This project focused on assessing the level of site characterization needed to support quality decisions regarding remedial strategies and long term stewardship of contaminated sites using a flux and mass balance based approach. Site characterization efforts were aimed at understanding the link between dense nonaqueous phase liquid (DNAPL) source zones mass discharge under natural hydrological conditions and modified as a result of source zone treatment. The transition between forward and back-diffusion was also explored. This project provides an overview of methodologies that can be used to characterize source zone mass discharge functions and how this controls plume response and back diffusion.
Specific objectives of this project were to: 1) Develop source-strength functions for site management purposes using existing historical site data supplemented with limited flux- and core-based sampling; 2) Extend to the field-scale the ability to predict DNAPL source depletion through dissolution, based on a priori characterization of the source zone architecture; 3) Characterize the near-source plume response to source-mass depletion at selected field sites in order to provide the understanding needed to predict long-term plume responses; 4) Link the characterization of the near-source, short-term responses to likely long-term behavior of the dissolved plume including back diffusion; and 5) Provide recommended guidance on the level of source zone characterization needed to adequately predict source-strength functions and plume response.
The research effort developed source-strength functions, MD(t), using existing historical data at field sites. Researchers selected five primary field sites and up to ten secondary sites with available sets of high quality historical data. At the primary sites, additional data was collected.
Characterization of the source-strength function were based on a quality historical record of concentration and head measurements, particularly in the near-source region. For each site considered, currently available data alone was used to provide the best possible description of the source function. Following completion of this exercise on secondary sites, primary sites were selected. Sites were evaluated to identify what type of additional data will be obtained including flux-based measurements such as integral pump tests and passive flux meter (PFM) well screen profiles. The data collected from the primary sites was used to improve the estimated source-strength function and associated error estimates.
Advanced characterization of the DNAPL architecture included current source zone mass (M ), contaminant fluxes, J(x,y,z), and mass discharge rate, MD; high-resolution DNAPL saturation characterization, SN(x,y,z); and trajectory-integrated DNAPL saturation. Mass flux measurements made at down-gradient control planes, and tracer-based trajectory-integrated data were Lagrangian characteristics that contain information representing the actual flow paths taken by water traversing the source zone. Point-based spatial data such as soil cores contain different information, and both data types will be used to predict dissolution.
At the Alameda California, primary site selected for further evaluation, the down-gradient response to changing source zone strength was quantified through a controlled field experiment. The experiment was conducted by containing a portion of an existing source zone using three injection wells and four extraction wells forming a hydraulic control cell. Within this flow cell, the source-strength function behavior under enhanced groundwater flow conditions was observed. Reactive and nonreactive tracers were used to characterize transport conditions within the flow cell including diffusion into lower conductivity zones with the aquifer. The subsequent response of the contaminant plume to a source-strength function perturbation was monitored for “back diffusion” from lower conductivity zones or DNAPL contaminated zones.
The field data collected in this project was used to evaluate several approaches for estimating the source strength function, and the uncertainties associated with the estimated model parameters. Cost-effective approaches to DNAPL site characterization allows Department of Defense (DoD) users and site managers to more accurately assess the benefits of costly aggressive source zone treatment technologies. Accurate assessment of near-source flux changes and the magnitude of "back diffusion" expected will help the DoD avoid costly remedial efforts with inadequate benefits.
The results of the project can be used by DoD site managers and consultants to better characterize sites to make informed decisions regarding implementing aggressive source zone remedial efforts. Guidelines on site characterization efforts aimed at understanding the link between the site DNAPL source strength function under natural hydrological conditions and as modified by source treatment. These guidelines provide an established methodology for defining the source strength function and how this function defines predicted plume responses. The ability to better characterize these relationships can reduce site management and clean-up costs by enabling quantitative prediction of the benefits of costly source zone treatment. In addition, this project developed and disseminated, through journal articles, recommended guidelines and workshops, an approach for quantifying "back diffusion" processes at contaminated sites.