The Gunma Prefectural Government said Sunday that formaldehyde levels in the upper part of the Tone River, a major source of Tokyo drinking water, are within normal limits.
Excessive levels of the hazardous substance cut off water to three prefectures last week by triggering alarms at filtration plants.
The formaldehyde level in the river was found in seven locations to be below the national standard of 0.08 mg per liter, Gunma officials said.
Meanwhile, the Tokyo Metropolitan Government said it halted water from one of the capital’s filtration plants after detecting abnormal levels of the chemical and said water is being supplied from other plants.
The chemical prompted Chiba, Saitama, and Gunma prefectures to shut some of their water filtration plants Friday and Saturday, cutting off water to five Chiba cities populated by some 340,000 households.
The contamination was believed to have originated upstream in a river system that covers a wide swath of the Kanto region. Saitama officials said Saturday that the upper part of the Karasu River, a Tone River tributary in Gunma Prefecture, could potentially be the source of the contamination because there are businesses in the area dealing with substances that generate formaldehyde.
The Gunma government said it also checked runoff from a chemical plant in the city of Takasaki that uses a chemical that generates formaldehyde but found levels there within norms.
Saitama halted water intake and supply at one of its filtration plants after abnormally high levels of formaldehyde were found in tap water. Neighboring Chiba halted water intake at three plants after detecting the substance, and Gunma followed suit.
Most of the plants, except one in the city of Noda, Chiba Prefecture, resumed operation after formaldehyde levels were lowered to safe levels by filtration or dilution using reserve water, the prefectural governments said.
Chiba later said water supply had resumed in Noda as of 4 a.m. Sunday and that it would restart the halted filtration plant.
Gunma said it collected water from seven parts of the Karasu, its branches and its runoff and combined it with chlorine to generate formaldehyde. But all of the levels in the sample were below the national standard.