Degradation and Transport of the Chiral Herbicide S-Metolachlor at the Catchment Scale: Combining Observation Scales and Analytical Approaches

Modelling the impacts of climate change on agrochemical fate and transport by water on a catchment scale

The export of agrochemicals and their transformation products (TPs) following their application in the agricultural fields poses a threat to water quality. Future changes in climatic conditions (e.g. extreme weather events such as heavy rainfall or extended dry periods) could alter the degradation and mobility of agrochemicals. In this research, we use an integrated modelling framework to understand the impact of extreme climate events on the fate and transport of the agrochemical S-Metolachlor and two of its TPs (M-OXA, Metolachlor Oxanilic Acid and M-ESA, Metolachlor Ethyl Sulfonic Acid). This is done by coupling climate model outputs to the Zin-AgriTra agrochemical reactive transport model in four simulation scenarios. 1) Reference (2015-2018), 2) Very dry (2038-2041), 3) Very wet (2054-2057) and 4) High temperature (2096-2099) conditions of a selected RCP8.5 based regional climate scenario. The modelling framework is tested on an agricultural catchment, Wulka, in Burgenland, Austria. The model results indicate that 13-14 % of applied S-Metolachlor is retained in the soil, and around 85 % is degraded into TPs in the different scenarios. In very dry and high-temperature scenarios, degradation is higher, and hence, there is less S-Metolachlor in the soil. However, a large share of formed M-OXA and M-ESA are retained in the soil, which is transported via overland and groundwater flow, leading to a build-up effect in M-OXA and M-ESA river concentrations over the years. Though a small share of S-Metolachlor and TPs are transported to rivers, their river export is affected by the intensity and amount of rainfall. The very wet and high-temperature scenarios show higher S-Metolachlor and TP concentrations at the catchment outlet due to higher river discharge. The reference scenario shows higher river peak concentrations associated with higher overland flow caused by measured hourly rainfall compared to disaggregated daily precipitation data in the other scenarios.

Occurrence and seasonal variation of organochlorine pesticides in selected vegetable farmlands in Lagos State, Nigeria

Pest infestation in crop production have increased farmers' interest in pesticides use with short and long term consequences. This study investigated the occurrence and seasonal variations of organochlorine pesticide residues in vegetable farms in selected areas of Lagos State. Non carcinogenic and carcinogenic risk assessment was also evaluated. Soil samples were collected during the wet and dry seasons at soil depth of 0-30 cm. Gas chromatography coupled with an Agilent mass spectrometer was used to analyse organochlorine residues (alpha-lindane,lindane, delta.-lindane, aldrin, heptachlor epoxide, alpha.-endosulfan, p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE), endrin, endosulfan, m,p'-dichlorodiphenyldichloroethane (m,p'-DDD), endosulfan sulfate, o,p'-dichlorodiphenyltrichloroethane (o,p'-DDT) and endrin ketone) in soil. Heptachlor epoxide showed maximum concentration of 43.03 mg/kg in Station 19 in Western zone during the dry season while m,p'-DDD and endosulfan had minimum value of 0.004 mg/kg in Station 2 and Station 5 respectively during the wet season in the Far eastern zone. The concentrations of organochlorine residues were intermediate in the Eastern zone in both seasons. There was significant (p < 0.05) increase in dry season concentrations when compared to wet season. The risk assessment indicated Hazard Quotient (HQ) > 1 for non-cancer risk and cancer risk > 10-6. Thus a need for stringent monitoring programs for pesticides.

Innovative Arylimidazole-Fused Phytovirucides via Carbene-Catalyzed [3+4] Cycloaddition: Locking Viral Cell-To-Cell Movement by Out-Competing Virus Capsid-Host Interactions

The control of potato virus Y (PVY) induced crop failure is a challengeable issue in agricultural chemistry. Although many anti-PVY agents are designed to focus on the functionally important coat protein (CP) of virus, how these drugs act on CP to inactivate viral pathogenicity, remains largely unknown. Herein, a PVY CP inhibitor -3j (S) is disclosed, which is accessed by developing unusually efficient (up to 99% yield) and chemo-selective (> 99:1 er in most cases) carbene-catalyzed [3+4] cycloaddition reactions. Compound -3j bears a unique arylimidazole-fused diazepine skeleton and shows chirality-preferred performance against PVY. In addition, -3j (S) as a mediator allows ARG191 (R191) of CP to be identified as a key amino acid site responsible for intercellular movement of virions. R191 is further demonstrated to be critical for the interaction between PVY CP and the plant functional protein NtCPIP, enabling virions to cross plasmodesmata. This key step can be significantly inhibited through bonding with the -3j (S) to further impair pathogenic behaviors involving systemic infection and particle assembly. The study reveals the in-depth mechanism of action of antiviral agents targeting PVY CP, and contributes to new drug structures and synthetic strategies for PVY management.

