Dnapl Site Remediation: Status and Research Needs (Invited)

Remediation of sites impacted by dense, non-aqueous phase liquids (DNAPLs) such as chlorinated solvents remains technically challenging despite significant advances over the past 30 years. Contaminants are difficult to locate in the subsurface, and it is difficult to deliver remedial agents to the contamination effectively. If lower permeability media are present, these can act as diffusive sinks for aqueous and sorbed phase constituents, further complicating characterization and cleanup. DNAPL source zones are particularly difficult to remediate, and even after treatment these sources can persist for many decades, if not centuries, and it is difficult to transition sites to a passive management strategy. A recent expert panel on source zone remediation identified three overriding objectives for future remediation - to be more surgical, more sustainable, and more certain. Surgical remediation refers to precise delineation of contaminants and hydrogeology, with more targeted remediation efforts. Sustainable remediation refers to the growing need to consider all environmental impacts when developing remediation strategies, including energy use, greenhouse gas emissions, lifecycle impacts, and the increasing demand for clean water. Although considerable uncertainty is inherent in subsurface remediation, there is potential to reduce this uncertainty through improved monitoring and modeling. Specific characterization and remediation needs will be summarized separately. Improved technologies for source characterization are critical because inadequate characterization is common given the costs and limitations of current techniques. As a result, the performance of field-scale remediation technologies is frequently disappointing. Specific research needs to improve source zone characterization include: (i) better delineation and mass estimation, (ii) source zone architecture characterization methods, and (iii) increased resolution and fine-scale mapping of geologic properties and contaminant distribution. Specific remediation research needs include (i) understanding the long-term impacts of source zone remediation, (ii) improving remedial fluid delivery, (iii) developing combined remedies, (iv) improving performance assessment techniques, (v) improving containment and ex-situ treatment, (vi) developing better technology performance models, (vii) improving treatment of challenging sites such as fractured media or highly heterogeneous aquifers. The panels also emphasized the need for effective technology transfer to promote the adoption of new technologies and practices. These efforts should target technology users (practitioners, regulators, managers, and the public) using several different avenues, but particularly through web-based tools that allow users to quickly find focused information specific to their issues. Finally, the funding for remediation research is decreasing. To leverage the limited funds available, research partnerships and collaborations between investigators and between different agencies are encouraged.