Using compound-specific isotope analysis to assess the degradation of chloroacetanilide herbicides in lab-scale wetlands

Removal, transformation and ecological risk assessment of pesticide in rural wastewater by field-scale horizontal flow constructed wetlands of treated effluent

Constructed wetlands (CWs) are widely used in sewage treatment in rural areas, but there are only a few studies on field-scale CWs in treating wastewater-borne pesticides. In this study, the treatment and metabolic transformation of 29 pesticides in rural domestic sewage by 10 field-scale horizontal flow CWs (HF-CWs), each with a treatment scale of 36‒5000 m3/d and operated for 2‒10 years, in Guangzhou, Southern China was investigated. The risk of pesticides in treated effluent and main factors influencing such risk were evaluated. Results demonstrated that HF-CWs could remove pesticides in sewage and reduce their ecological risk in effluent, but the degree varied among types of pesticides. Herbicides had the highest mean removal rate (67.35 %) followed by insecticides (60.13 %), and the least was fungicides (53.22 %). In terms of single pesticide compounds, the mean removal rate of butachlor was the highest (73.32 %), then acetochlor (69.41 %), atrazine (68.28 %), metolachlor (58.40 %), and oxadixyl (53.28 %). The overall removal rates of targeted pesticides in each HF-CWs ranged from 11 %‒57 %, excluding two HF-CWs showing increases in pesticides in treated effluent. Residues of malathion, phorate, and endosulfan in effluent had high-risks (RQ > 5). The pesticide concentration in effluent was mainly affected by that in influent (P = 0.042), and source control was the key to reducing risk. The main metabolic pathways of pesticide in HF-CWs were oxidation, with hydroxyl group to carbonyl group or to form sulfones, the second pathways by hydrolysis, aerobic condition was conducive to the transformation of pesticides. Sulfones were generally more toxic than the metabolites produced by hydrolytic pathways. The present study provides a reference on pesticides for the purification performance improvement, long-term maintenance, and practical sustainable application of field-scale HF-CWs.

Using compound-specific isotope analysis to identify the mechanism of acetochlor degradation during oxygenation of hyporheic zone sediment

Biodegradation is recognized as the main pathway for acetochlor attenuation in aquatic environments. However, the potential abiotic degradation of acetochlor by hydroxyl radicals (•OH) generated during oxygenation of hyporheic zone sediments has not been investigated. This study aims to examine the production of •OH during oxygenation of hyporheic zone sediments and its effects on acetochlor attenuation. A significant decrease of acetochlor, ranging from 77.9% to 100%, was observed in the water-sediment systems with extensive •OH production. The primary sources of •OH production were found to be the oxidation of Fe(II) and reduced humic acids. Furthermore, a •OH quenching experiment suggests that •OH driven oxidation is the dominant pathway for acetochlor attenuation. Carbon isotope fractionation of acetochlor degradation during oxygenation of sediments (εbulk,C ranged from -1.5‰ to -0.5 ± 0.3‰) was close to that during acetochlor degradation by •OH in a H2O2-Fe3O4 Fenton system (εbulk,C = -0.5 ± 0.1‰), but significantly smaller than that during acetochlor biodegradation (εbulk,C = -5.8 ± 0.9‰). Compound-specific isotope analysis (CSIA) further suggests that •OH produced by sediment oxygenation plays a critical role in acetochlor attenuation in aquatic environments. Results of calculated apparent kinetic isotope effect of carbon (AKIEC) and transformation products indicate that SN1 and SN2-type nucleophilic substitution are the first steps in acetochlor attenuation through •OH driven oxidation (AKIEC = 1.007 ± 0.001) and aerobic biodegradation (AKIEC = 1.088 ± 0.013), respectively. Our findings highlight the potential of CSIA to assess the acetochlor degradation in water-sediment system, which can help to elucidate the fate of herbicide in aquatic environments.

Using a retention pond to capture agricultural contaminants from surface waters

To meet the demand of a constantly growing population, agriculture is intensifying, causing an increased use of fertilizers and pesticides. Excessive nutrients transfer to aquatic ecosystems can disrupt the water quality and impact the aquatic life. Pesticides can also have toxic effects on non-target organisms from aquatic systems. The purpose of this study was to evaluate the efficiency of an agricultural retention pond in reducing the supply of nutrients, pesticides and suspended solids to the Nicolet River, a tributary of Lake St. Pierre in the St. Lawrence River. Research combining the study of the fate of a wide range of contaminants in both pond water and sediments, their toxicity to microcrustaceans, microalgae and amphipods, and the effectiveness of contaminant removal, has rarely been carried out in the past. Peak contaminant concentrations occurred one to two months after pesticide and fertilizer applications, and during the months with the highest rainfall. Toxic effects were only observed on microalgae, with suspended solids apparently responsible for this effect through light inhibition on growth rates. However, the pond was not effective in removing this toxicity even if suspended solids were largely removed. Pesticides removal varied widely among sampling dates and pesticide types, with an efficiency reaching 95 % for thiamethoxam, but generally remaining low and often negative (higher concentrations in outflowing water) for other pesticides. On the other hand, the mean fractional removal of suspended solids, phosphorus, and nitrogen based on concentrations was 71 %, 44 % and 22 %, respectively. These are conservative estimations since the removal rates based on loads were above 94 %. The use of retention ponds thus seems an efficient approach to reduce the quantity of fertilizers in rivers draining agriculture areas, but the studied pond was not systematically effective in removing pesticides.

