Remediation of nonaqueous phase liquid (NAPL)-impacted sites is difficult and costly. Even with enhancements (e.g., thermal, chemical), mass transfer constraints of NAPL dissolution govern control of sources and the attainment of cleanup goals. To better manage expenditures, the Department of Defense (DoD) needs a scientifically-based, process-centric method to evaluate the extent of control provided by past NAPL remediation and the potential benefit of additional treatment. Current approaches to predict the impact of NAPL remediation include (1) screening models, which lack a physical basis and are overly simplistic, and (2) numerical transport models, which are immensely complex and costly. The objective of this project is to establish a practical and cost-effective method to assess source control at NAPL sites by applying a volume-averaged model using site- and technology-specific NAPL dissolution rates.

This project will demonstrate a practical approach for efficient, quantitative assessment of changes in NAPL source concentration and mass flux over time. Published mass transfer coefficients describing NAPL dissolution specific to remedial technologies and post-remediation source depletion will be compiled and incorporated within a volume-averaged model that includes coupled processes. Using volume-averaging with physically-based mass transfer coefficients minimizes spatial specificity, limits the required site-specific inputs, and reduces the burden of parameter estimation and calibration. Further, the complexity of this method is adaptable to the available data and processes under consideration and therefore can be adjusted as needed. Solutions obtained from this framework are derived from mass balance principles, thus maintaining a physical basis. This approach will be validated and demonstrated through comparisons with numerical transport modeling at sites with various NAPL architectures exposed to multiple remedial processes, including well-documented field demonstrations and implementations and synthetically-generated data.

This effort will result in a stand-alone source control calculation tool that will estimate site-specific remedial impact given a modest amount of site characterization data. Available as web-based modules, the tool will be at a level of complexity accessible to environmental managers and practitioners, allowing for broad adoption. In addition, the method can be modified to assess source zone control of other emerging and recalcitrant contaminants, such as per- and polyfluoroalkyl substances (PFAS) and energetics.

This project will provide DoD with a technically-defensible methodology to rapidly assess source zone control for a range of complex NAPL architectures and remedial technologies. This approach is expected to benefit site managers by providing them with greater certainty on outcomes from additional source treatment, potential reductions of remedial timeframes, and impacts on current expenditures and long-term obligations. The tool has applicability to hundreds of DoD sites: (1) where remedial decisions are pending, (2) for evaluation of ongoing remedial actions, and (3) for assessing additional technologies to meet response action objectives or address regulatory concerns regarding source control and remedial lifetime. (Anticipated Project Completion - 2023)

Stewart, L.D., J.C. Chambon, M.A. Widdowson, and M.C. Kavanaugh. 2022. Upscaled Modeling of Complex DNAPL Dissolution. Journal of Contaminant Hydrology, 244:103920. doi.org/10.1016/j.jconhyd.2021.103920.