Strategies for mitigation of pesticides from the environment through alternative approaches: A review of recent developments and future prospects

Chemical-based peticides are having negative impacts on both the healths of human beings and plants as well. The World Health Organisation (WHO), reported that each year, >25 million individuals in poor nations are having acute pesticide poisoning cases along with 20,000 fatal injuries at global level. Normally, only ∼0.1% of the pesticide reaches to the intended targets, and rest amount is expected to come into the food chain/environment for a longer period of time. Therefore, it is crucial to reduce the amounts of pesticides present in the soil. Physical or chemical treatments are either expensive or incapable to do so. Hence, pesticide detoxification can be achieved through bioremediation/biotechnologies, including nano-based methodologies, integrated approaches etc. These are relatively affordable, efficient and environmentally sound methods. Therefore, alternate strategies like as advanced biotechnological tools like as CRISPR Cas system, RNAi and genetic engineering for development of insects and pest resistant plants which are directly involved in the development of disease- and pest-resistant plants and indirectly reduce the use of pesticides. Omics tools and multi omics approaches like metagenomics, genomics, transcriptomics, proteomics, and metabolomics for the efficient functional gene mining and their validation for bioremediation of pesticides also discussed from the literatures. Overall, the review focuses on the most recent advancements in bioremediation methods to lessen the effects of pesticides along with the role of microorganisms in pesticides elimination. Further, pesticide detection is also a big challenge which can be done by using HPLC, GC, SERS, and LSPR ELISA etc. which have also been described in this review.

Coupling multifactor dominated the biochemical response and the alterations of intestinal microflora of earthworm Pheretima guillelmi due to typical herbicides

The excessive application of herbicides on farmlands can substantially reduce labor costs and increase crop yields, but can also have undesirable effects on terrestrial ecosystems. To evaluate the ecological toxicity of herbicides, metolachlor and fomesafen, two typical herbicides that are extensively used worldwide were chosen as target pollutants, and the endogeic earthworm Pheretima guillelmi, which is widely distributed in China, was selected as the test organism. A laboratory-scale microcosmic experiment was set, and energy resources, enzymes, and the composition and connections of intestinal microorganisms in earthworms were determined. Both herbicides depleted the energy resources of the earthworms, especially glycogen contents; increased the levels of antioxidant enzymes; and inhibited acetylcholinesterase. Moreover, the richness and diversity of the intestinal bacterial community of the earthworms were suppressed. Additionally, the bacterial composition at the genus level changed greatly and the connections between dominant bacteria increased dramatically. Most interactions among the bacterial genera belonging to the same and different phyla showed mutualism and competition, respectively. Importantly, metolachlor with higher toxicity had a transitory effect on these indicators in earthworms, whereas fomesafen, with lower toxicity but stronger bioaccumulation potential, exerted a sustaining impact on earthworms. Collectively, these results indicate that the toxic effects of herbicides on terrestrial organisms should be comprehensively considered in combination with biological toxicity, persistence, bioaccumulation potential, and other factors.

Insights into the metabolic pathways and biodegradation mechanisms of chloroacetamide herbicides