Evaluating pesticide degradation in artificial wetlands with compound-specific isotope analysis: A case study with the fungicide dimethomorph

Pesticide degradation in wetland systems intercepting agricultural runoff is often overlooked and mixed with other dissipation processes when assessing pesticide concentrations alone. This study focused on the potential of compound-specific isotope analysis (CSIA) to estimate pesticide degradation in a stormwater wetland receiving pesticide runoff from a vineyard catchment. The fungicide dimethomorph (DIM), with diastereoisomers E and Z, was the prevalent pesticide in the runoff entering the wetland from June to September 2020. DIM Z, the most commonly detected isomer, exhibited a significant change (Δ(13C) > 3 ‰) in its carbon isotopic composition in the wetland water compared to the runoff and commercial formulation, which indicated degradation. Laboratory DIM degradation assays, including photodegradation and biodegradation in oxic wetland water with and without aquatic plants and in anoxic sediments, indicated that DIM degradation mainly occurred in the wetland sediments. The rapid degradation of both DIM isomers (E:t1/2 = 1.2 ± 0.6, Z: t1/2 = 1.5 ± 0.8 days) in the wetland sediment led to significant carbon isotopic fractionation (εDIM-E = -3.0 ± 0.6 ‰, εDIM-Z = -2.0 ± 0.2 ‰). In contrast, no significant isotope fractionation occurred during DIM photodegradation, despite the rapid isomerization of the E isomer to the Z isomer and a half-life of 15.3 ± 2.2 days for both isomers. DIM degradation was slow (E: t1/2 = 56-62 days, Z: t1/2 = 82-103 days) in oxic water with plants, while DIM persisted (120 days) in water without plants. DIM CSIA was thus used to evaluate the in situ biodegradation of DIM Z in the wetland. The DIM Z degradation estimates based on a classical concentration mass balance (86-94 %) were slightly higher than estimates based on the isotopic mass balance (61-68 %). Altogether, this study shows the potential of CSIA to conservatively evaluate pesticide degradation in wetland systems, offering a reliable alternative to classical labor-intensive mass balance approaches.).

Reactive transport of micropollutants in laboratory aquifers undergoing transient exposure periods

Groundwater quality is of increasing concern due to the ubiquitous occurrence of micropollutant mixtures. Stream-groundwater interactions near agricultural and urban areas represent an important entry pathway of micropollutants into shallow aquifers. Here, we evaluated the biotransformation of a micropollutant mixture (i.e., caffeine, metformin, atrazine, terbutryn, S-metolachlor and metalaxyl) during lateral stream water flow to adjacent groundwater. We used an integrative approach combining concentrations and transformation products (TPs) of the micropollutants, compound-specific isotope analysis (δ¹³C and δ¹⁵N), sequencing of 16S rRNA gene amplicons and reactive transport modeling. Duplicate laboratory aquifers (160 cm × 80 cm × 7 cm) were fed with stream water and subjected over 140 d to three successive periods of micropollutant exposures as pulse-like (6000 μg L^-1) and constant (600 μg L^-1) injections under steady-state conditions. Atrazine, terbutryn, S-metolachlor and metalaxyl persisted in both aquifers during all periods (<10 % attenuation). Metformin attenuation (up to 14 %) was only observed from 90 d onwards, suggesting enhanced degradation over time. In contrast, caffeine dissipated during all injection periods (>90 %), agreeing with fast degradation rates (t_1/2 < 3 d) in parallel microcosm experiments and detection of TPs (theobromine and xanthine). Significant stable carbon isotope fractionation (Δδ¹³C ≥ 6.6 ‰) was observed for caffeine in both aquifers, whereas no enrichment in ¹⁵N occurred. A concentration dependence of caffeine biotransformation in the aquifers was further suggested by model simulations following Michaelis-Menten kinetics. Changes in bacterial community composition reflected long-term bacterial adaptation to micropollutant exposures. Altogether, the use of an integrative approach can help to understand the interplay of subsurface hydrochemistry, bacterial adaptations and micropollutants biotransformation during stream-groundwater interactions.

Simple extraction methods for pesticide compound-specific isotope analysis from environmental samples

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Extractions from soil, plants, and water were tested for pesticide C and N CSIA.

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Pesticide recoveries strongly varied among compounds and matrices properties.

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Tested extraction methods caused no effect on δ¹³C and δ¹⁵N of pesticides.

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C and N pesticide CSIA can be applied in situ to agricultural water samples.

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Pesticide CSIA for soil and sediment samples are limited to source areas.

Compound-specific isotope analysis (CSIA) is a powerful approach to evaluate the transformation of organic pollutants in the environment. However, the application of CSIA to micropollutants, such as pesticides, remains limited because appropriate extraction methods are currently lacking. Such methods should address a wide range of pesticides and environmental matrices, while recovering sufficient mass for reliable CSIA without inducing stable isotope fractionation. Here, we present simple extraction methods for carbon and nitrogen CSIA for different environmental matrices and six commonly used herbicides, i.e., atrazine, terbutryn, acetochlor, alachlor, butachlor, and S-metolachlor, and three fungicides, i.e., dimethomorph, tebuconazole, and metalaxyl. We examined the potential of several extraction methods for four types of soils or sediments, three types of environmental waters and aerial and root plant samples for multielement (ME)-CSIA.•

Pesticide extraction recoveries varied depending on the physical characteristics of the pesticides and matrix properties for environmental water (77 to 87%), soil and sediment (35 to 82%), and plant (40 to 59%) extraction.

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The tested extraction methods did not significantly affect the carbon and nitrogen stable isotope signatures of pesticides (Δ(¹³C) <0.9‰ for Δ(¹⁵N) <1.0‰).

Do pesticides degrade in surface water receiving runoff from agricultural catchments? Combining passive samplers (POCIS) and compound-specific isotope analysis

Pesticides lead to surface water pollution and ecotoxicological effects on aquatic biota. Novel strategies are required to evaluate the contribution of degradation to the overall pesticide dissipation in surface waters. Here, we combined polar organic chemical integrative samplers (POCIS) with compound-specific isotope analysis (CSIA) to trace in situ pesticide degradation in artificial ponds and agricultural streams. The application of pesticide CSIA to surface waters is currently restricted due to environmental concentrations in the low μg.L^-1 range, requiring processing of large water volumes. A series of laboratory experiments showed that POCIS enables preconcentration and accurate recording of the carbon isotope signatures (δ¹³C) of common pesticides under simulated surface water conditions and for various scenarios. Commercial and in-house POCIS did not significantly (Δδ¹³C < 1 %) change the δ¹³C of pesticides during uptake, extraction, and δ¹³C measurements of pesticides, independently of the pesticide concentrations (1-10 μg.L^-1) or the flow speeds (6 or 14 cm.s^-1). However, simulated rainfall events of pesticide runoff affected the δ¹³C of pesticides in POCIS. In-house POCIS coupled with CSIA of pesticides were also tested under different field conditions, including three flow-through and off-stream ponds and one stream receiving pesticides from agricultural catchments. The POCIS-CSIA method enabled to determine whether degradation of S-metolachlor and dimethomorph mainly occurred in agricultural soil or surface waters. Comparison of δ¹³C of S-metolachlor in POCIS deployed in a stream with δ¹³C of S-metolachlor in commercial formulations suggested runoff of fresh S-metolachlor in the midstream sampling site, which was not recorded in grab samples. Altogether, our study highlights that the POCIS-CSIA approach represents a unique opportunity to evaluate the contribution of degradation to the overall dissipation of pesticides in surface waters.

