River baseflow age information inferred from environmental tracers (³H–⁸⁵Kr): a new tool for the description of surface water resources vulnerability

Groundwater is the main contributor to most surface waters bodies worldwide providing baseflow that sustains surface ecosystems as well as human societies all year round. Understanding and predicting the adverse effects of anthropogenic forcings (urbanisation, global climate change), and particularly the time lags associated with contaminant transport from recharge areas to discharge areas (typically streams) is crucial for a sustainable management of the water resources. River baseflow age information is particularly relevant for assessing shallow groundwater vulnerability to surface perturbations. However, determining groundwater age using gaseous tracers from stream measurements is challenging due to the progressive loss of tracer signals from stream water to the atmosphere. In Eastern Canada, the presence of an ice cover in winter is thought to reduce air/water exchange rates, enhancing both the precision and the scale of application of tracer studies. A multi-tracer approach (CFCs, SF₆,⁸⁵Kr, noble gases and³H) was adopted in this study to determine baseflow age information in a medium sized catchment (Nelson River, Québec, 70 km²). A dual inert gas injection setup allowed to continuously inject He and SF₆ into the partially ice-covered river. The deliberate injection of artificial gas tracers, under the unique field conditions, showed that air/water exchanges were limited compared to other studies. Due to a local contamination of the aquifer, CFCs and SF₆ could not be used for groundwater dating. However,³H and⁸⁵Kr proved to be valuable tracers of the discharging groundwater and allowed to determine a mean transit time of roughly 10 years with the exponential and the dispersion models. A new vulnerability assessment tool based on both baseflow long-term quantity and baseflow age information is proposed. The application of this methodology is very promising for helping to identify vulnerable catchments, in many areas worldwide.