Compatibility of polar organic chemical integrative sampler (POCIS) with compound specific isotope analysis (CSIA) of substituted chlorobenzenes

Compound specific isotope analysis (CSIA) is a powerful technique to demonstrate in situ degradation of traditional groundwater contaminants when concentrations are typically in the mg/L range. Currently, an efficient preconcentration method is lacking to expand CSIA to low aqueous concentration environmental samples. Specially for the H- and N-CSIA of heteroatom-bearing non-traditional compounds, the CSIA analytical detection limits are significantly higher than that of the C-CSIA. This work demonstrates the compatibility of polar organic chemical integrative sampler (POCIS) with C-, H-, and N-CSIA using four nitro- and amino-substituted chlorobenzenes that are common industrial feedstocks for numerous applications and are commonly detected in the environment at mg/L to μg/L range. Using lab experiments, we showed isotopic equilibrium in POCIS was achieved after 30 days with either a negligible (<0.5 ‰) or a constant shift for C (<1 ‰) and N (<2 ‰). Similar negligible (<5 ‰) or constant shift (<20 ‰) was evident for H isotope except for 3,4-dichloroaniline. The method quantification limits for the combined sorbent and membrane of one POCIS were comparable to that of the solid phase extraction (SPE) using 10 L water. Next, we demonstrated the field applicability of POCIS for C- and N-CSIA after a 60-day deployment in a pilot constructed wetland by showing <1 ‰ difference between the δ13C and δ15N obtained from POCIS and SPE. Finally, we evaluated whether the biofilm development on POCIS membrane could affect the isotope signature of the sampled compounds during field deployment. Although a diverse microbial community was identified on the membrane after a 60-day deployment, we did not observe significant isotope fractionation. This was likely due to either slower diffusion in the biofilm or microbial degradation of the sampled compounds. This work demonstrates the potential of using POCIS-CSIA as a simple, fast, and sensitive method for low-concentration contaminants, such as pesticides, pharmaceuticals, and flame-retardants.

Modeling multi-year phosphorus dynamics in a bioretention cell: Phosphorus partitioning, accumulation, and export

Phosphorus (P) export from urban areas via stormwater runoff contributes to eutrophication of downstream aquatic ecosystems. Bioretention cells are a Low Impact Development (LID) technology promoted as a green solution to attenuate urban peak flow discharge, as well as the export of excess nutrients and other contaminants. Despite their rapidly growing implementation worldwide, a predictive understanding of the efficiency of bioretention cells in reducing urban P loadings remains limited. Here, we present a reaction-transport model to simulate the fate and transport of P in a bioretention cell facility in the greater Toronto metropolitan area. The model incorporates a representation of the biogeochemical reaction network that controls P cycling within the cell. We used the model as a diagnostic tool to determine the relative importance of processes immobilizing P in the bioretention cell. The model predictions were compared to multi-year observational data on 1) the outflow loads of total P (TP) and soluble reactive P (SRP) during the 2012-2017 period, 2) TP depth profiles collected at 4 time points during the 2012-2019 period, and 3) sequential chemical P extractions performed on core samples from the filter media layer obtained in 2019. Results indicate that exfiltration to underlying native soil was principally responsible for decreasing the surface water discharge from the bioretention cell (63 % runoff reduction). From 2012 to 2017, the cumulative outflow export loads of TP and SRP only accounted for 1 % and 2 % of the corresponding inflow loads, respectively, hence demonstrating the extremely high P reduction efficiency of this bioretention cell. Accumulation in the filter media layer was the predominant mechanism responsible for the reduction in P outflow loading (57 % retention of TP inflow load) followed by plant uptake (21 % TP retention). Of the P retained within the filter media layer, 48 % occurred in stable, 41 % in potentially mobilizable, and 11 % in easily mobilizable forms. There were no signs that the P retention capacity of the bioretention cell was approaching saturation after 7 years of operation. The reactive transport modeling approach developed here can in principle be transferred and adapted to fit other bioretention cell designs and hydrological regimes to estimate P surface loading reductions at a range of temporal scales, from a single precipitation event to long-term (i.e., multi-year) operation.

Compound-Specific Carbon, Hydrogen, and Nitrogen Isotope Analysis of Nitro- and Amino-Substituted Chlorobenzenes in Complex Aqueous Matrices

Compound-specific isotope analysis (CSIA) is an established tool to study the fate of legacy groundwater contaminants but is only emerging for nonconventional contaminants, e.g., nitro- and amino-substituted chlorobenzenes that are widely used as industrial feedstock and the target of this work. To date, CSIA of the target compound groups used special combustion interfaces and the potential matrix interferences in environmental samples has not been assessed. We validated CSIA methods for δ¹³C, δ²H, and δ¹⁵N of four analytes from each chemical group and developed a solid-phase extraction (SPE) method to minimize matrix interferences during preconcentration of complex aqueous samples. The SPE recovery was >80% and the method quantification limits of SPE-CSIA for δ¹³C, δ²H, and δ¹⁵N were 0.03-0.57, 1.3-2.7, and 3.4-10.2 μM aqueous-phase concentrations, respectively, using 2 L of spiked MQ water. The SPE-CSIA procedure showed negligible isotope fractionation for δ¹³C (≤0.5‰), δ¹⁵N (≤0.5‰), and δ²H (≤5‰ for nitroaromatics and ≤10‰ for aminoaromatics). In addition, solvent evaporation, water sample storage up to 7 months, and SPE extract storage for 1.5 years did not change analytes' δ¹³C signatures beyond ±0.5‰. However, to avoid significant δ²H and δ¹⁵N fractionation of aminoaromatics, cartridge breakthrough should be avoided and SPE preconcentration must be conducted at pH > pK_a + 2. Application of the method at a contaminated site showed excellent precision, at ≤0.3‰ for C and N, and ≤1.5‰ for H. The methods validated here now allow the use of multielement CSIA to track the environmental fate of nitro- and amino-substituted chlorobenzenes in complex aqueous samples.