Removal of organochlorine pesticides and metagenomic analysis by multi-stage constructed wetland treating landfill leachate

Constructed wetlands (CWs) can effectively treat landfill leachate (LL). However, there is limited research on the removal of organochlorine pesticides (OCPs) refractory organics during LL treatment in CWs. In this study, multi-stage subsurface flow CWs was used to treat LL, and the removal fate of hexachlorocyclohexane (HCH) and dichlorodiphenyltrichloroethane (DDT) in CWs was investigated. The structural differences between plant roots and substrate microbial communities were compared and the Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathway of organic matter was analyzed based on metagenomic analysis. The results showed that substrate adsorption (50.55%-72.74%) and microbial degradation (20.38%-27.89%) were the main ways to remove OCPs. The Proteobacteria occupied a dominant position in the CWs system, among which Betaproteobacteria (34.37%-35.90%) were contained in the substrate, and Alphaproteobacteria (21.19%-23.84%) was a more dominant microorganism in plant roots. Formaldehyde assimilation and serine pathway were the main pathways of methane metabolism. This study provides a reference for the removal mechanism of OCPs to promote the application of CWs technology in LL treatment.

A Literature Review of Wetland Treatment Systems Used to Treat Runoff Mixtures Containing Antibiotics and Pesticides from Urban and Agricultural Landscapes

Wetland treatment systems are used extensively across the world to mitigate surface runoff. While wetland treatment for nitrogen mitigation has been comprehensively reviewed, the implications of common-use pesticides and antibiotics on nitrogen reduction remain relatively unreviewed. Therefore, this review seeks to comprehensively assess the removal of commonly used pesticides and antibiotics and their implications for nitrogen removal in wetland treatment systems receiving non-point source runoff from urban and agricultural landscapes. A total of 181 primary studies were identified spanning 37 countries. Most of the reviewed publications studied pesticides (n = 153) entering wetlands systems, while antibiotics (n = 29) had fewer publications. Even fewer publications reviewed the impact of influent mixtures on nitrogen removal processes in wetlands (n = 16). Removal efficiencies for antibiotics (35–100%), pesticides (−619–100%), and nitrate-nitrogen (−113–100%) varied widely across the studies, with pesticides and antibiotics impacting microbial communities, the presence and type of vegetation, timing, and hydrology in wetland ecosystems. However, implications for the nitrogen cycle were dependent on the specific emerging contaminant present. A significant knowledge gap remains in how wetland treatment systems are used to treat non-point source mixtures that contain nutrients, pesticides, and antibiotics, resulting in an unknown regarding nitrogen removal efficiency as runoff contaminant mixtures evolve.

Pesticide residues in drinking water, their potential risk to human health and removal options

The application of pesticides in agricultural and public health sectors has resulted in substantially contaminated water resources with residues in many countries. Almost no reviews have addressed pesticide residues in drinking water globally; calculated hazard indices for adults, children, and infants; or discussed the potential health risk of pesticides to the human population. The objectives of this article were to summarize advances in research related to pesticide residues in drinking water; conduct health risk assessments by estimating the daily intake of pesticide residues consumed only from drinking water by adults, children, and infants; and summarize options for pesticide removal from water systems. Approximately 113 pesticide residues were found in drinking water samples from 31 countries worldwide. There were 61, 31, and 21 insecticide, herbicide, and fungicide residues, respectively. Four residues were in toxicity class IA, 14 residues were in toxicity class IB, 55 residues were in toxicity class II, 17 residues were in toxicity class III, and 23 residues were in toxicity class IV. The calculated hazard indices (HIs) exceeded the value of one in many cases. The lowest HI value (0.0001) for children was found in Canada, and the highest HI value (30.97) was found in Egypt, suggesting a high potential health risk to adults, children, and infants. The application of advanced oxidation processes (AOPs) showed efficient removal of many pesticide classes. The combination of adsorption followed by biodegradation was shown to be an effective and efficient purification option. In conclusion, the consumption of water contaminated with pesticide residues may pose risks to human health in exposed populations.

Triple-Element Compound-Specific Stable Isotope Analysis (3D-CSIA): Added Value of Cl Isotope Ratios to Assess Herbicide Degradation

Multielement isotope fractionation studies to assess pollutant transformation are well-established for point-source pollution but are only emerging for diffuse pollution by micropollutants like pesticides. Specifically, chlorine isotope fractionation is hardly explored but promising, because many pesticides contain only few chlorine atoms so that "undiluted" position-specific Cl isotope effects can be expected in compound-average data. This study explored combined Cl, N, and C isotope fractionation to sensitively detect biotic and abiotic transformation of the widespread herbicides and groundwater contaminants acetochlor, metolachlor, and atrazine. For chloroacetanilides, abiotic hydrolysis pathways studied under acidic, neutral, and alkaline conditions as well as biodegradation in two soils resulted in pronounced Cl isotope fractionation (ε_Cl from -5.0 ± 2.3 to -6.5 ± 0.7‰). The characteristic dual C-Cl isotope fractionation patterns (Λ_C-Cl from 0.39 ± 0.15 to 0.67 ± 0.08) reveal that Cl isotope analysis provides a robust indicator of chloroacetanilide degradation. For atrazine, distinct Λ_C-Cl values were observed for abiotic hydrolysis (7.4 ± 1.9) compared to previous reports for biotic hydrolysis and oxidative dealkylation (1.7 ± 0.9 and 0.6 ± 0.1, respectively). The 3D isotope approach allowed differentiating transformations that would not be distinguishable based on C and N isotope data alone. This first data set on Cl isotope fractionation in chloroacetanilides, together with new data in atrazine degradation, highlights the potential of using compound-specific chlorine isotope analysis for studying in situ pesticide degradation.

