PFAS in tap water: Kumamoto 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

• The prefecture says it tested 14 wells within about 500 m of the Uki City source in February 2025 and that all were below the guideline value of 50. Interviews with 14 nearby businesses also found no record of having or using products containing PFOS or PFOA, and in a May 2026 document the prefecture says, “at present, it is difficult to determine the cause.” → What happened in areas where high values were found?
• Koshi City had 40 in FY2023, 37 in FY2024 and 43 in FY2025, staying at 37–43 ng/L for three years in a row. In the November 2024 table, Koshi City is a utility whose testing point was “tap only” (the table does not list the testing points for values from October 2024 onward or for FY2025).
• Kumamoto City gets all of its tap water from groundwater (according to the city’s page). The national table values for the Kumamoto City Waterworks and Sewerage Bureau (熊本市上下水道局) were 20–29 in FY2021–FY2025 (in the November 2024 table, its testing points were the tap, the treatment plant outlet and raw water). The city says it has tested tap water regularly since FY2021 and that it has never exceeded the provisional value (50).
• The 38 (FY2024) for the Ubuyama Village (産山村) small water supply system was measured only in raw water (water before treatment). In the November 2024 table, Yamaga City (山鹿市) is a utility whose testing point was “raw water only” (the table does not list the testing point for its FY2025 value of 39). In the November 2024 table, 45 of the prefecture’s 112 rows were measured only in raw water, and those values are not necessarily the values of the water delivered to homes.
• Of the 112 rows in the national table, 14 have no testing records, and all of them are small water supply systems. Kumamoto Prefecture has published PFOS and PFOA test results (FY2024 and FY2025) for monitoring points under the prefecture’s water quality management plan for tap water, and says, “there were no points that exceeded the standard value.”
• There are 238 private water supplies (for a facility: in-house water systems for company housing, hospitals, schools and so on). In the national count, none of the 180 that were tested exceeded 50 (FY2020 to August 2025). Separately, in August 2024 the prefecture announced that 82 ng/L had been found in the source well of the private water supply at Tokai University’s Aso Kumamoto Rinku Campus (東海大学 阿蘇くまもと臨空キャンパス) in Mashiki Town (益城町). All of the prefecture’s follow-up tests (9 points in total: 7 wells and 2 points on the Kiyama River (木山川); a resample of the Tokai University well was 15) were 50 or below, and even after interviews with 11 nearby business sites, the cause “could not be identified” (September 27, 2024).

Timeline

Rivers and groundwater (environmental monitoring)

These measure rivers or groundwater themselves, not tap water. The water is not necessarily used for drinking.

• According to Kumamoto City’s page, in the city’s first survey of rivers and groundwater in FY2022, 2 of 44 points (5 river and 39 groundwater) exceeded the guideline value (provisional), both of them groundwater (55–110 ng/L). In a follow-up survey published in May 2023 (22 points within 250 m of the wells that exceeded the value), 12 groundwater points exceeded it. These are well water measurements, not tap water values.
• Kumamoto City says that water from wells exceeding the guideline value is not suitable for drinking and asks people to avoid drinking it, for example by using the public water supply instead. On December 11, 2024, the city published a summary of the detailed survey results, and on February 1, 2025, it set up an expert meeting on organic fluorine compound measures (“熊本市有機フッ素化合物対策専門家会議”). → High levels in rivers or groundwater = high levels in tap water?

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
• Do high PFAS values in rivers or groundwater mean high values in tap water? How guideline values differ from the tap water standard
• How do you check PFAS in your local tap water? Reading the national tables and working out “what percent of the TDI”
• 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: List of results of the follow-up survey of PFOS and PFOA in water supplies (published December 25, 2025)
• Ministry of the Environment and MLIT: Results of the follow-up survey (December 25, 2025)
• Kumamoto Prefecture: “Response to and investigation of the cause of PFOS and PFOA exceeding the target value at a tap water source in Uki City” (May 2026)
• Kumamoto Prefecture: “Water quality test results (PFOS and PFOA) under the Kumamoto Prefecture Water Quality Management Plan for tap water”
• Kumamoto Prefecture: “Response to the case of PFOS and PFOA exceeding the target value in the private water supply at Tokai University’s Aso Kumamoto Rinku Campus”
• Kumamoto City: “Organic fluorine compounds (PFOS and PFOA)”
• Kumamoto City: “Kumamoto, a groundwater city the world can be proud of”

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

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