Occurrence and Risk of Systemic Insecticides and Metabolites in Source and Tap Water of High-Rice-Producing Regions in Japan

The extensive application of persistent systemic insecticides in Japanese rice paddies has raised concerns about contamination of surrounding water bodies, including their metabolites. Nevertheless, data on their presence in source and tap water, which may represent human exposure pathways, are scarce. We conducted monthly monitoring of the source and tap water from two major rice-growing regions, Akita City and Ogata Village, analyzing 11 systemic insecticides and six neonicotinoid metabolites using liquid chromatography–tandem mass spectrometry (LC-MS/MS). Detected concentrations were compared with regulatory benchmarks and target values to estimate potential risks based on hazard quotient (HQ) and hazard index (HI) values. Systemic insecticides were detected in all samples, with maximum total concentrations of 943 ng/L in the source water and 572 ng/L in the tap water in August. Dinotefuran was the predominant compound, while its metabolite dinotefuran-UF, known for higher toxicity than dinotefuran, appeared in higher concentrations in tap water than in source water. Although all HQ and HI values were less than 1, potential chronic and mixture effects, along with the absence of metabolite-specific regulatory standards, highlight the need for continued monitoring and risk assessment to protect drinking water safety.