Application of Compound-Specific Isotope Analysis in Environmental Forensic and Strategic Management Avenue for Pesticide Residues

Unintended pesticide pollution in soil, crops, and adjacent environments has caused several issues for both pesticide users and consumers. For users, pesticides utilized should provide higher yield and lower persistence while considering both the environment and agricultural products. Most people are concerned that agricultural products expose humans to pesticides accumulating in vegetation. Thus, many countries have guidelines for assessing and managing pesticide pollution, for farming in diverse environments, as all life forms in soil are untargeted to these pesticides. The stable isotope approach has been a useful technique to find the source of organic matter in studies relating to aquatic ecology and environmental sciences since the 1980s. In this study, we discuss commonly used analytical methods using liquid and gas chromatography coupled with isotopic ratio mass spectrometry, as well as the advanced compound-specific isotope analysis (CSIA). CSIA applications are discussed for tracing organic pollutants and understanding chemical reactions (mechanisms) in natural environments. It shows great applicability for the issues on unintended pesticide pollution in several environments with the progress history of isotope application in agricultural and environmental studies. We also suggest future study directions based on the forensic applications of stable isotope analysis to trace pesticides in the environment and crops.

The Role of Ponds in Pesticide Dissipation at the Agricultural Catchment Scale: A Critical Review

Ponds in agricultural areas are ubiquitous water retention systems acting as reactive biogeochemical hotspots controlling pesticide dissipation and transfer at the catchment scale. Several issues need to be addressed in order to understand, follow-up and predict the role of ponds in limiting pesticide transfer at the catchment scale. In this review, we present a critical overview of functional processes underpinning pesticide dissipation in ponds. We highlight the need to distinguish degradative and non-degradative processes and to understand the role of the sediment-water interface in pesticide dissipation. Yet it is not well-established how pesticide dissipation in ponds governs the pesticide transfer at the catchment scale under varying hydro-climatic conditions and agricultural operation practices. To illustrate the multi-scale and dynamic aspects of this issue, we sketch a modelling framework integrating the role of ponds at the catchment scale. Such an integrated framework can improve the spatial prediction of pesticide transfer and risk assessment across the catchment-ponds-river continuum to facilitate management rules and operations.

Phase Transfer and Biodegradation of Pesticides in Water-Sediment Systems Explored by Compound-Specific Isotope Analysis and Conceptual Modeling

Current approaches are often limited to evaluating the contribution of pesticide dissipation processes in water-sediment systems as both degradation and phase transfer, that is, sorption-desorption, contribute to the apparent decrease of pesticide concentration. Here, the dissipation of widely used herbicides acetochlor and S-metolachlor was examined in laboratory by water-sediment microcosm experiments under oxic and anoxic conditions. Compound-specific isotope analysis (CSIA) emphasized insignificant carbon isotope fractionation in the sediment, indicating prevailing pesticide degradation in the water phase. Conceptual modeling accounting for phase transfer and biodegradation indicated that biodegradation may be underestimated when phase transfer is not included. Phase transfer does not affect carbon isotope fractionation for a wide spectrum of molecules and environmental conditions, underscoring the potential of pesticide CSIA as a robust approach to evaluate degradation in water-sediment systems. CSIA coupled with the identification of transformation products by high-resolution tandem mass spectrometry suggests the degradation of acetochlor and S-metolachlor to occur via nucleophilic substitution and the predominance of oxalinic acids as transformation products under both anoxic and oxic conditions. Altogether, combining the pesticide CSIA, the identification of transformation products, and the use of conceptual phase-transfer models improves the interpretation of pesticide dissipation in water-sediment systems.

Do rainfall characteristics affect the export of copper, zinc and synthetic pesticides in surface runoff from headwater catchments?

Rainfall and runoff characteristics may influence off-site export of pesticides into downstream aquatic ecosystems. However, the relationship between rainfall characteristics and pesticide export from small headwater catchments remains elusive due to confounding factors including the application dose and timing and the variation of pesticide stocks in soil. Here we examined the impact of rainfall characteristics on the export of copper (Cu), zinc (Zn) and 12 legacy and currently used synthetic pesticides in surface runoff from a headwater vineyard catchment. Cluster analysis of rainfall intensity, depth and duration of 78 events revealed four distinct rainfall categories, i.e., Small, Long, Moderate and Intense (p < 0.001). Event mean concentrations of pesticides did not differ among rainfall categories (p > 0.05). In contrast, event loads of both dissolved and solid-bound Cu and Zn significantly differed among rainfall categories (p < 0.001). Rainfall depth and intensity significantly correlated with both Cu and Zn loads in runoff (ρ_s = 0.33 to 0.92, p < 0.002), and might be the main drivers of Cu and Zn export at the catchment scale. In contrast, rainfall depth, intensity or duration did not influence the loads of synthetic pesticides in runoff, even when weekly variations of pesticide stocks in the soil were accounted for. However, intense rainfall-runoff events, that can fragment soil, may control the export of persistent and hydrophobic legacy pesticides stocks in the soil, such as simazine and tetraconazole. Our results show that rainfall characteristics controlled the off-site export of Cu, Zn and legacy synthetic pesticides in a small headwater catchment, whereas the application timing drove the export of currently used synthetic pesticides in runoff. We anticipate our results to be a preliminary step to forecast the influence of regional rainfall patterns on the export of both metallic and synthetic pesticides by surface runoff from small agricultural headwater catchments.

