Résumé
Assessing the role of agricultural lands in pesticide contamination of water ecosystems is critical for water management agencies and policymakers when formulating effective mitigation strategies. Current approaches based on concentration measurements are often insufficient to evaluate the contribution of pesticide dissipation processes in complex agroecosystems. This study focuses on the dissipation of profenofos insecticide within plots subject to intensive agriculture in the Berambadi watershed (India). We examined profenofos dissipation kinetics and related carbon isotopic fractionation in laboratory volatilisation, hydrolysis, photolysis and soil biodegradation experiments, and in a field plot experiment. Process-specific isotope fractionation analyses revealed significant carbon isotope fractionation, with epsilon(C) =- 2.0 f 0.8 %o during UV photolysis, and epsilon(C) =- 0.9 f 0.4 %o during biodegradation of profenofos in the soil. Accordingly, the formation of 4-bromo-2-chlorophenol and another profenofos transformation product indicated the cleavage of O-P and C-Br bonds in soil experiments. By integrating dissipation kinetics, compound-specific isotope analysis (CSIA), transformation products analysis and modelling results, biodegradation was identified as the dominant dissipation process in the agricultural plot, accounting for > 90 % of profenofos dissipation. Model predictions were consistent with the observed dissipation kinetics and isotopic data, confirming the fast degradation (T-1/2 = 1.1 +/- 0.6 day) and low (< 0.02 %) leaching potential of profenofos, which was not detected in the local groundwater monitored by passive samplers (POCIS). Overall, these results highlight the usefulness of profenofos CSIA to identify and unravel dissipation processes in tropical agroecosystems for improving contamination risk assessment.