Formation of trifluoroacetic acid from common trifluoromethyl pesticides in agricultural soils

The short-chain per- and polyfluoroalkyl substances trifluoroacetic acid (TFA) has been recognized as a degradation product of only three pesticides, but many other trifluoromethyl pesticides (C-CF₃ pesticides) may potentially produce TFA based on their molecular structure. We determined TFA formation from seven C-CF₃ pesticides with high sales rates (trifluralin, fluazifop-P-butyl, fluazinam, tau-fluvalinate, diflufenican, fluopyram, and mefentrifluconazole) over 52 weeks in a laboratory experiment with three agricultural soils. TFA was formed from all seven pesticides, but TFA formation varied both among compounds and among soils. Fluopyram had the highest TFA formation, at 10.7% of the pesticide amount added in one soil, but lower formation in the two other soils (3.3% and 7.9%). Fluazinam also had relatively high TFA formation (5.3%-6.5%), but unlike fluopyram, only minor differences were seen between soils. TFA formation was lower for the other pesticides with 2.4%-5.2% for diflufenican, 3.0%-5.3% for fluazifop-P-butyl, 1.1%-2.2% for trifluralin, 0.8%-3.3% for mefentrifluconazole, and 0.4%-1.0% for tau-fluvalinate. The measured TFA formation was used to estimate how spraying with C-CF₃ pesticides may increase the annual average TFA concentrations in net precipitation infiltrating through arable topsoil (vadose zone water). The estimated TFA concentrations depended on crop type and growth stage and showed that the yearly TFA contribution from some C-CF₃ pesticides may equal the atmospheric contribution in infiltrating soil water below sprayed fields.