Long-term spatiotemporal changes of 15 water-quality parameters in Japan: An exploratory analysis of countrywide data during 1982-2016.

We conducted an exploratory analysis of 15 water-quality parameters collected countrywide during 1982-2016 to better understand human impacts on aquatic ecosystems in Japan. We used the Mann-Kendall test to identify temporal trends. On the basis of this analysis, we identified and ranked the sites for future management where there were trends toward lower water quality. The study showed general improvement of dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, suspended solids, Escherichia coli counts, n-hexane extracts, total nitrogen, total phosphorus, and total zinc. We concluded that management of wastewater has been effective throughout Japan, but with the caveat that conditions have deteriorated at some sites, which should be the focus of studies aimed at identifying the causes of the lower water quality. Concentrations of chloroform, formaldehyde, nonylphenol, and linear alkylbenzene sulfonates, which are toxic environmental pollutants that can have adverse effects on human and ecosystem health, showed significant increasing or decreasing temporal trends at only a few monitoring sites. Sites where concentrations of these toxicants increased should be targeted for further study to determine whether remedial actions are needed.