The overall objective of this project is to implement the physical process-based volume-averaged (VA) model for non-aqueous phase liquid (NAPL) dissolution developed and validated as part of ESTCP project ER19-5223 (Source Control and Remedial Performance Evaluation [SCARPE]), into the Sequential Electron Acceptor Model, 3-Dimensional (SEAM3D) NAPL Dissolution Package within the widely used commercial groundwater modeling graphical user interface Department of Defense (DoD) Groundwater Modeling System (GMS). This work will also provide the framework and approach for implementation within any numerical transport modeling family without utilizing a multi-phase flow simulation. Numerical transport models provide a powerful tool to evaluate chemical plume downgradient of NAPL source zones. This work will address the critical need for incorporating representative mathematical models of site-specific source zones into numerical transport models to support management decisions at complex NAPL sites, where fate of NAPL source zone and downgradient plume transport and attenuation need to be evaluated under a range of remedial alternatives. The goal is to bridge the gap between complex and costly numerical models needed to represent NAPL dissolution and validated physical process-based VA NAPL dissolution model to enable improved site models where source zone dynamics are coupled to plume transport and attenuation. The result will be a cost-effective numerical model tool, which can consider complex NAPL dissolution processes without the computational expense of very fine grid and/or reliance on empirically projected future source zone concentrations with no consideration of mass balance or physical-chemical properties or laws governing mass transfer, for evaluating solute transport and attenuation process combined with source zone depletion to support remedial alternative evaluation.

 

The new VA modeling approach used in SCARPE is a physical process-based method incorporating source zone metrics (i.e., geometrical dimensions of characteristic NAPL accumulations into upscaled mass transfer functions). The effectiveness of the VA modeling approach for replicating complex NAPL dissolution behavior measured in laboratory, numerical, and field studies was demonstrated by incorporating system properties into model parameters without undertaking model calibration. It has been shown that the VA modeling approach offers a level of simulation accuracy of mass discharge from NAPL source zone that is comparable to very fine-grid numerical simulations, but at a tiny fraction of the computational and user effort. This project will modify the NAPL Dissolution Package currently included in SEAM3D, which is compatible with public domain modular finite-difference flow model (MODFLOW) groundwater flow and transport codes through SERDP project ER20-1079 and is available within DoD GMS. The modification will consist in utilizing the VA source zone model developed under ESTCP project ER19-5223, which includes a relative permeability allowing its use in single phase models without the need to compute small scale changes in velocity associated with diminishing mass. The outcome will be a manageable, more straightforward model with the best of both worlds: a NAPL source zone that fits between over-simplified and overly complex combined with a numerical model for plume transport and attenuation.

The new numerical model capabilities will allow existing flow and transport models to be upgraded to include a realistic, transient NAPL source and mass transfer effects without the need for very-fine grid or complex, multi-phase numerical modeling. Many site models already have MODFLOW models within the GMS platform; therefore, this new capability will be readily accessible to the site modelers, and easy to implement in the existing site models. The project will also facilitate the development of models accounting for NAPL dissolution at sites without existing models, using the same platform as the majority of sites. Including NAPL dissolution and mass transfer limitations within these single-phase models will make them more reliable for making remediation decisions, which will ultimately benefit the Department of Defense by improving the ability to maintain operational readiness and warfighter preparedness. (Anticipated Project Completion - 2026)