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Groundwater aquifers worldwide experience unsustainable depletion, compounded by population growth, economic development, and climate forcing. Managed aquifer recharge provides one tool to alleviate flood risk and replenish groundwater. However, concerns grow that intentional flooding of farmland for groundwater recharge, a practice known as Ag-MAR, may increase the leaching of pesticides and other chemicals into groundwater. This study employs a physically based unsaturated flow model to determine the fate and transport of residues of four pesticide in three vadose zone profiles characterized by differing fractions of sand (41 %, 61 %, and 84 %) in California's Central Valley. Here, we show that the complex heterogeneity of alternating coarse and fine-grain hydrogeologic units controls the transit times of pesticides and their adsorption and degradation rates. Unsaturated zones that contain a higher fraction of sand are more prone to support preferential flow, higher recharge rates (+8 %), and faster (42 %) water flow and pesticide transport, more flooding-induced pesticide leaching (about 22 %), as well as more salt leaching correlating with increased risks of groundwater contamination. Interestingly, considering preferential flow predicted higher degradation and retention rates despite shorter travel times, attributed to the trapping of pesticides in immobile zones where they degrade more effectively. The findings underscore the importance of considering soil texture and structure in Ag-MAR practices to minimize environmental risks while enhancing groundwater recharge. The study also highlights that selecting less mobile pesticides can reduce leaching risks in sandy areas.

Study region: Artificially drained agricultural land in northwest Europe. Study focus: The use of application timing as a mitigation tool for pesticide leaching to drains was investigated by simulating pesticide fate after application every day in a pesticide-specific application window, using the agro-hydrological model DAISY. The simulations were carried out for six combinations of pesticide-crop-seasons using three synthetically generated climate series and 800 soil profiles. The simulated drain concentrations were transformed to normalized pesticide concentrations in a hypothetical adjacent stream. Each application day was then characterized by the maximum normalized hourly pesticide concentration in the stream, expressed as the maximum hourly toxic unit (mTU), occurring within 300 days after application. New hydrological insights for the region: The result showed that if the pesticide was applied consistently every year at the best application day, pesticide leaching, in terms of the 90th percentile of mTU, could be reduced by up to 62% compared to a random application. If the pesticide application day was restricted to vary in a five-day period, the 90th percentile of mTU could be reduced by up to 21%. Thus, our study shows that there is a significant mitigation potential for reducing pesticide leaching to drains by tailoring the timing of pesticide application to weather conditions.

To protect human health, wildlife and the aquatic environment, "safe uses" of pesticides are determined at the EU level while product authorization and terms of use are established at the national level. In Sweden, extra precaution is taken to protect drinking water, and permits are therefore required for pesticide use within abstraction zones. This paper presents MACRO- DB , a tool for assessing pesticide contamination risks of groundwater and surface water, used by authorities to support their decision-making for issuing such permits. MACRO-DB is a meta-model based on 583,200 simulations of the physically-based MACRO model used for assessing pesticide leaching risks at EU and national level. MACRO-DB is simple to use and runs on widely available input data. In a qualitative comparative assessment for two counties in Sweden, MACRO-DB outputs were in general agreement with groundwater monitoring data and matched or were more protective than the national risk assessment procedure for groundwater. • MACRO-DB is a pesticide risk assessment tool for drinking water abstraction zones. • Web-based meta-model (look-up tables) of the physics-based simulation model MACRO. • Novel algorithms for parameters controlling site hydrology and preferential flow. • All input data are widely available which facilitates local risk assessment. • Model results are in qualitative agreement with groundwater monitoring data. [ABSTRACT FROM AUTHOR]

