Seasonal and stormflow chloride loads in an urban-agricultural watershed in central Illinois, USA

Manual and high-frequency measurements (n=535) of chloride (Cl-) along a low-order stream in an urban-agricultural watershed (8% urban, 87% agriculture) were conducted to investigate the importance of stormflow to Cl- transport. Sampling was conducted from February 2018 to February 2019; manual sampling occurred weekly, while stormflows were sampled at high frequency. Total Cl- export was nearly 780,000 kg, of which 42.1% occurred during winter. Stormflows, which represented 19% of the period, contributed 57.6% of the total Cl- load. The importance of stormflows varied seasonally; winter and spring storms exported nearly half of total Cl-, 29.1% and 18.2% respectively, while summer (8.2%) and fall storm (2.1%) events account for only similar to 10% of total export. A substantial portion of Cl- export, 43.4%, also occurred during baseflow, likely from accumulation of Cl- related to road salt and KCl fertilizer in groundwater. The results identified two periods of elevated Cl- concentrations: (1) flushing of road salt from impervious surfaces during and immediately following the cold season and (2) transport of salt accumulated from the dry season. Elevated discharges associated with summer and fall stormflows when road salt was not present did not generate similar Cl- loads, despite peak discharge values larger than those measured in the winter and spring. The lack of deicing application and a shift to stored Cl- during the summer stormflows created a discharge-load relationship that reached an asymptote where further increases in discharge do not correspond with increased Cl- load.

Automated summary

The importance of stormflow to chloride transport varied seasonally, with winter and spring storms contributing the majority of total chloride load, while summer and fall storm events accounted for a smaller proportion. Additionally, a significant portion of chloride export occurred during baseflow periods, likely due to accumulation of chloride related to road salt and KCl fertilizer in groundwater.