Radon as a Natural Partitioning Tracer for Locating and Quantifying DNAPL Saturation in the Subsurface

The inability to locate and quantify dense nonaqueous phase liquid (DNAPL) saturation in the subsurface presents obstacles to site characterization and remediation. The objective of this study is to evaluate the use of naturally occurring radon as an in-situ, partitioning tracer to locate and quantify DNAPL saturation. In the saturated zone, radon emanating from aquifer solids occurs as a dissolved gas and, due to its non-polarity, partitions into DNAPL. Partitioning between the DNAPL and aqueous phases results in retarded radon transport during groundwater flow. The radon retardation factor can be determined using single-well 'push-pull' tracer tests, enabling the calculation of the DNAPL saturation. Radon can also be used as a 'static' partitioning tracer, whereby grab samples of radon from monitoring wells in contaminated and non-contaminated portions of an aquifer are collected and compared to calculate the DNAPL saturation and to monitor saturation changes as remediation proceeds. The utility of these methods was investigated in the laboratory using a physical aquifer model (PAM). Static and push-pull tests were performed before and after contamination of a portion of the PAM sediment pack with trichloroethene (TCE). The PAM was then remediated using alcohol cosolvent and tap water flushes, and static and push-pull tests were performed to assess the efficacy of remediation. Numerical simulations were used to estimate the retardation factor for radon in the push-pull tests. Radon partitioning was observed in static and push-pull tests conducted after TCE contamination. Calculated TCE saturations ranged up to 1.4 % (static test) and 14.1 % (push-pull test), based on the numerical method modeling approach used to analyze the results. Post-remediation tests showed decreases in TCE saturations. The results show that radon is sensitive to changes in DNAPL (e.g., TCE) saturation in space and time. Recent advances in numerical modeling of radon in push-pull tests have shown the influence of TCE saturation distribution and initial radon concentrations on radon breakthrough curves and calculated TCE saturations. These advances have led to more accurate predictions of the TCE saturation in the PAM. The push-pull method was applied at a field site at Dover Air Force Base, Delaware. The site consists of an aquifer 'test cell' 27 ft long and 18 ft wide surrounded by steel pilings to a clay confining unit 40 ft below grade. Push-pull tests were performed before and after contamination of the test cell with perchloroethene (PCE). Push-pull tests performed before contamination showed no evidence of radon retardation, while tests performed after contamination showed evidence of retardation and suggested the presence of PCE.