In the northern area of Japan, stro

In the northern area of Japan, strongly acidic mine drainage discharged to the River Akagawa from the abandoned Matsuo mine had been causing the serious pollution in the downstream reaches, the River Kitakami. The influence extended as far as 160km downstream from the point receiving the mine drainage (Terui, 1978,1979). After the treatment plant to neutralize the mine drainage and remove iron and arsenic mainly was
constructed in 1982, the River Kitakami was restored to the sound water environment. However, Kikuchi et al. (1998) showed from the investigation during one year that the upper and middle reaches of the River Akagawa have remained acidic, the riverbed has been covered with the deposit of iron, and the individual number of benthic macroinvertebrates was very small even in the lower reaches. McKnight and Feder