Stable Isotopes As Tracer Of Water Cycling From Precipitation To Tap Water Across Canada

Abstract:

There is a growing need to address water security issues in Canada in relation to global warming effects and water management issues. In the field of geohydrology, stable isotopes of hydrogen and oxygen (𝛿²H and 𝛿¹⁸O) have become a popular tool to identify regional water resources issues and their underlying causes. In this study, we built the first national level maps of isotopic values in tap water using 576 tap water samples that were collected from across Canada during the summer. Observed isotopic values in tap water follow similar spatial variation patterns as precipitation with higher isotopic values in the coastal regions and the Great Lakes regions and a progressive decrease as continentality increases. The Rocky Mountains display the lowest isotopic values as expected from the rainout and temperature effect associated with mountain regions. We separated the tap water based on their sources including groundwater, river and lake. The isotopic values of tap water sourced from groundwater are well-predicted by those of local annual precipitation. Conversely, the isotopic values of tap water sourced from rivers and even more those of lakes are poorly predicted by the annual or seasonal isotopic composition of precipitation. To further analyze the dominant controls of isotopic variability in tap waters sourced from different sources (i.e., groundwater, river and lake), we constructed a total of 7 water balance models to predict the 𝛿²H and 𝛿¹⁸O variability of river water across Canada. We used a DEM to accumulate the isotopic composition of local precipitation weighed by precipitation (with or without evapotranspiration) either monthly, seasonally or annually. These water balance models are better than precipitation-only models to predict the isotopic values of tap waters sourced from rivers and lakes but do not improve the predictions of isotopic values of tap water sourced from groundwater. We argue that most of the isotopic composition of tap water sourced from groundwater represents an annual average of the isotopic composition of local precipitation. These water sources are less sensitive to seasonal isotopic variability though some seasonal recharge can occur in some regions of Canada. Conversely the isotopic composition of tap water sourced from rivers and lakes is well-predicted by the summer isotopic composition of local rivers. More regionally, we find that winter precipitation, snow melt and glaciers contribute to a large part of tap water sources around the Rockies making them more vulnerable to water balance changes. In eastern Canada, evaporative water loss associated with natural and water management processes strongly impact the tap water isotopic values. Our study highlights the cycling of water from precipitation to tap water across Canada for different tap water sources with implications for water managements, contamination risks and vulnerability to climate change. Our models and databases also provide an isotopic baseline for human forensic cases.