Pesticide fate on catchment scale: conceptual modelling of stream CSIA data

Stefanie R Lutz,Sylvain Payraudeau,Boris M Van Breukelen,Ype Van Der Velde,Gwenaël Imfeld,Marie Lefrancq,Omniea F Elsayed

doi:10.5194/hess-21-5243-2017

Abstract

Abstract. Compound-specific stable isotope analysis (CSIA) has proven beneficial in the characterization of contaminant degradation in groundwater, but it has never been used to assess pesticide transformation on catchment scale. This study presents concentration and carbon CSIA data of the herbicides S-metolachlor and acetochlor from three locations (plot, drain, and catchment outlets) in a 47 ha agricultural catchment (Bas-Rhin, France). Herbicide concentrations at the catchment outlet were highest (62 µg L−1) in response to an intense rainfall event following herbicide application. Increasing δ13C values of S-metolachlor and acetochlor by more than 2 ‰ during the study period indicated herbicide degradation. To assist the interpretation of these data, discharge, concentrations, and δ13C values of S-metolachlor were modelled with a conceptual mathematical model using the transport formulation by travel-time distributions. Testing of different model setups supported the assumption that degradation half-lives (DT50) increase with increasing soil depth, which can be straightforwardly implemented in conceptual models using travel-time distributions. Moreover, model calibration yielded an estimate of a field-integrated isotopic enrichment factor as opposed to laboratory-based assessments of enrichment factors in closed systems. Thirdly, the Rayleigh equation commonly applied in groundwater studies was tested by our model for its potential to quantify degradation on catchment scale. It provided conservative estimates on the extent of degradation as occurred in stream samples. However, largely exceeding the simulated degradation within the entire catchment, these estimates were not representative of overall degradation on catchment scale. The conceptual modelling approach thus enabled us to upscale sample-based CSIA information on degradation to the catchment scale. Overall, this study demonstrates the benefit of combining monitoring and conceptual modelling of concentration and CSIA data and advocates the use of travel-time distributions for assessing pesticide fate and transport on catchment scale.

Highlights

Diffuse pollution of groundwater and rivers is a recurrent issue in agricultural catchments due to the extensive application of pesticides to arable land
This study presents the first measurements of herbicide Compound-specific stable isotope analysis (CSIA)-data on catchment scale
The simulation results underlined that assessment of pesticide degradation on catchment scale with a constant half-life might lead to overestimation of pesticide degradation in deeper soil layers and overly optimistic expectations of environmental protection

Summary

Introduction

Diffuse pollution of groundwater and rivers is a recurrent issue in agricultural catchments due to the extensive application of pesticides to arable land. Pesticide degradation on catchment scale removes pesticides from the environment, which reduces their potential impact on the ecosystem provided that pesticide transformation products are nontoxic. Pesticide concentrations vary due to degradation, and depending on, for example, the amount and timing of pesticide application Lutz et al.: Pesticide fate on catchment scale: conceptual modelling of stream CSIA data

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