“The REMChlor-MD Groundwater Transport and Remediation Model for Sites with Matrix Diffusion” by Dr. Ron Falta, Dr. Chuck Newell, Dr. Shahla Farhat

Matrix diffusion occurs when groundwater contaminants present in high permeability zones diffuse into adjacent low-permeability zones. Contaminants in the low permeability zones can represent a long-term source of contamination following remediation of high permeability zones due to the phenomena of back diffusion. Current numerical modeling approaches are not able to accurately resolve the local-scale matrix diffusion effects without resorting to extremely fine grids, which are not practical for real-world problems.

An alternative approach has been developed using funding from ESTCP (ER-201426). This approach uses semi-analytical approximations inside each normal (large) gridblock to represent local-scale matrix diffusion. With this method, only the high permeability zone is discretized in the numerical model, and the interaction with the low permeability zone is accounted for in a time-dependent source/sink term that is computed analytically in each gridblock at each time-step.

The new method is extremely efficient, and it compares well with laboratory experiments and exact analytical solutions for transient matrix diffusion in aquifer/aquitard systems and in fractured porous media. This modeling technique has been implemented in a new version of the contaminant transport screening model REMChlor, called REMChlor-MD, and it is available for download at no cost from the ESTCP website.

The REMChlor-MD model retains the remediation capabilities of the original REMChlor model, while allowing for matrix diffusion in the plume. The model considers first order decay and production of daughter products in both the high and low permeability parts of the domain, with independent decay rates and retardation factors for the individual species in the two zones. Contaminant reaction rates can be varied in space and time in either or both domains to simulate plume remediation actions.

Dr. Ron Falta is a Professor of Environmental Engineering and Earth Sciences at Clemson University in South Carolina. Ron has served as Principal Investigator on multiple research and development grants focusing on subsurface remediation and mathematical model development for sites contaminated by chlorinated solvents. He has authored more than 60 peer-reviewed research papers and book chapters in these and related areas. He is the developer of the REMChlor and REMChlor-MD transport and remediation models, and co-developer of the USDOE T2VOC thermal remediation model. Ron is currently working on ESTCP and SERDP projects related to modeling matrix diffusion at sites contaminated by chlorinated solvents. He earned his bachelor’s and master’s degrees in civil engineering from Auburn University, and his doctoral degree in mineral engineering from the University of California, Berkeley in 1990.

Dr. Charles Newell is a Vice President at GSI Environmental in Houston, Texas. He is a member of the American Academy of Environmental Engineers, a NGWA Certified Ground Water Professional, and an Adjunct Professor at Rice University. His professional expertise over the past 30 years includes site characterization, groundwater modeling, non-aqueous phase liquids, risk assessment, natural attenuation, remediation, environmental software development, and long-term monitoring projects. He has been awarded the Hanson Excellence of Presentation Award by the American Association of Petroleum Geologists, the Outstanding Presentation Award by the American Institute of Chemical Engineers, the 2001 Wesley W. Horner Award by the American Society of Civil Engineers, the 2008 Outstanding Alumni Award from Rice University, the Strategic Research and Development Program (SERDP) 2014 Project of the Year as a Co-PI, and the ITRC Environmental Excellence Award in 2016. Chuck has served on ITRC’s Integrated DNAPL Site Strategy and Remediation Management of Complex Sites teams. He has a bachelor’s degree in chemical engineering, and master’s and doctoral degrees, all from Rice University.

Dr. Shahla Farhat is a Managing Scientist at GSI Environmental Inc. in Houston, Texas.   She specializes in environmental research and engineering, with a focus in assessment and remediation of local and regional groundwater aquifer systems, fate and transport modeling, and decision support systems. She is the primary author of the “Matrix Diffusion Toolkit”, "Mass Flux Toolkit", and “Source History Tool” developed by GSI for ESTCP, the "SourceDK Remediation Timeframe Decision Support System Model" developed by GSI for the Air Force Civil Engineer Center, the “GSI Mann-Kendall Toolkit”, and several proprietary software for petrochemical industries.  She is also the main developer of the user interface for ESTCP’s REMChlor-MD.  She has served as co-PI for several ESTCP projects over the past 10 years.