In situ bioremediation (ISB) and monitored natural attenuation (MNA) are two widely used approaches to treat and control persistent matrix diffusion sources at chlorinated solvent sites. Such source zones represent a significant liability to the Department of Defense (DoD). Research has suggested that processes may be active at both ISB and MNA sites that could provide additional benefits to their application near or within low-permeability (low-K) matrix diffusion zones. The objectives of this project were: i) to develop new process knowledge on how to measure and demonstrate sustained treatment following application of ISB and ii) to evaluate and quantify MNA processes in low-K matrix diffusion zones.
Sustained treatment is a term used to describe the enhanced attenuation capacity within an ISB treatment zone that can prolong the benefits of ISB treatment after the depletion of the primary organic substrate. Post-ISB sustained treatment could be an important mechanism for controlling chlorinated solvent concentrations and preventing rebound, particularly at sites where matrix diffusion is expected. Low-K MNA is characterized by natural processes such as biodegradation and sorption that serve to control migration of chemicals that have diffused into a low permeability matrix. The extent to which these compounds undergo natural attenuation in the low-K zone is an emerging area of research and quantifying MNA processes in the low-K zone is an integral component for the application of this approach as a management tool. To assess the occurrence and extent of these processes, data mining efforts and field demonstration studies were conducted. The results were analyzed to provide guidance on how to assess and quantify post-ISB sustained treatment and low-K MNA. Fact Sheets were prepared to help guide remedial site managers on the application of these tools at their sites.
Post-ISB Sustained Treatment at Mulch Biowalls: Results demonstrated the ongoing and long-term efficacy of mulch biowalls 10 years after their installation. Given the sustained treatment, augmentation of the biowalls with supplemental substrate does not appear warranted.
Post-ISB Sustained Treatment at Substrate Injection Sites: Parent chlorinated volatile organic compound (CVOC) concentration reductions following ISB at two sites indicated sustained concentration reductions without evidence of rebound 4 to 5 years after treatment. The data mining study, which included 34 sites with long post-treatment monitoring periods, indicated that sustained treatment of parent concentrations is observed at about 75% of sites. These results suggest that a generally well designed and implemented ISB project often will benefit from sustained treatment, at a minimum in terms of rebound suppression for 3 to more than 15 years.
Low-K MNA: A lines-of-evidence approach was established to assess the occurrence of MNA in low-K zones, along with a data mining study to establish “benchmark” decay rates. High-resolution chemical profiling of CVOCs in the aquitard indicated that biodegradation daughter products were present at each site. Fraction organic carbon was found to be approximately 2.6 times higher in the low-K zone than the transmissive zone, with a median of 1.1% in the low-K zone. Fact Sheets are provided to allow cost effective application of these concepts at other sites using existing site data.
The DoD has recognized that chlorinated solvent source zones in complex hydrogeologic settings can cause difficult to treat, persistent groundwater plumes, with the potential to result in costly and incomplete treatment. Back diffusion from low-K zones can sustain plumes long after the source has been treated and the importance of these processes for site management has become more apparent in recent years. Few remedial technologies can overcome the challenges associated with matrix diffusion, and those which have proven effective at some sites, such as in situ thermal treatment or deep soil mixing, may not be cost effective in many situations.
Recognizing the challenges associated with matrix diffusion, it is important to identify and better understand remedial approaches and mechanisms that may provide residual, low-cost benefits at matrix diffusions sites. Sustained treatment following ISB and MNA in low-K zones are two remedial approaches that have the potential to provide such benefits. This project better quantified these processes and provides guidance to remedial project managers on how to evaluate their occurrence and incorporate the findings into site conceptual models and remediation planning.
McGuire, T.M., D.T. Adamson, C.J. Newell, and M., Rysz. 2016. MNA in Low-K Zones: Fact or Fiction? Proceedings of the Tenth International Conference on Remediation of Chlorinated and Recalcitrant Compounds. Palm Springs, California. 22-26 May.
Walker, K.L., T.M. McGuire, D.T. Adamson, and C.J., Newell. 2016. Sustained Treatment of Chlorinated Solvents in Mulch Biowalls: A 10-Year Anniversary Analysis?. Proceedings of the Tenth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Palm Springs, California. 22-26 May.
Walker, K.L., T.M. McGuire, D.T. Adamson, and R.H. Anderson. 2020. Long-Term Evaluation of Mulch Biowall Performance to Treat Chlorinated Solvents. Groundwater Monitoring & Remediation, 40(1):35-46.