PFAS in tap water database (Japan)ExplainerChecked 2026.10.04
Do high PFAS values in rivers or groundwater mean high values in tap water? How guideline values differ from the tap water standard
The “guideline value” for PFOS and PFOA in rivers and groundwater and the “water quality standard” for tap water are both 50 ng/L, but they are different things. In FY2024, 629 sites in 26 prefectures exceeded the guideline value, but most of them are not drinking water. Including the case of Settsu City (摂津市), we explain how to read environmental values and tap water values separately.
Short answer
The guideline value for rivers and groundwater (PFOS + PFOA 50 ng/L) is a guide for stepping up monitoring, not a standard that must be met. The tap water water quality standard is a mandatory standard for water at the tap (from April 2026).
Of 3,941 sites measured in FY2024, 629 sites in 26 prefectures (496 of them groundwater) exceeded the guideline value. But these include many sites measured intensively around places where contamination was suspected, so this can’t be read as “X% of Japan’s wells exceed it.”
Even when rivers or groundwater are high, water suppliers choose their sources and treat the water at plants before supplying it. Environmental values and tap values need to be looked at separately. If you drink household well water, check your local government’s releases and its guidance on testing.
Guideline value and water quality standard: the same 50, but different things
| Guideline value for rivers and groundwater | Water quality standard for tap water | |
|---|---|---|
| Covers | Public water bodies (rivers, lakes, sea) and groundwater (including well water) | Water supplied by the water supply system (in principle, at the tap) |
| Law | Item requiring monitoring under the Water Pollution Prevention Act | Water quality standard under the Water Supply Act |
| Value | PFOS + PFOA 50 ng/L | PFOS + PFOA 50 ng/L |
| Binding force | None. A guide for stepping up monitoring (not an environmental quality standard) | Yes. Testing and compliance are mandatory |
| Timing | Guideline value (provisional) in May 2020; “provisional” removed on June 30, 2025 | Provisional target value from FY2020; water quality standard from April 1, 2026 |
- Raw water (water before treatment at the plant) is not subject to the tap water standard. Raw water above 50 does not by itself mean “tap water exceeded the standard.”
FY2024 measurements: 629 sites exceeded the guideline value
| Category | Sites measured | Sites exceeding the guideline value |
|---|---|---|
| Rivers | 1,469 | 132 |
| Lakes | 37 | 1 |
| Sea areas | 115 | 0 |
| Groundwater | 2,320 | 496 |
| Total | 3,941 (47 prefectures) | 629 (26 prefectures) |
- The 629 exceeding sites break down into 282 that exceeded in the past and are monitored continuously, 217 surveyed to check the surrounding area, and 130 newly found to exceed.
- In FY2023, 2,078 sites in 39 prefectures were measured, and 242 sites in 22 prefectures exceeded. The number of exceeding sites has risen because more sites are being measured. The increase can’t be read directly as evidence that contamination has spread.
- Measurement sites include many surveys around places where contamination is suspected. The share that exceeded can’t be written as “X% of Japan’s wells.”
- For wells that exceed, the local government instructs and advises owners not to drink the water (Ministry of the Environment, “Guide to responding to PFOS and PFOA”).
Rivers may be high, but the tap is separate: the gap between sources and treatment plant outlets
| Area | Source, raw water | Treated water, tap |
|---|---|---|
| Okinawa Prefectural Enterprise Bureau, Chatan treatment plant (北谷浄水場) | Hija River (比謝川): max 143 in FY2025 (Dakujaku River (大工廻川) 266; not a water source) | Treated water fiscal-year max: 13 in FY2025, 6 in FY2026 (April–August) |
| Ogawa water purification station (小川浄水所), Tokyo (July 2020) | Raw water 280 → 100 after the response | Tap 64 → 9 after the response |
| Akashigawa treatment plant (明石川浄水場) and Toba treatment plant (鳥羽浄水場), Akashi City (明石市), Hyogo | Above 50 in raw water (a mix of Akashi River water and groundwater) | Treated water did not exceed |
- Water suppliers lower the values of the water they supply by adding low-concentration sources, stopping high-concentration wells, treating with activated carbon and so on.
- Conversely, there are cases with almost no difference between plant inlet and outlet. At the Osaka Water Supply Authority (大阪広域水道企業団)’s Murano treatment plant (村野浄水場), values in May 2026 were 12 at the inlet (raw water) and 12 and 13 ng/L at the outlet (both well below 50). The reliable way to check the tap value is the supplier’s releases. See the guide to checking.
- Websites selling water filters and water servers sometimes present river and groundwater values as “danger zones” or “worst rankings.” Environmental values are not tap water values.
Groundwater in Settsu City, Osaka
- In the Ministry of the Environment’s FY2019 nationwide survey, groundwater in Settsu City was PFOS 43.6 + PFOA 1,812.0 = 1,855.6 ng/L (sampled January 2020), the highest in that survey.
- Osaka Prefecture continues to measure groundwater and waterways around Daikin Industries’ Yodogawa Plant (ダイキン工業淀川製作所). In the July 2025 survey, waterways were 110–9,000 and groundwater 120–35,000 ng/L, and all 8 sites exceeded the guideline value. PFOA at the prefecture’s continuous monitoring site has fallen over the long term from 26,000 in November 2007, and has been flat since FY2019.
- The prefecture’s document says “the city has confirmed that groundwater in Settsu City is not used for drinking and that raw water for the water supply is below the provisional target value for tap water.” In the national table, Settsu’s tap water was 11–15 ng/L from FY2020 to FY2025 (treatment plant outlet and raw water).
- The prefecture’s documents describe the company’s measures (pumping up and treating groundwater, construction of an impermeable wall begun in November 2023, etc.) and the prefecture’s requests to the company.
- We found no blood testing of residents by public bodies.
If you drink household well water
- Household wells are not part of the water supply system, so they are not in the national tap water tables. In June 2025, the “Guidelines for Sanitary Measures for Drinking Wells” were revised to add testing for PFOS and PFOA.
- When a nearby site exceeds the guideline value, local governments may notify users of surrounding wells or provide guidance on testing (as in Shimotsuke City (下野市), Tochigi). Check your local government’s releases and guidance on testing.
- The performance of water filters as a way to reduce it at home is compared on the water filter page.
Sources
- Ministry of the Environment press release, “Results of FY2024 water quality measurements of public water bodies and groundwater” (March 27, 2026)
- Ministry of the Environment, “Overview of the amendment to the ministerial ordinance on water quality standards”
- Ministry of the Environment press release (June 11, 2020; FY2019 nationwide survey of PFOS and PFOA occurrence)
- Osaka Prefecture, survey results for PFOA and related substances in Settsu City (July 2025)
- Okinawa Prefectural Enterprise Bureau, detection status of PFOS and related substances
- MLIT, “PFOS and PFOA response cases by water suppliers to date”
- Ministry of the Environment, FAQ on PFAS
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.
This page lists levels published by national and local government bodies and facts from standards, assessments and papers. It does not judge whether tap water is good or bad or whether it affects health. Check the latest values and measures with your local water utility.