PFAS in tap water: Osaka 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

Values published by local governments and utilities

Good to know

• At Shijonawate’s Tawara treatment plant (340 m³ per day), 79 ng/L was found at the treatment plant outlet on October 6, 2020, and 73 ng/L at a tap in a test on October 30. On November 5 the plant was shut down and supply switched to treated water from the Osaka Water Supply Authority’s bulk supply; because levels in the raw water did not fall afterward, the plant was closed at the end of FY2022 (MLIT case studies; the Osaka Water Supply Authority). Shijonawate now buys all its water from the authority, and the national table shows 10 ng/L for FY2025.
• Of the 43 suppliers in the prefecture, 15 buy all their water from a bulk supplier (they have no water source of their own and receive all their water from the Osaka Water Supply Authority). These are Sakai City (堺市), Neyagawa City (寝屋川市), Kadoma City (門真市), Daito City (大東市), Matsubara City (松原市) and Izumiotsu City (泉大津市), plus the Yao (八尾), Takaishi (高石), Sennan (泉南), Shijonawate (四條畷), Osakasayama (大阪狭山), Hannan (阪南), Tadaoka (忠岡), Kumatori (熊取) and Tajiri (田尻) waterworks run by the authority. In the FY2025 national table they ranged from 6 to 12 ng/L. The authority is a body formed by the 42 municipalities in the prefecture other than Osaka City, and it publishes values for each treatment plant (see the table below).
• Suppliers with their own wells or other water sources have somewhat higher values: Katano City 37 (FY2023), the authority’s Fujiidera waterworks (藤井寺水道事業) 37 (FY2022), Habikino City 33 (FY2024), the authority’s Kashiwara waterworks (柏原水道事業) 32 (FY2022) and Ibaraki City (茨木市) 32 (FY2023), all 50 or below. For Katano, Habikino, Ibaraki and Fujiidera, the testing points in the national table include raw water (before treatment).
• The Osaka City Waterworks Bureau (大阪市水道局) is a separate supplier from the authority and has three treatment plants, Kunijima (柴島), Niwakubo (庭窪) and Toyono (豊野), which draw water from the Yodo River. The national table shows 19 (FY2020) to 12 (FY2025), with testing points at the treatment plant outlet and raw water. In the city’s research survey (FY2024, 4 rounds), treated water had PFOA at 4–12 and PFOS at below 1 to 6 ng/L, one of the few cases where PFOS and PFOA are reported separately.
• Sakai City (which buys all its water from a bulk supplier) began testing at 12 taps in the city (at the ends of each distribution line) in FY2024, moved to four times a year in FY2025, and since FY2026 has done the testing itself with the city’s own analyzer (LC-MS/MS). Values through August 4, 2026 were 7–10 ng/L, less than 20% of the water quality standard of 50 ng/L. Values for the same fiscal year differ by source: for FY2025 the national table (through the end of August 2025) shows 10, while the city’s test on August 5, 2025 had a maximum of 12 ng/L. The public documents do not explain the difference.
• Among private water supplies (for a facility, such as company housing, factories and hospitals), 3 of the 99 testing points in the prefecture exceeded the provisional target value (FY2020–FY2024, Osaka Prefecture summary): a facility in Osaka City at 232 ng/L (FY2024), a facility in Ibaraki City at 94 (FY2023, since closed) and a facility in Hirakata City (枚方市) at 59 (FY2024). Under guidance from the city in charge, they stopped using that water and switched to city tap water. These are not values for the city water supplies.
• Settsu City Waterworks (摂津市水道事業) shows 11–15 ng/L in the national table (FY2020–FY2025; testing points: treatment plant outlet and raw water). The high values reported for groundwater in Settsu come from environmental measurements and are separate from tap water (see “Rivers and groundwater” below). In FY2025, all 43 rows in the prefecture’s table had a value, and none was nd (below the quantification limit).

Timeline

Rivers and groundwater (environmental monitoring)

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

• In the Ministry of the Environment’s nationwide survey (FY2019, published June 2020), groundwater in Settsu City (摂津市) had PFOS 43.6 + PFOA 1,812.0 = 1,855.6 ng/L (sampled January 22, 2020), the highest value in that survey.
• In Osaka Prefecture’s survey of July 29, 2025 (8 points in Settsu: 5 waterways and 3 groundwater), groundwater was 120–35,000 ng/L and waterways were 110–9,000 ng/L, and every point exceeded the guideline value (PFOS + PFOA 50 ng/L, a benchmark for monitoring rivers and groundwater). PFOA in the groundwater the prefecture has tested continuously since FY2007 (survey point A) fell from 26,000 in November 2007 to 8,800 in August 2012 and 4,800 ng/L in August 2015, but the prefecture says it has been “flat since FY2019” (1,700 ng/L in July 2025).
• According to Osaka Prefecture’s documents, groundwater in Settsu is not used for drinking, and Settsu City has confirmed that the raw water for its tap water is below the provisional target value. Settsu’s tap water is 11–15 ng/L in the national table.
• The same prefectural documents state that Daikin Industries’ Yodogawa Plant (ダイキン工業淀川製作所) stopped using PFOA in October 2012, that it pumps up groundwater to prevent the spread (the pumped water is treated with activated carbon and ion exchange resin and sent to the sewer), and that it began building a cut-off wall along the site boundary in November 2023 (about 20% done as of the end of August 2025). The prefecture is asking the company to speed up permanent measures to prevent outflow and to publish its own data on levels in groundwater and elsewhere on the site, among other requests.
• The Ministry of the Environment’s FAQ (Q11) cites Settsu City as a case in which the city compared regional health indicators using existing statistics and reportedly found no difference from the areas compared. → How does PFAS affect the body?
• PFOS and PFOA levels in rivers in Sakai City are published in the Sakai City Environment Bureau’s (堺市環境局) “Public Water Body Water Quality Survey Results” (linked from the Sakai City Water and Sewerage Bureau’s page). For how river and groundwater values differ from tap water values → High levels in rivers or groundwater = high levels in tap water?

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
• What happened in areas where high PFAS values were found in tap water? A timeline of Kibichuo, Tama, Okinawa and more
• 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?
• 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)
• Osaka Prefecture, “About the national ‘Survey on PFOS and PFOA in Water Supply’”
• Sakai City Water and Sewerage Bureau, “PFOS and PFOA in tap water” (updated August 31, 2026)
• Osaka Water Supply Authority, “About organic fluorine compounds (PFAS) in tap water” (updated July 21, 2026)
• Osaka City Waterworks Bureau, “FY2024 Annual Report of the Water Quality Management Research Center (No. 76),” survey results for organic fluorine compounds (PFASs) (FY2024)
• 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 9: Shijonawate waterworks
• Osaka Prefecture, “Water quality survey results for PFOA and related substances (July 2025)”
• Ministry of the Environment, “Results of the FY2019 nationwide survey on the presence of PFOS and PFOA” (June 11, 2020)

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

• Mie Prefecture
• Shiga Prefecture
• Kyoto Prefecture
• Hyogo Prefecture
• Nara Prefecture
• Wakayama Prefecture