Four priority pesticides (isoproturon, thifensulfuron methyl, glyphosate and propyzamide) and their main transformation products (TPs; monodesmethyl isoproturon, thifensulfuron acid, aminomethyl phosphonic acid and a ketone metabolite) were investigated for leaching behaviour from sandy clay loam soil contained in polyvinyl chloride pipes under laboratory conditions. Pesticides and their TPs were applied separately to different soil cores at the maximum recommended rates. The soils were irrigated and leachates were collected each week. Analysis of pesticides and TPs were performed via liquid chromatography mass spectrometry. It was found that isoproturon, monodesmethyl isoproturon, glyphosate, and aminomethyl phosphonic acid showed significant leaching behaviour, and were detected in leachates after a lag phase of 3-4 weeks. Propyzamide and thifensulfuron methyl were not detected in leachates nor were their TPs. Aminomethyl phosphonic acid showed a weak leaching tendency by comparison with glyphosate. It is evident that polarity as well as the ionic properties of the pesticides is important for leaching in clay loam soil. Being more polar, TPs should also be considered during risk assessment and monitoring studies of surface and ground waters. [ABSTRACT FROM AUTHOR]

Caption: Pesticides in a tile drained landscape. • hydrological conditions are key factor for total drainage loads. • wet weather conditions lead to 10-fold increase in drainage loss. • timing and frequency of pesticide peaks are controlled by sorption properties. • leaching of flufenacet is not only controlled by high amounts of precipitation. • highest loss of pendimethalin and diflufenican during high discharges in winter. The input of harmful substances into surface waters are of major concern since their side effects may negatively affect the chemical state of surface waters. This study investigated the drainage loss of pesticides into surface waters in a small agricultural catchment in Northern Germany. The pesticides flufenacet, diflufenican, and pendimethalin were monitored at a daily resolution for 154 days in 2016 (dry period) and 111 days in 2017 (wet period) for two consecutive years at both field- (10 ha) and catchment-scale (100 ha). Highly contrasting weather conditions led to extremely high differences in loads between both monitoring periods. Regarding both scales and campaigns, flufenacet was released often in considerably higher amounts and faster than diflufenican and pendimethalin. The very mobile pesticide, flufenacet, is not exclusively leached during high precipitation events but also continuously discharged from soils to the drainage system during low precipitation. Pendimethalin had the lowest recovery rate in comparison to its application amount and showed a lower total loss rate than the less sorptive pesticide diflufenican. Pendimethalin and diflufenican showed high retarded loads caused by increased drainage discharge and sediment transport during late winter induced by freezing and thawing processes in the upper soil. Hence, leaching of the pesticides was controlled by the sorption properties of the investigated compounds and, to a large extent, by hydrological boundary conditions, which were highly variable from an (inter-) annual perspective. Our study identified antecedent and prevailing precipitation and soil moisture conditions as the key impact of pesticide leaching to the drainage system in the long-term. [ABSTRACT FROM AUTHOR]

Abstract Modelling of pesticide leaching is paramount to managing the environmental risks associated with the chemical protection of crops, but it involves large uncertainties in relation to climate, agricultural practices, soil and pesticide properties. We used Latin Hypercube Sampling to estimate the contribution of these input factors with the STICS-MACRO model in the context of a 400 km2 catchment in France, and two herbicides applied to maize: bentazone and S-metolachlor. For both herbicides, the most influential input factors on modelling of pesticide leaching were the inter-annual variability of climate, the pesticide adsorption coefficient and the soil boundary hydraulic conductivity, followed by the pesticide degradation half-life and the rainfall spatial variability. This work helps to identify the factors requiring greater accuracy to ensure better pesticide risk assessment and to improve environmental management and decision-making processes by quantifying the probability and reliability of prediction of pesticide concentrations in groundwater with STICS-MACRO. Highlights • Uncertainty analysis on the pesticide outputs of STICS-MACRO model was performed. • Climate, agricultural practices, soil and pesticide properties were considered. • Influential factors were climate temporal variability, Kf, hydraulic conductivity. • Small spatial variation in rainfall led to high variation in pesticide concentrations. • The results help to improve environmental management and decision-making processes. [ABSTRACT FROM AUTHOR]

