Using leaky sewers to map urban groundwater chloride contamination

In many snow-affected regions, road salt application (primarily sodium chloride) has caused long-term chloride increases in streams, lakes and groundwater, negatively impacting freshwater biodiversity and drinking water quality. While regional-scale studies in Ontario, Canada have documented strong relationships between chloride concentrations and urban land use, local drivers of longitudinal stream chloride patterns within individual watersheds remain poorly understood. In Toronto’s highly urbanized Black Creek catchment, in-stream sensors reveal spatial variability in chloride, but insufficient groundwater monitoring wells prevent similar high-resolution mapping of subsurface concentrations.

Here, we leveraged leaking stormwater sewers as sampling points for shallow groundwater throughout the watershed. Water samples collected during inter-event periods in summer 2025 from storm sewer outfalls (n = 111) were analyzed for stable isotopes of oxygen and hydrogen in water and major ions. Using historical isotope signatures for groundwater, municipal water, and precipitation, we identified 70 % of the samples as likely groundwater. Among these, chloride concentrations ranged from 126 to 4,241 mg/L, all exceeding  Canada’s chronic guideline (120 mg/L). Spatial patterns indicate highest concentrations near multi-lane highways crossing the catchment, corroborating previous modelling studies. This demonstrates that leaky sewer sampling offers a novel, accessible approach for mapping shallow groundwater chloride, which is critical for understanding surface water patterns and identifying areas that are vulnerable to elevated salinity.