Implications of polar organic chemical integrative sampler for high membrane sorption and suitability of polyethersulfone as a single-phase sampler

Polar organic chemical integrative sampler (POCIS) contains sorbent, which is typically enclosed between two polyethersulfones (PES) membranes. A significant PES uptake is reported for many contaminants, yet, aqueous concentration is mainly correlated with the sorbent uptake using first-order kinetics. Under high PES sorption, the first-order kinetics often provide erroneous sampling rate for the sorbent phase due to increased membrane resistance. This work evaluated the uptake of four high PES sorbing chemicals, i.e., three Cl- and CH₃-substituted nitrobenzenes and one chlorinated aniline using POCIS and the potential of a single-phase PES sampler using laboratory experiments. POCIS calibration results demonstrated that both sorbent and membrane had similar affinity for the target compounds. A rapid PES sorption occurred in the earlier days (<7 days) followed by a gradual increase in the PES phase concentration (equilibrium not achieved after 60 days). Especially, the membrane was the primary sink for 3,4-dichloroaniline and 3,4-dichloronitrobenzene for up to 14 and 31 days, respectively. On the other hand, the single-phase PES sampler showed similar mass uptake as POCIS and reached equilibrium within 19 days under static condition, indicating its potential suitability in the equilibrium regime. PES-water partition coefficient of the target compounds was between 1.2 and 6.5 L/g. Finally, we present a poly-parameter linear-free energy relationship (pp-LFER) using published data to predict the PES-water partition coefficients. The pp-LFER models showed moderate predictability as indicated by R²_adj values between 0.7 and 0.9 for both internal and external data set consisting of a wide range of hydrophobic and hydrophilic compounds (-0.1 ≤ logK_OW ≤ 7.4). The proposed pp-LFER model can be used to screen high PES-sorbing chemicals to increase the reliability and accuracy of aqueous concentration prediction from POCIS sampling and to select the most appropriate sampling approach for new compounds.

Is In-Service Granular Activated Carbon Biologically Active? An Evaluation of Alternative Experimental Methods to Distinguish Adsorption and Biodegradation in GAC

In-service granular activated carbon (GAC) may transform into biological activated carbon (BAC) and remove contaminants through both adsorption and biodegradation, but it is difficult to determine its biodegradative capacity. One approach to understand the GAC biodegradative capacity is to compare the performance between unsterilized and sterilized GAC, but the sterilization methods may not ensure effective microbial inhibition and may affect adsorption. This study identified the ¹⁴C-glucose respiration rate as the best metric to evaluate the effectiveness of three sterilization methods: sodium azide addition, autoclaving, and γ irradiation. The sterilization protocols were refined, including continuously feeding 300 mg/L of sodium azide, three cycles of autoclaving, and 10-12 kGy of γ irradiation. Parallel minicolumn tests were conducted to identify sodium azide addition as the most broadly effective sterilization method with an insignificant effect on adsorption in most cases, except for the adsorption of anionic compounds under certain conditions. Nevertheless, this problem was solved by decreasing the azide dosage as long as it is still sufficient to provide effective microbial inhibition. This study helps to develop an approach that differentiates adsorption and biodegradation in GAC, which could be used by future studies to advance our understanding of BAC filtration.

Evaluating perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) removal across granular activated carbon (GAC) filter-adsorbers in drinking water treatment plants

Granular activated carbon (GAC) was harvested from six filter-adsorbers that are used for taste and odour control in three drinking water treatment plants in Ontario, Canada, and evaluated for the removal of perfluorooctanic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) using minicolumn tests under different operational conditions. Parallel column tests were conducted using unsterilized GAC and sterilized GAC to distinguish adsorption from potential biodegradation of PFOA and PFOS across the GAC. It was observed that the GAC could achieve approximately 20% to 55% of PFOA and PFOS removal even after a long period of GAC operation (e.g., 6 years). There was no evidence of PFOA and PFOS biodegradation, so the removal in GAC can be attributed solely to adsorption under the conditions tested. However, in one location, there was evidence suggesting both removal and formation of PFOS and PFOA across the GAC, with the formation presumably due to the biotransformation of pre-existing precursors in the source water. Additionally, GAC service time and empty bed contact time (EBCT) were identified to be important factors that could affect the removal of PFOA and PFOS. Based on this information, an empirical model was proposed to predict PFOA and PFOS removal in GAC filter-adsorbers as a function of GAC service time and EBCT. This study provides useful information for utilities that have installed GAC for taste and odour control but may consider per- and polyfluoroalkyl substances (PFAS) removal as an additional voluntary objective or due to more stringent guidelines.

Harnessing plant-microbiome interactions for bioremediation across a freshwater urbanization gradient

Urbanization impacts land, air, and water, creating environmental gradients between cities and rural areas. Urban stormwater delivers myriad co-occurring, understudied, and mostly unregulated contaminants to aquatic ecosystems, causing a pollution gradient. Recipient ecosystems host interacting species that can affect each others' growth and responses to these contaminants. For example, plants and their microbiomes often reciprocally increase growth and contaminant tolerance. Here, we identified ecological variables affecting contaminant fate across an urban-rural gradient using 50 sources of the aquatic plant Lemna minor (duckweed) and associated microbes, and two co-occurring winter contaminants of temperate cities, benzotriazole and salt. We conducted experiments totalling >2,500 independent host-microbe-contaminant microcosms. Benzotriazole and salt negatively affected duckweed growth, but not microbial growth, and duckweeds maintained faster growth with their local, rather than disrupted, microbiota. Benzotriazole transformation products of plant, microbial, and phototransformation pathways were linked to duckweed and microbial growth, and were affected by salt co-contamination, microbiome disruption, and source sites of duckweeds and microbes. Duckweeds from urban sites grew faster and enhanced phytotransformation, but supported less total transformation of benzotriazole. Increasing microbial community diversity correlated with greater removal of benzotriazole, but taxonomic groups may explain shifts across transformation pathways: the genus Aeromonas was linked to increasing phototransformation. Because benzotriazole toxicity could depend on amount and type of in situ transformation, this variation across duckweeds and microbes could be harnessed for better management of urban stormwater. Broadly, our results demonstrate that plant-microbiome interactions harbour manipulable variation for bioremediation applications.

