Do lab-derived distribution coefficient values of pesticides match distribution coefficient values determined from column and field-scale experiments? A critical analysis of relevant literature

Tailor-made polymer tracers reveal the role of clay minerals on colloidal transport in carbonate media

HYPOTHESIS

Host rock weathering and incipient pedogenesis result in the exposition of minerals, e.g., clay minerals in sedimentary limestones. Once exposed, these minerals provide the surfaces for fluid-solid interactions that control the fate of dissolved or suspended compounds such as organic matter and colloids. However, the functional and compositional diversity of organic matter and colloids limits the assessment of reactivity and availability of clay mineral interfaces. Such assessment demands a mobile compound with strong affinity to clay surfaces that is alien to the subsurface.

EXPERIMENT

We approached this challenge by using poly(ethylene glycol) (PEG) as interfacial tracer in limestone weathering experiments.

FINDINGS

PEG adsorption and transport was dependent on the availability of clay mineral surfaces and carbonate dissolution dynamics. In addition, PEG adsorption featured adsorption-desorption hysteresis which retained PEG mass on clay mineral surfaces. This resulted in different PEG transport for experiments conducted consecutively in the same porous medium. As such, PEG transport was reconstructed with a continuum-scale model parametrized by a Langmuir-type isotherm including hysteresis. Thus, we quantified the influence of exposed clay mineral surfaces on the transport of organic colloids in carbonate media. This renders PEG a suitable model colloid tracer for the assessment of clay surface exposition in porous media.

Mechanistic modelling of amphibian body burdens after dermal uptake of pesticides from soil

Amphibians are currently considered to be covered by pesticide Environmental Risk Assessment schemes by surrogacy assumptions of exposure and susceptibility based on typical laboratory test species such as fish, mammals, and birds. While multiple reviews have shown for this approach to be adequate in the case of aquatic stages, the same cannot be definitively stated for terrestrial stages. Concerns have risen that exposure of amphibians is likely to be highly influenced by dermal absorption, primarily due to the high permeability of their skin and the lack of a protective layer, such as fur or feathers. It is thus hypothesized that dermal uptake could be a significant route of exposure. Consequently, it is necessary to determine the relative importance of different exposure routes that might affect the integrated toxicity outcome for terrestrial amphibian life-stages. Here, a one-compartment Toxicokinetic model was derived and tested using a publicly available dataset containing relevant exposure and uptake information for juvenile anurans exposed to 13 different pesticides. Modelled body burdens were then compared to measured burdens for a total of 815 individuals. Overall, a good concordance between modelled and measured values was observed, with the predicted and measured body burdens differing by a factor of 2 on average (overall R2 of 0.80 and correlation coefficient of 0.89), suggesting good predictivity of the model. Accordingly, the model predicts realistic body burdens for a variety of frog and toad species, and overall, for anurans. As the model includes rehydration (implicit in the evaluated studies) but currently does not account for metabolism, it can be seen as a worst-case assessment. We suggest toxicokinetic models, such as the one here presented, could be used to characterize dermal exposure in amphibians, screen for pesticides of concern, and prioritize risk assessment efforts, whilst reducing the need for de novo vertebrate testing.

A machine learning based approach for estimating site-specific partition coefficient Kd of organic compounds: Application to nonionic pesticides

The partitioning coefficient K_d for a specific compound and location is not only a key input parameter of fate and transport models, but also critical in estimating the safety environmental concentration threshold. In order to reduce the uncertainty caused by non-linear interactions among environmental factors, machine learning based models for predicting K_d were developed in this work based on literature datasets of nonionic pesticides including molecular descriptors, soil properties, and experimental settings. The equilibrium concentration (C_e) values were specifically included for the reason that a varied range of K_d corresponding to a given C_e occurred in a real environment. By transforming 466 isotherms reported in the literature, 2618 paired equilibrium concentrations of liquid-solid (C_e-Q_e) data points were obtained. Results of SHapley Additive exPlanations revealed that soil organic carbon, C_e, and cavity formation were the most important. The distance-based applicability domain analysis was conducted for the 27 most frequently used pesticides with 15952 pieces of soil information from the HWSD-China dataset by setting three C_e scenarios (i.e., 10, 100, and 1000 μg L^-1). It was revealed the groups of compounds showing log K_d < 0.06 and log K_d > 1.19 were composed mostly of those with log K_ow of -0.800 and 5.50, respectively. When log K_d varied between 0.100 and 1.00, it was impacted by interactions among soil types, molecular descriptors, and C_e comprehensively, which accounted for 55% of the total 2618 calculations. It could be concluded that site-specific models developed in this work are necessary and practicable for the environmental risk assessment and management of nonionic organic compounds.

