Quantification of residual DNAPL within aquifer system using atmospheric noble gases and radon: Partitioning behavior...

Dense non-aqueous phase liquids (DNAPLs) are characterized by low solubility and high density, leading to extensive groundwater contamination over both spatial and temporal scales. Identification of sources and quantification of residual DNAPL is therefore critical for designing effective remediation strategies. Radon has been widely used as a tracer for DNAPL quantification due to its preferential phase partitioning behavior towards DNAPL. Building on this concept, noble gases have been proposed as potential tracers; however, they have not been generally applied, as the coefficients of Ar, Kr, and Xe between water and DNAPL are not known. Thus, this study aimed to evaluate the use of noble gases, alongside radon, as partitioning tracers for identifying and quantifying residual DNAPL in groundwater. Partition coefficients for Ar, Kr, and Xe were derived, while Xe was found most sensitive, indicating the strongest affinity for DNAPL. Correspondingly, Fe_Xe was most depleted in regions near the highest PCE concentrations. Using the derived partition coefficients, the potential residual PCE zone ("contaminated zone") was quantified within the PCE contaminated study site at the Namdong Industrial Complex, South Korea. The residual PCE inferred from noble gases agreed with radon-based estimates yet yielded a more tightly constrained range, and remained of the same order of magnitude as soil core derived values. Overall, this study demonstrates the potential of using noble gases as a partitioning tracer for DNAPL quantification, and the derived noble gas partition coefficients opens a novel window to apply noble gases in groundwater.