Numerical simulation of a multitracer study in a basaltic aquifer

This paper describes field observations and numerical simulations of the environmental tracers 3H, 85Kr, CFC-113 (C2Cl3F3) and SF6 in groundwate- r. The field site is a well-characterized shallow basalt aquifer in central Germany overlain by loess deposits of up to 15 m thickness. A two-dimensional numerical model was used to simulate tracer transport at the study site. Simulated and observed concentrations show acceptable agreement for most wells and tracers. Due to the strongly varying thickness of the loess cover, residence times of 3H in the unsaturated zone are highly variable with values ranging from 1 year to more than 30 years. Simulation results indicate that CFC-113 transport is retarded in the aquifer. This is presumably due to adsorption of CFC-113 on aquifer material. Assuming linear fast adsorption a retardation factor of R = 1.5 is needed to simulate CFC-113 measurements with the numerical model calibrated using measured concentrations of 85Kr, SF6 and 3H.