Well-defined poly(ethylene glycol) polymers as non-conventional reactive tracers of colloidal transport in porous media

HYPOTHESIS

A prominent fraction of mobile organic matter in natural aqueous soil solutions is formed by molecules in sizes that seamlessly exceed the lower end of what is defined as a colloid. The hydrodynamics and the functional diversity of these molecules result in a transport behavior that is fundamentally different from smaller compounds. However, there is a lack of "reactive tracers" that allow for the study of colloidal transport phenomena appropriately. We hypothesize that tailor-made and well-defined synthetic polymers can overcome this limitation.

EXPERIMENTS

We prepared and characterized the hydrodynamic properties of water-soluble poly(ethylene glycol)s (PEG) and studied their adsorption to mixtures of quartz, illite, and goethite in batch and column experiments.

FINDINGS

We used this information to independently predict the transport of PEG with striking agreement to the observed mean breakthrough times in all porous media. As PEG transport can be comprehensively and quantitatively reconstructed, we conclude that functionalized PEGs are promising candidates to be used as tailorable and non-toxic tracers available in the size range of natural organic (macro-)molecules.

Modelling herbicides mobility in amended soils: Calibration and test of PRZM and MACRO

Addition of organic residues to soil is a current farming practice but it is not considered in the modelling studies for pesticide risk assessment at regulatory level despite its potential impact on the pesticide dynamics in soil. Thus, the objective of this work was to examine and to compare the ability of PRZM and MACRO pesticide fate models to simulate soil water content, and bromide (Br^-, tracer), chlorotoluron and flufenacet concentrations in the soil profiles (0-100 cm) of one agricultural soil, unamended (control soil, S), amended with spent mushroom substrate (S + SMS) or amended with green compost (S + GC). Based on a two-year field-scale dataset, the models were first calibrated against measurements of water and solutes contents in the soil profiles (first year) and then tested without any further model calibration by comparison with the field observations of the second year. In general, the performance of MACRO to simulate the whole dataset in the three soil treatments was higher than that of PRZM. MACRO simulated satisfactorily the water dynamics along the soil profiles whereas it was poorly described by the capacity model PRZM. Both models predicted very well the Br^- mobility in control and amended soils after dispersion parameters were fitted to observations. No calibration was necessary to reproduce correctly herbicides vertical distribution in the control soil profile. In the amended soils, MACRO simulations were highly correlated to the observed vertical distribution of flufenacet and chlorotoluron, but calibration of the K_d of chlorotoluron was needed. On the contrary, modelling with PRZM required calibration of K_d and DT₅₀ of both herbicides to obtain an acceptable agreement between observations and predictions in the amended soils. K_d and DT₅₀ calibration was based on the initial dissolved organic carbon contents (DOC) of amended soils. It allowed to take into account the processes that decrease the herbicides sorption on the soil and enhance their bioavailability, but that are not described in PRZM and MACRO (such as the formation of herbicide-DOC mobile complexes). This work showed that models such as PRZM and MACRO are able to simulate the fate of pesticides in amended soils. However, before using these models as predictive tools in large amended soil conditions, and especially in the regulatory context, further modelling studies should focus on other pedoclimatic-pesticides-organic residues combinations, and on longer periods.

Sorption and transport of trenbolone and altrenogest photoproducts in soil-water systems

This study evaluated the sorption and transport potential of seven phototransformation products of 17α-trenbolone, 17β-trenbolone, trendione, and altrenogest, along with the parent trienone steroids in batch and column soil-water systems. In batch systems, the target solutes exhibited linear isotherms, with values for sorption coefficients (log Koc) of parent steroids (2.46-2.76) higher than those for photoproducts (1.92-2.57). In column systems, the estimated retardation factors (Rsol) for parents (2.7-5.1) were ∼2-5 times higher than those for photoproducts (0.84-1.7). The log Koc (R2 = 0.75) and Rsol (R2 = 0.89-0.98) were well correlated with measured log Kow values, indicating that hydrophobic partitioning governed the soil-solute interaction of these biologically potent compounds in soil-water systems. These data indicated that photoproducts exhibited reduced sorption affinity and increased transport potential relative to more hydrophobic parent structures. In agroecosystems, traditional runoff management practices would be expected to exhibit reduced treatment effectiveness for photoproducts relative to the parent compounds of commonly used trienone steroids.