HYDRUS-1D is a well-established reliable instrument to simulate water and pesticide transport in soils. It is, however, a point-specific model which is usually used for site-specific simulations. Aim of the investigation was the development of pesticide accumulation and leaching risk maps for regions combining HYDRUS-1D as a model for pesticide fate with regional data in a geographical information system (GIS). It was realized in form of a python tool in ArcGIS. Necessary high resolution local soil information, however, is very often not available. Therefore, worldwide interpolated 250-m-grid soil data ( SoilGrids.org ) were successfully incorporated to the system. The functionality of the system is shown by examples from Thailand, where example regions that differ in soil properties and climatic conditions were exposed in the model system to pesticides with different properties. A practical application of the system will be the identification of areas where measures to optimize pesticide use should be implemented with priority. [ABSTRACT FROM AUTHOR]

STICS-MACRO is a process-based model simulating the fate of pesticides in the soil-plant system as a function of agricultural practices and pedoclimatic conditions. The objective of this work was to evaluate the influence of crop management practices on water and pesticide flows in contrasted environmental conditions. We used the Morris screening sensitivity analysis method to identify the most influential cropping practices. Crop residues management and tillage practices were shown to have strong effects on water percolation and pesticide leaching. In particular, the amount of organic residues added to soil was found to be the most influential input. The presence of a mulch could increase soil water content so water percolation and pesticide leaching. Conventional tillage was also found to decrease pesticide leaching, compared to no-till, which is consistent with many field observations. The effects of the soil, crop and climate conditions tested in this work were less important than those of cropping practices. STICS-MACRO allows an ex ante evaluation of cropping systems and agricultural practices, and of the related pesticides environmental impacts. [ABSTRACT FROM AUTHOR]

Leaching, the movement of water and chemicals into deeper soil layers and groundwater is a subject of worldwide interest because a high percentage of drinking water is extracted from groundwater. The objective of this study was to evaluate the potential leaching and persistence of sixteen pesticides (one fungicide, three nematicides/insecticides, and twelve herbicides) for three Mediterranean agricultural soils with similar texture (clay loam) but different organic matter content (1.2-3.1%). Adsorption was studied in batch experiments and leaching was tested using disturbed soil columns (40 cm length × 4 cm i.d.). Degradation studies were carried out during 120 days under laboratory conditions. Mobility experiments showed that pesticides can be grouped according to their potential leaching. Thus, pesticides showing medium leachability were included in group 1 (referred as G1) while those with high leachability were termed as G2. The differences observed in the leachability can be attributed to the different organic carbon (OC) content in the soils (0.7-1.8%). Values of log KOC were higher in the order: soil C > soil B > soil A, which agrees with the OC content in each soil. The calculated half-lives ranged from 4.2 days for carbofuran in soil A to 330 days for prometon in soil C. As a general rule, when higher OC content in the soil the greater persistence of the pesticide was observed as a consequence of the increased adsorption. The first order kinetics model satisfactorily explains the disappearance of the studied pesticides in the soil.

• Weather conditions near application day affect pesticide leaching to drains. • Machine-learning models trained on simulated data can predict the safest app. day. • Safest days are predicted with an 80% accuracy with accurate weather and field data. Pesticides applied on drained fields can be transported through drain-connected macropores to surface waters, as affected by weather and field conditions around spraying time. We investigated whether field conditions and past and near-future weather could be predictors of the ecotoxicological risk of pesticide leaching to surface waters and help identify the safest application day. We developed a methodological approach for one climate-soil setting and two herbicides. The agro-hydrological model Daisy and a 3,000-year series of generated weather were used to simulate a total of 369,326 pesticide application days and their resulting hourly drain concentrations, used in the risk calculation. Recurrent neural networks were trained on the simulated data. The trained meta -models selected the safest application day within a 5-day period with an average accuracy of 60–80%. The effective risk reductions were only of 3–11% for the investigated climate-soil-pesticide settings. Nevertheless, they represented 46–86% of the achievable reductions according to Daisy. [ABSTRACT FROM AUTHOR]

