Bottom sediment

Japanese: 底質 - ていしつ（英語表記）bottom sediment

The characteristics of the bottom of the water. It consists of sediments, bedrock, lava, mudflows, etc. In rivers, lakes, and coastal areas, pollutants accumulate, which can lead to water pollution, contamination of fish and shellfish, and a decline in purification capacity. Causes include sedimentation from surface water, dumping of pollutants, and the mass settling of dead organisms, such as red tides. Article 2 of the Basic Law for Environmental Pollution Control includes this as water pollution as "deterioration of bottom sediment." In the case of Tagonoura, a large amount of pollutants from pulp waste accumulated, causing a large amount of hydrogen sulfide, which not only harmed fish and shellfish but also ships and crew members, and this became a problem. After that, mercury sludge, cadmium, PCBs, and other pollution problems occurred one after another. In this way, substances are circulated between the bottom sediment and the water quality, and water pollution pollutes the bottom sediment, and harmful substances are concentrated in the bottom sediment, becoming a secondary source of pollution. Currently, provisional standards have been established to remove total mercury from rivers and lakes when it is 25 ppm or more, from the sea when it is above the danger level calculated based on the elution rate, and when PCBs are 10 ppm or more. Note that the "pollution of water quality" in the Water Pollution Control Law does not include deterioration of bottom sediments.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

水底の形質。堆積物，岩盤，溶岩，泥流などから成る。河川，湖沼，沿海では汚染物質が堆積して水質の汚濁，魚介類の汚染，浄化能の低下をきたすことがある。原因は表流水からの沈殿，汚染物質の投棄などのほか，赤潮など生物の死骸の大量沈降による場合もある。公害対策基本法 2条では「水底の底質の悪化」として水質汚濁のなかに含めている。田子ノ浦でパルプ廃液の汚濁物質が大量に堆積して硫化水素が大量に発生するなど，魚介類だけでなく船舶や乗組員にまで被害が及んで問題になり，その後，水銀へどろ，カドミウム，PCBなどの汚染が次々と問題となった。このように底質と水質との間では物質の循環が行われ，水質汚濁が底質を汚染し，底質で有害物質が濃縮されて2次汚染源ともなる。現在，総水銀が河川，湖沼で 25ppm以上，海域では溶出率などにより算出された危険度以上ある場合，また PCBが 10ppm以上ある場合は，除去するよう，暫定基準が定められている。なお水質汚濁防止法の「水質の汚濁」には，底質の悪化は含まれていない。

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