Tritium and helium: 3 as groundwater age tracers in the Borden Aquifer

Abstract

Vertical profiles of 3 H and He isotope ratios have been measured in groundwater from the well‐characterized Borden aquifer, Ontario. The sum of 3 H and tritiogenic 3 He ( 3 He * ) is used as an equivalent nondecaying tracer, while the ratio of 3 He * to 3 H is used to compute groundwater ages. The mid‐1960s 3 H peak is not always apparent using the 3 H data because both dispersion and radioactive decay have significantly reduced the magnitude of the 3 H bomb peak. However, the sum of 3 H and 3 He * clearly defines the bomb peak at several locations. The accuracy of the 3 H/ 3 He dating method depends on the ability of the saturated zone to retain 3 He * against diffusive loss at the water table and on the amount of dispersive mixing that occurs within the saturated zone of shallow unconfined aquifers. Helium 3 confinement is strong while dispersive mixing is weak in the Borden aquifer, resulting in an excellent delineation of groundwater travel times. Computed 3 H/ 3 He age profiles are compared with travel times predicted using a previously calibrated flow model. Although the 3 H/ 3 He age profiles are vertically offset from the modeled travel times, the travel time and 3 H/ 3 He age gradients compare exceptionally well. Recharge rates have been computed using the 3 H/ 3 He age gradients and vary from 62 cm yr −1 beneath the Borden landfill to 14 cm yr −1 north of the landfill. The 3 H/ 3 He‐computed recharge agrees well with the recharge function used in previous flow modeling.