日本海における重油流出事故とダム堆積物問題 : 日本海の環境保全と展望

The Russian Tanker Nakhodka spilled C-type heavy oil in the sea of Japan in 1997．Heavy oil came ashore in the Hokuriku district，Ishikawa Prefecture，Japan．Many scientists cooperated to quickly assess the environmental impacts．We carried out immediate site investigations，measured the level of air pollution，studied C-type heavy oil ingredients and their toxicity，and developed methods of removing heavy oil．Soluble substances in seawater，sediment movements，effects on intertidal organisms and bacterial bioremediations were also investigated．In this paper，we report the polluted field situation，and discuss what we have done for future action．The effectiveness of the intensive beach cleaneup was ensured by the huge amount ofwork by volunteers．
The Dashidaira Dam was built on the Kurobe River in Toyama Prefecture，Japan in1985．This dam is equipped with a flushing gate．Since the dam sediments were first flushed in December 1991，benthic fishes have decreased in quantity year by year due to repeated flushing events．
In June 2001，the flushed dam water contained a low oxygen concentration（1 to 2 mg/l）and high concentrations of organic particles．To establish the cause ofthe death of the benthic fishes，living flatfish were collected from Toyama Bay to examine their gills．These showed tissue damage and chemical changes caused by adhering expandable clays and organic matters，derived from the flushed dam sediments．For comparison，exposure experiments on rainbow trout were carried out using smectite suspensions．Observation by optical microscopy revealed that the gills had shrunk because of the smectite concentrations．Low oxygen concentrations in water and large volumes of expanding clays and organic matters have a significant effect on downstream benthic life．