Direct simulation of ground water age in the Rabis Creek Aquifer, Denmark

The steady-state ground water age distribution in the Rabis Creek Aquifer in Denmark is simulated directly and compared to the age estimated from observed tritium concentrations at three wells. The simulation code is based on an existing solute transport code, modified to include a zero-order rate term of unity to account for the aging of ground mater by one day per one day elapsed. The directly simulated ages of the observed tritium peaks agree to within less than four years with the observations. The tritium peaks would have been located to within 2 m vertically if the directly simulated ages had been used to select sampling depths in the three wells. An analytical solution that assumes that the geology is uniform and rectangular is shown to be a useful tool as a firsthand estimate of the age distribution. However, ages from a numerical direct age simulation that account for nonuniform aquifer properties agree better with the observed ages.