Mark Widdowson is a Professor in the Charles E. Via, Jr. Department of Civil and Environmental Engineering at Virginia Tech. His expertise is in reactive contaminant transport modeling in groundwater systems. He has more than 20 years of experience developing and applying computer models for subsurface remediation of contaminants of concern. He is the co-author and principal investigator of the solute transport code SEAM3D (Sequential Electron Acceptor Model for 3D Transport) for simulating biodegradation and transport of contaminants with NAPL dissolution in aquifers and NAS (Natural Attenuation Software). Dr. Widdowson is a registered professional engineer. He received a Ph.D. in civil engineering from Auburn University, an M.S. in water resource engineering from the University of Kansas, and B.S.C.E. from the University of Cincinnati.
Presentation Description
Tools for Evaluating the Impact of Source Zone Mass Reduction on Remediation Timeframes
The concept of plume reduction – source zone reduction combined with monitored natural attenuation (MNA) – as a remedial strategy has been demonstrated at a limited number of sites with promising results. In this case, partial source removal using appropriate innovative technologies is coupled with a reduction in the size and concentration of the contaminant plume. Regulatory closure may be facilitated using risk-based cleanup goals or predetermined levels of contaminant concentrations at a regulatory reference boundary.
Computational tools have been developed to assist in decision-making and to estimate remediation timeframes using this approach. It is useful and often necessary at sites to estimate target source concentration (or mass discharge) required for a plume extent to contract to regulatory limits based on site-specific remediation objectives. In addition, estimates of the time required for source zone contaminants to attenuate to a predetermined target source concentration and the time required for a plume extent to contract to regulatory limits after source reduction become important inputs in decision-making on the level of source zone treatment in conjunction with MNA.
A key component of this analysis is the estimation the mass discharge reduction and long-term source zone depletion over time. Source zone functions combined with solute transport models of varying levels of sophistication show promise in this area, but application and field validation has been limited to simple, well-characterized sites. Considerable uncertainty in estimates using computational tools is expected at sites with complex hydrogeology and multi-component, non-aqueous phase liquid (NAPL) source zones.
One approach relies on long-term time-series data to calibrate the mass discharge over time. The range in time of remediation estimates is based in part on the range in source mass estimates and NAPL composition. An alternative approach to estimating the impact of source zone reduction on remediation timeframes is to combine innovative field tests with source zone models to estimate source zone properties and to facilitate model parameter estimation. One example is the implementation of mass-transfer tests conducted in conjunction with a pilot study of thermal enhanced extraction in a multi-component NAPL source.
