Modeling Subsurface Fate of S‐Metolachlor and Metolachlor Ethane Sulfonic Acid in the Westliches Leibnitzer Feld Aquifer

Abstract

Pesticides and their metabolites have been increasingly detected in groundwater bodies in southeastern Austria in recent years. The main objective of this study was to model the fate of the herbicide S-metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]acetamide; SMET) and the main metabolite metolachlor ethane sulfonic acid (MESA) at the Westliches Leibnitzer Feld (WLF) aquifer. For this purpose, a modeling approach based on coupling the one-dimensional vadose zone model PEARL and the two-dimensional groundwater flow and solute transport model FEFLOW was developed. To calibrate the one-dimensional pesticide fate model, we used leachate concentrations of SMET and MESA from lysimeter experiments. Additionally, samples of representative soil types in the WLF aquifer were analyzed to infer SMET-and MESA-specific fate parameters (e.g., half-life DT 50 , Freundlich sorption coefficient K foc ), which were used for the PEARL model. The results show that using SMET fate parameters derived from the lysimeter data considerably improved the fit of the simulation results with the field observations compared with the application of standard laboratory-derived fate parameters accounting for soil type differences. Although locally an overestimation of the monitoring data prevailed, the description of the subsurface fate of pesticides will improve the interpretation of concentration data and the design of mitigation measures.Abbreviations: DT 50 , half-life; MESA, metolachlor ethane sulfonic acid; NSE, Nash-Sutcliffe model efficiency; OC, organic carbon; PPDB, Pesticide Property Database; SMET, S-metolachlor; WLF, Westliches Leibnitzer Feld.In Austria, almost all drinking water is supplied by untreated groundwater.Approximately half of it originates from springs out of karstified or fractured rocks, while the other half is provided by pumping wells from sand and gravel aquifers. Because of the Austrian topography, sediment-filled river valleys and basins are also intensively used by numerous human activities such as settlements, manufacturing, and in particular agriculture. Monitoring results show that the greatest threats to groundwater quality in Austria and at the European scale originate from the application of fertilizers and plant protection products as well as the emergence of corresponding metabolites in agriculture (e.g., Loos et al., 2010).Among the vast number of plant protection products, in our present research we focused on the environmental fate of the herbicide SMET, which is often applied to maize (Zea mays L.) to combat the emergence of grass weeds. It transforms into the main metabolite MESA, which is classified as irrelevant in Austria. Thus, rather than the European drinking water limit of 0.1 mg L −1 , no general groundwater concentration limits for MESA apply (European Commission, 2003), although Austria has specified a threshold concentration in groundwater of 3 mg L −1 . Core Ideas • Lysimeter experiments allow sitespecific knowledge about the fate of pesticides. • Lysimete...

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