VARIATIONS OF WATER QUALITY ALONG ACIDIC RIVERS IN VOLCANIC AREAS OF EASTERN JAPAN

There are many acidic rivers in Japan, due to acid mine drainage (AMD) and drainage from acidic hot springs. Low pH values and relatively high concentrations of heavy metals and other trace elements are often detected in the acidic rivers. In this study, water quality parameters including pH and concentrations of major cations and anions, heavy metals, and other trace elements were investigated in nine acidic river basins located in volcanic areas of Eastern Japan. The study included both untreated river basins and basins where neutralization treatment is carried out. Variations of water quality along two acidic river basins and the mechanisms controlling the variations in water quality were also discussed. Generally, the pH values ranged from 1.31 to 5.45 in the nine river basins. Positive effects from neutralization treatment were observed along the Agatsuma river basin where pH values markedly increased and the concentrations of As, Pb, F, Fe, and Al dramatically decreased. In comparison, along the untreated Nagase river basin, the pH values gradually increased and the concentrations of As, Pb, F, Fe, and Al steadily decreased, mainly due to the dilution effect caused by mixing with other rivers. In these two river basins, harmful elements such as As, Pb, and F were probably coprecipitated with Fe and Al hydroxides. The sediments deposited in the Nagase river bed and Lake Inawashiro may therefore contain much higher concentrations of harmful elements including As, Pb, and F as were previously reported on the Shinaki Dam of the Agatsuma river basin.