Road salt application has caused long-term chloride increases in cold-region streams, lakes and groundwater, negatively impacting freshwater biodiversity and drinking water quality. Identifying salt vulnerable areas is useful for prioritizing winter maintenance best management practices. While inter-watershed variability in stream chloride is well-explained by urban land use extent, understanding intra-watershed variability is critical for management decisions. In Toronto's highly urbanized Black Creek catchment, in-stream sensors reveal spatial variability in chloride, but insufficient groundwater monitoring wells prevent high-resolution mapping of subsurface concentrations, limiting our ability to separate surface and subsurface chloride transport pathways. 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 samples that were 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. The spatial relationship between stream and shallow groundwater chloride patterns was also evaluated. This study 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 salt vulnerable areas.