighting the importance of utilizing isotopic data to determine dechlorination rates in complex systems. Results also emphasized the importance of high-resolution characterization and monitoring in facilitating improved design and performance monitoring. https://www.serdp-estcp.org/content/download/47280/451089/file/ER-201428%20Final%20Report.pdf