We validated a SPE-CSIA method that allows analysis of isotope ratios of mixtures of pesticides that commonly occur together in environmental water samples.

Widespread application of herbicides impacts surface water and groundwater. Their metabolites (e.g., desphenylchloridzon from chloridazon) may be persistent and even more polar than the parent herbicide, which increases the risk of groundwater contamination. When parent herbicides are still applied, metabolites are constantly formed and may in addition be degraded. Evaluating their degradation based on concentration measurements is, therefore, difficult. This study presents compound-specific stable isotope analysis (CSIA) of nitrogen and carbon isotope ratios at natural abundances as alternative analytical approach to track origin, formation and degradation of desphenylchloridazon (DPC), the major degradation product of the herbicide chloridazon. Methods were developed and validated for carbon and nitrogen isotope analysis (δ 13 C and δ 15 N) of DPC by liquid chromatography-isotope ratio mass spectrometry (LC-IRMS) and derivatization-gas chromatography-IRMS (GC-IRMS), respectively. Injecting standards directly onto an Atlantis LC-column resulted in reproducible δ 13 C isotope analysis (standard deviation < 0.5 ‰) by LC-IRMS with a limit of precise analysis of 996 ng DPC on-column. Accurate and reproducible δ 15 N analysis with a standard deviation < 0.4 ‰ was achieved by GC-IRMS after derivatization of > 100 ng DPC with 160-fold excess of (trimethylsilyl)diazomethane. Application of the method to environmental seepage water indicated that newly formed DPC could be distinguished from "old" DPC by different isotopic signatures of the two DPC sources.

Core Ideas Atrazine preferential flow shortly after application was governed by soil type. Injection of atrazine at depth enhanced preferential flow. Uranine realistically illustrated the rapid and significant atrazine breakthrough. In reserve, bromide mimicked early atrazine breakthrough only with moraine soil. Using dye tracers as pesticide surrogates might assist in making sustainable decisions. The suitability of two different tracers to mimic the behavior of pesticides in agricultural soils and to evidence the potential for preferential flow was evaluated in outdoor lysimeter experiments. The herbicide atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine] was used as a model compound. Two tracers were used: a nonadsorbing tracer (bromide) and a weakly adsorbing dye tracer (uranine). Two soils that are expected to show a different extent of macropore preferential flow were used: a well‐drained sandy‐loamy Cambisol (gravel soil) and a poorly drained loamy Cambisol (moraine soil). Conditions for preferential flow were promoted by applying heavy simulated rainfall shortly after pesticide application. In some of the experiments, preferential flow was also artificially simulated by injecting the solutes through a narrow tube below the root zone. With depth injection, preferential leaching of atrazine occurred shortly after application in both soil types, whereas with surface application, it occurred only in the moraine soil. Thereafter, atrazine transport was mainly through the porous soil matrix, although contributions of preferential flow were also observed. For all the application approaches and soil types, after 900 d, <3% of the applied amount of atrazine was recovered in the drainage water. Only uranine realistically illustrated the early atrazine breakthrough by transport through preferential flow. Uranine broke through during the first intense irrigation at the same time as atrazine. Bromide, however, appeared earlier than atrazine in some cases. The use of dye tracers as pesticide surrogates might assist in making sustainable decisions with respect to pesticide application timing relative to rainfall or soil potential for preferential flow.