Interim Results

Our research group has been conducting mass balance analysis on a list of 20 sites selected for the research study. All 20 sites are considered secondary sites meaning evaluation of source and plume mass balance are conducted based on existing historical data. Five of the 20 sites were identified as primary sites where new data has been collected. Those sites are Alameda Naval Air Station (NAS), California; Hangar-K, Cape Canaveral AS, Florida; Calf Pasture Point Rhode Island, an industrial site in Gainesville FL; and an industrial site in Perth Australia (no SERDP funding for data collection). We have collected data at sites with evidence of back–diffusion to characterize the magnitude of this problem through collection of flux profiles (e.g. Brandywine site in Maryland). These profiles are compared to core data collected during well installation to look at the mass associated with low permeability units and the magnitude of mass flux as measured by passive flux meter deployments.

The Alameda NAS site, Plume 4-1, was identified as a primary site for extensive site data collection.  An active dissolution test was conducted at Alameda in collaboration with Shaw Engineering and completed in June 2011. That data has been modeled using source strength functions. The dissolution data shows a good declining trend with time which fits the concept of a source strength function. The model fit was used to look at long term dissolution predictions to reach site cleanup objectives. The next phase consists of enhanced bio-treatment of the source zone and is currently underway. Following the bio-treatment, mass discharge from the site will be quantified for comparison to data collected at the beginning of this study. 

At the Calf Pasture Point site in Rhode Island, a former Navy facility, passive flux meters (PFMs) were deployed in a series of wells with a total of 59 PFMs forming three discrete transects within the plume.  PFM analysis was completed in 2012 and evaluation of mass discharge at each of the three transects proceeds. The site source strength function has been fit to both temporal data available for the site and the new three transects location spatially along the plume axis. This demonstrates two approaches that can be used to estimate source strength functions.  Installation of new wells is planned for 2013 to improve the estimated mass discharge at the third transect.

A back-diffusion model that considers different source strength functions has been developed and applied over a wide range of realistic conditions. The one-dimensional diffusion model was used to investigate the effects of DNAPL source zone dissolution and remediation on the storage and release of contaminants from aquitards. Source zone dissolution was represented by a power law source depletion model, which served as a time variable boundary condition to the diffusion equation used to describe mass transport in the aquitard. The model indicated that the mass per unit area stored in the aquitard was 3 or more orders of magnitude less than the initial DNAPL source zone mass per unit area, and the back diffusion flux from the aquitard was typically 4 or more orders of magnitude less than the initial source zone flux. Additionally, the effects of partial source zone remediation were investigated, and the results suggest that source remediation can have a favorable effect on long-term back-diffusion risk. In the remaining period of the project, back-diffusion will be characterized in laboratory aquifer models and at three field sites using PFMs. The model developed will be applied to these systems.

In modeling, laboratory and field efforts, dissolution of DNAPL source zones was accurately predicted based on appropriate characterization of the source zone architecture, which controls the rate of mass discharge or source strength function. However, the architecture changes temporally as the source zone mass is depleted by dissolution. To generalize comparisons between contaminated sites with different porewater velocities or contaminant solubilities, site age was defined in terms of the fraction of contaminant mass that has been eluted from the source zone by aqueous dissolution. In laboratory studies, DNAPL architecture during dissolution of a source zone were measured by light transmission visualization in laboratory flow chambers. Architectures measured at ages corresponding to initial conditions, 20, 50, and 90% mass removal were used in an equilibrium streamtube (EST) model to accurately predict subsequent dissolution. It is shown both experimentally and theoretically that as DNAPL contaminated sites age, fractional reductions in contaminant discharge and mass converge to become equal, regardless of the initial architecture. This behavior is a consequence of convergence from lognormal to exponential behavior. Applications of the EST model to field data demonstrated the importance of flow field conditions between the characterization though partitioning tracers and the remedial flow field applied. The design of the flow field and how this impacts dissolution will be evaluated during the remaining period of the project.