PFAS in tap water: Chiba Prefecture

Levels by utility (PFOS + PFOA combined, ng/L)

These are the highest values in each fiscal year reported by each utility to the national government (MLIT and the Ministry of the Environment). A utility with several sources or treatment plants can have different levels in different parts of its area. Samples are taken at the tap in principle, but some utilities only measured raw water (water before treatment), and raw-water values are not covered by the tap-water standard. nd = below the detection or quantification limit; — = no value reported. *FY2025 covers results up to the end of August 2025. Utility names are left in Japanese as in the official table.

>50 a year above 50 ng/L (for utilities that also sampled raw water, the value may be from raw water)　raw above 50 ng/L, but measured in raw water only

Good to know

• Yachimata City’s annual maximums were 2, 36, 28, 6, 12 and 4 (FY2020–FY2025). It measures both at the tap and in raw water (water before treatment at the plant), and the table does not show which one the listed values come from. Kamogawa City was 25 (FY2023), 23 (FY2024) and 7 (FY2025).
• The Chiba Prefectural Public Enterprise Bureau (千葉県企業局) (prefectural waterworks) was 7–8 (treatment plant outlet and raw water), the Chiba City Waterworks Bureau (千葉市水道局) 6–10, and Kashiwa City (柏市) nd–10. Among the bulk water suppliers (utilities that sell water wholesale to cities and towns), the Kita-Chiba Regional Water Supply Authority (北千葉広域水道企業団) was 5–6, the Kujukuri Area Water Supply Authority (九十九里地域水道企業団) 3–14, and the Minami-Boso Regional Water Supply Authority (南房総広域水道企業団) nd–14.
• Seven utilities buy all their water from a bulk supplier (they have no water sources of their own and buy water from another utility): the Hasso Water Supply Authority (八匝水道企業団), the Sanbu District Regional Water Supply Authority (山武郡市広域水道企業団), Shiroi City (白井市), Asahi City (旭市), two utilities of Tonosho Town (東庄町), and the Inba Regional Bulk Water Supply (印旛広域水道用水供給事業). For years in which they did not test the water themselves, check the supplier’s values. → How to read the national table
• The Isumi District Regional Association Waterworks Bureau began operating in FY2025, so its only value is 11 for FY2025.
• Chiba Prefecture has 948 operators of private water supplies (for facilities such as company housing, hospitals and schools); 665 of them were tested, and 4 exceedances have been confirmed (none national-government, through the end of August 2025).
• Well water is not included in the national water supply table. In the national summary for FY2024, 629 sites in 26 prefectures (including 496 groundwater sites) exceeded the guideline value (50 ng/L). Many of them are sites that exceeded in the past and are still being monitored, or sites around an exceedance where the extent of contamination was investigated. → High levels in rivers or groundwater = high levels in tap water?
• The national table shows “the highest value for each utility in that fiscal year.” From April 2026 this becomes a water quality standard, and periodic testing is, in principle, required roughly once every three months or more often (small water supply systems can test less often based on past results, and utilities that buy all their water from a bulk supplier can skip it under certain conditions).

Put it in terms of your own intake

Related explainers

• How do you check PFAS in your local tap water? Reading the national tables and working out “what percent of the TDI”
• Do high PFAS values in rivers or groundwater mean high values in tap water? How guideline values differ from the tap water standard
• Japan’s PFAS standard for tap water, “50 ng/L”: how was it set, and how does it differ from the U.S. and EU?
• How does PFAS affect the body? Assessments by different bodies, and what a blood test can tell you
• Water filters and PFAS

Sources

• Ministry of the Environment and MLIT, “Results of the follow-up survey on PFOS and PFOA in water supplies” (December 25, 2025)
• Same, list of survey results (updated), Excel (annual maximums, FY2020–FY2025)
• Ministry of the Environment and MLIT, list of survey results, Excel (published November 29, 2024; sampling point column)
• Same, follow-up survey annex (status of private water supplies by prefecture)
• Ministry of the Environment, “Results of FY2024 water quality monitoring of public water bodies and groundwater” (March 27, 2026)
• Ministry of the Environment, notice on the enforcement of the partial revision of the Ministerial Ordinance on Water Quality Standards, etc. (No. 2506301, June 30, 2025)

Last checked 2026.10.04 (against the original government documents and papers, collected and cross-checked in Gemini Notebook). Utility values are as in the national table; Urayomi sorted and summarized them.

Other prefectures

• Ibaraki Prefecture
• Tochigi Prefecture
• Gunma Prefecture
• Saitama Prefecture
• Tokyo
• Kanagawa Prefecture