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

We describe: field observations and numerical simulations of the environmen tal tracers H-3, Kr-85, CFC-113 (C2Cl3S1), SF6 and Ne in groundwater. The f ield site is a well-characterised shallow aquifer in central Germany, consi sting 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 acceptabl e agreement for most wells and tracers. Due to the variable thickness of th e loess cover, residence times of H-3 in the unsaturated zone are highly va riable with values ranging from I to more than 30 years. This effect explai ns the observed variability of H-3 in the saturated zone. Excess air in gro undwater requires correction of measured concentrations of the dissolved ga s tracers. A maximum excess of the SF, content in water compared to the the oretical 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 tra cer exploring mean sorption characteristics of the aquifer material. Extrap olation of CFC-113 sorption characteristics to organic contaminants with co mparable physicochemical properties (e.g. chlorinated hydrocarbons) offers an opportunity for improved assessment of the behaviour of this important g roup of contaminants in groundwater. (C) 2001 Elsevier Science B.V. All rig hts reserved.