Chloroacetamide herbicides are widely used around the world due to their high efficiency, resulting in increasing levels of their residues in the environment. Residual chloroacetamides and their metabolites have been frequently detected in soil, water and organisms and shown to have toxic effects on non-target organisms, posing a serious threat to the ecosystem. As such, rapid and efficient techniques that eliminate chloroacetamide residues from the ecosystem are urgently needed. Degradation of these herbicides in the environment mainly occurs through microbial metabolism. Microbial strains such as Acinetobacter baumannii DT, Bacillus altitudinis A16, Pseudomonas aeruginosa JD115, Sphingobium baderi DE-13, Catellibacterium caeni DCA-1, Stenotrophomonas acidaminiphila JS-1, Klebsiella variicola B2, and Paecilomyces marquandii can effectively degrade chloroacetamide herbicides. The degradation pathway of chloroacetamide herbicides in aerobic bacteria is mainly initiated by an N/C-dealkylation reaction, followed by aromatic ring hydroxylation and cleavage processes, whereas dechlorination is the initial reaction in anaerobic bacteria. The molecular mechanisms associated with bacterial degradation of chloroacetamide herbicides have been explored, with amidase, hydrolase, reductase, ferredoxin and cytochrome P450 oxygenase currently known to play a pivotal role in the catabolic pathways of chloroacetamides. The fungal pathway for the degradation of these herbicides is more complex with more diversified products, and the degradation enzymes and genes involved remain to be discovered. However, there are few reviews specifically summarizing the microbial degrading species and biochemical mechanisms of chloroacetamide herbicides. Here, we briefly summarize the latest progress resulting from research on microbial strain resources and enzymes involved in degradation of these herbicides and their corresponding genes. Furthermore, we explore the biochemical pathways and molecular mechanisms for biodegradation of chloroacetamide herbicides in depth, thereby providing a reference for further research on the bioremediation of such herbicides.

Comparative insight of pesticide transformations between river and wetland systems

The widespread use of pesticides threatens the environment and ecosystems. Despite the positive effects of plant protection products, pesticides also have unexpected negative effects on nontarget organisms. The microbial biodegradation of pesticides is one of the major pathways for reducing their risks at aquatic ecosystems. The objective of this study was to compare the biodegradability of pesticides in simulated wetland and river systems. Parallel experiments were conducted with 17 pesticides based on the OECD 309 guidelines. A comprehensive analytical method, such as target screening combined with suspect and non-target screening, was performed to evaluate the biodegradation via identification of transformation products (TPs) using LC-HRMS. As evidence of biodegradation, we identified 97 TPs for 15 pesticides. Metolachlor and dimethenamid had 23 and 16 TPs, respectively, including Phase II glutathione conjugates. The analysis of 16S rRNA sequences for microbials characterized operational taxonomic units. Rheinheimera and Flavobacterium, which have the potential for glutathione S-transferase, were dominant in wetland systems. Estimation of toxicity, biodegradability, and hydrophobicity using QSAR prediction indicated lower environmental risks of detected TPs. We conclude that the wetland system is more favorable for pesticide degradation and risk mitigation mainly attributed to the abundance and variety of the microbial communities.

Enantioselectivity and mechanisms of chiral herbicide biodegradation in hydroponic systems

This study demonstrates the enantioselective removal dynamics and mechanisms of the chiral herbicide metolachlor in a hydroponic system of Phragmites australis. It presents the first work to elucidate plant-microbial driven enantioselective degradation processes of chiral chemicals. The results showed a degradation efficiency of up to 95.07 ± 2.81% in the hydroponic system driven by a notably high degradation rate constant of 0.086 d^-1. P. australis was demonstrated to rapidly increase the contribution of biodegradation pathways in the hydroponic system to 82.21 ± 4.81% within 4 d with an enantiomeric fraction (EF) drop to 0.26 ± 0.02 to favour the enantioselective degradation of S-Metolachlor (k_{S-Metolachlor} = 0.568 d^-1 and k_{R-Metolachlor} = 0.147 d^-1). Comparatively, the biodegradation pathways in the control constituted less than 25%, with an EF value of circa 0.5. However, the enantioselective biodegradation pathways exhibited complete reversal after about 4 d to favour R-Metolachlor. Plants promoted the degradation of R-Metolachlor, evidenced by an increase in EF to 0.59 ± 0.03. Nonetheless, metolachlor showed an inhibitory effect on plants reflected by the reduction of plant growth rate, chlorophyll content, and electron transport rate to -7.85 ± 1.52%, 1.33 ± 0.43 mg g^-1, 4.03 ± 1.33 μmol (m² s)^-1, respectively. However, rhizosphere microorganisms aided plants to catalyze excessive reactive oxygen species production by the antioxidant enzymes to protect plants from oxidative damage and restore their physiological activities. High-throughput analysis of microbial communities demonstrated the enrichment of Massilia (40.63%) and Pseudomonas (8.16%) in the initial stage to promote the rapid degradation of S-Metolachlor. By contrast, the proliferation of Brevundimonas (32.29%) and Pseudarthrobacter (11.03%) in the terminal stage was closely associated with the degradation of R-Metolachlor. Moreover, as symbiotic bacteria of plants, these bacteria aided plants protection from reactive oxygen damages and promoted the recovery of plant metabolic functions and photosynthesis. Overall, these results demonstrate biodegradation mediated by plant-microbe mechanisms as the main driver for the enantioselective degradation of metolachlor in hydroponic systems.

