Development of Radon-222 as Natural Tracer for Monitoring the Remediation of NAPL in the Subsurface

Abstract

EXECUTIVE SUMMARYNaturally occurring 222-radon in ground water can potentially be used as an in situ partitioning tracer to characterize dense nonaqueous phase liquid (DNAPL) saturations. The static method involves comparing radon concentrations in water samples from DNAPL-contaminated and non-contaminated portions of an aquifer. During a push-pull test, a known volume of test solution (radon-free water containing a conservative tracer) is first injected ("pushed") into a well; flow is then reversed and the test solution/groundwater mixture is extracted ("pulled") from the same well. In the presence of NAPL radon transport is retarded relative to the conservative tracer. Assuming linear equilibrium partitioning, retardation factors for radon can be used to estimate NAPL saturations. The utility of this methodology was evaluated in laboratory and field settings. Laboratory push-pull tests were conducted in both non-contaminated and trichloroethene NAPL (TCE)-contaminated sediment packs before-and after alcohol cosolvent flushing and pump-and-treat remediation; field push-pull tests were conducted in wells located in non-contaminated and light non-aqueous phase liquid (LNAPL)-contaminated portions of an aquifer at a former petroleum refinery. The laboratory and field push-pull tests demonstrated that radon retardation does occur in the presence of TCE and LNAPL and that radon retardation can be used to calculate TCE saturations. However, nonequilibrium radon partitioning and heterogeneous TCE distributions may affect the retardation factors and TCE saturation estimates. Numerical simulations were used to further investigate the influence of 1) initial radon concentration, which varies as a function of NAPL saturation and 2) heterogeneity in NAPL saturation distribution within the radius of influence of the push-pull test.A method is described for determining the partition coefficient for radon in the presence of NAPL. The method uses sequential extractions of radon into equal volume aliquots of organic solvent. The radon-laden organic liquid is then counted on a liquid scintillation analyzer with alpha-beta separation. The high quench resistance and counting efficiency of alpha particles by liquid scintillation methods are ideal for counting a variety of aromatic, aliphatic, and cyclic organic solvent and scintillation cocktail mixtures. Accurate knowledge of the instrument counting efficiency, quench, and standard solution activity are not required. Replicate measurements of the aqueous-organic radon partition coefficient on benzene, toluene, o-xylene, n-hexane, cyclohexane, trichloroethene, and perchloroethene showed excellent agreement with theoretical radon partition coefficients derived from Ostwald solubility coefficients. The method was also used to determine the aqueous-organic radon partition coefficient for several commercial liquid scintillation solutions. 4 LITERATURE REVIEW CHLORINATED ALIPHATICS AND DNAPLsChlorinated solvents have seen prolific use in the industrialized world throughout the twentieth...