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

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 δ 13 C 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 δ 13 C 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 traveltime distributions for assessing pesticide fate and transport on catchment scale.

Solid-phase extraction method for stable isotope analysis of pesticides from large volume environmental water samples

“…16 Various analytical methods have been recently developed. [17][18][19][20][21][22][23][24][25][26][27][28][29] Nonetheless, application of CSIA of pesticides to field studies remain scarce 24,[30][31][32][33][34] because of two major challenges. First, isotope effect-free extraction and pre-concentration methods are required to allow CSIA at low environmentally relevant concentrations by gas chromatography-isotope ratio mass spectrometry (GC/IRMS) or -in the case of compounds like desphenylchloridazon for which GC/IRMS-based carbon isotope analysis does not work -by liquid chromatography-IRMS (LC/IRMS).…”

<i>tran</i>-SAS v1.0: a numerical model to compute catchment-scale hydrologic transport using StorAge Selection functions

“…The field of hydrologic transport focuses on how water flows through a watershed and mobilizes solutes towards the catchment outlets. The proper representation of transport processes is important for a number of purposes such as understanding streamflow generation processes (Weiler et al, 2003;McGuire and McDonnell, 2010;McMillan et al, 2012), modeling the fate of nutrients and pollutants (Jackson et al, 2007;Hrachowitz et al, 2015), characterizing how watersheds respond to change (Kauffman et al, 2003;Oda et al, 2009;Danesh-Yazdi et al, 2016;Wilusz et al, 2017), and estimating solute mass export to stream (Destouni et al, 2010;Maher, 2011). The spatiotemporal evolution of a solute is typically expressed (Rinaldo and Marani, 1987;Hrachowitz et al, 2016) as a combination of displacements, due to the carrier motion, and biogeochemical reactions, due to the interactions with the surrounding environment.…”

Carbon and nitrogen stable isotope fractionation during abiotic hydrolysis of pesticides

“…Because substantial biodegradation can occur without significant deviations of the enantiomeric fractions (EFs), ESIA has become a promising new approach that can provide insight into enantioselective fates and source apportionment of environmental organic contaminants (Badea and Danet, 2015). Although high detection limits of isotope ratio mass spectrometry and small contribution of nitrogen (i.e., only one N heteroatom in the chloroacetanilides) challenge multi-element CSIA of anilide pesticides at environmental concentrations (Lutz et al, 2017), sample preconcentration (>1000 times) recently allowed dual N and C-CSIA of S-Metolachlor at an agricultural catchment (Alvarez-Zaldívar et al, 2018). However, reference C and N isotopic fractionation ε for transformation reactions involving anilides are lacking so far.…”