Real-time forecasting of pesticide concentrations in soil

Forecasting pesticide residues in soils in real time is essential for agronomic purposes, to manage phytotoxic effects, and in catchments to manage surface and ground water quality. This has not been possible in the past due to both modelling and measurement constraints. Here, the analytical transient probability distribution (pdf) of pesticide concentrations is derived. The pdf results from the random ways in which rain events occur after pesticide application. First-order degradation kinetics and linear equilibrium sorption are assumed. The analytical pdfs allow understanding of the relative contributions that climate (mean storm depth and mean rainfall event frequency) and chemical (sorption and degradation) properties have on the variability of soil concentrations into the future. We demonstrated the two uncertain reaction parameters can be constrained using Bayesian methods. An approach to a Bayesian informed forecast is then presented. With the use of new rapid tests capable of providing quantitative measurements of soil concentrations in the field, real-time forecasting of future pesticide concentrations now looks possible for the first time. Such an approach offers new means to manage crops, soils and water quality, and may be extended to other classes of pesticides for ecological risk assessment purposes.

An integrated approach for assessing the migration behavior of chlorpyrifos and carbaryl in the unsaturated soil zone

Chlorpyrifos (O, O-diethyl O-3,5,6-trichloropyridin-2-yl phosphorothioate) and carbaryl (1-naphthyl methylcarbamate) are often applied concurrently as insecticides in food production. The aim of this study was to research their migration behavior in a real environment. We researched the leaching of both pesticides by setting up field lysimeters on a farm with the typical soil used in fruit production today. In order to analyze the variables involved in this process, we performed complementary adsorption studies, we performed complementary adsorption studies using batches and undisturbed soil laboratory columns for both compounds. The results for pesticide transport through the lysimeters showed that less than 1% of chlorpyrifos was recovered in the leachates, while almost 17% was recovered for carbaryl. Having completed the experiment in undisturbed laboratory columns, soil analysis showed that chlorpyrifos mainly remained in the first 5 cm, while carbaryl moved down to the lower sections. These results can be explained in view of the sorption coefficient values (K_D) obtained in horizons A and B for chlorpyrifos (393 and 184 L kg^-1) and carbaryl (3.1 and 4.2 L kg^-1), respectively. By integrating the results obtained in the different approaches, we were able to characterize the percolation modes of these pesticides in the soil matrix, thus contributing to the sustainable use of resources.

Experimental and modeling of the unsaturated transports of S-metolachlor and its metabolites in glaciofluvial vadose zone solids

The transport of pesticides to groundwater is assumed to be impacted by flow processes and geochemical interactions occurring in the vadose zone. In this study, the transport of S-metolachlor (SMOC) and its two metabolites ESA-metolachlor (MESA) and OXA-metolachlor (MOXA) in vadose zone materials of a glaciofluvial aquifer is studied at laboratory scale. Column experiments are used to study the leaching of a conservative tracer (bromide) and SMOC, MESA and MOXA under unsaturated conditions in two lithofacies, a bimodal gravel (Gcm,b) and a sand (S-x). Tracer experiments showed water fractionation into mobile and immobile compartments more pronounced in bimodal gravel columns. In both lithofacies columns, SMOC outflow is delayed (retardation factor>2) and mass balance reveals depletion (mass balance of 0.59 and 0.77 in bimodal gravel and sand, respectively). However, complete mass elution associated with retardation factors close to unity shows that there is no adsorption of MESA and MOXA in either lithofacies. SMOC transport is characterized by non-equilibrium sorption and sink term in both bimodal gravel and sand columns. Batch experiments carried out using agitation times consistent with column water residence times confirmed a time-dependence of SMOC sorption and high adsorption rates (>80%) of applied concentrations. Desorption experiments confirm the irreversibility of a major part of the SMOC adsorption onto particles, corresponding to the sink term in columns. In the bimodal gravel column, SMOC adsorption occurs mainly on reactive particles in contact with mobile water because of flow regionalization whereas in the sand column, there is pesticide diffusion to the immobile water. Such results clearly show that sorption mechanisms in the vadose zone solids below the soil are both solute and contact-time-dependent and are impacted by hydrodynamic conditions. The more rapid transport of MESA and MOXA to the aquifer would be controlled mainly by water flow through the unsaturated zone whereas SMOC transport is retarded by sorption processes within the vadose zone.

Extended sorption partitioning models for pesticide leaching risk assessments: Can we improve upon the koc concept?