Current status of pesticide effects on environment, human health and it's eco-friendly management as bioremediation: A comprehensive review

Pesticides are either natural or chemically synthesized compounds that are used to control a variety of pests. These chemical compounds are used in a variety of sectors like food, forestry, agriculture and aquaculture. Pesticides shows their toxicity into the living systems. The World Health Organization (WHO) categorizes them based on their detrimental effects, emphasizing the relevance of public health. The usage can be minimized to a least level by using them sparingly with a complete grasp of their categorization, which is beneficial to both human health and the environment. In this review, we have discussed pesticides with respect to their global scenarios, such as worldwide distribution and environmental impacts. Major literature focused on potential uses of pesticides, classification according to their properties and toxicity and their adverse effect on natural system (soil and aquatic), water, plants (growth, metabolism, genotypic and phenotypic changes and impact on plants defense system), human health (genetic alteration, cancer, allergies, and asthma), and preserve food products. We have also described eco-friendly management strategies for pesticides as a green solution, including bacterial degradation, myco-remediation, phytoremediation, and microalgae-based bioremediation. The microbes, using catabolic enzymes for degradation of pesticides and clean-up from the environment. This review shows the importance of finding potent microbes, novel genes, and biotechnological applications for pesticide waste management to create a sustainable environment.

Ageing of copper, zinc and synthetic pesticides in particle-size and chemical fractions of agricultural soils

The transformation and mobility of heavy metals and synthetic pesticides in soil depend on ageing, involving their chemical and physical distributions among soil fractions over time. Heavy metals and synthetic pesticides often co-occur in soil, although their ageing is usually evaluated separately and in bulk soil. Here, contrasting vineyard and crop soils were spiked with copper (Cu; 700 mg kg^-1) and zinc (Zn; 200 mg kg^-1) a and/or synthetic pesticides (5 mg kg^-1), i.e., the fungicide metalaxyl (MTY) and herbicide S-metolachlor (SMET), to evaluate within 200 days their distribution among soil physical and chemical fractions. More than 90% of MTY and SMET in soil was released into the water phase, even 200 days after spiking. This emphasizes the potential mobilization of MTY and SMET from the soil following field application. MTY, SMET, Cu and Zn were associated mainly with the silt fraction and to a lesser extent (<30%) with the sand and clay fractions. Overall, the ageing of MTY, SMET, Cu and Zn in agricultural soil was affected mainly by the soil type and sterilization and only to a minor extent by their co-occurrence. Sorption controlled the dissipation of MTY and SMET in soil, while biodegradation contributed to less than 10%. A large fraction (37 ± 2%) of Cu was associated with Fe oxides after 200 days of ageing, while Zn was found (33 ± 2%) in the residual soil fraction. The silt fraction of the nonsterile vineyard soil became enriched in ⁶⁵Cu over time (Δδ⁶⁵Cu = 0.25 ± 0.07‰), whereas the clay fraction was depleted in ⁶⁵Cu (Δδ⁶⁵Cu = -0.20 ± 0.07‰). Cu isotope fractionation mirrored the Cu distribution in soil chemical fractions, suggesting that Cu stable isotopes may help to follow-up Cu ageing. In contrast, no significant Zn isotope fractionation was observed among soil experiments or over time. Overall, our study emphasizes the variability in ageing of synthetic pesticides and heavy metals co-occurring in agricultural soils and their interplay in physical and chemical fractions of the soil.

