Applicability and limitations of the radon-deficit technique for the preliminary assessment of sites contaminated with complex mixtures of organic chemicals: A blind field-test

A blind field test with 136 independent measurements of radon (Rn-222) in soil air retrieved from a depth of 0.8 m in a decommissioned lindane (gamma-hexachlorocyclohexane) production plant was undertaken to evaluate the performance of the Rn-222-deficit technique as a screening methodology for the location and delineation of subsurface accumulations of complex mixtures of organic contaminants. Maps of Rn-222 iso-concentrations were drawn and interpreted before direct analytical information regarding concentrations of hexachlorocyclohexanes, chlorobenzenes and BTEX compounds in soil, groundwater and soil air were disclosed to the authors. The location and extension of pollution hot spots inferred from the Rn-222 campaigns agrees remarkably well with the analytical data obtained from the intrusive sampling campaigns and with the location of contaminant source zones (chemical reactor and waste-storage area) and geological sinks of those contaminants (paleochannel). Two main limitations to the applicability of the Rn-222-deficit technique were identified and assessed: The statistically significant variation of Rn-222 concentrations with diurnal changes of ground-level air temperature and the maximum depth of investigation in the absence of significant advective and co-advective transport of radon. If the influence of those two factors is accounted for and/or minimized (by averaging replicated measurements during the workday and in different days), the Rn-222-deficit technique has the potential to be an efficient technique which delivers information in quasi-real time, with a much higher spatial density than that of intrusive techniques, at a much faster rate and at a significantly lower cost. Main findings: The Rn-222-deficit technique is an effective tool for real-time site characterization only limited by diffusion length of radon and diurnal temperature variations.