Dissipation of S-metolachlor and butachlor in agricultural soils and responses of bacterial communities: Insights from compound-specific isotope and biomolecular analyses

The soil dissipation of the widely used herbicides S-metolachlor (SM) and butachlor (BUT) was evaluated in laboratory microcosms at two environmentally relevant doses (15 and 150 μg/g) and for two agricultural soils (crop and paddy). Over 80% of SM and BUT were dissipated within 60 and 30 days, respectively, except in experiments with crop soil at 150 μg/g. Based on compound-specific isotope analysis (CSIA) and observed dissipation, biodegradation was the main process responsible for the observed decrease of SM and BUT in the paddy soil. For SM, biodegradation dominated over other dissipation processes, with changes of carbon isotope ratios (Δδ¹³C) of up to 6.5‰ after 60 days, and concomitant production of ethane sulfonic acid (ESA) and oxanilic acid (OXA) transformation products. In crop soil experiments, biodegradation of SM occurred to a lesser extent than in paddy soil, and sorption was the main driver of apparent BUT dissipation. Sequencing of the 16S rRNA gene showed that soil type and duration of herbicide exposure were the main determinants of bacterial community variation. In contrast, herbicide identity and spiking dose had no significant effect. In paddy soil experiments, a high (4:1, V/V) ESA to OXA ratio for SM was observed, and phylotypes assigned to anaerobic Clostridiales and sulfur reducers such as Desulfuromonadales and Syntrophobacterales were dominant for both herbicides. Crop soil microcosms, in contrast, were associated with a reverse, low (1:3, V/V) ratio of ESA to OXA for SM, and Alphaproteobacteria, Actinobacteria, and Bacillales dominated regardless of the herbicide. Our results emphasize the variability in the extent and modes of SM and BUT dissipation in agricultural soils, and in associated changes in bacterial communities.

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

Compound-specific isotope analysis (CSIA) is a valuable tool for assessing the fate of organic pollutants in the environment. However, the requirement of sufficient analyte mass for precise isotope ratio mass spectrometry combined with prevailing low environmental concentrations currently limits comprehensive applications to many micropollutants. Here, we evaluate the upscaling of solid-phase extraction (SPE) approaches for routine CSIA of herbicides. To cover a wide range of polarity, a SPE method with two sorbents (a hydrophobic hypercrosslinked sorbent and a hydrophilic sorbent) was developed. Extraction conditions, including the nature and volume of the elution solvent, the amount of sorbent and the solution pH, were optimized. Extractions of up to 10 L of agricultural drainage water (corresponding to up to 200 000-fold pre-concentration) were successfully performed for precise and sensitive carbon and nitrogen CSIA of the target herbicides atrazine, acetochlor, metolachlor and chloridazon, and metabolites desethylatrazine, desphenylchloridazon and 2,6-dichlorobenzamide in the sub-μg L^-1-range. ¹³C/¹²C and ¹⁵N/¹⁴N ratios were measured by gas chromatography-isotope ratio mass spectrometry (GC/IRMS), except for desphenylchloridazon, for which liquid chromatography (LC/IRMS) and derivatization-GC/IRMS were used, respectively. The method validated in this study is an important step towards analyzing isotope ratios of pesticide mixtures in aquatic systems and holds great potential for multi-element CSIA applications to trace pesticide degradation in complex environments.

13C- and 15N-Isotope Analysis of Desphenylchloridazon by Liquid Chromatography-Isotope-Ratio Mass Spectrometry and Derivatization Gas Chromatography-Isotope-Ratio Mass Spectrometry

The widespread application of herbicides impacts surface water and groundwater. Metabolites (e.g., desphenylchloridazon 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 also be degraded. Evaluating their degradation on the basis of concentration measurements is, therefore, difficult. This study presents compound-specific stable-isotope analysis (CSIA) of nitrogen- and carbon-isotope ratios at natural abundances as an alternative analytical approach to track the 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 (δ¹³C and δ¹⁵N) 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 δ¹³C-isotope analysis (standard deviation <0.5‰) by LC-IRMS with a limit of precise analysis of 996 ng of DPC on-column. Accurate and reproducible δ¹⁵N analysis with a standard deviation of <0.4‰ was achieved by GC-IRMS after derivatization of >100 ng of 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 the different isotopic signatures of the two DPC sources.

Carbon and nitrogen stable isotope fractionation during abiotic hydrolysis of pesticides

Compound-specific Stable Isotope Analysis (CSIA) has been recently established as a tool to study pesticide degradation in the environment. Among degradative processes, hydrolysis is environmentally relevant as it can be chemically or enzymatically mediated. Here, CSIA was used to examine stable carbon and nitrogen isotope fractionation during abiotic hydrolysis of legacy or currently used pesticides (chloroacetanilide herbicides: Acetochlor, Alachlor, S-Metolachlor and Butachlor, acylalanine fungicide: Metalaxyl, and triazine herbicide: Atrazine). Degradation products analysis and CN dual-CSIA allowed to infer hydrolytic degradation pathways from carbon and nitrogen isotopic fractionation. Carbon isotopic fractionation for alkaline hydrolysis revealed similar apparent kinetic isotope effects (AKIE_C = 1.03-1.07) for the 6 pesticides, which were consistent with S_N2 type nucleophilic substitutions. Neither enantio-selectivity (EF ≈ 0.5) nor enantio-specific isotope fractionation occurred during hydrolysis of R (AKIE_C = 1.04 ± 0.01) and S (AKIE_C = 1.04 ± 0.02) enantiomers of a racemic mixture of Metalaxyl. Dual element isotope plots enabled to tease apart CCl bond breaking of alkane (Λ ≈ ε_N/ε_C ≈ 0, Acetochlor, Butachlor) and aromatic π-system (Λ ≈ 0.2, Atrazine) from CO bond breaking by dealkylation (Λ ≈ 0.9, Metalaxyl). Reference values for abiotic versus biotic S_N2 reactions derived from carbon and nitrogen CSIA may be used to untangle pesticide degradation pathways and evaluate in situ degradation during natural and engineered remediation.