Numerous pesticide fate models are available, but few of them are able to take into account specific agricultural practices, such as catch crop, mixing crops or tillage in their predictions. In order to better integrate crop management and crop growth in the simulation of diffuse agricultural pollutions, and to manage both pesticide and nitrogen pollution, a pesticide fate module was implemented in the crop model STICS. The objectives of the study were: (i) to implement a pesticide fate module in the crop model STICS; (ii) to evaluate the model performance using experimental data from three sites with different pedoclimatic contexts, one in The Netherlands and two in northern France; (iii) to compare the simulations with several pesticide fate models; and (iv) to test the impact of specific agricultural practices on the transfer of the dissolved fraction of pesticides. The evaluations were carried out with three herbicides: bentazone, isoproturon, and atrazine. The strategy applied in this study relies on a noncalibration approach and sensitivity test to assess the operating limits of the model. To this end, the evaluation was performed with default values found in the literature and completed by sensitivity tests. The extended version of the STICS named STICS-Pest, shows similar results with other pesticide fate models widely used in the literature. Moreover, STICS-Pest was able to estimate realistic crop growth and catch crop dynamic, which thus illustrate agricultural practices leading to a reduction of nitrate and a change in pesticide leaching. The dynamic plot-scale model, STICS-Pest is able to simulate nitrogen and pesticide fluxes, when the hydrologic context is in the validity range of the reservoir (or capacity) model. According to these initial results, the model may be a relevant tool for studying the effect of long-term agricultural practices on pesticide residue dynamics in soil and the associated diffuse pollution transfer. [ABSTRACT FROM AUTHOR]

The large spatial heterogeneity in soil physico-chemical and microbial parameters challenges our ability to predict and model pesticide leaching from agricultural land. Microbial mineralization of pesticides is an important process with respect to pesticide leaching since mineralization is the major process for the complete degradation of pesticides without generation of metabolites. The aim of our study was to determine field-scale variation in the potential for mineralization of the herbicides glyphosate, bromoxyniloctanoate, diflufenican, and bentazone and to investigate whether this variation can be predicted by variations in basic soil parameters. Sixty-five soil samples were sampled from an agricultural, loamy field in Silstrup, Denmark, from a 60 × 165 m rectangular grid. The mineralization potential of the four pesticides was determined using a 96-well microplate C-radiorespirometric method. Initial mineralization rates were determined using first-order kinetics for glyphosate and bromoxyniloctanoate and zero-order kinetics for diflufenican and bentazone. The mineralization rates of the four pesticides varied between the different pesticides and the different soil samples, but we could not establish correlations between the pesticide mineralization rates and the measured soil parameters. Only the glyphosate mineralization rates showed slightly increasing mineralization potentials towards the northern area of the field, with increasing clay and decreasing OC contents. The mineralization potentials for glyphosate and bentazone were compared with 9-years leaching data from two horizontal wells 3.5 m below the field. The field-scale leaching patterns, however, could not be explained by the pesticide mineralization data. Instead, field-scale pesticide leaching may have been governed by soil structure and preferential flow events. [ABSTRACT FROM AUTHOR]

The European Union authorization procedure for pesticides includes an assessment of the leaching risk posed by pesticides and their degradation products (DP) with the aim of avoiding any unacceptable influence on groundwater. Twelve-year's results of the Danish Pesticide Leaching Assessment Programme reveal shortcomings to the procedure by having assessed leaching into groundwater of 43 pesticides applied in accordance with current regulations on agricultural fields, and 47 of their DP. Three types of leaching scenario were not fully captured by the procedure: long-term leaching of DP of pesticides applied on potato crops cultivated in sand, leaching of strongly sorbing pesticides after autumn application on loam, and leaching of various pesticides and their DP following early summer application on loam. Rapid preferential transport that bypasses the retardation of the plow layer primarily in autumn, but also during early summer, seems to dominate leaching in a number of those scenarios. [ABSTRACT FROM AUTHOR]

