Direct and indirect photodegradation of atrazine and S-metolachlor in agriculturally impacted surface water and associated C and N isotope fractionation

Knowledge of direct and indirect photodegradation of pesticides and associated isotope fractionation can help to assess pesticide degradation in surface waters. Here, we investigated carbon (C) and nitrogen (N) isotope fractionation during direct and indirect photodegradation of the herbicides atrazine and S-metolachlor in synthetic agriculturally impacted surface waters containing nitrates (20 mg L-1) and dissolved organic matter (DOM, 5.4 mg(C) L-1). Atrazine and S-metolachlor were quickly photodegraded by both direct and indirect processes (half-lives <5 and <7 days, respectively). DOM slowed down photodegradation while nitrates increased degradation rates. The analysis of transformation products showed that oxidation mediated by hydroxyl radicals (HO) predominated during indirect photodegradation. UV light (254 nm) led to significant C and N isotope fractionation, yielding isotopic fractionation values epsilon(C) = 2.7 +/- 0.3 and 0.8 +/- 0.1 parts per thousand, and epsilon(N) = 2.4 +/- 0.3 and -2.6 +/- 0.7 parts per thousand for atrazine and S-metolachlor, respectively. In contrast, photodegradation under simulated sunlight led to negligible C and slight N isotope fractionation, emphasizing the effect of the radiation wavelengths on the isotope fractionation induced by direct photodegradation. Altogether, these results highlight the importance of using simulated sunlight to obtain environmentally-relevant isotopic fractionation values and to distinguish photodegradation and other dissipation pathways in surface waters.

Automated summary

Knowledge of direct and indirect photodegradation of pesticides and associated isotope fractionation can help to assess pesticide degradation in surface waters. In this study, we investigated carbon and nitrogen isotope fractionation during direct and indirect photodegradation of herbicides atrazine and S-metolachlor in agriculturally impacted surface waters. The results showed that DOM slowed down photodegradation while nitrates increased degradation rates. UV light led to significant C and N isotope fractionation, while simulated sunlight resulted in negligible C and slight N isotope fractionation, highlighting the importance of radiation wavelengths on isotope fractionation induced by direct photodegradation.