Models used to assess leaching of pesticides to groundwater still rely on the sorption koc value, even though its limitations have been known for several decades, especially for soils of low organic carbon content (i.e. subsoils). This is mainly because the general applicability of any improved model approach that is also simple enough to use for regulatory purposes has not been demonstrated. The objective of this study was to test and compare alternative models of sorption that could be useful in pesticide risk assessment and management. To this end, a database containing the results of batch sorption experiments for pesticides was compiled from published studies in the literature, which placed at least as much emphasis on measurements in subsoil horizons as in topsoil. The database includes 785 data entries from 34 different published studies and for 21 different active substances. Overall, the apparent koc value, koc(app), roughly doubled as the soil organic carbon content decreased by a factor of ten. Nevertheless, in nearly half of the individual datasets, a constant koc value proved to be an adequate model. Further analysis showed that significant increases in koc(app) in subsoil were found primarily for the more weakly adsorbing compounds (koc values<ca. 100-200Lkg(-1)) and that sorption to clay in loamy and clayey-textured subsoil horizons was the main cause. Tests with the MACRO model demonstrated that sorption to clay minerals may significantly affect the outcome of regulatory exposure and risk assessments for leaching to groundwater. The koc concept currently used in leaching models should therefore be replaced by an alternative approach that gives a more realistic representation of pesticide sorption in subsoil. The two alternative models tested in this study appear to have widespread applicability and are also simple enough to parameterize for this purpose.

Modeling fungicides mobility in undisturbed vineyard soil cores unamended and amended with spent mushroom substrates

The performance of the pesticide fate model PRZM to predict the fate of two fungicides, penconazole and metalaxyl, and the major metabolite of metalaxyl (CGA-62826), in amended and unamended vineyard soils was tested from undisturbed soils columns experiments. Three different treatments were tested in two soils: control soil (unamended), and soil amended with fresh or composted spent mushroom substrates, which correspond to common agricultural practices in Spain. Leaching experiments were performed under non-saturated flow conditions. The model was parameterized with laboratory and literature data, and using pedotransfer functions. It was first calibrated for water flow against chloride breakthrough curves. The key parameter was the hydrodynamic dispersion coefficient (DISP). No leaching of penconazole, the most hydrophobic fungicide, was observed. It remained in the top 0-8 cm of the column. In any case, simulations were highly correlated to the experimental results. On the contrary, metalaxyl and its metabolite were consistently found in the leachates. A calibration step of the Kd of metalaxyl and CGA-62826 and of DISP for CGA-62826 was necessary to obtain good prediction of the leaching of both compounds. PRZM generally simulated acceptable metalaxyl vertical distribution in the soil profiles although results were overestimated for its metabolite. Nevertheless, PRZM can be reasonably used to assess the leaching (through breakthrough curves) and vertical distribution of fungicides in amended soils, knowing their DISP values.

Solid-state UV-MALDI mass spectrometric quantitation of fluroxypyr and triclopyr in soil

The work presented here refers firstly to solid-state UV-MALDI-Orbitrap-mass spectrometric analysis of fluroxypyr (A) and triclopyr (B) in soils under laboratory conditions. The experimental design has involved the following: (a) determination of analytes A and B in polycrystalline composites of organic materials 1-7, based on 2-piperidine (pyrrolidine or piperazine)-1-yl-ethyl ammonium salts in order to determine the effect of sample preparation techniques on method performance using commercial herbicide formulations and (b) analysis of non-(X j,k,l (i) ) and sterilized (Y j,k,l (i) ) soil samples (i-fold rate 1, 10, 100, or 1,000; j-pesticide type A or B; k-time (0, 5, 10, 20, and 50 days) and l = 1-3 replicated samples) having clay content ∈ 5.0-12.0 %, silt ∈ 23.0-51.1 %, sand ∈ 7.2-72.0 %, and pH ∈ 4.0-8.1. In order to obtain a high representativeness of the data toward real-field experiments, the pollution scheme has involved 1-, 10-, 100-, and 1,000-fold rates. The firstfold rate has concentration of pollutant A of 2.639 × 10(-4) g in 625 cm(2) soil horizon of 0-25 cm(2) (5 cm depth) according to registration report (PSM-Zulassungbericht) of German Federal Office of Consumer Protection and Food Safety (Bundesamt für Verbraucherschutz und Lebensmittelsicherheit) 6337/26.10.2009. The experimental design has involved quincunx systematic statistical approach for collection of soil samples. The performance has been compared with the corresponding statistical variable obtained, using an independent HPLC-ESI-(APCI-)-MS/MS analysis.

