Japan's Ministry of the Environment has opened a month of public comment on a draft revision to guideline concentrations for two agricultural fungicides that can persist in treated tap water: thiophanate-methyl and benomyl. The draft would tighten the thiophanate-methyl target from 0.3 milligrams per litre to 0.2 mg/L, and loosen the benomyl target from 0.02 mg/L to 0.07 mg/L, more than tripling that non-binding guideline concentration.
| Pesticide | Current target (mg/L) | Proposed target (mg/L) | Direction |
|---|---|---|---|
| Thiophanate-methyl | 0.3 | 0.2 | Tightened |
| Benomyl | 0.02 | 0.07 | Loosened |
These are not the binding water quality standards set under Article 4 of the Waterworks Law. They sit in a separate category the ministry calls "water quality management target-setting items," created by a 2003 Health Bureau Director notification to flag substances worth systematic monitoring even though they fall short of a legal standard. Individual water suppliers choose which pesticides to test based on local conditions, but the ministry's supplementary list singles out pesticides likely to turn up in finished water, including these two.
The measurement method is changing too. Under the draft, benomyl concentrations would be measured as methyl-2-benzimidazole carbamate (MBC) and then converted to a benomyl-equivalent figure. The ministry's note flags a wrinkle for testing labs: MBC can also form when thiophanate-methyl breaks down, so a lab reading for benomyl may be picking up residue from the other fungicide as well.
The ministry says the revision follows a food health impact assessment by the Food Safety Commission, a Cabinet Office body, though the notice does not detail the assessment's specific findings. Comments can be submitted in Japanese only, either through the e-Gov portal or by mail, from September 24 through October 23, 2026, under case number 195260058. The ministry plans to issue formal notification in late March 2027 and set an effective date of April 1, 2027.