2020 Excellence in Environmental Engineering and Science™ Awards Competition Winner

Honor Award - University Research

REMChlor-MD Groundwater Containment Transport and Remediation Computer Model

Entrant: Ronald W. Falta, Ph.D.
Person in Charge: Ronald W. Falta, Ph.D.
Location: Clemson, South Carolina
Media Contact: David L. Freedman, Ph.D., BCEEM

Entrant Profile

Dr. Falta's primary teaching and research interests are in hydrogeology, contaminant transport/remediation, and multiphase heat and fluid flow in porous media. His research projects have largely focused on environmental remediation of hazardous waste sites, geologic sequestration of carbon dioxide, and mathematical modeling of contaminant transport and remediation. Dr. Falta earned his B.S. and M.S. degrees in Civil Engineering and his Ph.D. in Material Science & Mineral Engineering from the University of California at Berkeley.

Dr. Falta is internationally recognized for his research on modeling of contaminant fate and transport. He created the widely used software Remediation Evaluation Model for Chlorinated Solvents (REMChlor). REMChlor has been used extensively by practitioners and as such has had a major impact on the groundwater remediation community. Most recently, Dr. Falta has developed a new mathematical method to incorporate the effects of matrix diffusion on predicting the behavior of groundwater contaminant plumes. The updated model is entitled REMChlor-MD, and it is available free of charge from the Department of Defense Environmental Security Technology Certification Program (ESTCP). REMChlor-MD is the project under consideration for the AAEES University Research Award. Dr. Falta has collaborated on REMChlor-MD with colleagues at GSI Environmental Inc. (Drs. S. K. Farhat and C. J. Newell), the U.S. Environmental Protection Agency (K. Lynch), and several Clemson University graduate students (N. Muskus, W. Wang).

Project Description

Groundwater has been contaminated at thousands of sites around the world as a result of leaks and spills of hazardous materials. These primary sources of contamination are often referred to as source zones, and they can lead to extensive dissolved contamination plumes. Secondary sources of contamination, such as dissolved chemicals in low permeability zones result in plume persistence and limitations for plume remediation. The process of matrix diffusion has emerged as a primary cause of plume persistence during remediation efforts. Matrix diffusion is the process of mass transfer of solutes between high permeability zones and surrounding low permeability zones due to a concentration gradient. This process is known as forward diffusion when the transport is from high to low permeability zones. Forward diffusion takes place during a "loading period", usually when contaminants reach the aquifer. After the contaminant source has been removed, the concentration gradient reverses and diffusion goes from low to high permeability zones, resulting in back diffusion. Therefore, low permeability zones act as contaminant sinks first and later serve as contaminant sources to transmissive zones with flowing groundwater due to matrix diffusion. Back diffusion of contaminants from low permeability areas can result in plume persistence and severe limitations in remediation.

There are analytical solutions available to model matrix diffusion, but they are constrained to simple geometries corresponding to ideal cases. Numerical simulations studies of matrix diffusion have shown the ability to reproduce the transient matrix diffusion effects. However, it has been demonstrated that very fine discretization is required in order to reproduce the diffusive fluxes at the high permeability/low permeability interfaces, often controlled by concentration gradients at the scale of centimeters or less. High resolution grids result in an excessive computational effort that makes it impossible to use this approach for 3-dimensional field scale problems.

A new, practical modeling approach for simulating field-scale groundwater matrix diffusion has been developed and implemented in the contaminant transport model REMChlor- MD. REMChlor-MD incorporates a semi-analytical/numerical method for modeling matrix diffusion in heterogeneous and fractured groundwater systems. The new matrix diffusion method allows for the low permeability matrix to be embedded within a numerical gridblock, having finite average thickness, a specified volume fraction and a specified interfacial area with the high permeability domain. The new formulation also allows for coupled parent-daughter decay reactions with multiple species that each have independent retardation factors, decay rates, and yield coefficients in both the high and low permeability parts of the system. The method uses a dynamic fitting function to approximate the transient concentration profile in the low permeability part of each gridblock at each time-step so that the matrix diffusion flux into or out of the high permeability part of the gridblock can be computed as a concentration dependent source-sink term. This semi-analytic approach is extremely efficient because the only unknowns in each gridblock are the concentrations in the high permeability domain, so there is practically no increase in computational effort compared to a conventional groundwater transport simulation.

The matrix diffusion method has been shown to compare well with an analytical solution for matrix diffusion in fractured media with parallel fractures, with an analytical solution for matrix diffusion with parent-daughter decay reactions, with laboratory experiments of matrix diffusion in a layered system, with a laboratory experiment involving lens shaped inclusions, and with fine grid numerical simulations of transport in highly heterogeneous systems.