The extensive use of pesticides for increasing the agricultural production is affecting the quality of groundwater. The objectives of this article are to (i) develop pesticide relative leaching ranks for well sites, (ii) develop maps for human health risks due to pesticide applications, and (iii) identify the most significant parameters in pesticide simulations for groundwater vulnerability assessment. The methods include (i) development of acifluorfen relative leaching ranks for 25 well sites using ArcPRZM-3, (ii) development of health risk maps using model simulated maximum dissolved bentazon concentrations on the basis of USA drinking water quality guidelines, (iii) sensitivity analysis for 14 ArcPRZM-3 input parameters using the Plackett-Burman method. ArcPRZM-3 is a user-friendly system for spatial modeling of pesticide leaching from surface to groundwater. Thirteen acifluorfen relative leaching potential ranks were developed in which the pesticide leaching decrease from 1 to 13. The model predicted ranks for well 34 and well 9 were 2nd and 3rd, respectively, and acifluorfen was detected in both wells during the physical monitoring. The percentages of high health risks in the agricultural areas were 48.38 and 72.72% for Randolph and Independence Counties, respectively. The most significant parameters were thickness of horizon compartment, runoff curve number of antecedent moisture condition II for cropping, soil bulk density, and total application of pesticide. The irrigation, soil permeability, and numerical dispersion could impact the pesticide leaching in soils toward groundwater. The ArcPRZM-3 system could be efficiently applied for spatial modeling and mapping of pesticide concentrations for groundwater vulnerability assessment on a large scale. [ABSTRACT FROM AUTHOR]

Abstract: Since the 90s an increasing number of assessment methods using operational tools like indicators have been proposed for environmental issues linked to pesticides, among them, groundwater contamination by pesticide transfer. To our knowledge none of these indicators address preferential flow, an important process determining pesticide leaching. The objective of this study is twofold: (i) to develop a new groundwater sub indicator for an existing indicator, I-Phy (former Ipest), that explicitly take preferential flow into account, and (ii) to test the possibility of developing an indicator by means of data-mining methods using simulations of a mechanistic model. The groundwater sub indicator developed is in the form of decision trees based on fuzzy inference systems. It was derived through neuro-adaptive learning on data sets from simulations running the process-based MACRO model. Unlike the previous version, the new indicator considers preferential flow, climatic differences and differences in soil texture with depth. Other benefits are less dependency on expert knowledge and the possibility to integrate a broad range of conditions. [Copyright &y& Elsevier]

Abstract: Pore water flow velocity can influence the processes involved in the contaminant transport between relative stagnant zones of porous media and their adjacent mobile zones. A particular case of special interest is the occurrence of high flow rates around the controlled release granules containing pesticides buried in soil. The release of the pesticides carbofuran and fenamiphos from commercial controlled release formulations (CRFs) was studied, comparing release tests in a finite volume of water with water flow release tests in saturated packed sand at different seepage velocities. For water release kinetics, the time taken for 50% of the pesticide to be released (T 50) was 0.64hours for carbofuran and 1.97hours for fenamiphos. In general, the release rate was lower in the porous matrix than in the free water tests. The faster release rate for carbofuran was attributed to its higher water diffusivity. The seepage velocity has a strong influence on the pesticide release rate. The dominant release mechanism varies with the progress of release. The evolution of the mechanism is discussed on the basis of the successive steps that involve the moving boundary of the dissolution front of the pesticide inside the granule, the concentration gradient inside the granule and the flow boundary layer resistance to solute diffusion around the granule. The pore water velocity influences the overall release dynamics. Therefore, seepage velocity should be considered in pesticide release to evaluate the risk of pesticide leaching, especially in scenarios with fast infiltration. [Copyright &y& Elsevier]

