Chloride concentrations in Lake Simcoe have increased steadily over recent decades, primarily due to winter de-icing practices associated with urban development. Understanding how chloride accumulates within different regions of the lake, and how future land-use changes may influence long-term concentrations, is essential for effective watershed and lake management. In this study, a spatially resolved chloride mass-balance model is developed that combines observed in-lake chloride concentrations with a five-segment lake mass-balance model driven by watershed chloride loads estimated using a SPARROW (SPAtially Referenced Regression On Watershed attributes) framework and spatially assigned to lake segments based on the terrestrial stream network. This structure enables explicit simulation of chloride transport and accumulation across key lake regions, including Cook’s Bay, Kempenfelt Bay, and the central basin. The model was applied to evaluate long-term chloride concentration trajectories through 2100 under a Current Conditions (baseline) loading scenario, as well as future land-use scenarios reflecting full urban build-out and improved parking-lot salt management. Under baseline scenario, model results indicate that chloride concentrations in urban-influenced bays approach or exceed the 120 mg/L chronic toxicity threshold by the latter half of the century, with earlier exceedance in Cook’s Bay and Kempenfelt Bay relative to the main lake. Improved parking-lot management scenarios substantially reduce long-term chloride accumulation and delay threshold exceedance in highly urbanized segments. This study provides a robust, policy-relevant framework for linking watershed chloride sources to in-lake concentrations and for assessing the long-term implications of land-use and management decisions on Lake Simcoe water quality.