Oak Ridges Moraine, Ontario: Groundwater Sustainability, Age and Mixing Scenarios Employing Environmental Isotopic Tracers

The Oak Ridges Moraine (ORM) is recognized as a regionally significant groundwater recharge area and is an important source of domestic water for the Greater Toronto Region, along with providing baseflow to hundreds of streams.  Groundwater protection and management are therefore vital for the entire region.

In complex geological environments such as the ORM, isotopic tracers and age dating are being utilized to provide an understanding of the flow system, the impacts of urbanization and municipal groundwater usage.  Changes in the residence time of waters may be correlated to usage and provide insight on groundwater sustainability. This study utilizes isotopes tracers: δ²H, δ¹⁸O, tritium and ¹⁴C. The presence of tritium and its decay over time provides another clear indicator of infiltration of younger or modern water (<60 yrs) into older systems, and can provide evidence of the vulnerability of an aquifer to anthropogenic contamination.  Radiocarbon (corrected) ages show significant amounts of very old waters (6-<44 kya) particularly in the deep tunnel channels. Low tritium waters found at the surface may indicate discharge of older waters.  Future analysis will include chlorofluorocarbons (CFCs), sulphur hexafluoride (SF₆) and rare gases.

Stable isotopic results from the study have defined four distinct groundwater types: post-glacially recharged waters with no tritium and depleted oxygen isotopic signatures, waters with a preserved climatic signal showing significant deuterium excess (18 to 22‰) at depth, a minority of samples are recharged from waters having undergone evaporative processes and represent monitoring wells with a direct connection to surface waters, and finally the majority of samples have measurable tritium (up to 183.5 TU), suggesting a large component of recently recharged meteoric water. As with Fritz et al (1987), these samples show isotopic change with geographical distribution across the study region. Mixing between these groundwater groups is prevalent as a result of usage.