Salt to stream: a process-based integrated watershed model for urban stream chloride from winter salts using SWMM

ABSTRACT Infographic showing a 30% increase in stream chloride loads in Etobicoke Creek from 2011 to 2020, driven by a 33.87% rise in impervious surfaces, illustrated through a conceptual model and subcatchment map. Freshwater salinization (FS) has led stream chloride (Cl) to be a ubiquitous threat in urban watersheds with cold climates. Currently, there are no urban watershed models that integrate long-term hydrological processes with Cl transport for response to changes in climate, land use, and watershed management. We created such a model for a heavily urbanized watershed. We compared the role of sampling frequency on the model's outcomes, and tested its ability to capture exceedances over chronic and acute toxicity thresholds. Using data from daily streamflow and monthly Cl, we calibrated for 2018–2020, then validated with the different climate and land use conditions over 2011–2013. The hydrologic model performed satisfactorily throughout calibration and validation periods. The water quality model using monthly samples was able to consistently capture monthly averages in stream Cl. The model showed a 30% increase (comparable to the observed 31%) in Cl loads corresponding to an increase of total impervious surfaces by 34% from 2011–2013 to 2018–2020. Using hourly Cl data from 2021 to 2022, it captured chronic exceedances within 3% of observations, but failed to model acute events. We discussed the implications of such models for future scenarios in climate, land use, and management, current challenges in urban FS modeling, and recommendations.