Assessing the toxicity and risk of salt-impacted winter road runoff to the early life stages of freshwater mussels in the Canadian province of Ontario

In temperate urbanized areas where road salting is used for winter road maintenance, the level of chloride in surface waters has been increasing. While a number of studies have shown that the early-life stages of freshwater mussels are particularly sensitive to salt; few studies have examined the toxicity of salt-impacted winter road runoff to the early-life stages of freshwater mussels to confirm that chloride is the driver of toxicity in this mixture. This study examines the acute toxicity of field-collected winter road runoff to the glochidia of wavy-rayed lampmussels (Lampsilis fasciola) (48 h exposure) and newly released juvenile fatmucket mussels (Lampsilis siliquoidea) (<1 week old; 96 h exposure) under different water hardness. The chronic toxicity (28 d) to older juvenile L. siliquoidea (7-12 months old) was also investigated. The 48-h EC50 and 96-h LC50 for L. fasciola glochidia and L. siliquoidea juveniles exposed to different dilutions of road run-off created with moderately hard synthetic water (∼80 mg CaCO₃/L) were 1177 (95% confidence interval (CI): 1011-1344 mg Cl^-/L) and 2276 mg Cl^-/L (95% CI: 1698-2854 mg Cl^-/L), respectively. These effect concentrations correspond with the toxicity of chloride reported in other studies, indicating that chloride is likely the driver of toxicity in salt-impacted road-runoff, with other contaminants (e.g., metals, polycyclic aromatic hydrocarbons) playing a de minimis role. Toxicity data from the current study and literature and concentrations of chloride in the surface waters of Ontario were used to conduct a probabilistic risk assessment of chloride to early-life stage freshwater mussels. The assessment indicated that chronic exposure to elevated chloride levels could pose a risk to freshwater mussels; further investigation is warranted to ensure that the most sensitive organisms are protected.