Management of sites that are impacted by chlorinated solvent dense nonaqueous phase liquids (DNAPLs) is a major challenge for the Department of Defense (DoD). There are many DoD facilities that have DNAPL source areas present in unconsolidated aquifers. Recently, several laboratory and field demonstrations have indicated that bioaugmentation is a viable remedial option for DNAPL source areas. The intensive DNAPL characterization and bioaugmentation field demonstration previously performed at Alameda Point, California, in cooperation with the Strategic Environmental Research and Development Program (SERDP) Project ER-1613, which utilized many advanced tools for assessing a DNAPL source area in overburden, provided an excellent opportunity to perform a detailed long-term performance assessment following bioaugmentation. This site was highly characterized and monitored both before and during bioaugmentation and remained instrumented to provide high-resolution groundwater monitoring data for the current demonstration. Performing such an assessment following bioaugmentation of a highly characterized DNAPL source area now provides the DoD with much needed information regarding the long-term effectiveness of bioaugmentation for DNAPL sources, and will facilitate improved design, implementation, monitoring, and management of DNAPL-impacted aquifers.

The overall objective of this project was to perform a long-term performance assessment at a site where bioaugmentation was used to treat a highly characterized overburden DNAPL source area. The long-term treatment impacts with respect to groundwater quality, DNAPL mass, contaminant flux, reductive dechlorination, geochemistry, and microbial structure were assessed.  Specific objectives, which were all met during the demonstration, included (1) assess the effectiveness of DNAPL mass removal in low permeability materials using pre- and post- bioaugmentation sampling data and partitioning tracer data, (2) assess the long-term dechlorination activity (biotic/abiotic) following active remediation, (3) determine downgradient impacts using groundwater sampling data and contaminant mass flux measurements, and (4) identify characterization and monitoring tools that were most critical for designing and assessing treatment.

This demonstration took advantage of the highly characterized DNAPL demonstration site at Alameda to perform a long-term (starting at two years following cessation of active treatment and extending through almost four years post-treatment) detailed assessment of bioaugmentation performance. With the source area and downgradient Multi-Level Sampling (MLS) well transects still in place, there was a unique opportunity to perform an intensive long-term post treatment evaluation to assess contaminant rebound, extended treatment due to exogenous biomass decay, contaminant flux, geochemical conditions, and microbial communities within and downgradient of the source area. Three rounds of groundwater monitoring from up to 106 monitoring locations were performed during the 2-year monitoring period of this project, thereby allowing for observation in long-term trends following treatment with respect to dechlorination rates, geochemistry, groundwater quality, and microbial community. This assessment included monitoring at wells immediately downgradient of the treated DNAPL source area, which was used to assess treatment, biogeochemical impacts, Dehalococcoides (DHC) migration, and microbial community shifts on the near downgradient plume long term.

In addition, using the existing well network, Passive Flux Meters (PFMs) were deployed to measure flux both within and downgradient of the source area. PFM data collected prior to bioaugmentation (as part of SERDP Project ER-1613) was used to provide a direct measure of the contaminant flux reduction resulting from bioaugmentation treatment. Partitioning Tracer Testing (PTT) and soil sampling were also performed and compared to pre-treatment results to determine the extent of DNAPL mass removal, and to further evaluate the relationship between DNAPL mass removal, groundwater quality in high and low permeability zones, and contaminant flux.

Results showed that, despite the absence of lactate, lactate fermentation transformation products, or hydrogen, biogeochemical conditions remained favorable for the reductive dechlorination of chlorinated ethenes. In locations where soil data showed that Trichloroethene (TCE) DNAPL sources persisted, local contaminant rebound was observed in groundwater, whereas no rebound or continuous decreases in chlorinated ethenes were observed in locations where DNAPL sources were treated. While ethene levels measured 3.7 years after active treatment suggested relatively low (2 to 30%) dechlorination of the parent TCE and daughter products, compound specific isotope analysis (CSIA) for carbon showed that the extent of complete dechlorination was much greater than indicated by ethene generation, and that the estimated first-order rate constant describing the complete dechlorination of TCE at 3.7 years following active bioremediation was approximately 3.6 yr^-1.

Results of the push-pull tracer testing (using bromide and partitioning tracers) confirmed that DNAPL remained in a portion of the source area. The tracer testing was consistent with the results of the soil and groundwater data and showed that DNAPL removal in one portion of the site had been minimal (compared to another portion of the site where DNAPL had been effectively removed).

Overall, results of this study suggest that biological processes may persist to treat TCE for years after cessation of active bioremediation, thereby serving as an important component of remedial treatment 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 provided a more reliable estimate; this result highlights the importance of utilizing isotopic data to determine dechlorination rates in complex systems. Results of this study also emphasize the need for high-resolution characterization and monitoring to facilitate improved design and performance monitoring (short- and long-term) to optimize resources needed to achieve remedial goals.

PFM data interpretation must be carefully performed, as the Darcy velocity varied spatially and temporally, which had an impact on contaminant flux rates. Not surprisingly, biofouling of the injection wells previously utilized during the bioaugmentation activities was a challenge during this demonstration.

Schaefer, C.E., G.M. Lavorgna, A.A. Haluska, and M.D. Annable. 2018. Long‐Term Impacts on Groundwater and Reductive Dechlorination Following Bioremediation in a Highly Characterized Trichloroethene DNAPL Source Area. Groundwater Monitoring & Remediation, 38(3):65-74.