A ³ H/ ³ He Study of Ground Water Flow in a Fractured Bedrock Aquifer

Abstract

Abstract The concentrations of tritium, helium isotopes, and neon have been measured in ground water 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 3 He. The identification of the composition of these excess components allows reliable separation of the tritiogenic 3 He concentration, and hence the calculation of the 3 H/ 3 He ages of the ground water. Comparison of the combined tritium plus tritiogenic 3 He concentrations with the historical record of tritium input from precipitation confirms that the 3 H/ 3 He data are self‐consistent and provides constraints on the degree of mixing or helium loss. The distribution of 3 H/ 3 He ages is related to the large‐scale topography of the study area and the depth of the wells. Furthermore, correlations between the 3 H/ 3 He ages and concentrations of total dissolved solids and total CO 2 show that the ages contain meaningful information related to the temporal changes of the ground water chemistry.