In Japan, the target value for radioactive cesium (cesium-134 and -137) in tap water is 10 Bq/kg. Monitoring inspections are conducted regularly to measure the levels of radioactive cesium in the drinking water of Fukushima Prefecture and the neighboring prefectures of Miyagi, Ibaraki, and Tochigi, and the results are publicly disclosed.

Within Fukushima Prefecture, radionuclides in the drinking water have not been found to exceed the target values since May 5, 2011.

Reference
Ministry of Health, Labour and Welfare : Detection of radioactive materials in tap water --Fukushima prefecture-- Fukushima Prefectural Government: Fukushima Revitalization Station - Safety of Drinking Water