Fertilizers and pesticides applied to home lawns can contaminate surface and groundwater making them unfit for aquatic organisms and human consumption. Application of fertilizers and pesticides can improve the aesthetics of a home lawn, improve turfgrass shoot and root density, water use, organic matter accumulation, and other factors known to be influential in urban hydrology. A field study determined the impact of applying home landscape pesticides with or without supplemental irrigation of leaching into groundwater or runoff to surface water. Free-draining lysimeters having a sandy loam soil (coarse-loamy, mixed, active, mesic Lamellic Hapludalf) were used to monitor total volume and nutrients and pesticides losses. Samples from natural precipitation events were analysed for pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2, 6-dinitrobenzenamine] applied in spring, mecoprop [2-(4-Chloro-2-methylphenoxy) propionic acid]+2, 4-D (2,4-dichlorophenoxyacetic acid) applied both in late spring and early fall, and bifenthrin [(2-methyl-1,1-biphenyl-3-y1)-methyl-3-(2-chloro-3,3,3-trifluoro-1-propenyl)-2,2-dimethyl cyclopropanecarboxylate] applied in late summer. Pesticide concentrations fluctuated over time, but no differences in concentrations were observed from irrigated or non-irrigated turfgrass for any pesticide. Mecoprop and 2,4-D were the most mobile and values found in runoff during establishment were above maximum contaminant levels (MCL) for U.S. standards. Surface runoff of lawn pesticides were observed at concentrations several magnitudes higher than leachate. The total extent of pesticide runoff was 0.71%, 1.3%, 6.0%, and 0.003% of the amount applied for 2,4-D, mecoprop, pendimethalin and bifenthrin, respectively. With the exception of one sampling period, pendimethalin concentrations in runoff and leachate were low; however, when applied during a period of minimal turf growth and high precipitation, pendimethalin could contribute significantly to surface water pollution. [ABSTRACT FROM PUBLISHER]

It is not straightforward to sample and demonstrate the presence and transport of pesticides in heterogeneous soil. Following leaching experiments with four differently structured 50-cm-long soil columns (tilled and untilled soil), the objective of this study was to investigate the extent that visual tracing of the dye Brilliant Blue could support in soil sampling for two strongly sorbing pesticides (14C-labeled glyphosate and pendimethalin). About 830 samples were collected. No pesticide was found below 10– 25 cm depth by random sampling, even though 0.21–0.31% of the applied amounts were leached, and 0.18% of the soil volume was sampled. With similar sampling efforts, the pesticides could generally be traced throughout the columns by sampling from stained soil volumes, only. None of the two particular sampling strategies for pesticides produced accurate mass balances or balances that were obviously better than the other. No pesticide was detected outside stained soil volumes, except for glyphosate in one sample. Below 30 cm, stained soil comprized on average 5% of the total soil volume, leaving 95% as expectedly pesticide-free. The results suggest that much more efficient sampling for sorbing pesticides can be obtained by using the dye and focusing on stained soil volumes. [ABSTRACT FROM AUTHOR]

Abstract: Surfactants, which are emitted to soils mainly through wastewater disposal and also for remediation purposes, can alter pesticide sorption/desorption as well as the water flow through the soil. As a consequence, pesticide mobility may change and affect groundwater quality. Therefore, a deep insight into the coupled transport of pesticides and surfactants is necessary and helpful for contamination prevention and remediation. Dimethoate leaching was performed after flowing wastewater and surfactant solutions (Tween 80 and sodium dodecyl sulfate, SDS) through disturbed soil columns. To identify relevant parameters related to dimethoate mobility, breakthrough curves were fitted with convection dispersion (CDE) transport model. In all cases, the elution peaks showed a rapid release in the early tailing followed by a much slower release in the extended tailing with a retention factor close to 1, as corresponds to a weakly retained compound. CDE non-equilibrium parameters showed that, independently from the leaching solution employed, most of the pesticide was transported in the solution phase (convective transport). Infiltrated volume for wastewater was the highest (2× Tween and 5× SDS) for the same infiltration period, which increases the risk of early groundwater pollution. On the other hand, SDS decreased soil permeability providing a slight retention of dimethoate on soil, mainly in the lower column layer. [Copyright &y& Elsevier]

