Chlorinated solvents are the primary contaminants of concern for Department of Defense (DoD) sites. A clear understanding of the mass transfer and mass removal behavior of chlorinated solvents in subsurface systems is critical to accurately assess the human health risks associated with sites contaminated by chlorinated solvents, and to design effective remediation systems for such contamination. The overall goal of this project was to enhance our understanding of the relationships between source zone architecture, mass transfer dynamics, and contaminant mass removal for heterogeneous dense non-aqueous phase liquid (DNAPL) source zones, and the associated impacts on plume response. The specific objectives that were addressed in this project are:
- Investigate the impact of source zone aging on the relationship between mass removal and mass flux reduction.
- Investigate the impact of source zone architecture and mass transfer dynamics on contaminant removal, mass flux, and plume response at the field scale.
- Apply mathematical models to evaluate the impact of porous medium heterogeneity, nonuniform DNAPL distribution, and mass transfer processes on mass flux behavior at multiple scales.
- Assess the efficacy of methods for estimating mass flux reduction/mass removal behavior.
This project was designed to accomplish a systematic study of the mass transfer behavior of chlorinated solvent immiscible liquids in heterogeneous porous media at multiple scales, and to investigate the impact of source zone architecture on mass discharge and plume response. The project involved both bench-scale and field-scale investigations, as well as mathematical modeling analysis. A unique component of the project involved comprehensive long-term studies conducted at a field site. The analyses employed contaminant mass discharge as an integrative measure of the performance and effectiveness of remediation efforts. The standard approach of characterizing discharge at the source zone scale was expanded to provide characterization at the plume scale, which was evaluated by examining the change in contaminant mass discharge associated with plume-scale pump-and-treat systems. This approach allows linking the impacts of source zone remediation to effects on site-wide risk.
In total, the results of the studies demonstrated that the configuration of the source zone with respect to spatial distributions of subsurface properties (porous media type, permeability, contaminant) exerts a significant control on the magnitudes and rates of mass removal and associated contaminant mass discharge. In particular, it was observed that source zone age was a critical factor mediating system behavior.
Temporal profiles of contaminant mass discharge, combined with knowledge of initial contaminant mass, can be used to evaluate the relationship between reductions in contaminant mass discharge (CMDR) and reductions in contaminant mass (MR). In this project, the team developed CMDR-MR relationships for a well detailed field site, and also for several other prior field projects. These data represent the first comprehensive field-based analysis of this entity. The results indicate that the CMDR-MR relationship is a defining characteristic of system behavior, and is mediated by system properties and conditions such as permeability distribution, contaminant distribution, and mass transfer processes.
The response of the plume after implementation of source zone management is a critical issue for chlorinated solvent sites. This project included two field studies of plume response, one wherein aggressive source remediation efforts were implemented and one for which source control was achieved via hydraulic containment. In total, the results of the project demonstrated that management of source zones can reduce contaminant mass discharge to the groundwater plume. The analyses indicated that this action can, in turn, reduce the time required to clean up the groundwater plume. However, the results also showed that the plumes can be expected to persist for an extended time due to the contributions of back diffusion associated with contaminant mass stored in extensive lower permeability units within the plume.
The results of this project support the growing consensus that most sites with large groundwater plumes will require many decades before cleanup will be achieved under current methods and standards. The continued use of pump-and-treat to manage plumes at these sites for the many additional decades anticipated will result in enormous aggregate operations and maintenance cost. Hence, there is a critical need to evaluate alternative methods that can be used to cost effectively manage these plumes while continuing to meet all remedial objectives. More effective methods for treating poorly accessible contaminant also are needed. In addition, there is a critical need to develop methods that can provide higher resolution site characterization that: (1) are cost effective, (2) are specifically designed to minimize disruption of operating remedial systems, and (3) can readily provide temporal updates of site conditions, thus supporting adaptive responses of the remedial systems to changing site conditions.
This project demonstrated that time continuous measurements of contaminant mass discharge can provide useful information to characterize mass transfer processes, assess mass removal magnitudes and conditions, and estimate contaminant distributions and quantities, as well as quantify mass discharge. Time continuous profiles of contaminant mass discharge can be obtained by conducting extended contaminant mass discharge tests or by capturing operational fluid extraction data. This project demonstrated the utility of both methods, and presented methods of data analysis and interpretation. Specifically, the project showed that the approach for comprehensive analysis of operational pump-and-treat data is a powerful, cost-effective method for providing higher resolution, value added characterization of contaminated sites. Advantages of the method include: (1) use of data that typically exists for operating sites, thus minimizing data collection costs, (2) no disruption of the operating remediation system, and (3) ability to update the analysis at any time, providing a means to periodically revise the site conceptual model, and the operation of the remediation system.
The studies investigated the impact of source zone remediation efforts on contaminant mass discharge and associated plume response at an unprecedented resolution and scale. The results provide a significant, unique contribution to our understanding of source zone dynamics and plume persistence for DNAPL sites. It is anticipated that application of the concepts and tools developed from the project to other DoD sites would provide substantial benefits, including improving the assessment and cost-effective operation of remediation systems for, and enhancing long term management of, sites contaminated by chlorinated solvents.