Indigenous functional microbial communities for the preferential degradation of chloroacetamide herbicide S-enantiomers in soil

This study investigated indigenous functional microbial communities associated with the degradation of chloroacetamide herbicides acetochlor (ACE), S-metolachlor (S-MET) and their enantiomers in repeatedly treated soils. The results showed that biodegradation was the main process for the degradation of ACE, S-MET and their enantiomers. Eight dominant bacterial genera associated with the degradation were found: Amycolatopsis, Saccharomonospora, Mycoplasma, Myroides, Mycobacterium, Burkholderia, Afipia, and Kribbella. The S-enantiomers of ACE and S-MET were preferentially degraded, which mainly relied on Amycolatopsis, Saccharomonospora and Kribbella for the ACE S-enantiomer and Amycolatopsis and Saccharomonospora for the S-MET S-enantiomer. Importantly, the relative abundances of Amycolatopsis and Saccharomonospora increased by 146.3%-4467.2% in the S-enantiomer treatments of ACE and S-MET compared with the control, which were significantly higher than that in the corresponding R-enantiomer treatments (25.3%-4168.2%). Both metagenomic and qPCR analyses demonstrated that four genes, ppah, alkb, benA, and p450, were the dominant biodegradation genes (BDGs) potentially involved in the preferential degradation of the S-enantiomers of ACE and S-MET. Furthermore, network analysis suggested that Amycolatopsis, Saccharomonospora, Mycoplasma, Myroides, and Mycobacterium were the potential hosts of these four BDGs. Our findings indicated that Amycolatopsis and Saccharomonospora might play pivotal roles in the preferential degradation of the S-enantiomers of ACE and S-MET.

Understanding the origins of herbicides metabolites in an agricultural watershed through their spatial and seasonal variations

The aim of this study is to understand the spatial and seasonal variations of persistent herbicides metabolites and to determine their origins in the Vilaine River watershed, Britany-France. Improving knowledge on herbicides metabolites sources and seasonality is important for drinking water resource management. Data were collected at 13 sampling stations during five sampling campaigns in 2016 and 2017. Relations between water quality parameters, herbicides and metabolites were analyzed using statistical methods. The influence of land use and wastewater treatment plants (WWTP) on streams water quality has been identified. Cluster Analysis revealed that two groups of sampling stations can be described as "urban" with stations downstream the urban area and as "agricultural" with stations located downstream of the watershed. Chloroacetamids metabolites have been associated together with nitrates and agricultural areas as could be expected. Thus, the drinking water treatment plant located in the estuary of the Vilaine River is exposed to high metolachlor ESA and nitrate loads all year long. Aminomethylphosphonic acid (AMPA) is associated to anthropogenic urban contamination and nutrient loads. AMPA has its major sources in both glyphosate and phosphonate detergents issued from WWTP. This can help to adapt surface water treatment process and water management policies concerning herbicides metabolites.

Agriculture Development, Pesticide Application and Its Impact on the Environment

Pesticides are indispensable in agricultural production. They have been used by farmers to control weeds and insects, and their remarkable increases in agricultural products have been reported. The increase in the world's population in the 20th century could not have been possible without a parallel increase in food production. About one-third of agricultural products are produced depending on the application of pesticides. Without the use of pesticides, there would be a 78% loss of fruit production, a 54% loss of vegetable production, and a 32% loss of cereal production. Therefore, pesticides play a critical role in reducing diseases and increasing crop yields worldwide. Thus, it is essential to discuss the agricultural development process; the historical perspective, types and specific uses of pesticides; and pesticide behavior, its contamination, and adverse effects on the natural environment. The review study indicates that agricultural development has a long history in many places around the world. The history of pesticide use can be divided into three periods of time. Pesticides are classified by different classification terms such as chemical classes, functional groups, modes of action, and toxicity. Pesticides are used to kill pests and control weeds using chemical ingredients; hence, they can also be toxic to other organisms, including birds, fish, beneficial insects, and non-target plants, as well as air, water, soil, and crops. Moreover, pesticide contamination moves away from the target plants, resulting in environmental pollution. Such chemical residues impact human health through environmental and food contamination. In addition, climate change-related factors also impact on pesticide application and result in increased pesticide usage and pesticide pollution. Therefore, this review will provide the scientific information necessary for pesticide application and management in the future.