Pesticide degradation and export losses at the catchment scale: Insights from compound-specific isotope analysis (CSIA)

Although pesticides undergo degradation tests prior to use, determining their export, degradation and persistence under field conditions remains a challenge for water resource management. Compound specific isotope analysis (CSIA) can provide evidence of contaminant degradation extent, as it is generally independent of non-destructive dissipation (e.g., dilution, sorption, volatilization) regulating environmental concentrations. While this approach has been successfully implemented in subsurface environments, its application to pesticides in near-surface hydrological contexts at catchment scale is lacking. This study demonstrates the applicability of CSIA to track pesticide degradation and export at catchment scale and identify pesticide source areas contributing to changes in stable isotope signature in stream discharge under dynamic hydrological contexts. Based on maximum shifts in carbon stable isotope signatures (Δδ¹³C  = 4.6 ± 0.5‰) of S-metolachlor (S-met), a widely used herbicide, we estimate maximum degradation to have reached 96 ± 3% two months after first application. Maximum shifts in nitrogen isotope signatures were small and inverse (Δδ¹⁵N=-1.3±0.6‰) indicating potential secondary isotope effects during degradation. In combination with a mass balance approach including S-met main degradation products, total catchment non-destructive dissipation was estimated to have reached 8 ± 7% of the applied product. Our results show that CSIA can be applied to evaluate natural attenuation of pesticides at catchment scale. By providing a more detailed account of pesticide dissipation and persistence under field conditions we anticipate the contribution of pesticide CSIA to the improvement of regulatory and monitoring strategies.

Estimation of soil-specific microbial degradation of alpha-cypermethrin by compound-specific stable isotope analysis

In this study, the microbial degradation of alpha-cypermethrin was investigated in 14 different soil samples by compound-specific stable isotope analysis. After 40 and 80 days of microbial degradation, the stable carbon isotope ratios of alpha-cypermethrin shifted from - 32.18 to - 31.87‰ ~ - 31.12 and - 31.45‰ ~ - 29.75‰ in the soils, respectively. The microbial degradation percentages of alpha-cypermethrin were calculated as 15.6~44.3% after 40 days of incubation and 33.3~73.9% after 80 days of incubation. The effects of physicochemical properties of soil samples were then explored on the microbial degradation of alpha-cypermethrin. The results revealed that the microbial degradation percentages were positively correlated with the contents of organic carbon in soils with Pearson correlation coefficients of 0.794 and 0.843 for 40 and 80 days of microbial degradation, respectively, indicating that the microbial degradation of alpha-cypermethrin was faster in the soil with higher content of organic carbon. Accordingly, soil-specific use should be considered in practical agricultural application of alpha-cypermethrin to reduce the residue of the pesticide.

Enantiomer-specific stable carbon isotope analysis (ESIA) to evaluate degradation of the chiral fungicide Metalaxyl in soils

Chiral pesticides are often degraded enantioselectively in soils, leading to disparity among enantiomers that may display different toxicity levels. Monitoring pesticide degradation extents and processes remains out of reach in the field using conventional bulk and enantiomer concentration analyses. Enantioselective stable carbon isotope analysis (ESIA) combines compound specific isotope analysis (CSIA) and enantioselective analysis, and bears potential to distinguish enantiomer degradation from non-destructive dissipation. We developed ESIA of the fungicide Metalaxyl, providing the ¹³C/¹²C ratios for S-Metalaxyl and R-Metalaxyl separately, and applied it to follow degradation in soil incubation experiments. Significant enantioselective degradation (k_S-MTY = 0.007-0.011 day^-1 < k_R-MTY = 0.03-0.07 day^-1) was associated with isotope fractionation (Δδ¹³C_S-MTY ranging from 2 to 6‰). While R-Metalaxyl degradation was rapid (T_1/2≈10 days), concomitant enrichment in heavy isotopes of the persistent S-Metalaxyl occurred after 200 days of incubation (ε_S-Metalaxyl ranging from -1.3 to -2.7‰). In contrast, initial racemic ratios and isotopic compositions were conserved in abiotic experiments, which indicates the predominance of microbial degradation in soils. Degradation products analysis and apparent kinetic isotope effect (AKIE) suggested hydroxylation as a major enantioselective degradation pathway in our soils. Altogether, our study underscores the potential of ESIA to evaluate the degradation extent and mechanisms of chiral micropollutants in soils.

Aqueous photodegradation of substituted chlorobenzenes: Kinetics, carbon isotope fractionation, and reaction mechanisms

Substituted chlorobenzenes are the basic substructure of many surface water contaminants. In this study, the isotope fractionation and reaction mechanisms involved during the aqueous direct and indirect photodegradation of CH₃-, Cl-, and NO₂- substituted chlorobenzenes were investigated in laboratory experiments. Only 4-nitrochlorobenzene showed slow but isotopically fractionating direct photolysis. During indirect photodegradation using UV/H₂O₂-generated OH radicals, the pseudo first-order reaction rate constants increased in the order of the NO₂- < Cl- < CH₃- substituted chlorobenzenes. The most pronounced carbon enrichment factors were observed for nitrochlorobenzenes (up to -4.8 ± 0.5‰), whereas the lowest were for chlorotoluenes (≤-1.0 ± 0.1‰). As the substituents became more electron-withdrawing, the activation energy barrier increased, leading to slower reaction rates, and the transition state changed to a more symmetrical or less reactant-like structure, resulting in larger apparent kinetic isotope effects. The results suggest that the rate-determining step in the reaction with OH radicals was the addition of the electrophile to the benzene ring. Even though further research is needed to quantify isotope fractionation during other transformation processes, these results showed evidence that compound specific isotope analysis can be used as a diagnostic tool for the fate of substituted chlorobenzenes in water.

Potential of vegetated ditches to manage organic pollutants derived from agricultural runoff and domestic sewage: A case study in Sinaloa (Mexico)

This case study presents the fate of selected organic, priority and emerging pollutants along a 3.6km sector of a vegetated, agricultural ditch situated in Sinaloa (Mexico). The ditch receives runoff of agriculture and domestic wastewater from an adjacent community. During 2013, the occurrence of 38 organic pollutants (pesticides, polycyclic aromatic hydrocarbons (PAHs), artificial sweeteners and pharmaceutical residues) was monitored monthly at five selected points in the ditch water. Additionally, sediment and Typha domingensis (cattail) plants were collected in March, June, and September 2013 and investigated concerning their ability to absorb and accumulate pollutants. The concentrations of the selected pollutants in the ditch water ranged from sub ngL^-1 (metolachlor, atrazine) to μgL^-1 (metalaxyl, acesulfame). The metabolites endosulfan sulfate and endosulfan lactone exceeded mostly the concentration of the precursor insecticide endosulfan. Sorption on sediments was of minor relevance for accumulation of pollutants in the ditch system. Concentrations in the sediments varied seasonally and ranged from 0.2 to 12,432μgkg^-1 dry weight (d.w.). T. domingensis accumulated ten of the studied pollutants mainly in roots (5-1065μgkg^-1 d.w.). Overall, the monitoring results of the ditch compartments indicated that downstream the concentrations of the target pollutants decreased. Under no-flow conditions in the hot season, the ditch revealed a noticeable potential to mitigate pollutants. Among the high microbial activity in the water and the subtropical climate conditions, the ditch vegetation contributed to natural attenuation of the selected pollutants.

