There is a critical need to conduct accurate, quantitative performance assessments over longer time scales. This is especially important for the many complex sites that contain dense nonaqueous phase liquid (DNAPL) source zones and large groundwater contaminant plumes comprising persistent contamination, which are recognized to present the most difficult challenges to site closure. The implementation of longer-term performance assessments is expected to reduce the risk and costs associated with remediation and management of these complex sites.
The overall goal of this project was to demonstrate the effectiveness of longer-term performance assessments for evaluating the benefits of remedial actions. The specific objectives of the project were as follows:
- Assess the long-term performance of fracturing-enhanced in situ biological reduction (ISBR) remediation for a specific application site.
- Compare the observed performance of the fracturing-enhanced in situ biological reduction (FE-ISBR) project to the prior in situ chemical oxidation (ISCO) project conducted at the same site.
- Compare the observed performance of the FE-ISBR project to other projects that have employed fracturing to enhance amendment delivery for ISBR or in situ chemical reduction.
- Compare the observed performance of the FE-ISBR project to other ISBR projects.
- Evaluate the effectiveness of alternative methods for assessing performance.
- Develop an integrated approach for conducting contaminant elution and tracer tests to support site characterization and performance assessments.
The project was conducted using data and information obtained for remedial actions (RA) conducted at the Air Force Plant 44 site, which is part of the Tucson International Airport Area (TIAA) federal Superfund site located in Tucson, AZ. The TIAA site comprises several primary source zones and a large, several kilometer long, groundwater contaminant plume that resides in the regional aquifer. Trichloroethene and 1,4-dioxane are the primary contaminants of concern (COC) for the regional groundwater plume. Chromium is also present in localized zones.
Two separate remediation projects have been conducted at the three designated DNAPL source zones of the Air Force plant 44 (AFP44) site, ISCO and fracturing-enhanced ISBR. Both projects focused on the interface between the vadose zone and saturated zone. This interface region, which consists of primarily lower-permeability (clay) media, has been identified as the primary location for remaining COC. Slow release of COC from this domain is considered to be the primary cause of the observed delayed attainment of cleanup objectives. The two remediation projects were conducted by different contractors working with the U.S. Air Force.
The project was designed to accomplish a robust, long-term assessment of the performance of ISCO and fracturing-enhanced ISBR remediation technologies, and to compare performance to other RA projects. The objectives of the project were met. Performance monitoring data were obtained for a period of greater than three years after completion of ISBR RAs at AFP44. The percent reductions obtained for the RAs conducted at AFP44 are consistent with results reported for prior RAs.
The project has generated information that will benefit remedial project managers (RPM) and other end-users at sites impacted by persistent groundwater contamination. The dissemination of this information will improve the effective implementation of these technologies, which has the potential to substantially reduce the Department of Defense cost burden for management of groundwater contaminated sites. It is anticipated that the project outcomes will be relevant to RPMs, regulators, and other stakeholders for these sites.
Brusseau, M.L. and Z. Guo. 2018. The Integrated Contaminant Elution and Tracer Test Toolkit, ICET3, for Improved Characterization of Mass Transfer, Attenuation, and Mass Removal. Journal of Contaminant Hydrology, 208:17-26.
Guo, Z. and M.L. Brusseau. 2017. The Impact of Well-Field Configuration and Permeability Heterogeneity on Contaminant Mass Removal and Plume Persistence. Journal of Hazardous Materials, 333:109–115.
Guo, Z. and M.L. Brusseau. 2017. The Impact of Well‐Field Configuration on Contaminant Mass Removal and Plume Persistence for Homogeneous versus Layered Systems. Hydrological Processes, 31(26):4748-4756.