Effects of pesticides and road salt on freshwater quality and nutrient dynamics: A review

Pesticides and road salts increasingly threaten freshwater ecosystems. These stressors are not independent, and the interactions among them are complex, making it difficult to predict their ecological responses. Current management approaches tend to focus on a single stressor abatement, and this may not address the cumulative impacts of these co-occurring contaminants. The widespread application of road de-icing salts, including sodium chloride, calcium chloride, and magnesium chloride, has led to elevated salt concentrations in historically freshwater bodies that induce a range of physicochemical changes such as altered dissolved oxygen concentrations, reduced benthic respiration, occurrence of hypoxic zones in stratified lakes, increased turbidity, and shifts in pH. Such changes influence the degradation, bioavailability, and toxicity of pesticides entering aquatic systems via agricultural runoff and urban effluents, which in turn disrupt aquatic nutrient cycles and degrade habitat quality. The interactions between pesticides and road salts in freshwater systems are complex, exhibiting synergistic, additive, or antagonistic effects. This review systematically evaluates the mechanistic bases of independent and combined interactions of these stressors, assessing their influence on ecosystem processes while integrating considerations of environmental justice related to freshwater resource protection. Additionally, the review identifies significant knowledge gaps, such as limited data on long-term combined effects and insufficient integration of multiple stressor dynamics in risk assessment frameworks. By synthesizing current knowledge on these interactions and highlighting critical research gaps, this review aims to inform and enhance management policies targeted at mitigating the cumulative impacts of pesticides and road salts on freshwater ecosystems.