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.