Leaching of unexpected cyazofamid degradation products into groundwater demonstrates gaps in current pesticide risk assessment

To investigate the formation and leaching potential of degradation products N,N-dimethylsulfamide (DMS) and dimethylsulfamic acid (DMSA) from cyazofamid under real-world agricultural conditions, the fungicide cyazofamid was applied in a potato crop as part of the Danish Pesticide Leaching Assessment Programme (PLAP). Leaching of DMS, DMSA, 4-chloro-5-(4-methylphenyl)-1H-imidazole-2-carbonitrile (CCIM), and 4-chloro-5-(4-methylphenyl)-1H-imidazole-2-carboxylicacid (CTCA) was monitored in water from the variably saturated zone (suction cups) and groundwater for more than two years following the applications. In total, 424 samples were analyzed for the content of the four degradation products. An additional laboratory study was executed in parallel with the field monitoring study. Here, cyazofamid was applied to soil columns and leaching of the four degradation products was studied under controlled conditions. In the EFSA conclusion on cyazofamid, CCIM and CTCA are mentioned as major relevant metabolites; DMS is not mentioned in the risk assessment and DMSA is only included in acute oral toxicity studies and an in vitro bacterial mutation assay. In contrast to the EFSA conclusion on cyazofamid, our studies showed no leaching of the two major metabolites, CTCA and CCIM, but instead, major leaching of DMS and DMSA in both the field and laboratory studies was observed. That is, both DMS and DMSA leached to the groundwater in concentrations >0.1 μg/L for more than half a year. Based on this, we suggest improvements to the current pesticide risk assessment.