Compound-specific isotope analysis (CSIA) of pesticide residues in soil to evaluate in situ degradation over space and time

Contamination of soils by organic pollutants such as pesticides, hydrocarbons or chlorinated solvents in agricultural, urban and industrial soils is a widespread issue. Knowledge on the occurrence, extent and pathways of (bio)degradation of persistent pollutants in soil is crucial to improve the monitoring of their persistence and predict ecotoxicological risks. One of the latest important analytical developments is the coupling of gas/liquid-chromatography to continuous-flow isotope ratio mass spectrometry allowing to measure various stable isotopes ratios specific to each pollutant molecule. Starting from about the year 2000, compound-specific isotope analysis (CSIA), based on natural abundance, has successfully been applied to evaluate the occurrence and transformation pathways of industrial pollutants in groundwaters. However, the need of a sufficient mass of analyte for CSIA combined with low pesticide concentrations (sub-ug g^-1) and the co-enrichment of non-volatile soil components, leading to the so-called ‘matrix effect’ during chromatographic separation, currently challenge CSIA application to pesticide residues in soil. Here, we examined preparation procedures of soil samples to maximize the analytical performance for precise and sensitive CSIA without altering the isotope ratio of the target pesticides. Overall, our results emphasize the versatility of QuEChERS approaches as a standard preparation method for pesticide CSIA from soil samples and possible adaptations for specific matrix-analyte combinations to reach more selective extraction. Different families of pesticides with contrasted physico-chemical properties were extracted from various types of soil for CSIA from microcosms, mesocosms and field studies. No significant isotope fractionation for carbon (Δδ¹³C ≤ 1‰) and nitrogen (Δδ¹⁵N ≤ 0.5‰) was observed, despite variable extraction efficiencies. CSIA coupled to enantioselective analysis (ESIA) enabled to evaluate the degradation extent and mechanisms in soil of the chiral fungicide metalaxyl (i.e., S-MTY and R-MTY enantiomers). Significant enantioselective degradation (k_S-MTY= 0.007 – 0.011 day⁻¹ < k_R-MTY=0.03 – 0.07 day⁻¹) was associated with significant carbon stable isotope fractionation (Δδ¹³C_S-MTY from 2 to 6‰). Column mesocosm experiments showed that biodegradation of anilide herbicides and fungicides (i.e. acetochlor, alachlor, S-metolachlor, butachlor and metalaxyl) was favored in the soil solution of soil-plant systems, independently of the soil type, whereas degradation in soil remained limited. CSIA of terbutryn, an urban biocide commonly added in facade paints and renders, highlighted its persistence in outdoor soil lysimeters and its potential transport into groundwater. In a field study, we demonstrated the applicability of CSIA to track at the catchment scale the degradation and export of the pre-emergence herbicide S-metolachlor from soil to water and identify the contributing source areas. Based on maximum shifts in carbon stable isotope signatures (Δδ¹³C = 4.6 ± 0.5‰) of S-metolachlor we estimated maximum degradation in soil to have reached 96 ± 3% two months after first application. Altogether, this study emphasizes the variability degradation of different pesticides in soils and proposes a framework using CSIA to examine the contribution of pesticide dissipation processes in polluted urban and agricultural soils.