A multi-tracer study in a shallow aquifer using age dating tracers 3H, 85Kr, CFC-113 and SF 6 — indication for retarded transport of CFC-113

Abstract

We describe field observations and numerical simulations of the environmental tracers 3H, 85Kr, CFC-113 (C 2Cl 3F 3), SF 6 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 3H in the unsaturated zone are highly variable with values ranging from 1 to more than 30 years. This effect explains the observed variability of 3H in the saturated zone. Excess air in groundwater requires correction of measured concentrations of the dissolved gas tracers. A maximum excess of the SF 6 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 SF 6, 85Kr and 3H transport, a retardation factor of R=1.5 for CFC-113 with respect to SF 6, 3H and 85Kr can be derived. Together with non-adsorbing tracers, such as 85Kr or SF 6, 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.