How combining multi-scale monitoring and compound-specific isotope analysis helps to evaluate degradation of the herbicide S-metolachlor in agro-ecosystems?

Abstract. The presence of pesticides in surface water poses a significant risk to the quality of drinking water resources. A critical challenge in water quality management involves quantifying the export, degradation, and persistence of pesticides at the catchment scale. Compound-specific isotope analysis (CSIA) may help to evaluate the contribution of pesticide biodegradation in topsoil and water, as it is generally unaffected by non-degradative processes such as dilution, sorption, and volatilisation. In this study, multi-scale monitoring with CSIA was combined with a mass balance approach to determine the source apportionment and degradation contribution to the overall dissipation of S-metolachlor, a widely used herbicide, in the Souffel catchment (115 km²) during a corn and sugar beet growing season. The mass balance, including topsoil, river water, sediment, and wastewater treatment plant (WWTP) effluent, showed that 98.9 ± 4.7 % (𝑥̅ ± SD) of S-metolachlor applied during the study period was degraded over the five-month growing season. Most degradation occurred in the topsoil, with only 12.3 ± 3.1 % degraded in the river. CSIA-based estimates of S-metolachlor degradation corroborated the mass balance results, indicating that 98 ± 20 % of S-metolachlor was degraded over the growing season. WWTPs contributed to 52 ± 18 % of the input mass based on daily discharges. However, S-metolachlor from non-point and point sources could not be clearly distinguished due to similar stable isotope signatures. Despite this limitation, our results demonstrate that pesticide CSIA, applied from upstream to downstream, enabled robust estimation of pesticide degradation across an entire catchment with relatively low sampling and analytical effort. We anticipate that CSIA will enhance surface water management by improving the diagnosis of pesticide off-site transport and degradation. This approach can support the development of efficient regulatory strategies aimed at preserving and restoring aquatic ecosystems.