PFAS in tap water: Nagano 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

• In August 2020, Nagano City tested taps at 3 points in the city and found 58 ng/L at Matsushiro-machi Terao (松代町寺尾). Checking the water source, it found high levels in some of the 6 wells at the Kawaishinden water source (川合新田水源) and stopped drawing water from the two with the highest levels (Wells No. 1 and No. 4) (MLIT case studies).
• In March 2021, Nagano City set its own management target of “below 25 ng/L” and tests the wells, the water source outlet and taps every month. In July 2023 it set up an expert panel to study how to draw water, installed observation wells (wells for measuring groundwater) inside the water source area, and in March 2024 added more outside the area (case studies, as of November 2024).
• The city’s environment department tested wells and irrigation water within a 500 m radius of the Kawaishinden water source and reported that no well or environmental water exceeded the guideline value at the time (50 ng/L) (case studies).
• Nakano’s 19–20 ng/L (FY2023 and FY2024) is a “raw water only” value in the table published in 2024. Of the 178 rows for the prefecture, 52 are “raw water only,” so check the testing point when you read a value. → How to read the national table
• Among private water supplies (for a facility, such as company housing or hospitals), none of the 61 tested in the prefecture was found to exceed the provisional target value (through the end of August 2025).

Timeline

Put it in terms of your own intake

Related explainers

• What happened in areas where high PFAS values were found in tap water? A timeline of Kibichuo, Tama, Okinawa and more
• 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
• Water filters and PFAS

Sources

• Ministry of the Environment and MLIT, “Survey on PFOS and PFOA in Water Supply,” follow-up survey: list of results (published December 25, 2025)
• Ministry of the Environment and MLIT, list of results (published November 29, 2024; testing point column)
• MLIT, “Cases of PFOS and PFOA responses by water suppliers to date” (November 29, 2024), Case 1: Nagano City
• Same follow-up survey, attachment (status of private water supplies by prefecture)

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

• Niigata Prefecture
• Toyama Prefecture
• Ishikawa Prefecture
• Fukui Prefecture
• Yamanashi Prefecture
• Gifu Prefecture
• Shizuoka Prefecture
• Aichi Prefecture