The objective of this project was to develop practical and efficient mathematical methods for simulating the effects of matrix diffusion in groundwater transport and remediation models. These methods apply to various types of heterogeneous settings, including fractured porous media and sites with extensive low permeability layers and lenses. The new mathematical methods were then implemented in the U.S. Environmental Protection Agency groundwater remediation screening model REMChlor to develop a new generation model that considers matrix diffusion in the plume: REMChlor-MD.
The problem of matrix diffusion of chlorinated volatile organic compounds (CVOCs) in groundwater plumes is mathematically analogous to heat conduction in fractured rocks and heterogeneous reservoirs. Elegant numerical and semi-analytical methods for modeling the heat conduction processes that occur during fluid injection or extraction were developed in the 1980s by geothermal reservoir engineers. With some modifications, these methods are directly applicable to the problem of CVOC matrix diffusion. They offer accurate solutions that do not require explicit discretization of the low permeability zones. Instead, the low permeability zones that occur at scales smaller than a normal numerical gridblock are represented with sub-gridblock scale analytical or numerical approximations. In most cases of interest, these approximations are expected to offer a level of simulation accuracy comparable to the fine-grid numerical simulations, but at a tiny fraction of the computational and user effort.
REMChlor-MD attempts to assist site managers and site consultants to better understand matrix diffusion and help site stakeholders determine if matrix diffusion processes are significant enough to cause “rebounding” of downgradient plume concentrations above remediation goals after plume remediation or isolation is complete. Having this information readily available before a remedy is implemented, could assist site stakeholders select more appropriate remedies and improve effective risk communication with regulators and the public.
REMChlor-MD is intended to be used as a screening level tool for simulating matrix diffusion effects. REMChlor-MD has the following assumptions and limitations:
REMChlor-MD will enable site managers and stakeholders to quickly assess the likely impacts of different source and plume remediation schemes (including natural attenuation) with a comprehensive treatment of matrix diffusion effects. This will reduce overall costs of remediating these sites, and it will help ensure that limited resources for site remediation are used most effectively.
Falta, R.W. and W. Wang. 2017. A Semi-Analytical Method for Simulating Matrix Diffusion in Numerical Transport Models, Journal of Contaminant Hydrology, 197:39-49.
Muskus, N., and R.W. Falta. 2018. Semi-Analytical Model for Matrix Diffusion in Heterogeneous and Fractured Systems with Parent-Daughter Reactions. Journal of Contaminant Hydrology, 218:94-109.
Muskus, N. 2017. Evaluation of a Semi-Analytical/Numerical Method for Modeling Matrix Diffusion Effects in Groundwater Chemical Transport, MS Thesis, Clemson University, December 2017.
Wang, W. 2014. Comparison of Analytical, Numerical and Semi-Analytical Methods for Modeling Matrix Diffusion Effects in Aquitards, MS Thesis, Clemson University, December 2014.