Given the complex hydrogeology and the limited success of prior remediation efforts at many federal and state hazardous waste-impacted groundwater sites, particularly those contaminated with chlorinated solvents, current nationwide estimates of costs and likely cleanup times needed for restoration may be significantly underestimated. The realization that many of these sites are unlikely to achieve restoration goals (e.g., maximum contaminant levels [MCLs]) and closure in acceptable time frames (i.e., < 30-50 years) has led to a shift in the remediation paradigm from one focused primarily on aggressive source zone treatment to a more holistic approach incorporating source removal (when applicable) to the extent practicable and long term management. For this new remediation paradigm to achieve cost reductions while managing long term risks effectively, conceptual site models (CSMs) must more accurately account for natural abiotic and biotic attenuation mechanisms and processes that control contaminant mass storage and persistence of chemicals of concern (COCs) in groundwater.

To support this remediation approach consistent with Department of Defense (DoD) long term goals for contaminated sites, the overall goals of this project are:

1. To improve our fundamental understanding of the natural attenuation (NA) of chlorinated solvents in complex groundwater systems over long (i.e., > 30 years) time frames,
2. To identify conditions where transition from active to more passive remediation (i.e., monitored natural attenuation [MNA]) can be accelerated, and
3. To develop predictive models and tools that can be used to support site transition decisions leading to more efficient and cost effective management of sites impacted by chlorinated solvents.

This research program couples detailed laboratory experiments, numerical modeling, and field-scale screening to estimate aquifer natural attenuation capacity (NAC) and improve long-term plume management through quantification of risks associated with reliance on NA. The project is structured around three technical tasks that will:

1. Perform fundamental studies of coupled physical, chemical, and biological processes governing NAC in heterogeneous aquifer formations;
2. Validate mathematical models that describe coupled processes in complex hydrogeological systems and conduct numerical experiments to quantify NAC for conditions observed at complex sites; and
3. Evaluate existing site performance monitoring data to develop guidance for quantifying NAC based on site characterization data that may not be routinely measured.

Task 1 will focus on experimental measurements of coupled reactions that contribute to chlorinated ethene attenuation in heterogeneous aquifer cells, which will provide the necessary rate parameters, correlated with site characteristics, for mathematical model development under Task 2. The numerical model subroutines from Task 2 will also be used to identify biogeochemical conditions conducive to NA and the relative value of field-measured parameters in determining NAC at real-world sites that will inform Task 3 site selection. The work will culminate in Task 3, which will combine mining of data from representative field sites with mathematical models to develop and demonstrate predictive tools and protocols that provide guidance to stakeholders on the viability and long-term risk reduction of MNA for long-term management at chlorinated solvent-impacted groundwater sites and resolution of the spatial and temporal variability in the NAC allowing quantification of short- and long-term risks of reliance on NA with appropriate confirmation monitoring programs.

The outcomes of this research will provide site managers, regulatory officials, the scientific community and other stakeholders with: (1) a more complete and quantitative understanding of the relationships between the coupled processes that govern long-term NA of chlorinated solvent groundwater plumes, and (2) mathematical models and predictive tools that are capable of estimating aquifer NAC and long-term chlorinated solvent behavior in complex hydrogeological settings. The research team anticipates that the outcomes of this project will serve to guide the selection of scientifically-sound strategies for sustainable long-term management of complex sites impacted by chlorinated solvents and support recommendations on when transition from active to passive treatment is justified. (Anticipated Project Completion - 2024)