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

Abstract

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