Groundwater-surface water interaction in Frenchman's Bay watershed, Ontario: Implications for urban recharge, contaminant storage and migration

Urban development has, historically, been more concerned with the hydraulic characteristics of the drainage systems and technological solutions for controlling floods and pollution than understanding the hydrological characteristics and sustainability of urban catchments. Given the complexity in evaluating the hydrologic behaviour of urban catchments, these systems remain poorly understood and an under-researched field of hydrogeology. Conventional models that solely concentrate on urban runoff as the primary cause of reduced natural recharge do not effectively deal with new sources or pathways of recharge created specifically by urbanization.;Analytical and numerical models were used to quantify and validate the elements of the urban water cycle. A modified soil moisture balance for the watershed highlighted the significance of localized recharge through preferential pathways. It was found that localized recharge through bypass flow has increased net recharge by about 36%. Moreover, infiltration of roof runoff and physical losses from water mains have essentially maintained urban recharge at preurban levels. Numerical simulations verified increased recharge that occurs following urbanization through onsite infiltration of roof runoff and bypass flow.;A chloride mass balance revealed that approximately 50% of road salt applied to the catchment is stored in the shallow subsurface. The presence of oxygen-18 depleted groundwaters with higher bicarbonate concentrations in the most urbanized part of the catchment further substantiates the existence of a highly responsive urban aquifer.;The findings of this study have important implications in our understanding of the complex behaviour of urban aquifers including groundwater-surface water interactions and contaminant flow paths.;The hydrologic cycle of Frenchman's Bay watershed, a 27 km2 highly urbanized catchment just east of Toronto, Ontario, Canada has undergone a fundamental change in the last 50 years due to increased urban cover. Isotopic hydrograph separation of groundwater and surface waters in stormflow showed that only about 15% of rainfall appears as overland flow and that groundwater accounts for 21% of the total stormflow. Furthermore, it was estimated that 35% of the total rainfall contributes to groundwater recharge.