TRITIUM AND HE-3 AS GROUNDWATER AGE TRACERS IN THE BORDEN AQUIFER

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