TOKYO, May 20 (Kyodo) — The government of Gunma Prefecture said Sunday that the level of hazardous formaldehyde in the upstream of the Tone River was below the national standard, after the substance was detected in concentrations above the limit at filtration plants in three eastern Japan prefectures.

The level of the substance in the river waters in seven locations was below the 0.08 milligram per liter allowed under the water quality standards set by the central government, according to the local government.

Chiba, Saitama, and Gunma prefectures halted operations of some water filtration plants Friday and Saturday after formaldehyde was detected, leading to the water supply being cut off in five cities in Chiba Prefecture, affecting some 340,000 households.

The contamination was believed to have originated from the upstream area of a river system covering a wide swath of the Kanto region centering on Tokyo. Saitama Prefecture officials said Saturday the upstream area of the Karasu River, a Tone River branch in Gunma Prefecture, could be a potential source of the contamination, as there are businesses in the area dealing with substances that generate formaldehyde.

The Gunma government said it also checked the runoff from a chemical plant in the city of Takasaki which is dealing with a chemical that generates formaldehyde, but the level of the hazardous formaldehyde was below the designated standard.

Saitama Prefecture halted water intake and supply at one of its filtration plants after formaldehyde exceeding permitted limits was found in tap water there, while neighboring Chiba stopped water intake at three plants after detecting the substance. Gunma Prefecture has followed suit.

Operations at most of the affected plants, except one in Noda city in Chiba Prefecture, resumed after levels of the substance dropped and remained below the national standard as a result of filtration or diluting with water held in reserve, according to the local governments.

At 4 a.m. Sunday, water supply resumed in Noda, the Chiba prefectural government said, adding that it will soon restart operation at the filtration plant in the city that has halted operation.

The Gunma prefectural government said it collected water from the Karasu River and its branches in seven locations, as well as from the runoff, and combined with chlorine to generate formaldehyde. But the levels of the substance in the water from all locations and runoff were below the national standard.

The local government said it will continue to investigate the source of the contamination.

Formaldehyde could be generated by a combination of organic substances present in the runoff from chemical plants and chlorine used at filtration plants to sterilize river water.

Formaldehyde can cause cancer, but the local governments said the amount of the substance detected did not pose a health risk.