Please see my report in the attached pdf. 

This study investigates the degradation of the widely applied herbicide S-metolachlor at the catchment scale through a nine-month field campaign. Two complementary approaches were employed: a conventional mass balance method based on extensive water and soil sampling and concentration measurements, and compound-specific isotope analysis (CSIA). Both methods gave consistent results, indicating that approximately 98% of the applied S-metolachlor is degraded during the growing season. The degradation occurs predominantly in topsoils, while in-stream degradation is limited, primarily due to short water residence times.

This manuscript provides a significant contribution to the field by showcasing the potential of CSIA in assessing pesticide dissipation under field conditions at the catchment scale, and is well presented. It clearly reflects an extensive effort and presents a large volume of data, although navigating the SI is not always straightforward.

The study is well aligned with the scope of HESS, but several comments must be addressed:

1) Isotope fractionation associated with biodegradation in topsoils had to be modeled due to matrix-related analytical issues (I assumed this refers to coelution or high background signal? It would be valuable to explicitly mention and maybe discuss these limitations, as they are relevant for the broader CSIA community).

• It is unclear where the fractionation factor of –1.4 ‰ used in the model is coming from, as Droz et al. (2021) reported values of either –1.2 or –1.9 ‰. Clarification is needed.
• Additionally, Torrento et al. 2021 (https://doi.org/10.1021/acs.est.1c03981) reported a fractionation factor for carbon of -2.4 ‰ for S-metolachlor biodegradation in soils. It would strengthen the analysis to include a sensitivity test or alternative model run using this value for comparison.

2) Section SI 1.7 mentions that three piezometers were installed toward the end of the sampling campaign. Could the authors clarify the rationale behind this installation? What was the intended purpose, and how were the data used in the context of the study? Aside from a brief mention of groundwater electrical conductivity (P12L331), no groundwater data are presented or discussed.

Specific comments

P2L60-61: “tracking pesticide degradation under environmental conditions remains challenging due to limitations in current approaches.” Approaches are described but their limitations are not.

P3L65: “although its application has not previously been employed” please reformulate.

P3L69-70: enables and facilitates.

P5L126: this approach was employed.

P6L143: “water samples from eight monitoring sites”. It is not clear to me whether these sampling points are those shown in Figure 1 (that shows nine sampling points), or different sampling points.

P7L158: electrical conductivity.

P11L286: please correct “for the same month was the five time drier”

P11L286-287: for every month, or just for some of them (in this case which ones)?

P11L298-299: Figure S6a doesn’t really show that up to 100% of the flow comes from WWTP effluents during low-flow periods. Figure S6b does.

P12L330: conductivity observed.... compared to A1.... suggesting

P17L445-446: " However, due to a minimum carbon mass required for accurate GC-IRMS analysis ... only a subset of dissolved water samples (Fig. 4) was measurable.” Does this introduce a bias in the reported isotope values, and is this a limitation of the CSIA approach in general? The lowest concentrations are often expected to exhibit the highest levels of degradation.

P18L454: The uncertainty associated with the extent of biodegradation estimated by CSIA is high compared to that obtained from the mass balance approach, and, according to the authors, this is due to analytical limitations. This point warrants further development.

Figure 4: it would be helpful to add the Δδ13C = 0 ‰ line.

P19L492: "the apportionment of S-metolachlor with an isotopic signature distinct from that of agricultural sources". This contradicts P15L390-395, which state that the most plausible explanation for the occurrence of S-metolachlor in WWTPs is related to “releases during pesticide preparation ... or sprayer clean-out at farmyards”.

P20L498-499: “This indicates that S-metolachlor biodegradation likely occurred between downstream and upstream regions of the Souffel”. Again, this contradicts earlier discussions in the paper, which state that most of the degradation occurs in topsoils. Moreover, a substantial degree of degradation would be required to produce a measurable and significant shift in isotope values.

P20L519-520: “Currently, datasets characterising isotopic fractionation associated with the key pesticide degradation processes, such as biodegradation, photolysis, and hydrolysis, in WWTPs remain scarce”. While this is generally true for many pesticides and micropollutants, it is less true for S-metolachlor, particularly concerning carbon isotopes. See for example Torrento et al., 2021 for hydrolysis and biodegradation (https://doi.org/10.1021/acs.est.1c03981) and Levesque-Vargas et al., 2025 for photodegradation (https://doi.org/10.1016/j.chemosphere.2024.144010). Although some processes are specific to WWTPs, others are not, and enrichment factors can probably still be applied beyond their original context.

P21L526-527 and L548: Water residence times in this catchment are very short. Would this statement remain valid in a watershed with ponds, where longer residence times are expected?

Supporting information

The table of contents should be more detailed to facilitate navigation.

The Y-axis unit should be revisited in Figure S1.

Section S1.9: The text in this section suggests that fractionation factors for two different elements, C and N, were used in Equation S13 to derive a single effective fractionation factor (P10L191-192). This point requires clarification.

P15L281-282: Levesque-Vargas et al., 2025 reported isotope fractionation for S-metolachlor photodegradation, although it was limited (-0.4 ± 0.1‰ during indirect photodegradation).

Figure S6, X axis: it should be “sept” instead of “oct”.