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 water, mimicking agriculturally impacted surface waters containing nitrates (20 mg L–1) and dissolved organic matter (DOM, 5.4 mgC L–1). Atrazine and S-metolachlor were quickly photodegraded by both direct and indirect pathways (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•) predominates during indirect photodegradation. UV light (254 nm) caused significant C and N isotope fractionation, yielding isotope enrichment factors ε_C = 2.7 ± 0.3 and 0.8 ± 0.1‰, and ε_N = 2.4 ± 0.3 and –2.6 ± 0.7‰ for atrazine and S-metolachlor, respectively. In contrast, photodegradation under simulated sunlight led to negligible C and slight N isotope fractionation, indicating the influence of the radiation wavelength on the direct photodegradation-induced isotope fractionation. Altogether, this study highlights the relevance of using simulated sunlight to evaluate photodegradation pathways in the environment and the potential of CSIA to distinghuish photodegradation from other dissipation pathways in surface waters.