Presented April 21, 2016- Presentation Slides
“A Practical Approach for Modeling Matrix Diffusion Effects in Low Permeability Zones at Chlorinated Solvent Sites” by Ron Falta
Matrix diffusion occurs when groundwater contaminants present in high permeability zones diffuse into adjacent low permeability zones. Contaminants that have diffused into the low permeability zones can represent a long term source of contamination following remediation of the high permeability zones due to the phenomena of back diffusion. Current numerical modeling approaches are not able to accurately resolve 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 ESTCP funding (ER-201426) that 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.
The new method is extremely efficient, and it compares well with exact analytical solutions for transient matrix diffusion in aquifer/aquitard systems and in fractured porous media. This modeling technique is being implemented in a new version of the contaminant transport model REMChlor.
“Methods for Minimization and Management of Variability in Long Term Groundwater Monitoring Results” by Poonam Kulkarni and Thomas McHugh
The primary purpose of groundwater monitoring is to determine the long term reduction in contaminant concentrations due to natural attenuation or active remediation. However, short term variability in contaminant concentrations limits the ability to accurately quantify attenuation rates which may increase monitoring costs and limit the ability to make appropriate site management decisions.
As such, the objectives of project ER-201209 were to (1) validate the use of alternative field sampling methods for the collection of groundwater samples in order to minimize variability in results, (2) develop and validate an improved method to optimize monitoring frequency by evaluating the site-specific short term variability and long term attenuation rate, and (3) develop and validate an improved method to identify long term concentration trends that better account for the effects of short term variability.
The project included a field demonstration that was conducted at two sites to evaluate five sample collection methods (three variations of low-flow purge, SNAP Samplers and HydraSleeve) that minimize variability in monitoring results. Additionally, a toolkit was developed in order to help project managers assess (1) the amount of monitoring needed to determine a site’s long term attenuation rate with a defined level of accuracy or confidence, and (2) the trade-offs between monitoring frequency versus the time required for trend identification.
Dr. Ron Falta is a Professor of Environmental Engineering and Earth Sciences at Clemson University in South Carolina. He has served as Principal Investigator on multiple research and development grants that focused 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 was the developer of the USEPA REMChlor transport and remediation model, and was a co-developer of the USDOE T2VOC thermal remediation model. He is currently working on ESTCP and SERDP projects related to modeling matrix diffusion at sites contaminated by chlorinated solvents. Dr. Falta 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.
Ms. Poonam Kulkarni is an environmental engineer with GSI Environmental in Houston, Texas. She has served as Co-PI on multiple research projects on evaluating and reducing variability in groundwater monitoring data, applying novel enhanced bioattenuation technologies at chlorinated solvent sites, and demonstrating passive soil vapor extraction technologies. Her project experience also includes remediation system design, methods to evaluate and enhance natural source zone depletion at hydrocarbon sites, and large database analyses. Ms. Kulkarni earned a B.S. in Chemical Engineering from the University of Texas, Austin (2006), an M.C.E.E. in Environmental Engineering from Rice University (2010). She is a licensed professional engineer in Texas.
Dr. Thomas McHugh is a toxicologist with GSI Environmental in Houston, Texas. Dr. McHugh has extensive project experience in environmental site investigation, site restoration, human health and ecological risk assessment, data mining, and statistical analysis. He has served as the principal investigator for several research projects on vapor intrusion, groundwater monitoring and other topics. He has developed training classes on a number of topics including the Texas Risk Reduction Program (TRRP) and has contributed to the development of state and federal guidance documents. Dr. McHugh received a B.A. in Biochemistry and Environmental Science from Rice University (1990), an M.S. in Environmental Engineering from Stanford University (1993), and a Ph.D. in Toxicology from the University of Washington (1997). He is a Diplomate of the American Board of Toxicology.