Triclosan uptake and transformation by the green algae Euglena gracilis strain Z

Triclosan is an antimicrobial chemical present in consumer products that is frequently detected in aquatic environments. In this research, we investigated the role of a common freshwater microalgae species, Euglena gracilis for triclosan uptake and transformation in open-water treatment wetlands. Lab-scale wetland bioreactors were created under various conditions of light (i.e., continuous (white) light, red light, and in the dark), media (i.e., wetland, autoclaved wetland, Milli-Q, and growth media water), and presence or absence of algae. Triclosan and its potential transformation products were identified in the water and algae phases. Triclosan transformation occurred most rapidly with reactors that received continuous (white) light, with pseudo first-order rate constants, k, ranging from 0.035 to 0.292 day^-1. This indicates that phototransformation played a major role in triclosan transformation during the day, despite light screening by algae. Algae contributed to the uptake and transformation of triclosan in all reactors, and algae and bacteria both contributed to triclosan biotransformation under dark conditions, representative of nighttime conditions. Some transformation products were formed and further transformed, e.g., triclosan-O-sulfate, methoxy and diglucosyl conjugate of hydroxylated triclosan, and dimethoxy and glucosyl conjugate of 2,4-dichlorophenol, suggesting their minimal accumulation over the 25 days of the experiments. This study shows that the combined action of light, microbes, and algae allows the safe transfer and transformation of triclosan in open-water treatment wetlands.

Automatic quantification and classification of microplastics in scanning electron micrographs via deep learning

Microplastics quantification and classification are demanding jobs to monitor microplastic pollution and evaluate the potential health risks. In this paper, microplastics from daily supplies in diverse chemical compositions and shapes are imaged by scanning electron microscopy. It offers a greater depth and finer details of microplastics at a wider range of magnification than visible light microscopy or a digital camera, and permits further chemical composition analysis. However, it is labour-intensive to manually extract microplastics from micrographs, especially for small particles and thin fibres. A deep learning approach facilitates microplastics quantification and classification with a manually annotated dataset including 237 micrographs of microplastic particles (fragments or beads) in the range of 50 μm-1 mm and fibres with diameters around 10 μm. For microplastics quantification, two deep learning models (U-Net and MultiResUNet) were implemented for semantic segmentation. Both significantly outmatched conventional computer vision techniques and achieved a high average Jaccard index over 0.75. Especially, U-Net was combined with object-aware pixel embedding to perform instance segmentation on densely packed and tangled fibres for further quantification. For shape classification, a fine-tuned VGG16 neural network classifies microplastics based on their shapes with high accuracy of 98.33%. With trained models, it takes only seconds to segment and classify a new micrograph in high accuracy, which is remarkably cheaper and faster than manual labour. The growing datasets may benefit the identification and quantification of microplastics in environmental samples in future work.

Stormwater Bioretention Cells Are Not an Effective Treatment for Persistent and Mobile Organic Compounds (PMOCs)

Bioretention cells are a stormwater management technology intended to reduce the quantity of water entering receiving bodies. They are also used to reduce contaminant releases, but their performance is unclear for hydrophilic persistent and mobile organic compounds (PMOCs). We developed a novel eight-compartment one-dimensional (1D) multimedia model of a bioretention cell ("Bioretention Blues") and applied it to a spike and recovery experiment conducted on a system near Toronto, Canada, involving PMOC benzotriazole and four organophosphate esters (OPEs). Compounds with (log D_OC) (organic carbon-water distribution coefficients) < ∼2.7 advected through the system, resulting in infiltration or underdrain flow. Compounds with log D_OC > 3.8 were mostly sorbed to the soil, where subsequent fate depended on transformation. For compounds with 2.7 ≤ log D_OC ≤ 3.8, sorption was sensitive to event size and compound-specific diffusion parameters, with more sorption expected for smaller rain events and for compounds with larger diffusion coefficients. Volatilization losses were minimal for all compounds tested. Direct uptake by vegetation also played a negligible role regardless of the compounds' physicochemical properties. Nonetheless, model simulations showed that vegetation could play a role by increasing transpiration, thereby increasing sorption to the bioretention soil and reducing PMOC release. Model results suggest design modifications to bioretention cells.

Evaluating the relative adsorption and biodegradation of 2-methylisoborneol and geosmin across granular activated carbon filter-adsorbers

This study investigated the relative contributions of adsorption vs. biodegradation towards 2-methylisoborneol (MIB) and geosmin removal in the granular activated carbon (GAC) harvested from six filter-adsorbers in three drinking water treatment plants in the Great Lakes region. Column tests using azide-treated (sterilized) and untreated GAC in parallel were used to isolate the two effects. It was identified that substantial MIB and geosmin biodegradation in the GAC was occurring in one location, and that GAC in some cases had significant adsorption capacity after as much as 9 years of operation. Four alternative biological parameters (adenosine triphosphate, esterase activity, phosphatase activity, and ¹⁴C-glucose respiration rate) were measured to quantify the biological activity of the GAC, and ¹⁴C-glucose respiration rate was identified to be a potential indicator for GAC biodegradative capacity in terms of MIB, geosmin, and dissolved organic carbon. Several potential MIB and geosmin biodegradation products were also identified using non-targeted screening analysis. By using the new tools identified in this study, we can begin to better understand where adsorption vs. biodegradation may predominate under real-world conditions (e.g., different temperatures, influent concentrations, and empty bed contact time), leading ultimately to more cost-effective use of GAC.

Understanding adsorption and biodegradation in granular activated carbon for drinking water treatment: A critical review

Drinking water treatment plants use granular activated carbon (GAC) to adsorb and remove trace organics, but the GAC has a limited lifetime in terms of adsorptive capacity and needs to be replaced before it is exhausted. Biological degradation of target contaminants can also occur in GAC filters, which might allow the GAC to remain in service longer than expected. However, GAC biofiltration remains poorly understood and unpredictable. To increase the understanding of adsorption and biodegradation in GAC, previous studies have conducted parallel column tests that use one column of GAC (potentially biologically active) to assess overall removal via both adsorption and biodegradation, and one column with either sterilized GAC or biological non-adsorbing media to assess adsorption or biodegradation alone. Mathematical models have also been established to give insight into the adsorption and biodegradation processes in GAC. In this review, the experimental and modeling approaches and results used to distinguish between the role of adsorption and biodegradation were summarized and critically discussed. We identified several limitations: (1) using biological non-adsorbing media in column tests might lead to non-representative extents of biodegradation; (2) sterilization methods may not effectively inhibit biological activity and may affect adsorption; (3) using virgin GAC coated with biofilm could overestimate adsorption; (4) potential biofilm detachment during column experiments could lead to biased results; (5) the parallel column test approach itself is not universally applicable; (6) competitive adsorption was neglected by previous models; (7) model formulations were based on virgin GAC only. To overcome these limitations, we proposed four new approaches: the use of gamma irradiation for sterilization, a novel minicolumn test, compound-specific isotope analysis to decipher the role of adsorption and biodegradation in situ, and a new model to simulate trace organic adsorption and biodegradation in a GAC filter .