Transformation of chlorpyrifos in integrated recirculating constructed wetlands (IRCWs) as revealed by compound-specific stable isotope (CSIA) and microbial community structure analysis

Carbon isotope analysis and 454 pyrosequencing methods were used to investigate in situ biodegradation of chlorpyrifos during its transport through three model integrated recirculating constructed wetlands (IRCWs). Results show that plant and Fe-impregnated biochar promoted degradation of chlorpyrifos and its metabolite 3,5,6-trichloro-2-pyridinol (TCP). Carbon isotope ratios in the IRCWs shifted to -31.24±0.58‰ (IRCW1, plant free), -26.82±0.60‰ (IRCW2, with plant) and -24.76±0.94‰ (IRCW3, with plant and Fe-biochar). The enrichment factors (Ɛ_bulk,c) were determined as -0.69±0.06‰ (IRCW1), -0.91±0.07‰ (IRCW2) and -1.03±0.09‰ (IRCW3). Microbial community analysis showed that IRCW3 was dominated by members of Bacillus, which can utilize and degrade chlorpyrifos. These results reveal that plant and Fe-biochar can induce carbon isotope fractionation and have a positive impact on the chlorpyrifos degradation efficiency by influencing the development of beneficial microbial communities.

Dynamics of chloroacetanilide herbicides in various types of mesocosm wetlands

Constructed wetlands (CWs) for pesticide mitigation from agricultural runoff became widespread in the last decade. However, comparison of different types of CWs at one location is missing. Therefore, site by site comparison of three different types (subsurface flow, surface flow and floating hydroponic root mat) of CWs treating four chloroacetanilide herbicides (acetochlor, s-metolachlor, metazachlor, dimethachlor) were carried out. All three planted systems are effective in removing the four herbicides with removal efficiency >92% after 9days. The metabolites ethane sulfonic acids (ESA) and oxanilic acids (OA) of the four herbicides peaked at 9days in the surface flow CWs with soil, but all the metabolites didn't peaked in the subsurface flow with gravel systems and the floating hydroponic root mat system after 21days. All the detected metabolites account about 20% of the mother compounds. There is no noticeable metabolites accumulation in the control system (no plants and no substrate), which indicate no microbial degradation taken place. Plant accumulation and soil adsorption are negligible for the removal of the four herbicides, which are <0.6%. In conclude, plants can enhance the removal of chloroacetanilide herbicides in all the CWs, and the floating hydroponic root mat is the most cost-efficient alternatives for chloroacetanilide herbicides removal due to the absence of substrate.

Joint interpretation of enantiomer and stable isotope fractionation for chiral pesticides degradation

Chiral pesticides are important contaminants affecting the health and functioning of aquatic systems. The combination of stable isotope and enantiomer analysis techniques has been recently proposed to better characterize the fate of these contaminants in natural and engineered settings. We introduce a modeling approach with the aim of unifying and integrating the interpretation of isotopic and enantiomeric fractionation. The model is based on the definition of enantiomer-specific isotopologues and jointly predicts the evolution of concentration, enantiomer fractionation, as well as changes in stable isotope ratios of different elements. The method allows evaluating different transformation pathways and was applied to investigate enzymatic degradation of dichlorprop (DCPP), enzymatic degradation of mecoprop methyl ester (MCPPM), and microbial degradation of α-hexachlorocyclohexane (α-HCH) by different bacterial strains and under different redox conditions. The model accurately reproduces the isotopic and enantiomeric data observed in previous experimental studies and precisely captures the dual-dimensional trends characterizing different reaction pathways. Furthermore, the model allows testing possible combinations of enantiomer analysis (EA), compound specific isotope analysis (CSIA), and enantiomer specific isotope analysis (ESIA) to identify and assess isotope and enantiomer selective reaction mechanisms.

Fate of mixed pesticides in an integrated recirculating constructed wetland (IRCW)

In this study, three model integrated recirculating constructed wetlands (IRCWs) planted with and without Cyperus alternifolius were used to investigate their ability to remove four pesticides (chlorpyrifos, endosulfan, fenvalerate, diuron). Iron (Fe)-impregnated biochar produced by Cyperus alternifolius was added as a primary substrate. Results showed that all four pesticides were efficiently removed in the three IRCWs. The highest pesticide removals were achieved when Fe-impregnated biochar was added to the IRCW (99%), followed by the planted (64-99%) and plant-free IRCW (45-99%). The removal of pesticides in IRCWs followed first-order kinetics, with half-lives of 1.5-11.6h. A mass balance study revealed that sorption (32.2-98.6%) and microbial degradation (1.3-52.8%) were the main removal processes in all IRCWs. This study suggests that the IRCW is a promising system to treat pesticide-contaminated water, and plant and Fe-impregnated biochar can enhance pesticide removal.

Compound-specific isotope analysis (CSIA) of micropollutants in the environment - current developments and future challenges

Over the last decade, the occurrence of micropollutants in the environment has become a worldwide issue of increasing concern. Compound-specific stable-isotope analysis (CSIA) of natural isotopic abundance may greatly enhance the evaluation of sources and transformation processes of micropollutants, such as pesticides, personal care products or pharmaceuticals. We summarize recent advances from laboratory studies, review current limitations and analytical challenges associated with low concentrations and high polarity of micropollutants, and delineate the potential of micropolluant CSIA for field applications. We highlight future challenges and prospects regarding source apportionment, identification of biotic and abiotic transformation reactions on a mechanistic level, as well as integrative evaluation of degradation hot spots on the catchment scale. Such advances may feed into a framework for risk assessment of micropollutants that includes CSIA.

