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The Kogi River in Osaka Prefecture has the worst water quality of any river in Japan, while the Yoichi River in Hokkaido has the purest, according to an Environment Agency study released Tuesday.

The agency’s latest annual study on water quality, which was conducted nationwide in fiscal 1997, found that 78 percent of about 3,244 bodies of water surveyed met the agency’s water quality standards, an improvement of 4.4 percent from the previous year’s survey.

About 80 percent of the river areas studied and 75 percent of the sea areas met requirements, but only 41 percent of the lakes did, the study found. The figure for sea areas was the worst recorded in the past 20 years.

Evaluations were made using the biochemical oxygen demand index for rivers and the chemical oxygen demand index for lakes and the sea. Water quality deteriorates as the indexes increase.

The worst water was found in the Kogi River, with 21 mg of BOD per liter, far exceeding the maximum recommended level of 10 mg per liter. Water in the lower stream of the Yoichi River had a BOD level of less than 0.5 mg, below the mandated ceiling of 1 mg.

The lake with the worst water quality is Lake Teganuma in Chiba Prefecture, with a COD level of 23 mg per liter against a target of less than 5 mg per liter. Lake Kuttara in Hokkaido was the best on the lake list, with a COD level of 0.8 mg per liter against the required level of less than 1 mg.

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