Trace Organic Contaminant Transfer and Transformation in Bioretention Cells: A Field Tracer Test with Benzotriazole

Bioretention cells can effectively infiltrate stormwater runoff and partly remove conventional water contaminants. A field tracer injection experiment in a conventionally designed bioretention cell was used to investigate the fate of benzotriazole, a model trace organic contaminant, during and between runoff events. Moderate (29%) benzotriazole load reductions were measured during the 6 h long injection experiment. The detection of 1-methyl benzotriazole, hydroxy benzotriazole, and methoxy benzotriazole provided in situ evidence of some rapid benzotriazole microbial transformation during the tracer test and more importantly between the events. The detection of benzotriazole alanine and benzotriazole acetyl alanine also showed fast benzotriazole phytotransformation to amino acid conjugates during the tracer test and suggests further transformation of phytotransformation products between events. These data provide conclusive full-scale evidence of benzotriazole microbial and phytotransformation in bioretention cells. Non-target chemical analysis revealed the presence of a diverse range of trace organic contaminants in urban runoff and exiting the bioretention cell, including pesticides and industrial, household, and pharmaceutical compounds. We have demonstrated the in situ potential of urban green infrastructure such as bioretention cells to eliminate polar trace organic contaminants from stormwater. However, targeted design and operation strategies, for example, hydraulic control and the use of soil amendments, should be incorporated for improved bioretention cell performance for such compounds.

Carbon, hydrogen and nitrogen stable isotope fractionation allow characterizing the reaction mechanisms of 1H-benzotriazole aqueous phototransformation

1H-benzotriazole is part of a larger family of benzotriazoles, which are widely used as lubricants, polymer stabilizers, corrosion inhibitors, and anti-icing fluid components. It is frequently detected in urban runoff, wastewater, and receiving aquatic environments. 1H-benzotriazole is typically resistant to biodegradation and hydrolysis, but can be transformed via direct photolysis and photoinduced mechanisms. In this study, the phototransformation mechanisms of 1H-benzotriazole were characterized using multi-element compound-specific isotope analysis (CSIA). The kinetics, transformation products, and isotope fractionation results altogether revealed that 1H-benzotriazole direct photolysis and indirect photolysis induced by OH radicals involved two alternative pathways. In indirect photolysis, aromatic hydroxylation dominated and was associated with small carbon (ε_C = -0.65 ± 0.03‰), moderate hydrogen (ε_H = -21.6‰), and negligible nitrogen isotope enrichment factors and led to hydroxylated forms of benzotriazole. In direct photolysis of 1H-benzotriazole, significant nitrogen (ε_N = -8.4 ± 0.4 to -4.2 ± 0.3‰) and carbon (ε_C = -4.3 ± 0.2 to -1.64 ± 0.04‰) isotope enrichment factors indicated an initial N-N bond cleavage followed by nitrogen elimination with a C-N bond cleavage. The results of this study highlight the potential for multi-element CSIA application to track 1H-benzotriazole degradation in aquatic environments.

Bioretention cells remove microplastics from urban stormwater

Microplastic pathways in the environment must be better understood to help select appropriate mitigation strategies. In this 2-year long field study, microplastics were characterized and quantified in urban stormwater runoff and through a bioretention cell, a type of low impact development infrastructure. Concentrations of microparticles ranged from below the detection limit to 704 microparticles/L and the dominant morphology found were fibers. High rainfall intensity and longer antecedent dry days resulted in larger microparticle concentrations. In addition, atmospheric deposition was a source of microplastics to urban runoff. Overall, these results demonstrate that urban stormwater runoff is a concentrated source of microplastics whose concentrations depend on specific climate variables. The bioretention cell showed an 84% decrease in median microparticle concentration in the 106-5,000 µm range, and thus is effective in filtering out microplastics and preventing their spread to downstream environments. Altogether, these results highlight the large contribution of urban stormwater runoff to microplastic contamination in larger aquatic systems and demonstrate the potential for current infiltration-based low impact development practices to limit the spread of microplastic contamination downstream.

Optimization of a solid-phase microextraction technique for chloro‑ and nitro- substituted aromatic compounds using design of experiments

A rapid and sensitive direct immersion solid-phase microextraction (SPME) technique for the analysis of seven chloro (Cl-) and nitro (NO₂-) substituted anilines, toluenes, and nitrobenzenes from small volume (1.5 mL) aqueous samples was optimized for gas chromatography using Design of Experiments (DoE). Screening of the SPME factors was performed by a fractional factorial DoE, and the optimization of influential factors was achieved with a central composite multi-response surface DoE. Extraction time, pre-SPME agitation speed, extraction temperature, and desorption temperature were identified as significant factors and their values were set using a desirability function that maximized the extraction of the seven target analytes. Extraction time and agitation speed showed significant interactions for most analytes (α = 0.05). The relative standard deviations (RSDs) for within-day and between-day analyses were below 8%, suggesting that the method was repeatable and reproducible. The obtained limits of detection were in the low μg/L range (1-10) using a Flame Ionization Detector, far below what is needed for industrial contaminated sites (usually >1 mg/L). The optimized SPME method increased the analyte concentration up to 2-3 orders of magnitude compared with direct GC injection. The optimized SPME method was applied to two groundwater samples from a contaminated site in which the concentrations of three of the target analytes were ranged from 0.06 to 9.42 mg/L with RSDs <11%. When the concentrations of the target analytes in the sample matrix were higher than 0.5 mg/L, a competition for the SPME extraction sites was observed where analytes with higher affinity for the fiber material replaced the analytes with lower affinity. As a result, dilution of highly contaminated samples is recommended. This study provided for the first time an analytical method for the quantification of frequently co-occurring contaminants from the chloro‑ and nitro- substituted aniline, toluene, and nitrobenzene families.

