Pesticide fate at catchment scale: conceptual modelling of stream CSIA data
Stefanie R. Lutz1, Ype van der Velde2, Omniea F. Elsayed3, Gwenaël Imfeld3, Marie Lefrancq3, Sylvain Payraudeau3, and Boris M. van Breukelen41UFZ Helmholtz Centre for Environmental Research, Department Catchment Hydrology, Theodor-Lieser-Str. 4, 06120 Halle (Saale), Germany 2Department of Earth Sciences, Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands 3Laboratoire d'hydrologie et de Géochimie de Strasbourg (LHyGeS), Université de Strasbourg/ENGEES, 1 rue Blessig, 67084 Strasbourg cedex, France 4Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Water Management, Stevinweg 1, Delft, The Netherlands
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 at 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 at 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 at 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 concentrations and CSIA data, and advocates the use of travel time distributions for assessing pesticide fate and transport at catchment scale.
Lutz, S. R., van der Velde, Y., Elsayed, O. F., Imfeld, G., Lefrancq, M., Payraudeau, S., and van Breukelen, B. M.: Pesticide fate at catchment scale: conceptual modelling of stream CSIA data, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2017-202, in review, 2017.