LAUSR

The Watarase River, running through Japan’s northern Kanto region, has a long history of trace-metal contamination originating from the Ashio Copper Mine. Given the historical importance of incidents at this mine, understanding spatiotemporal environmental changes in the river, including changes in water quality, is important. By using long-term water-quality monitoring data (1960–2010), we aimed to reconstruct the spatiotemporal changes in six water-quality variables—the concentrations of three metals (copper, zinc, arsenic), biochemical oxygen demand (BOD), chemical oxygen demand (COD), and concentration of nitrate-nitrogen—along the Watarase River using generalized additive mixed models. The modeling results clearly demonstrate that during the 1960 and 1970s, metal pollution levels (as represented by copper and zinc) greatly decreased (from 450–2300 to 8–39 μg Cu L−1 and from 490–1500 to 17–52 μg Zn L−1), whereas organic pollution levels, as represented by the BOD and COD increased. Unique changes were observed in the cases of arsenic and nitrate-nitrogen (e.g., marked increases in the 1960s). From the 1980s until 2010, gradual decreases in the levels of metal and organic pollution were generally observed. Only in the 2000s were annual mean concentrations of copper in the lower reaches of the Watarase River lower than the US Environmental Protection Agency (EPA) water-quality criterion.