Sulfidated nano zerovalent iron (S-nZVI) for in situ treatment of chlorinated solvents: A field study

Sulfidated nano zerovalent iron (S-nZVI), stabilized with carboxymethyl cellulose (CMC), was successfully synthesized on site and injected into the subsurface at a site contaminated with a broad range of chlorinated volatile organic compounds (cVOCs). Transport of CMC-S-nZVI to the monitoring wells, both downgradient and upgradient, resulted in a significant decrease in concentrations of aqueous-phase cVOCs. Short-term (0-17 days) total boron and chloride measurements indicated dilution and displacement in these wells. Importantly however, compound specific isotope analysis (CSIA), changes in concentrations of intermediates, and increase in ethene concentrations confirmed dechlorination of cVOCs. Dissolution from the DNAPL pool into the aqueous phase at the deepest levels (4.0-4.5 m bgs) was identifiable from the increased cVOCs concentrations during long-term monitoring. However, at the uppermost levels (∼1.5 m above the source zone) a contrasting trend was observed indicating successful dechlorination. Changes in cVOCs concentrations and CSIA data suggest both sequential hydrogenolysis as well as reductive β-elimination as the possible transformation mechanisms during the short-term abiotic and long-term biotic dechlorination. One of the most positive outcomes of this CMC-S-nZVI field treatment is the non-accumulation of lower chlorinated VOCs, particularly vinyl chloride. Post-treatment soil cores also revealed significant decreases in cVOCs concentrations throughout the targeted treatment zones. Results from this field study show that sulfidation is a suitable amendment for developing more efficient nZVI-based in situ remediation technologies.

Resilience to multiple stressors in an aquatic plant and its microbiome

PREMISE

Outcomes of species interactions, especially mutualisms, are notoriously dependent on environmental context, and environments are changing rapidly. Studies have investigated how mutualisms respond to or ameliorate anthropogenic environmental changes, but most have focused on nutrient pollution or climate change and tested stressors one at a time. Relatively little is known about how mutualisms may be altered by or buffer the effects of multiple chemical contaminants, which differ fundamentally from nutrient or climate stressors and are especially widespread in aquatic habitats.

METHODS

We investigated the impacts of two contaminants on interactions between the duckweed Lemna minor and its microbiome. Sodium chloride (salt) and benzotriazole (a corrosion inhibitor) often co-occur in runoff to water bodies where duckweeds reside. We tested three L. minor genotypes with and without the culturable portion of their microbiome across field-realistic gradients of salt (3 levels) and benzotriazole (4 levels) in a fully factorial experiment (24 treatments, tested on each genotype) and measured plant and microbial growth.

RESULTS

Stressors had conditional effects. Salt decreased both plant and microbial growth and decreased plant survival more as benzotriazole concentrations increased. In contrast, benzotriazole did not affect microbial abundance and even benefited plants when salt and microbes were absent, perhaps due to biotransformation into growth-promoting compounds. Microbes did not ameliorate duckweed stressors; microbial inoculation increased plant growth, but not at high salt concentrations.

CONCLUSIONS

Our results suggest that multiple stressors matter when predicting responses of mutualisms to global change and that beneficial microbes may not always buffer hosts against stress.

Optimizing Constructed Wetlands for Safe Removal of Triclosan: A Box-Behnken Approach

Traditional constructed wetland designs typically result in variable efficiencies for trace organic contaminant removal. In this work, we used a Box-Behnken experimental design for optimizing the conditions of pH, nitrate concentration, and dissolved organic carbon (DOC) concentration that would maximize the rate of triclosan phototransformation while minimizing the accumulation of toxic byproducts. Triclosan is a frequently detected and toxic antimicrobial agent present in many consumer and industrial products. The results showed that high pH values (9.9) and low DOC concentration (11 mg/L^-) would maximize triclosan phototransformation rate while minimizing the accumulation of toxic byproducts. As long as DOC concentrations were larger than 33 mg/L, nitrate concentration did not show a significant effect on triclosan phototransformation rate. The major transformation products detected were 2,4-dichlorophenol and compounds with chemical formulas C₁₂H₈Cl₂O₂ and C₁₂H₉ClO₃, resulting from a chlorine loss or replacement by a OH group. In addition, 4-chlorocatechol was mainly detected during direct photolysis and 2,8-dichlorodibenzo-p-dioxin was only found during direct photolysis at pH 8. This study showed that wetland efficiency at removing triclosan can theoretically be increased by limiting DOC-contributing factors, e.g., emergent vegetation, and supporting pH-increase processes, e.g., via algae growth or by incorporating alkaline geomedia.

Mechanisms of pharmaceutical and personal care product removal in algae-based wastewater treatment systems

The widespread distribution of pharmaceuticals and personal care products (PPCPs), particularly in the built environment, has led to increased concern about their effects on both human and ecosystem health. In this research, we investigated the role of algae species Scenedesmus obliquus and Chlorella vulgaris in governing PPCP transfer and transformation mechanisms in algae-containing environments. Lab-scale algal bioreactors were created under various conditions of light, water matrix, and sterilization method to isolate and elucidate reaction mechanisms affecting carbamazepine, ibuprofen, gemfibrozil, and triclosan. The parent compounds and their potential transformation products were analyzed in both the water and algae phases. The results showed that ibuprofen was primarily biotransformed due to synergistic relationships between the algae and the bacteria. Ibuprofen biotransformation products tentatively identified as hydroxy-ibuprofen, carboxy-ibuprofen, and 4-isobutylcatechol were detected in several samples. In all the reactors exposed to light, triclosan underwent both phototransformation and biotransformation. Triclosan biotransformation took place in Scenedesmus obliquus, as demonstrated by the presence of triclosan-O-sulfate in the algae extracts. No evidence of significant carbamazepine and gemfibrozil transfer or transformation was observed under the experimental conditions tested. These results suggest that microalgal-bacterial consortia can facilitate PPCP transformation in algae-based passive water treatment systems.

Bioretention cells under cold climate conditions: Effects of freezing and thawing on water infiltration, soil structure, and nutrient removal

Bioretention cells are a popular control strategy for stormwater volume and quality, but their efficiency for water infiltration and nutrient removal under cold climate conditions has been poorly studied. In this work, soil cores were collected from an active bioretention cell containing engineered soil material amended with a phosphate sorbent medium. The cores were used in laboratory column experiments conducted to obtain a detailed characterization of the soil's bioretention performance during six consecutive freeze-thaw cycles (FTCs, from -10 to +10 °C). At the start of each FTC, the experimental column undergoing the FTCs and a control column kept at room temperature were supplied with a solution containing 25 mg/L of bromide, nitrate and phosphate. Water saturated conditions were established to mimic the presence of an internal water storage zone to support anaerobic nitrate removal. At the end of each FTC, the pore solution was allowed to drain from the columns. The results indicate that the FTCs enhanced the infiltration efficiency of the soil: with each successive cycle the drainage rate increased in the experimental column. Freezing and thawing also increased the saturated hydraulic conductivity of the bioretention soil. X-ray tomography imaging identified a key role of macro-pore formation in maintaining high infiltration rates. Both aqueous nitrate and phosphate supplied to the columns were nearly completely removed from solution. Sufficiently long retention times and the presence of the internal water storage zone promoted anaerobic nitrate elimination despite the low temperatures. Dissolved phosphate was efficiently trapped at all depths in the soil columns, with ≤2% of the added stormwater phosphate recovered in the drainage effluent. These findings imply that, when designed properly, bioretention cells can support high infiltration rates and mitigate nutrient pollution in cold climates.

