A multi-tracer study in a shallow aquifer using age dating tracers 3H, 85Kr, CFC-113 and SF6 -: Indication for retarded transport of CFC-113

We describe: field observations and numerical simulations of the environmental tracers H-3, Kr-85, CFC-113 (C2Cl3S1), SF6 and Ne in groundwater. The field site is a well-characterised shallow aquifer in central Germany, consisting of basalts of Miocene age, overlain by up to 15 m of loess deposits. A two-dimensional numerical model was used to simulate tracer transport at the study site. Simulated and observed tracer concentrations show acceptable agreement for most wells and tracers. Due to the variable thickness of the loess cover, residence times of H-3 in the unsaturated zone are highly variable with values ranging from I to more than 30 years. This effect explains the observed variability of H-3 in the saturated zone. Excess air in groundwater requires correction of measured concentrations of the dissolved gas tracers. A maximum excess of the SF, content in water compared to the theoretical solubility equilibrium concentration of 28% was observed. A novel iterative method is used to correct for excess air. CFC-113 transport seems to be retarded. On the basis of the effective porosity for SF6, Kr-85 and H-3 transport, a retardation factor of R = 1.5 for CFC-113 with respect to SF6, H-3 and Kr-85 can be derived. Together with nonadsorbing tracers, such as Kr-85 or SF6, CFC-113 can therefore serve as a reactive retardation tracer exploring mean sorption characteristics of the aquifer material. Extrapolation of CFC-113 sorption characteristics to organic contaminants with comparable physicochemical properties (e.g. chlorinated hydrocarbons) offers an opportunity for improved assessment of the behaviour of this important group of contaminants in groundwater. (C) 2001 Elsevier Science B.V. All rights reserved.