DNAPL Detection and Delineation with the Radon-Deficit Technique: Analysis of Applicability and Limitations with a Real Case-Study

This study investigates the application of the radon-deficit technique as a real-time screening tool for detecting and delineating Dense Non-Aqueous Phase Liquids (DNAPLs) at a creosote-contaminated site. Traditional site characterization methods often fail to capture the spatial complexity of subsurface contamination. The radon-deficit method makes use of radon gas's preferential partitioning into DNAPL phases to identify contamination zones. A field study involving 558 measurements of ²²²Rn in soil gas conducted during two years at a former railroad tie treatment facility validated the technique's effectiveness, revealing previously undetected DNAPL accumulations beyond an impermeable barrier that was supposed to be containing the DNAPL accumulation. Subsequent campaigns demonstrated its predictive value for guiding monitoring well placement. Laboratory and modeling studies further clarified radon transport dynamics, suggesting that specific boundary conditions, such as deep geological radon sources, significantly influence the spatial range of applicability of the radon-deficit technique. These findings highlight the technique's effectiveness in refining sampling strategies and enhancing the understanding of DNAPL migration patterns in complex subsurface environments.