Determination of in situ biodegradation rates via a novel high resolution isotopic approach in contaminated sediments

A key challenge in conceptual models for contaminated sites is identification of the multiplicity of processes controlling contaminant concentrations and distribution as well as quantification of the rates at which such processes occur. Conventional protocol for calculating biodegradation rates can lead to overestimation by attributing concentration decreases to degradation alone. This study reports a novel approach of assessing in situ biodegradation rates of monochlorobenzene (MCB) and benzene in contaminated sediments. Passive diffusion samplers allowing cm-scale vertical resolution across the sediment-water interface were coupled with measurements of concentrations and stable carbon isotope signatures to identify zones of active biodegradation of both compounds. Large isotopic enrichment trends in ¹³C were observed for MCB (1.9-5.7‰), with correlated isotopic depletion in ¹³C for benzene (1.0-7.0‰), consistent with expected isotope signatures for substrate and daughter product produced by in situ biodegradation. Importantly in the uppermost sediments, benzene too showed a pronounced ¹³C enrichment trend of up to 2.2‰, providing definitive evidence for simultaneous degradation as well as production of benzene. The hydrogeological concept of representative elementary volume was applied to CSIA data for the first time and identified a critical zone of 10-15 cm with highest biodegradation potential in the sediments. Using both stable isotope-derived rate calculations and numerical modeling, we show that MCB degraded at a slower rate (0.1-1.4 yr^-1 and 0.2-3.2 yr^-1, respectively) than benzene (3.3-84.0 yr^-1) within the most biologically active zone of the sediment, contributing to detoxification.

Biogeochemical Controls on Strontium Fate at the Sediment-Water Interface of Two Groundwater-Fed Wetlands with Contrasting Hydrologic Regimes

Radioactive strontium (Sr) is a common groundwater contaminant at many nuclear sites. Its natural retention in groundwater-fed wetlands is an attractive remediation strategy. However, at present, the biogeochemical mechanisms controlling Sr transport at the sediment-water interface are poorly understood. In this field study, Sr fate was investigated in two wetlands with contrasting vegetation and hydrologic regimes. The marsh was an open-water wetland with constant water table and no emergent vegetation. The swamp was vegetated with fluctuating water levels and a thick mat of submerged cattail litter in the water column. High-resolution porewater Sr concentrations and solid-phase sediment Sr species revealed distinct profiles between the two wetlands. The marsh exhibited a strongly reduced environment and sharp concentration peaks at the sediment-water interface. In contrast, the smaller concentration gradients of the swamp resulted in a reduced flux of Sr to the surface water. The organic fraction of the sediment dominated Sr retention compared to the inorganic iron and manganese oxides. However, the marsh had a significant fraction of recalcitrant Sr presumably due to its incorporation into sulfur and/or carbonate minerals. These results suggest that vegetated wetlands with fluctuating hydrologic regimes could act as efficient sinks for Sr pollution.

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.

Strontium adsorption and desorption in wetlands: Role of organic matter functional groups and environmental implications

Strontium (Sr) is a chemical element that is often used as a tracer in hydrogeochemical studies, and is ubiquitously distributed as a radioactive contaminant in nuclear sites in the form of strontium-90 (Sr-90). At the interface between groundwater and surface water, wetlands possess unique hydrogeochemical properties whose impact on Sr transport has not been investigated thoroughly. In this study, the adsorption and desorption of Sr was investigated on six natural wetland substrates and two mixes of exogenous media and wetland sediment: winter and summer wetland sediments, decayed cattails, wood, leaf litter, moss, bone charcoal, and clinoptilolite. The composition of the organic matter was characterized using carbon-13, solid phase Nuclear Magnetic Resonance analysis. The range of the substrates' adsorption coefficients obtained could be explained by factors indicative of proteins in the organic matter, which were shown to support strong and poorly reversible Sr adsorption. In contrast, the proportion of carbohydrates and lignin were found to be indicative of lower adsorption coefficients and higher desorption. The implications of these results for Sr pollution remediation in wetlands are discussed.

Sediment Monitored Natural Recovery Evidenced by Compound Specific Isotope Analysis and High-Resolution Pore Water Sampling

Monitoring natural recovery of contaminated sediments requires the use of techniques that can provide definitive evidence of in situ contaminant degradation. In this study, a passive diffusion sampler, called "peeper", was combined with Compound Specific Isotope Analysis to determine benzene and monochlorobenzene (MCB) stable carbon isotope values at a fine vertical resolution (3 cm) across the sediment water interface at a contaminated site. Results indicated significant decrease in concentrations of MCB from the bottom to the top layers of the sediment over 25 cm, and a 3.5 ‰ enrichment in δ¹³C values of MCB over that distance. Benzene was always at lower concentrations than MCB, with consistently more depleted δ¹³C values than MCB. The redox conditions were dominated by iron reduction along most of the sediment profile. These results provide multiple lines of evidence for in situ reductive dechlorination of MCB to benzene. Stable isotope analysis of contaminants in pore water is a valuable method to demonstrate in situ natural recovery of contaminated sediments. This novel high-resolution approach is critical to deciphering the combined effects of parent contaminant (e.g., MCB) degradation and both production and simultaneous degradation of daughter products, especially benzene.

