Once released into the subsurface environment, dense nonaqueous phase liquids (DNAPLs) serve as long-term sources of groundwater contamination and are therefore a significant risk to water resources. Containment and removal are the two basic strategies that can be used in the management or restoration, respectively, of a DNAPL-contaminated site. Restoration can be pursued through in-situ destruction or extraction, and a number of aggressive DNAPL remediation techniques (e.g., alcohol or surfactant flushing, thermal treatment, air sparging, chemical oxidation) have been developed for these purposes. Using the techniques, complete DNAPL removal may be possible at some sites, but given current technology limitations and financial restrictions, partial mass depletion may most likely be the end result of aggressive source treatment at many DNAPL-contaminated sites. If it is neither practical nor economically feasible to achieve complete DNAPL mass depletion, are the aggregate benefits of partial DNAPL mass depletion sufficient to reduce risks to an acceptable level, and are the costs associated with partial DNAPL mass removal justified by the benefits received?

The objective of this project was to investigate the likely benefit from partial DNAPL mass depletion using an aggressive technology and to develop sufficient understanding of the linkage between source zone remediation and its impacts on dissolved plume behavior to permit optimizing the remedial process, balancing mass removal with plume attenuation. Critical to this objective was the interrelationship between DNAPL mass removal, contaminant flux from the source zone, and dissolved plume behavior to assess the long-term impacts of DNAPL removal from source zones. This holistic approach is needed to construct an appropriate framework in which to complete cost-benefit analysis required to make decisions concerning source zone treatment.

Hypothetical DNAPL Source and Dissolved Plume

The technical approach used to achieve the project objective was to (1) characterize the relationships between DNAPL mass reduction, contaminant mass flux, and plume behavior and (2) to use this information to develop a strategy for assessing the benefits of DNAPL source remediation. Three lines of investigation were employed in the technical approach: field site demonstrations, laboratory experiments, and numerical and analytical modeling. Using a combination of these applications allowed evaluation of varied hydrogeological settings and remediation scenarios. Specifically, field demonstrations and laboratory experiments were used to investigate the relationships between aggressive source treatment, mass removal, and flux response. Numerical and analytical modeling was likewise used to investigate specific linkages, as well as develop an overall framework encompassing all conceptual model components.

Contaminant mass flux and mass discharge data were collected before and after source treatment at four field sites. These data provide the first comprehensive field assessment of mass flux/discharge response to DNAPL source treatment. Three flux measurement techniques (transect method, passive flux meters, and integral pump tests) were used. Mass discharge reductions of approximately 90% and 99% were observed at the Hill Air Force Base and Fort Lewis sites, respectively. Passive flux meter and integral pump test data exhibited comparable results for these sites. Less dramatic mass discharge reductions were seen at the Borden and Sages sites, and significant differences were observed between measurement methods. Because the support volumes for these measurement techniques can be very different, heterogeneities in contaminant distribution and hydraulic properties could be responsible for these differences.

Relationships between contaminant architecture and mass flux were evaluated in the laboratory using 2-D flow chambers. Source strength function (relationship between DNAPL mass and contaminant mass flux) was observed to be primarily controlled by the DNAPL architecture, which can be characterized by the trajectory integrated NAPL content distribution. Based on laboratory results, beneficial changes in the associated plume following partial source removal at a real site are expected, such as a reduction in the spatial extent of the plume or the total contaminant mass in the plume. The extent to which the plume would change depends on both the magnitude of flux reduction and the natural attenuation capacity of the downgradient aquifer formation.

Predicting the effect of the source remediation on plume behavior has been limited by the lack of tools that explicitly link source and plume remediation. A new analytical model, called REMChlor, coupling source and plume remediation, was developed to evaluate the impact of source and plume remediation at a more generic and strategic level. This screening-level mass balance approach is not specific to any remediation technology. The contaminant source model is based on a power function relationship between source mass and source discharge, and it can consider partial source remediation at any time after the initial contaminant release. The source model serves as a time-dependent mass flux boundary condition to a new analytical plume model. The plume model simulates first-order sequential decay and production of several species, and the decay rates and parent/daughter yield coefficients are variable functions of time and distance.

This project developed an improved understanding of the linkage between source zone remediation and its impacts on dissolved plume behavior. The experimental data and modeling analyses provide a basis for developing appropriate flux-based remediation endpoints at DNAPL sites and help in the design of cost-effective remediation technologies. Thus, project results facilitate more comprehensive risk assessments and provide a scientific basis for developing regulatory and policy guidelines for DNAPL source zone remediation.