MAPPING TO RE-ESTABLISH TRITIUM AS A MODERN GROUNDWATER TRACER IN SOUTHERN ONTARIO

Priebe, Elizabeth H., Stewart M. Hamilton, Alex Lemieux, Ian D. Clark

Dans les comptes rendus d’articles de la conférence: GeoNiagara 2021: 74th Canadian Geotechnical Conference; 14th joint with IAH-CNC

ABSTRACT: The thermonuclear tritium peak of 1963 was a key hydrogeological tracer in the northern hemisphere for decades, used to estimate groundwater velocity and residence times. Today, the 1963 tritium peak has since been assimilated in most active groundwater systems by decay and dispersion. Long-term monthly tritium data collected in Ottawa show that annually-averaged concentrations in precipitation have been relatively stable since the early 1990's, with a median value of approximately 15 TU. With the assimilation of the 1963 tritium peak and the relative stability of concentrations in precipitation in eastern Ontario, we endeavor to investigate the future utility of tritium as a groundwater tracer in southern Ontario. Since its inception in 2007, the OGS Ambient Groundwater Geochemistry (AGGP) program has collected samples from 2,561 wells across southern Ontario. At every sample location, the aquifer type was identified and groundwater samples analyzed for tritium, in addition to a wide range of geochemical and isotopic parameters. The AGGP tritium dataset is globally one of the largest of its kind in spatial coverage and sample density. To investigate the utility of tritium as a present-day groundwater tracer, the AGGP data were used to interpolate a tritium surface that represents the most modern local recharge conditions. The approach was initiated by selecting a sub-set of AGGP wells that have the greatest probability of representing local recharge conditions based on well depth, aquifer type and tritium content itself. Tritium content from the sub-set of wells was then interpolated across southern Ontario using the Empirical Bayesian Kriging method. The quality of the interpolation was assessed and improved upon by reviewing and eliminating wells with large relative differences between predicted and measured tritium. The largest relative differences in measured and predicted tritium content were identified in areas of known brine upwelling on the Niagara Peninsula, in flowing artesian aquifers in the lowland areas of the Laurentian Valley and in upwelling areas near Ottawa where the groundwater chemistry is known to be influenced by Champlain Sea deposits. With anomalous samples removed, the final interpolated tritium surface delineates areas influenced by natural cosmogenic fallout, as well as areas influenced by the anthropogenic effects of nuclear power plants at Pickering, on the north shore of Lake Ontario, and at Kincardine on the eastern shore of Lake Huron. Although annually-averaged tritium levels are relatively stable in precipitation monitored at Ottawa, they are not near the nuclear power plants due to inconsistency in their operations and changing weather conditions. Despite inconsistent tritium levels in precipitation near nuclear power plants, long term monitoring shows that nearby shallow groundwater tritium levels have been stable for more than a decade. Such stability, even where precipitation inputs vary the most, supports the use of our interpolation for estimating recharge end-member tritium concentration for any location in southern Ontario.

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