Diffusion related isotopic fractionation effects with one-dimensional advective-dispersive transport

Aqueous phase diffusion-related isotope fractionation (DRIF) for carbon isotopes was investigated for common groundwater contaminants in systems in which transport could be considered to be one-dimensional. This paper focuses not only on theoretically observable DRIF effects in these systems but introduces the important concept of constraining "observable" DRIF based on constraints imposed by the scale of measurements in the field, and on standard limits of detection and analytical uncertainty. Specifically, constraints for the detection of DRIF were determined in terms of the diffusive fractionation factor, the initial concentration of contaminants (C0), the method detection limit (MDL) for isotopic analysis, the transport time, and the ratio of the longitudinal mechanical dispersion coefficient to effective molecular diffusion coefficient (Dmech/Deff). The results allow a determination of field conditions under which DRIF may be an important factor in the use of stable carbon isotope measurements for evaluation of contaminant transport and transformation for one-dimensional advective-dispersive transport. This study demonstrates that for diffusion-dominated transport of BTEX, MTBE, and chlorinated ethenes, DRIF effects are only detectable for the smaller molar mass compounds such as vinyl chloride for C0/MDL ratios of 50 or higher. Much larger C0/MDL ratios, corresponding to higher source concentrations or lower detection limits, are necessary for DRIF to be detectable for the higher molar mass compounds. The distance over which DRIF is observable for VC is small (less than 1m) for a relatively young diffusive plume (<100years), and DRIF will not easily be detected by using the conventional sampling approach with "typical" well spacing (at least several meters). With contaminant transport by advection, mechanical dispersion, and molecular diffusion this study suggests that in field sites where Dmech/Deff is larger than 10, DRIF effects will likely not be observable for common groundwater contaminants. Importantly, under most field conditions, Dmech/Deff≥10 is usually satisfied in the longitudinal direction, suggesting that DRIF is not likely to be observable in most groundwater systems in which contaminant transport is predominantly one-dimensional. Given the importance in the MDL it is recommended that MDL should always be explicitly reported in both modeling and field studies.

Compound specific isotope analysis of hexachlorocyclohexane isomers: a method for source fingerprinting and field investigation of in situ biodegradation

RATIONALE

The manufacturing and uses of hexachlorocyclohexane (HCH) have resulted in a serious environmental challenge and legacy. This study highlights the ability of compound specific isotope analysis (CSIA) to distinguish among various HCH sources and to support the evaluation of the potential for in situ biodegradation in contaminated groundwater.

METHODS

Tests were conducted to verify the absence of significant isotope fractionation during HCH sample pre-concentration including dichloromethane extraction, solvent exchange into iso-octane, and H2SO4 clean-up, and analysis by gas chromatography/combustion-isotope ratio mass spectrometry (GC/C-IRMS). The method was then applied to four Technical Grade (TG) HCH mixtures procured from different sources and to groundwater samples from a contaminated site.

RESULTS

The pre-concentration method enabled determination of carbon isotope ratios (δ(13)C values) of HCH isomers with no significant isotopic fractionation. The TG-HCH mixtures had significantly different δ(13)C values. Moreover, for any given TG-HCH, all isomers had δ(13)C values within 1.1‰ of each other - a distinctly uniform fingerprint. At the HCH-contaminated field site, compared with source wells, downgradient wells showed significant (up to 5.1‰) enrichment in (13)C and the δ(13)C values of the HCH isomers were significantly different from each other.

CONCLUSIONS

A method was successfully developed for the CSIA of HCH isomers that showed potential for HCH source differentiation and identification of HCH in situ biodegradation. At the HCH-contaminated site, the observed preferential isotopic enrichment of certain isomers relative to others for a given source allows differentiation between biodegraded and non-biodegraded HCH.

Diffusion sampler for compound specific carbon isotope analysis of dissolved hydrocarbon contaminants

Compound Specific Isotope Analysis (CSIA) is widely utilized to study the fate of organic contaminants in groundwater. To date, however, no method is available to obtain CSIA samples at a fine (cm) spatial scale across the sediment-surface water interface (SWI), a key boundary for discharge of contaminated groundwater to surface water. Dissolved contaminants in such discharged zones undergo rapid temporal and spatial changes due to heterogeneity in redox conditions and microbial populations. The compatibility of a passive sediment pore water sampler ("peeper") to collect 40 mL samples for CSIA of benzene, toluene, monochlorobenzene, and 1,2-dichlorobenzene at field-relevant concentrations (0.1-5 mg L(-1)) was evaluated in laboratory experiments. Results demonstrate that physical diffusion across the polysulfone membrane does not alter the carbon isotope values (±0.5‰). Measured δ(13)C values also remain invariant despite significant adsorption of the compounds on the peeper material, an effect which increased with higher numbers of chlorine atoms and sorption coefficient (Koc) values. In addition, isotope equilibrium between the peeper chamber and the sediment pore water occurred in less than a day, indicating the peeper method can be used to provide samples for CSIA analysis at fine spatial and temporal sampling resolutions in contaminated sediments.

Distinct carbon isotope fractionation during anaerobic degradation of dichlorobenzene isomers

Chlorinated benzenes are ubiquitous organic contaminants found in groundwater and soils. Compound specific isotope analysis (CSIA) has been increasingly used to assess natural attenuation of chlorinated contaminants, in which anaerobic reductive dechlorination plays an essential role. In this work, carbon isotope fractionation of the three dichlorobenzene (DCB) isomers was investigated during anaerobic reductive dehalogenation in methanogenic laboratory microcosms. Large isotope fractionation of 1,3-DCB and 1,4-DCB was observed while only a small isotope effect occurred for 1,2-DCB. Bulk enrichment factors (εbulk) were determined from a Rayleigh model: -0.8 ± 0.1 ‰ for 1,2-DCB, -5.4 ± 0.4 ‰ for 1,3-DCB, and -6.3 ± 0.2 ‰ for 1,4-DCB. εbulk values were converted to apparent kinetic isotope effects for carbon (AKIE) in order to characterize the carbon isotope effect at the reactive positions for the DCB isomers. AKIE values are 1.005 ± 0.001, 1.034 ± 0.003, and 1.039 ± 0.001 for 1,2-DCB, 1,3-DCB, and 1,4-DCB, respectively. The large difference in AKIE values between 1,2-DCB and 1,3-DCB (or 1,4-DCB) suggests distinct reaction pathways may be involved for different DCB isomers during microbial reductive dechlorination by the methanogenic cultures.

