Radon-222 as a Natural Tracer for Monitoring the Remediation of NAPL Contamination in the Subsurface

Nonaqueous phase liquids (NAPL), including chlorinated solvents, aromatic hydrocarbons, and other volatile organic chemicals (VOC), are common contaminants at Department of Defense (DoD) and other federal and non-federal sites. Residual or pooled NAPL contamination provides a long-term source of contamination as it slowly dissolves into groundwater. A major obstacle preventing cost-effective soil and groundwater cleanup at many DoD sites is the current inability to accurately and inexpensively locate and quantify NAPL contamination. This final report describes the use of naturally occurring radon-222 (Rn) as a partitioning tracer for locating and quantifying NAPL contamination in the subsurface and for monitoring changes in NAPL quantities resulting from remediation activities. Radon-222 possesses unique physical properties that make it a useful natural partitioning tracer for detecting and quantifying NAPL. Rn is produced in the subsurface by the continuous decay of naturally occurring radium-226. In the absence of NAPL contamination, the aqueous Rn concentration quickly reaches a site-specific equilibrium value determined by the mineralogy and porosity of the geologic formation. In the presence of NAPL, however, the Rn concentration is substantially reduced due to partitioning of Rn into the organic NAPL phase. Moreover, the reduction in Rn concentration of groundwater in contact with a NAPL phase is quantitatively correlated with the quantity of NAPL present, as described by simple equilibrium models. Thus, the method is based on measuring Rn in groundwater samples from existing monitoring wells.

The original document contains color images. Project No., ER-9916.