Stimulation of earthworms (Eisenia fetida) on soil microbial communities to promote metolachlor degradation

Degradation of metolachlor in surface soil is extremely important to its potential mobility and overall persistence. In this study, the effects of earthworms (Eisenia fetida) on the degradation of metolachlor at two concentration levels (5 and 20 mg kg^-1) in soil were investigated via the column experiment. The degradation kinetics of metolachlor indicate that addition of earthworms enhances metolachlor degradation significantly (P < 0.05), with the enhanced degradation rate of 30% and 63% in the low and high concentration treatments at the 15th day, respectively. Fungi rather than bacteria are primarily responsible for metolachlor degradation in soil, and earthworms stimulate metolachlor degradation mainly by stimulating the metolachlor-degrading functional microorganisms and improving fungal community structure. Earthworms prefer to promote the possible fungal degraders like order Sordariales, Microascales, Hypocreales and Mortierellales and the possible bacteria genus Rubritalea and strengthen the relationships between these primary fungi. Two metabolites metolachlor oxanilic (MOXA) and moetolachlor ethanesulfonic acid (MESA) are detected in soil and earthworms in the high concentration treatments. Earthworms stimulate the formation of MOXA and yet inhibit the formation of MESA in soil. Another metabolite metolachlor-2-hydroxy (M2H) is also detected in earthworms, which is reported firstly. The study provides an important information for the remediation of metolachlor-polluted soil.

New Photodegradation Products of the Fungicide Fluopyram: Structural Elucidation and Mechanism Identification

Identifying the fate of agrochemicals is important to understand their potential risk for living organisms. We report here new photodegradation products (PPs) of the fungicide fluopyram. The PPs were produced by irradiating a fluopyram standard in 0.1% acetonitrile aqueous media by a 150-W medium pressure Hg-lamp that emits wavelengths between 200⁻280 nm. The structural elucidation of PPs was achieved by combining the retention time, isotopic pattern, targeted fragmentation, and accurate mass measurements using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and high resolution-MS (HRMS). In addition to previously known PPs, seven new PPs of fluopyram were identified in this work: mainly dihydroxyl and hydroxylimide fluopyram as well as mono, di, and trihydroxyl lactam. Additionally, two PPs were found to be formed by rearrangement after the loss of H₂C=CH₂. Hence, the results of the work contribute to extending the current knowledge regarding the photoinduced fate of agrochemicals, and fluopyram in particular.

Toward Integrative Bacterial Monitoring of Metolachlor Toxicity in Groundwater

Common herbicides such as metolachlor (MET), and their transformation products, are frequently detected in groundwater worldwide. Little is known about the response of groundwater bacterial communities to herbicide exposure, and its potential use for ecotoxicological assessment. The response of bacterial communities exposed to different levels of MET from the Ariège alluvial aquifer (Southwest of France) was investigated in situ and in laboratory experiments. Variations in both chemistry and bacterial communities were observed in groundwater, but T-RFLP analysis did not allow to uncover a pesticide-specific effect on endogenous bacterial communities. To circumvent issues of hydrogeochemical and seasonal variations in situ, groundwater samples from two monitoring wells of the Ariège aquifer with contrasting records of pesticide contamination were exposed to different levels of MET in laboratory experiments. The standard Microtox® acute toxicity assay did not indicate toxic effects of MET, even at 5 mg L-1 (i.e., 1000-fold higher than in contaminated groundwater). Analysis of MET transformation products and compound-specific isotope analysis (CSIA) in laboratory experiments demonstrated MET biodegradation but did not correlate with MET exposure. High-throughput sequencing analysis (Illumina MiSeq) of bacterial communities based on amplicons of the 16S rRNA gene revealed that bacterial community differed mainly by groundwater origin rather than by its response to MET exposure. OTUs correlating with MET addition ranged between 0.4 to 3.6% of the total. Predictive analysis of bacterial functions impacted by pesticides using PICRUSt suggested only minor changes in bacterial functions with increasing MET exposure. Taken together, results highlight MET biodegradation in groundwater, and the potential use of bacterial communities as sensitive indicators of herbicide contamination in aquifers. Although detected effects of MET on groundwater bacterial communities were modest, this study illustrates the potential of integrating DNA- and isotopic analysis-based approaches to improve ecotoxicological assessment of pesticide-contaminated aquifers. GRAPHICAL ABSTRACTAn integrative approach was develop to investigate in situ and in laboratory experiments the response of bacterial communities exposed to different levels of MET from the Ariége alluvial aquifer (Southwest of France).

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.