Trends and legacy of freshwater salinization: untangling over 50 years of stream chloride monitoring

Abstract

Excessive use of road salts to maintain safe winter travel conditions leads to increasing chloride (Cl) concentrations in streams, damaging the structure and function of freshwater ecosystems. Long-term increasing stream Cl trends are generally attributed to increases in urban land cover, however recent research shows that even relatively rural streams can retain Cl and exceed water quality guidelines in summer after road salting has stopped. Untangling the relative influences of long-term changes in streamflow and urban growth on Cl trends is critical for making informed decisions about road salt management. The portion of Cl trends not explained by changes in streamflow or urban growth could be due to changes in road salt application rates and/or legacy Cl in groundwater that is slowly making its way to streams. This study assessed seasonal, long-term stream Cl trends across the Province of Ontario, Canada, where urbanization accelerated and road salt management plans started to develop since early 2000s. We compared stream Cl trends over salting and non-salting seasons with urban growth estimates from two independent time periods, 1965–1995 and 2002–2018. For a subset of sites with sufficient flow data in the periods analyzed, we parsed the seasonal trends into flow and management trend components. We found that most of the variance in the management trend component in the winter salting season could be explained by urbanization, while about half of it could be explained in the summer non-salting season. We further analyzed Cl estimates in low-flow conditions to explore the extent of subsurface contributions to Cl trends, and concluded with a summary of challenges and recommendations for future studies on road salt legacy in streams.

Spatial Heterogeneity and Temporal Stability of Baseflow Stream Chemistry in an Urban Watershed

“…Sources of high‐concentration anthropogenic chemicals coupled with infrastructure geometry interact with the critical zone and affect weathering processes in unknown ways. Long‐term residence times of subsurface solute masses drive the temporal persistence observed in baseflow chemistry patterns, for example, chloride in the urban critical zone (Mazumder et al., 2021) or nitrogen from agricultural land use (Van Meter et al., 2017). The persistence of stream chemistry patterns has been recognized in recent literature (Frei et al., 2021; Gu et al., 2021) but that persistence has not been directly tied to an array of specific solute sources.…”

Road Salt Legacies: Quantifying Fluxes of Chloride to Groundwater and Surface Water Across the Chicago Metropolitan Statistical Area

“…Surface‐water increases in chloride concentrations have commonly been thought to be due to direct runoff of winter‐applied road salt. However, there is increasing interest in the role of groundwater, first as a reservoir for chloride accumulation and then as a long‐term source of chloride to stream baseflow (Corsi et al., 2015; Kincaid & Findlay, 2009; Ledford et al., 2016; Mackie et al., 2022; Mazumder et al., 2021). Along the northern shore of Lake Ontario, for example, a study from the 1980s found average chloride concentrations of 380 mg L −1 being discharged into the lake from shallow groundwater and concentrations of 64 mg L −1 from the lower aquifer (Eyles & Howard, 1988; Mackie et al., 2022).…”

“…Along the northern shore of Lake Ontario, for example, a study from the 1980s found average chloride concentrations of 380 mg L −1 being discharged into the lake from shallow groundwater and concentrations of 64 mg L −1 from the lower aquifer (Eyles & Howard, 1988; Mackie et al., 2022). In a more recent study of more than 50 southern Ontario streams, an analysis of late‐summer trends in stream chloride concentrations suggest contributions from the subsurface, which could include legacy chloride in soil and groundwater (Mazumder et al., 2021). Soils adjacent to major roadways in snow‐affected areas are particularly susceptible chloride accumulation (Green et al., 2008; H. K. Robinson et al., 2017).…”

Lethal and sub-lethal implications of sodium chloride exposure for adult Unionid mussel species; Eurynia dilatata and Lasmigona costata

“…Secondary salinization of waterways via anthropogenic inputs originating from industrial e uent, oil and gas extraction, and road salt application (Elphick et al, 2011;Cañedo-Argüelles et al, 2013;Gillis et al, 2022;Hintz et al, 2022) have the potential to signi cantly affect freshwater systems if not mitigated (Hintz and Relyea, 2019). In southern Ontario (Canada), most salt inputs to freshwater systems have been attributed to road salt application (Evans and Frick, 2001), with high concentrations linked to impervious surfaces, road density, and urbanized areas (Mazumder et al, 2021). The increased use of road salt on roadways in the last few decades have been responsible for the salinization of surface waters in temperate regions in Canada and the Northern United States (Jackson and Jobbagy, 2005; Kaushal et al, 2005;Oswald et al, 2019).…”

“…In addition to the chloride spikes from road salt that occur in winter months, 40-90% of chloride inputs can be retained in soil and groundwater allowing for year-round release into freshwater systems and an increased baseline chloride concentration (Howard and Beck, 1993;Kelly et al, 2008;Roy et al, 2015;Oswald et al, 2019). This creates the potential for year-round chronic stress for salt-sensitive biota (Mazumder et al, 2021).…”