Evaluating the seasonal variability of groundwater chloride impacts on urban streams

ABSTRACT: Increasing salinization of surface waters due to excessive chloride inputs is a growing threat to aquatic ecosystems. One source of chloride is road salt used as a de-icing agent in temperate climates. Road salt can enter freshwater streams directly through surface runoff (including stormwater drainage systems) or through infiltration into the ground and subsequent groundwater transport to the stream. Historically, research has focused on the direct runoff pathway while there have been fewer studies on the role of the groundwater pathway on seasonal chloride concentrations in urban streams. The objective of this study is to evaluate the temporal and spatial variability of groundwater inputs of chloride to urban streams. This objective was addressed by conducting field work in two urban streams located adjacent to major arterial roadways in London, Ontario. The porewater streambed chloride concentrations were measured along ~100 m stream sections monthly starting November 2020. These concentrations were typically > 120 mg/L, the long-term freshwater aquatic life guideline for chloride in Canada, during all sampling events. These data also showed substantial spatial variation in chloride concentrations, with highs > 5000 mg/L generally nearest the roadways and only 20 m away from areas at ~ 150 mg/L. There are initial indications of a seasonal change to the porewater spatial pattern. Novel electrical resistivity tomography (ERT) surveys are also being performed at one of the stream sites every month to permit time-lapse monitoring of changes in electrical resistivity (reflecting changes in chloride concentrations) in the groundwater below the streambed between sampling events. Streambed temperature and electrical conductivity mapping are used to identify groundwater discharge patterns. Finally, continuous stream electrical conductivity and stage measurements will be used to evaluate the impact that groundwater has on the year-round chloride concentrations of these two urban streams.