Determination of radon partition coefficients between water and organic liquids and their utilization for the assessment of subsurface NAPL contamination

The inhomogeneous distribution of radon between water and non-aqueous phase liquid (NAPL) allows for utilizing naturally occurring radon as aqueous tracer for the assessment of residual NAPL contamination of aquifers (“NAPL source zones”). For the qualitative assessment (i.e. the localization) of NAPL source zones depleted radon concentrations that occur locally in the groundwater can be used as indicator. However, quantitative estimation of the NAPL saturation of the aquifer pore space requires the knowledge of the radon partition coefficient specific for the NAPL present. While radon partition coefficients are known for a wide range of pure substances, few reliable data is available on radon partitioning into complex NAPL mixtures. At the same time, widely used NAPL mixtures, such as diesel fuel, gasoline, and kerosene, have to be named as main contaminants at many NAPL contaminated sites. The paper presents radon partition coefficients for the three NAPL mixtures mentioned, achieved by application of an analytical method based on radon partitioning between air, water, and NAPL in a closed system, which is described in detail. Based on the experimental results a respective potential theoretical approach employing the regular-solution theory of Hildebrand and Scatchard is discussed and evaluated critically. Finally, the general practical applicability of naturally occurring radon as an indicator for the quantitative evaluation of NAPL source zones is assessed through laboratory experiments carried out in NAPL-contaminated sand columns. The distinct negative correlation between radon concentration and NAPL saturation of the pore space suggests the general applicability of radon for quantitative estimation of NAPL contamination of aquifers.