42-2 Chloride Contamination and Urban Water-Quality Degradation in a Second-Order Urban Stream: Evidence of Urban Stream Syndrome and Freshwater Salinization Syndrome

Abstract:

Urbanization alters hydrologic and biogeochemical processes in streams, producing characteristic degradation known as Urban Stream Syndrome (USS). In cold-climate regions, these effects are often accompanied by Freshwater Salinization Syndrome (FSS), which is characterized by increasing concentrations of dissolved ions (salts) from anthropogenic sources. This study investigated a second-order tributary of Sugar Creek in Normal, Illinois, to evaluate symptoms of USS and FSS, assess the severity of impairment, and identify likely contaminant sources. Weekly water samples were collected from upstream and downstream locations between fall 2023 and spring 2025 with any gaps in data resulting from brief pauses in research activities. Samples were analyzed for major anions, with emphasis on chloride (Cl⁻) and nitrate-nitrogen (NO₃⁻-N). Chloride concentrations were evaluated against U.S. Environmental Protection Agency aquatic-life criteria and compared with local weather conditions. Results revealed chronic chloride contamination throughout the study period at both sampling locations. Nearly all samples exceeded the EPA chronic threshold (230 mg L⁻¹), and several winter samples exceeded the acute threshold (860 mg L⁻¹). Chloride concentrations peaked during freezing conditions and snowmelt events meaning there is a probable connection between chloride concentration and winter road-salting events. While NO₃⁻-N concentrations remained below the EPA drinking-water standard, they closely tracked chloride patterns, indicating shared hydrologic pathways. Nitrate–chloride relationships suggest a mixture of urban contaminants, including road salt, septic leakage, pet waste, and other anthropogenic inputs. Road salt was identified as the dominant source of chloride and the primary driver of seasonal salinization. These findings demonstrate that even moderately urbanized Midwestern watersheds can experience severe winter salinization, chronic chloride impairment, and multiple symptoms of USS and FSS. The study highlights the disproportionate impact of urban land use on small streams and emphasizes the need for improved winter road-salt and stormwater management to protect downstream aquatic ecosystems.

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