Transport of bromide and pesticides through an undisturbed soil column: a modeling study with global optimization analysis

The fate of pesticides in tropical soils is still not understood as well as it is for soils in temperate regions. In this study, water flow and transport of bromide tracer and five pesticides (atrazine, imazaquin, sulfometuron methyl, S-metolachlor, and imidacloprid) through an undisturbed soil column of tropical Oxisol were analyzed using a one-dimensional numerical model. The numerical model is based on Richards' equation for solving water flow, and the advection-dispersion equation for solving solute transport. Data from a laboratory column leaching experiment were used in the uncertainty analysis using a global optimization methodology to evaluate the model's sensitivity to transport parameters. All pesticides were found to be relatively mobile (sorption distribution coefficients lower than 2 cm(3) g(-1)). Experimental data indicated significant non-conservative behavior of bromide tracer. All pesticides, with the exception of imidacloprid, were found less persistent (degradation half-lives smaller than 45 days). Three of the five pesticides (atrazine, sulfometuron methyl, and S-metolachlor) were better described by the linear kinetic sorption model, while the breakthrough curves of imazaquin and imidacloprid were more appropriately approximated using nonlinear instantaneous sorption. Sensitivity analysis suggested that the model is most sensitive to sorption distribution coefficient. The prediction limits contained most of the measured points of the experimental breakthrough curves, indicating adequate model concept and model structure for the description of transport processes in the soil column under study. Uncertainty analysis using a physically-based Monte Carlo modeling of pesticide fate and transport provides useful information for the evaluation of chemical leaching in Hawaii soils.

Illite spatial distribution patterns dictate Cr(VI) sorption macrocapacity and macrokinetics

This work examines the largely unexplored role of illite spatial distribution patterns in dictating the sorption of Cr(VI), a ubiquitously occurring contaminant. Flow-through experiments were carried out at 0.6, 3.0, and 15.0 m/day using columns packed with the same illite and quartz mass however with different spatial patterns and permeability contrasts. Column-scale sorption macrocapacity and macrorates were found to decrease with transport connectivity, a quantitative measure of heterogeneity characteristics. At 0.6 and 3.0 m/day, well-connected low permeability illite zones oriented in the flow-parallel direction lead to diffusion-controlled mass transport limitation for accessing sorption sites. This results in up to 1.4 order of magnitude lower macrocapacity and macrorates compared to those in minimally connected columns with well-mixed illite and quartz. At 15.0 m/day, effects of spatial heterogeneities are less significant (up to a factor of 2.8) owing to the close to chemical kinetics-controlled condition. Although the column-scale macrocapacity can reach full sorption capacity under low flow conditions, the macrorates are 10(-1) to 10(-3) of the microrates measured in well-mixed reactors. Insights gained here bridge gaps between laboratory observations and field applications and advance predictive understanding of reactive transport processes in the naturally heterogeneous subsurface.

Assessing the sorption and leaching behaviour of three sulfonamides in pasture soils through batch and column studies

We investigated the sorption potential and transport behaviour of three sulfonamides, namely, sulfamethoxazole (SMO), sulfachloropyridazine (SCP) and sulfamethazine (SM), and a conservative bromide tracer (Br(-)) in two undisturbed soil columns collected from the dairy farming regions in the North Island of New Zealand. Based on the low log Koc values obtained from the sorption study, all three sulfonamides are likely to have high mobility, making them a potential threat to surface and ground water. Soil column studies also showed that the mobility of the sulfonamides varied among soils and antibiotic type. Sulfonamides exhibited a mobility pattern similar to that of conservative Br(-) tracer. Considerable retardation was observed for the Hamilton soil, and the delayed peak arrival time (or maxima) was due to the role of sorption-related retention processes under saturated flow conditions. Residual antibiotic concentrations for SMO and SCP were detected in all soil sections including at 18 cm depth, while no resident concentration of SM was detected at any depth in the entire length of the core for both soils. The deterministic, physical equilibrium model (CXTFIT) described the peak arrival time as well as the maximum concentration of the antibiotic breakthrough curves reasonably, but showed some underestimation at the advanced stages of the leaching process. There was a significant difference in the model estimated retardation factors obtained from column study and the experimental retardation factors obtained from the conventional batch sorption experiments.

Impact of vegetated filter strips on sorbed herbicide concentrations and sorption equilibrium in agricultural plots

The objective was to investigate the impact of vegetated filter strips on exported atrazine and deethylatrazine concentrations [dissolved and sorbed to eroded sediments (>1.5 μm)], the deethylatrazine to atrazine ratio in water and sediments, the ratio of sorbed to dissolved herbicides in runoff and subsurface infiltration as well as field equilibrium state under natural climate during two seasons. We hypothesize that sorption equilibrium was not achieved in 2004 because of the short delay (<24 h) between herbicide application and the first rain event. In 2005, observations suggest that possible changing sorption equilibrium conditions were reached (20 days after atrazine application), especially for eroded sediments submitted to changing environmental conditions in subsurface. If confirmed by other experiments, this will raise the question of the representativeness of laboratory-determined soil sorption coefficients to predict the fate of pesticides.