Compound-Specific Stable Isotope Fractionation of Pesticides and Pharmaceuticals in a Mesoscale Aquifer Model

Compound-specific isotope analysis (CSIA) receives increasing interest for its ability to detect natural degradation of pesticides and pharmaceuticals. Despite recent laboratory studies, CSIA investigations of such micropollutants in the environment are still rare. To explore the certainty of information obtainable by CSIA in a near-environmental setting, a pulse of the pesticide bentazone, the pesticide metabolite 2,6-dichlorobenzamide (BAM), and the pharmaceuticals diclofenac and ibuprofen was released into a mesoscale aquifer with quasi-two-dimensional flow. Concentration breakthrough curves (BTC) of BAM and ibuprofen demonstrated neither degradation nor sorption. Bentazone was transformed but did not sorb significantly, whereas diclofenac showed both degradation and sorption. Carbon and nitrogen CSIA could be accomplished in similar concentrations as for "traditional" priority pollutants (low μg/L range), however, at the cost of uncertainties (0.4-0.5‰ (carbon), 1‰ (nitrogen)). Nonetheless, invariant carbon and nitrogen isotope values confirmed that BAM was neither degraded nor sorbed, while significant enrichment of (13)C and in particular (15)N corroborated transformation of diclofenac and bentazone. Retardation of diclofenac was reflected in additional (15)N sorption isotope effects, whereas isotope fractionation of transverse dispersion could not be identified. These results provide a benchmark on the performance of CSIA to monitor the reactivity of micropollutants in aquifers and may guide future efforts to accomplish CSIA at even lower concentrations (ng/L range).

Photodegradation of polycyclic aromatic hydrocarbons in soils under a climate change base scenario

The photodegradation of polycyclic aromatic hydrocarbons (PAHs) in two typical Mediterranean soils, either coarse- or fine-textured, was here investigated. Soil samples, spiked with the 16 US EPA priority PAHs, were incubated in a climate chamber at stable conditions of temperature (20 °C) and light (9.6 W m(-2)) for 28 days, simulating a climate change base scenario. PAH concentrations in soils were analyzed throughout the experiment, and correlated with data obtained by means of Microtox(®) ecotoxicity test. Photodegradation was found to be dependent on exposure time, molecular weight of each hydrocarbon, and soil texture. Fine-textured soil was able to enhance sorption, being PAHs more photodegraded than in coarse-textured soil. According to the EC50 values reported by Microtox(®), a higher detoxification was observed in fine-textured soil, being correlated with the outcomes of the analytical study. Significant photodegradation rates were detected for a number of PAHs, namely phenanthrene, anthracene, benzo(a)pyrene, and indeno(123-cd)pyrene. Benzo(a)pyrene, commonly used as an indicator for PAH pollution, was completely removed after 7 days of light exposure. In addition to the PAH chemical analysis and the ecotoxicity tests, a hydrogen isotope analysis of benzo(a)pyrene was also carried out. The degradation of this specific compound was associated to a high enrichment in (2)H, obtaining a maximum δ(2)H isotopic shift of +232‰. This strong isotopic effect observed in benzo(a)pyrene suggests that compound-specific isotope analysis (CSIA) may be a powerful tool to monitor in situ degradation of PAHs. Moreover, hydrogen isotopes of benzo(a)pyrene evidenced a degradation process of unknown origin occurring in the darkness.

Position-specific isotope modeling of organic micropollutants transformation through different reaction pathways

The degradation of organic micropollutants occurs via different reaction pathways. Compound specific isotope analysis is a valuable tool to identify such degradation pathways in different environmental systems. We propose a mechanism-based modeling approach that provides a quantitative framework to simultaneously evaluate concentration as well as bulk and position-specific multi-element isotope evolution during the transformation of organic micropollutants. The model explicitly simulates position-specific isotopologues for those atoms that experience isotope effects and, thereby, provides a mechanistic description of isotope fractionation occurring at different molecular positions. To demonstrate specific features of the modeling approach, we simulated the degradation of three selected organic micropollutants: dichlorobenzamide (BAM), isoproturon (IPU) and diclofenac (DCF). The model accurately reproduces the multi-element isotope data observed in previous experimental studies. Furthermore, it precisely captures the dual element isotope trends characteristic of different reaction pathways as well as their range of variation consistent with observed bulk isotope fractionation. It was also possible to directly validate the model capability to predict the evolution of position-specific isotope ratios with available experimental data. Therefore, the approach is useful both for a mechanism-based evaluation of experimental results and as a tool to explore transformation pathways in scenarios for which position-specific isotope data are not yet available.

Degradation of chloroacetanilide herbicides and bacterial community composition in lab-scale wetlands

Degradation of chloroacetanilide herbicides rac-metolachlor, acetochlor, and alachlor, as well as associated bacterial populations, were evaluated in vertical upflow wetland columns using a combination of hydrochemical and herbicide analyses, and DNA-based approaches. Mass dissipation of chloroacetanilides, continuously supplied at 1.8-1.9 μM for 112 days, mainly occurred in the rhizosphere zone under nitrate and sulphate-reducing conditions, and averaged 61±14%, 52±12% and 29±19% for acetochlor, alachlor and rac-metolachlor, respectively. Metolachlor enantiomer fractions of 0.494±0.009 in the oxic zone and 0.480±0.005 in the rhizosphere zone indicated preferential biodegradation of the S-enantiomer. Chloroacetanilide ethane sulfonic acid and oxanilic acid degradates were detected at low concentrations only (0.5 nM), suggesting extensive degradation and the operation of yet unknown pathways for chloroacetanilide degradation. Hydrochemical parameters and oxygen concentration were major drivers of bacterial composition, whereas exposure to chloroacetanilides had no detectable impact. Taken together, the results underline the importance of anaerobic degradation of chloroacetanilides in wetlands, and highlight the potential of complementary chemical and biological approaches to characterise processes involved in the environmental dissipation of chloroacetanilides.