Influence of groundwater discharge on stream chloride concentrations in mixed urban land use sub-watersheds receiving road salt applications

Freshwater salinization from elevated chloride (Cl) concentrations is a major threat to aquatic ecosystems in cold climate urban areas. Rising Cl levels in urban streams in summer suggest groundwater discharge is an important and increasing contributor, yet its role is poorly quantified. This study examines the influence of groundwater discharge on spatial and seasonal variations in Cl concentrations in three urban streams. Two approaches are used: (1) analysis of Cl concentration–discharge (C–Q) data over a 24-month period, and (2) assessment of longitudinal stream Cl and radon-222 inferred groundwater discharge patterns across seasons and flow conditions. Negative C–Q relationships in summer suggest groundwater is likely the dominant source of Cl, while flatter relationships during winter indicate decreased groundwater influence. Longitudinal data reveal that local groundwater contributions and surrounding land use changes can cause high spatial and temporal variation in Cl concentrations in small streams. For instance, stream Cl concentrations increased where high groundwater discharge coincided with urban land use, in contrast to declines observed in forested areas. Dilution in larger streams lessened the impact of groundwater discharge on stream Cl levels with substantial increases observed in an urbanized high groundwater discharge area in a small stream (>800 mg/L), but only minor increases observed in a larger stream (<30 mg/L). The findings indicate that sampling only at sub-watershed outlets may miss localized hotspots, potentially underestimating Cl contamination risks. More detailed spatial and temporal monitoring is essential to properly assess and manage urban freshwater salinization.