Chloride Enrichment in Small Rivers Receiving Highway Runoff and Potential Ecological Consequences in Takizawa, Iwate, Japan

Abstract

The upper reaches of the small and medium-sized rivers examined in this study, which flow through Takizawa City, Iwate Prefecture, Japan, are adjacent to the Tohoku Expressway. Concerns exist regarding the impact of expressway runoff on aquatic organisms, as the reproduction of the freshwater pearl mussel (Margaritifera laevis) inhabiting the lower reaches has stagnated. Accordingly, water quality measurements were conducted year-round for both expressway runoff and inflowing small to medium-sized rivers, focusing primarily on chloride (Cl^-) , which is known to exert ecotoxic effects on freshwater organisms. The results confirmed an increase in chloride concentration originating from deicing agents in the expressway runoff during the freezing period from December 1st to March 31st within the target river. The chloride in the expressway drainage was diluted approximately 10 to 20 times on average during the freezing period by the time it reached the river. In rivers receiving highway runoff, chloride concentrations exhibited diurnal peaks, with multiple instances of peaks ranging from 50 to 90 mg L^-1—a concentration range raising concerns about ecological impact to aquatic organisms such as freshwater pearl mussels. This result suggested that in rivers with low flow rates, highway runoff might flow at concentrations that pose a risk to aquatic organisms.