UPDATE2: Hazardous substance found at water plants in eastern Japan.

TOKYO, May 19 Kyodo

(EDS: RECASTING, UPDATING WITH INFORMATION)

Three eastern Japan prefectures halted operations of some of their filtration plants Friday and Saturday after hazardous formaldehyde was detected, leading to the water supply being cut off in four cities in Chiba Prefecture.

According to the local governments, the contamination is believed to have originated from the upstream area of a river system covering a wide swath of the Kanto region centering on Tokyo, but the incident will not pose a health risk.

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 also followed suit.

Operations at most of the plants have resumed after levels of the substance dropped and remained stable below the national standard as a result of filtration or diluting with water held in reserve, according to the local governments.

However, operations at water plants in the Chiba Prefecture cities of Noda and Matsudo remain suspended, although the plant at Nagareyama is back in operation.

In Chiba Prefecture, water supply was cut to the whole of Noda city covering 49,000 households and part of Kashiwa, Abiko, Nagareyama cities as of early Saturday afternoon, according to the local governments. As water has been scarce in the region, supply disruption could spread to wider area in the prefecture, they said.

Water intake and supply were also halted in the city of Gyoda in Saitama and the town of Chiyoda in Gunma.

The water at the five plants affected was taken from the Tone River, which runs through the Kanto region into the Pacific, and one of its branches, the Edo River, the prefectural governments said. The local governments are now examining the water quality at the head of the Tone River in Gunma to identify the source of the contamination.

Officials of Saitama Prefecture said Saturday that the upstream area of the Karasu River, also a Tone River branch, in Gunma Prefecture could be a potential source of the contamination, noting that there are local businesses dealing with substances that generate formaldehyde.

Formaldehyde could be generated by a combination of organic substances included in the runoff from chemical plants and chlorine. Vaporized formaldehyde can cause cancer and sick building syndrome, but the local governments said the amount of the substance detected will not be harmful to health.

According to the Saitama prefectural government, 0.096 to 0.168 milligram of formaldehyde per liter of water was detected in tap water at two filtration plants -- in Gyoda and Kasukabe -- between Thursday and Friday, while the city of Noda said 0.135 mg per liter was found in water at the plant. Gunma Prefecture said the level of the substance at its plant was 0.098 mg per liter of water.

The figures exceed the 0.08 mg per liter allowed under the water quality standards set by the central government.

The Kasukabe plant continued operations, but the Saitama prefectural government said it would be halted if formaldehyde exceeding the allowable level is detected again there.