A 3H/3He study of ground water flow in a fractured bedrock aquifer

The concentrations of tritium, helium isotopes, and neon have been measured in ground mater from a fractured bedrock aquifer in a densely populated suburban area near New York City, Samples were obtained from heavily pumped production wells of a regional water supply company. Helium and neon concentrations exceed the values for air-saturated water, which is explained by the addition of unfractionated atmospheric air, radiogenic helium, and tritiogenic He-3. The identification of the composition of these excess components allows reliable separation of the tritiogenic He-3 concentration, and hence the calculation of the H-3/He-3 ages of the ground water, Comparison of the combined tritium plus tritiogenic He-3 concentrations with the historical record of tritium input from precipitation confirms that the H-3/He-3 data are self-consistent and provides constraints on the degree of mixing or helium loss. The distribution of H-3/He-3 ages is related to the large-scale topography of the study area and the depth of the wells. Furthermore, correlations between the H-3/He-3 ages and concentrations of total dissolved solids and total CO2 show that the ages contain meaningful information related to the temporal changes of the ground water chemistry.