The approach to groundwater remediation has undergone dramatic shifts as a better understanding of source zones has developed. Initially, it was assumed that contaminants in the saturated zone occurred only in the aqueous or sorbed phases, so groundwater remediation was based on pump-and-treat systems. In the late 1980s and early 1990s, it was realized that dense non-aqueous phase liquids (DNAPL) are a secondary source of contamination. By the late 1990s, almost all chlorinated solvent sites were assumed to contain a continuing DNAPL source, even at sites where DNAPL was not directly observed. Better technologies that can remove or destroy residual DNAPL were developed in response. However, most of these methods are not commonly used as their benefits are difficult to assess and justify. Thus, there are two competing general site remediation options: (1) application of aggressive but emerging source treatment technologies to remove DNAPL or (2) application of long-term, low-intensity containment alternatives that leave the DNAPL in place and focus on long-term management of the DNAPL dissolution products. If DNAPL removal does not achieve a meaningful reduction in plume size or lifetime, then those resources may better be used for other risk control measures.
The objective of this project was to gather key knowledge and develop new, easy-to-use tools to help the groundwater community decide whether DNAPL source zone remediation is economically and technically justifiable at a particular site.
This project collected remedial cost and performance data from field sites to develop a decision support system for source zone treatment. Over 200 temporal records of concentration vs. time data were compiled from enhanced bioremediation, chemical oxidation, thermal treatment, and surfactant/cosolvent projects, as well as untreated sites. Results were used to develop planning-level source zone models that show potential relationships between the remediation timeframe and mass removal.
This project incorporated remediation performance data, cost data, and simple planning models in a decision support tool that users can consult in evaluating site-specific DNAPL source zone remediation. The decision support tool allows environmental professionals to explore the technical feasibility and cost effectiveness of intensive source remediation, allowing the most efficient use of limited remediation resources.
Performance data in the form of concentration vs. time records were compiled from 59 chlorinated solvent sites where in situ biodegradation, chemical oxidation, thermal treatment, or surfactant/cosolvent treatment of the source zone was applied. All four technologies exhibited median reductions in average source zone dissolved phase concentrations of 88% or greater for the parent chlorinated compound. Cost data from 36 field sites were compiled, with in situ biodegradation showing the lowest median unit cost of the four technologies ($29 per cubic yard). Long-term changes (i.e., over 5 - 15 years) in source zone concentrations also were evaluated at 23 untreated sites and suggested that, on average, these sites would experience a similar reduction in source zone concentrations over a 20-year period as would a site treated with a typical source depletion technology (project duration of 1-2 years). Finally, simple mass-balance models of DNAPL source zones (together with design curves, equations, and hypothetical examples) were developed to provide planning-level estimates of the change in remediation timeframe as a function of source mass removal. The results of this modeling exercise indicate that there is likely to be a nonlinear relationship between source mass removal and the reduction in remediation timeframe. All of the project’s data and results are presented in a Source Zone Decision Support System (free download at http://www.gsi-net.com/en/software/free-software/serdp-source-depletion-decision-support-system.html). (Project Completed – 2005)