Nonaqueous phase liquids (NAPL), including chlorinated solvents and other volatile organic compounds (VOC), are commonly found at Department of Defense (DoD) sites. A major obstacle preventing cost-effective soil and groundwater cleanup is the current inability to accurately and inexpensively locate and quantify NAPL contamination. Radon-222 (Rn), produced by the continuous decay of naturally occurring radium-226, has unique physical properties that make it a useful “natural” partitioning tracer for detecting and quantifying NAPL. In NAPL absence, aqueous Rn quickly reaches a site-specific equilibrium; however, in the presence of NAPL, the Rn concentration is reduced due to partitioning of Rn into the NAPL. This reduced groundwater Rn in contact with a NAPL phase is quantitatively correlated with the amount of NAPL present, as described by simple equilibrium models.
The objective of this project was to verify and describe the use of naturally occurring Rn as a partitioning tracer for locating and quantifying subsurface NAPL contamination as well as a strategic tool for monitoring changes in NAPL quantities resulting from remediation activities
Two methods of using Rn were evaluated at the Dover National Test Site—a “static” method involving spatial and temporal NAPL monitoring and a “dynamic” method using single-well push-pull tests. The static method provides a means to easily survey NAPL contamination and a method for remediation monitoring, and the more complicated dynamic push-pull method potentially eliminates the nonhomogeneic subsurface complexities. In push-pull tests, Rn-free groundwater was injected along with a conservative tracer (e.g., bromide) into a standard monitoring well, using the entire well screen or a packed section to probe a specific depth interval. Upon extraction at the same location, the bromide/Rn breakthrough was monitored, which ultimately provided NAPL saturation estimates in the test well vicinity.
The results indicate that a combination of static and dynamic push-pull tests might be used to monitor NAPL remediation progress. The Rn method is best employed at a NAPL residual saturation greater than 1% and where there are existing wells within a NAPL source zone. The most cost-effective use of the method employs the static method where groundwater samples are periodically collected and Rn is monitored at a specific location over time. To note, the field test for this push-pull technology was conducted in a controlled subsurface environment to better understand potential implementation limitations. There remains concern with the use of this technology at complex DNAPL sites, indicating additional technology development may be warranted.