Dynamics and mitigation of six pesticides in a "Wet" forest buffer zone

Pesticide pollution is one of the main current threats on water quality. This paper presents the potential and functioning principles of a "Wet" forest buffer zone for reducing concentrations and loads of glyphosate, isoproturon, metazachlor, azoxystrobin, epoxiconazole, and cyproconazole. A tracer injection experiment was conducted in the field in a forest buffer zone at Bray (France). A fine time-scale sampling enabled to illustrate that interactions between pesticides and forest buffer substrates (soil and organic-rich litter layer), had a retarding effect on molecule transfer. Low concentrations were observed for all pesticides at the forest buffer outlet thus demonstrating the efficiency of "Wet" forest buffer zone for pesticide dissipation. Pesticide masses injected in the forest buffer inlet directly determined concentration peaks observed at the outlet. Rapid and partially reversible adsorption was likely the major process affecting pesticide transfer for short retention times (a few hours to a few days). Remobilization of metazachlor, isoproturon, desmethylisoproturon, and AMPA was observed when non-contaminated water flows passed through the forest buffer. Our data suggest that pesticide sorption properties alone could not explain the complex reaction mechanisms that affected pesticide transfer in the forest buffer. Nevertheless, the thick layer of organic matter litter on the top of the forest soil was a key parameter, which enhanced partially reversible sorption of pesticide, thus retarded their transfer, decreased concentration peaks, and likely increased degradation of the pesticides. Consequently, to limit pesticide pollution transported by surface water, the use of already existing forest areas as buffer zones should be equally considered as the most commonly implemented grass buffer strips.

Pesticide contamination interception strategy and removal efficiency in forest buffer and artificial wetland in a tile-drained agricultural watershed

Pesticide pollution is a major threat to aquatic ecosystems that can be mitigated through complementary actions including buffer zones (BZs). This paper discusses the results of 3 yr of field-scale monitoring of the concentration and load transfer of 16 pesticides out of a tile-drained catchment (Bray, France) and their reduction through two BZ: an artificial wetland (AW) and a forest buffer (FB). Typically, the highest concentrations were measured in the first flows following pesticide applications or resuming after periods of low or no flow. An open/close water management strategy was implemented to operate the parallel BZ based on pesticide applications by the farmer. The strategy was efficient in intercepting molecules whose highest concentrations occurred during the first flows following application. Inlet vs. outlet pesticide load reductions ranged from 45% to 96% (AW) and from -32% to 100% (FB) depending on the pesticide molecule and the hydrological year. Partly reversible adsorption was a dominant process explaining pesticide removal; whereas, degradation occurred for sufficiently long water retention time. Apart from the least sorbing molecules (e.g., isoproturon), BZ can partially remove pesticide pollution.

Development of a constructed wetland water treatment system for selenium removal: use of mesocosms to evaluate design parameters

The Salton Sea in California is an important habitat for fish and waterfowl. Its ecosystem is threatened due to diminishing water supplies and increasing salinity. An alternative source of water to support species conservation habitat may be obtained from local rivers (e.g., Alamo or New Rivers), provided that a wetland treatment system can be developed to remove selenium (Se), fertilizer nutrients, and other contaminants. Here, we used mesocosms to evaluate a number of potential design options (e.g., plant species selection, sediment composition and arrangement, forced aeration, organic amendments, etc.) to improve the efficiency of Se removal using treatment wetlands. Our results show that, of five different substrate arrangements tested for Se removal, the most efficient was obtained for cattails growing in a substrate of cattail litter overlying sand and peat moss sediment (water column Se was reduced from 15 μg Se/L to <0.1 μg Se/L in 72 h). The addition of organic amendments in the form of alfalfa hay or alfalfa meal was also helpful in lowering Se levels. These results suggest that it may be possible to design constructed wetland water treatment systems capable of reducing Se concentrations in river water to values below 1 μg Se/L.

Selected pesticides adsorption and desorption in substrates from artificial wetland and forest buffer

Buffer zones such as artificial wetlands and forest buffers may help decrease non-point-source pesticide pollution from agricultural catchments. The present study focuses on understanding the role of the substrates mainly found in such buffer zones for pesticide adsorption and desorption. Radiolabeled [(14)C]isoproturon, [(14)C]metazachlor, and [(14)C]epoxiconazole were used to measure adsorption and desorption isotherms on wetland sediments and plants and forest soil and litter from two sites in France. Wetland sediments and forest soil exhibited the most important potential for pesticide adsorption. Wetland plants and forest litter also showed high adsorption coefficients and were associated with highly hysteretic desorption, particularly for the moderately mobile isoproturon and metazachlor. Adsorption of the highly hydrophobic epoxiconazole was strong and associated with weak desorption from all substrates. Calculated sorption coefficients were larger than those classically measured on soils. Isoproturon, metazachlor, and epoxiconazole K(OC) sorption coefficients ranged from 84 to 372, 131 to 255, and 1,356 to 3,939 L/kg, respectively. Therefore, specifically collecting buffer zone substrate sorption data is needed for modeling purposes. Results showed that forests and wetlands present potential for pesticide retention. This may be enhanced by planting vegetation and leaving dead vegetal material in buffer zone design.

Pesticide risk mitigation by vegetated treatment systems: a meta-analysis

Pesticides entering agricultural surface waters threaten water quality and aquatic communities. Recently, vegetated treatment systems (VTSs) (e.g., constructed wetlands and vegetated ditches) have been proposed as pesticide risk mitigation measures. However, little is known about the effectiveness of VTSs in controlling nonpoint source pesticide pollution and factors relevant for pesticide retention within these systems. Here, we conducted a meta-analysis on pesticide mitigation by VTSs using data from the scientific literature and the European LIFE ArtWET project. Overall, VTSs effectively reduced pesticide exposure levels (i.e., the majority of pesticide retention performances was >70%). A multiple linear regression analysis of 188 retention performance cases identified the two pesticide properties, organic carbon sorption coefficient value and water-phase 50% dissipation time, as well as the VTS characteristics overall plant coverage and hydraulic retention time for targeting high efficacy of pesticide retention. The application of a Tier I risk assessment (EU Uniform Principle) revealed a higher toxicity reduction for hydrophobic and nonpersistent insecticides compared with less sorptive and not readily degradable herbicides and fungicides. Overall, nearly half (48.5%) of all pesticide field concentrations ( = 130) failed Tier I standard risk assessment at the inlet of VTSs, and 29.2% of all outlet concentrations exceeded conservative acute threshold levels. We conclude that VTSs are a suitable and effective risk mitigation strategy for agricultural nonpoint source pesticide pollution of surface waters. Further research is needed to improve their overall efficacy in retaining pesticides.