Remediation of sites impacted with nonaqueous phase liquid (NAPL) 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 source control provided by past NAPL remediation and the potential benefit of future treatment. Current approaches to predict the impact of remediation include (1) screening models, which are simplistic, and (2) numerical transport models, which are complex and costly. The objective of this project was to establish a practical and cost-effective method to assess source control at NAPL sites using site- and technology-specific NAPL dissolution rates in a volume-averaged source zone.
This project implemented a volume-averaged source zone model based on upscaling of complex NAPL dissolution processes to characteristic dimensions of multiple NAPL accumulations. The volume-averaged model allows for incremental complexity (e.g., NAPL source architecture) to be easily incorporated as additional characterization data become available. Remediation of a NAPL-impacted source zone involves numerous interrelated, complex processes. Mathematical description of these processes was combined into a system of coupled, nonlinear, first order differential equations through volume-averaging. The output is an order-of-magnitude estimate for source zone discharge masses and concentrations over time subject to varied remedial processes, applicable to both design and performance evaluation. This effort resulted in beta versions of practical tools for multi- and single-component NAPL. The calculation tools estimate site-specific remedial impact given a modest amount of site characterization data.
This approach was validated and demonstrated through comparisons with data, including numerical, laboratory, and field-scale. The technology was applied to two demonstration sites, with various NAPL architectures exposed to multiple remedial processes. The outputs were compared to observed data and results from calibrated numerical transport models for validation. The technology was also used to evaluate past and future remedial strategies. Qualitative performance objectives for the project were ease of use and utility for supporting remedial decisions. The objectives were met based on feedback from remedial project managers, regulators, consultants, and other stakeholders. The utility of the results for remedial decisions was confirmed by remedial project managers. Based on the cost assessment, implementation of the volume-averaged model is a valuable tool to support remedial decision-making for a cost significantly lower than numerical models and comparable to qualitative screening models.
This project provides the 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 where remedial decisions are pending, for evaluation of ongoing remedial actions, and for assessing additional technologies to meet response action objectives or address regulatory concerns regarding source control and remedial lifetime.
During beta version testing, users cited the main implementation and utility issue as development of the conceptual source zone model and identification of input parameters. Based on this feedback, additional guidance was developed on estimating input parameters for the modeling. (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.