The area under investigation concerns a 0.3 km2 island in the Danube River near Passau (F.R.G.). Measurements of 18O-content were used to determine the portion of bank infiltrated river water making up the groundwater of the island. Values of 77 and 96% have been found for different measuring points. Applying mathematical flow models to the environmental tracer data, mean transit times of 48-120 days for the bank infiltrated water and dispersion parameter (D/vx) of about 0.12 have been calculated. These estimates permitted prediction of the migration of possible pollution from the river to the observation and production wells.