The presence of source zones containing dense non-aqueous phase liquid (DNAPL) is usually the single most important factor limiting the characterization and cleanup of organic-contaminated sites. Unfortunately, because of the complexities associated with the transport, retention, distribution, and mass transfer of DNAPLs, as well as the heterogeneity of subsurface environments, it is generally not possible to remove all DNAPL mass from the source zone. Thus, the issue for many sites becomes the relative costs/benefits of source reduction. This issue has been the focus of several recent reviews conducted under the auspices of the ITRC (2002), EPA (2003), and NRC (2004).
To address the source-reduction issue, as well as to accurately assess the human-health risks associated with chlorinated solvents in the subsurface, it is essential to gain a better understanding of the distribution and mass-transfer behavior of DNAPLs in subsurface systems. The source-reduction reviews mentioned above have noted the importance of improving our understanding of DNAPL processes, but did not provide detailed discussions. A workshop was convened in 2001 by the Department of Defense to evaluate the research and development needs for remediation of chlorinated-solvent contaminated sites. These three key issues were identified by the panel as critical for advancing the field:
- Research on source zones is a more pressing need at this point than research on plume restoration.
- The current scientific understanding of source zones is inadequate, and more information is needed regarding source-zone architecture, total contaminant mass, and the rates and mechanisms of contaminant release from source zones.
- It is important to better understand the processes occurring at the pore scale, especially at the interfaces surrounding NAPLs in the subsurface.
The goal of the workshop is to build upon the prior DOD review by providing specific, detailed analyses of the current state of the science and future research needs for DNAPL mass-transfer processes and associated source-zone mass-flux phenomena. The objectives of the proposed workshop include:
- Summarize the current state of knowledge with regard to DNAPL mass transfer and associated mass-flux phenomena.
- Identify major research needs in the next 5-10 years for enhancing the understanding of DNAPL dissolution and associated mass-flux phenomena.
- Identify emerging tools that will contribute to improving our understanding of source-zone architecture and mass-flux behavior.
- Produce a collaborative journal article, documenting the conclusions and recommendations generated by the workshop.
The two-day meeting was comprised of a series of presentations designed to summarize current knowledge and highlight research needs for several topic areas. The selected topic areas were: 1: The NAPL Source Zone Issue; 2: DNAPL Distribution in the Subsurface; 3: NAPL Dissolution: Pore-scale Experimentation; 4: NAPL Dissolution: Pore-scale Modeling; 5: NAPL Dissolution: Column-scale Investigations; 6: NAPL Dissolution: Intermediate-scale Investigations; 7: NAPL Dissolution: Field-scale Demonstrations; 8: NAPL Dissolution: Field-scale Modeling; 9: NAPL Mixtures; 10: Characterization of DNAPL Source Zones; 11: Evaluating and Predicting Source-zone Mass Flux; 12: Identified Research Needs. The workshop produced substantive discussion of these issues, and led to the identification of several critical research needs. A detailed summary of the workshop results is being developed for publication.
Funding for this workshop was provided by the NIEHS SBRP, with additional support provided by the UA SBRP Research Translation Core and the UA Department of Soil, Water, and Environmental Science.
Interstate Technology and Regulatory Council (ITRC). 2002. Regulatory Overview: DNAPL Source Reduction: Facing the Challenge.
Environmental Protections Agency (EPA). 2003. The DNAPL Remediation Challenge: Is there a Case for Source Depletion?
National Research Council. 2004. Source Removal of Contaminants in the Subsurface.