The new REMChlor-MD capability allows 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.

The REMChlor-MD program is available free of charge from the Department of Defense Environmental Security Technology Certification Program (ESTCP) website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426.

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With funding from ESTCP, Dr. Falta partnered with GSI Environmental to develop the newly released software REMChlor-MD. REMChlor-MD, developed for the Department of Defense ESTCP program, is an easy-to-use, free software tool that can assist site personnel 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.	Dr. Falta has given numerous seminars and in so doing he has had a major impact on the practice of groundwater remediation. He has collaborated extensively with consulting firms (e.g., GSI, Arcadis, and many others) and colleagues at the Savannah River National Laboratory.

Dr. Falta presented an overview of REMChlor-MD to ESTCP, which funded the research. This slide describes the motivation for developing the new modeling tool.	During a presentation to ESTCP, Dr. Falta explained the theoretical basis for REMChlor-MD.

During a presentation to ESTCP, Dr. Falta demonstrated how well REMChlor- MD reproduces simulations of matrix diffusion for tetrachloroethene (PCE) made with other, more complex, modeling approaches. The accuracy and simplicity of REMChlor-MD represent an important advance for the remediation community. The two citations listed in the title of the slide (Falta and Wang, 2017; Muskus and Falta, 2018) are presented at the end of the nomination package.	This is Figure 1 from "User's Manual: A Practical Approach for Modelling Matric Diffusion Effects in REMChlor," ESTCP Project ER-201426. It captures the very challenging problem created when contaminant mass within low permeability zones starts to diffuse out (i.e., "back-diffusion"), resulting in groundwater pollution that can persist for decades.

This is Figure A1-2 from "User's Manual: A Practical Approach for Modelling Matric Diffusion Effects in REMChlor," ESTCP Project ER-201426. The schematic demonstrates that contaminant discharge from a source zone is the product of the flowrate of water passing through the source zone, and the average concentration of contaminant in that water. Source discharge thus has units of mass per time, and is not to be confused with mass flux, which is a discharge divided by an area.	This is Figure A1-5 from "User's Manual: A Practical Approach for Modelling Matric Diffusion Effects in REMChlor," ESTCP Project ER-201426. Using REMChlor-MD, it depicts a simulation for contaminant concentration in a source zone in which no remediation is applied (red line), 90% source zone removal occurs 20 years after the source developed (blue line), and 90% source zone removal when the contaminant was initially discharged (yellow line). The benefit of source zone removal is apparent.

This is Figure 1.5 from "User's Manual: A Practical Approach for Modelling Matric Diffusion Effects in REMChlor," ESTCP Project ER-201426. It offers a comparison of REMChlor-MD (red line) and the more complex numerical model used by Chapman and Parker (2005) (blue line) over a 140 year time span. The dashed gray line is a REMChlor-MD simulation where the matrix diffusion process in the plume has been turned off. The gray line shows that without inclusion of the matrix diffusion process, the predicted plume cleanup time is too low (i.e., too optimistic).	This is Figure 10 from "A semi-analytical method for simulating matrix diffusion in numerical transport models," R. W. Falta and W. Wang, 2017, Journal of Contaminant Hydrology 197: 39-49. It offers a comparison of a MT3DMS (a fully numerical model) and the semi-analytical model for a thin sand layer bounded by clay during the loading period (a), and during the back diffusion period (b). The TCE source was present for 40 years. The semi-analytical model does an excellent job reproducing the numerical solutions, at a fraction of the complexity.

This is Figure 10 from "Semi-analytical method for matrix diffusion in heterogeneous and fractured systems with parent-daughter reactions," N. Muskus and R. W. Falta, 2018, Journal of Contaminant Hydrology 218: 94-109. It offers a comparison of MT3DMS (a numerical model) and the semi-analytical model for TCE concentration contours for lens case in the vertical xz plane for a) 10 years, b) 30 years, c) 130 years. The semi-analytical model does an excellent job simulating the contaminant plumes in comparison to the more complex numerical model.	This is a modified version of Figure 16 from "Semi-analytical method for matrix diffusion in heterogeneous and fractured systems with parent-daughter reactions," N. Muskus and R. W. Falta, 2018, Journal of Contaminant Hydrology 218: 94-109. It offers another comparison of MT3DMS (a numerical model) and the semi-analytical model for TCE mass discharge rate. Once again, we see how well the semi-analytical model does in recreating the results from the more complex numerical solution.