For the registration of pesticides in the European Union, model simulations for worst-case scenarios are used to demonstrate that leaching concentrations to groundwater do not exceed a critical threshold. A worst-case scenario is a combination of soil and climate properties for which predicted leaching concentrations are higher than a certain percentile of the spatial concentration distribution within a region. The derivation of scenarios is complicated by uncertainty about soil and pesticide fate parameters. As the ranking of climate and soil property combinations according to predicted leaching concentrations is different for different pesticides, the worst-case scenario for one pesticide may misrepresent the worst case for another pesticide, which leads to 'scenario uncertainty'. Pesticide fate parameter uncertainty led to higher concentrations in the higher percentiles of spatial concentration distributions, especially for distributions in smaller and more homogeneous regions. The effect of pesticide fate parameter uncertainty on the spatial concentration distribution was small when compared with the uncertainty of local concentration predictions and with the scenario uncertainty. Uncertainty in pesticide fate parameters and scenario uncertainty can be accounted for using higher percentiles of spatial concentration distributions and considering a range of pesticides for the scenario selection. Copyright © 2010 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Abstract: Previous research has suggested that pesticide losses at the field scale can be dominated by a small proportion of the field area. The objective of this study was to investigate whether site-specific applications (i.e. avoiding high-risk areas) at the field scale can contribute to a reduction of pesticide leaching despite uncertainty in the underlying model-based leaching risk map. Using a meta-model of the dual-permeability model MACRO, the annual average pesticide leaching concentrations were estimated for 162 sample sites on a 47 ha field. The procedure was repeated for different scenarios describing different patterns of spatial variation of degradation half-lives and the partition coefficient to soil organic carbon. To account for interpolation uncertainty, maps of predicted pesticide leaching risk were produced by the method of sequential Gaussian simulation. The results of the case study show that larger reductions of predicted leaching were achieved by site-specific application than by that of a comparable uniform dose reduction. Hence, site-specific-applications may be a feasible method to reduce pesticide leaching at the field-scale providing that the model approach gives reasonable estimates of the spatial pattern of pesticide leaching. [Copyright &y& Elsevier]

This article presents a study regarding the version 5.1 of the MACRO model for pesticide leaching in a Dutch clay soil. Testing of pesticide leaching models against comprehensive field-scale measurements is essential to increase confidence in their predictive ability when used as regulatory tools. The MACRO model version 5.1 was tested against measurements of water flow and the behavior of bentazone, bromide, and imidacloprid in a cracked clay soil. The model was calibrated against the bromide concentrations and the measured moisture profiles in soil and in drain water.

The article presents a study on the development of an isolation tool for site-particular vulnerability assessment of pesticide leaching to groundwater, based on the pesticide fate and transport model MACRO. In this study, the effects of uncertainty in the pedotransfer functions on simulation results were studied for 48 combination of soils, pesticides and application timings, by sampling pedotransfer function regression errors and spreading them through the simulation model in a Monte Carlo analysis. Relative to this, the pedotransfer function errors effected great variation in simulation results.

Several simple index methods that use easily accessible data have been developed and included in decision–support systems to estimate pesticide leaching across larger areas. However, these methods often lack important process descriptions (e.g. macropore flow), which brings into question their reliability. Descriptions of macropore flow have been included in simulation models, but these are too complex and demanding for spatial applications. To resolve this dilemma, a neural network simulation meta-model of the dual-permeability macropore flow model MACRO was created for pesticide groundwater exposure assessment. The model was parameterized using pedotransfer functions that require as input the clay and sand content of the topsoil and subsoil, and the topsoil organic carbon content. The meta-model also requires the topsoil pesticide half-life and the soil organic carbon sorption coefficient as input. A fully connected feed-forward multilayer perceptron classification network with two hidden layers, linked to fully connected feed-forward multilayer perceptron neural networks with one hidden layer, trained on sub-sets of the target variable, was shown to be a suitable meta-model for the intended purpose. A Fourier amplitude sensitivity test showed that the model output (the 80th percentile average yearly pesticide concentration at 1 m depth for a 20 year simulation period) was sensitive to all input parameters. The two input parameters related to pesticide characteristics (i.e. soil organic carbon sorption coefficient and topsoil pesticide half-life) were the most influential, but texture in the topsoil was also quite important since it was assumed to control the mass exchange coefficient that regulates the strength of macropore flow. [Copyright &y& Elsevier]

The Plant Protection Product Directive (91/414/EEC) stresses the need of validated models to calculate predicted environmental concentrations. The use of models has become an unavoidable step before pesticide registration. The main topic of the research presented here is the validation of the PEARL model for two well-instrumented sites located in the Netherlands and Sweden. First the water transport module was calibrated, and then the solute transport module, keeping unchanged the water transport parameters. The Dutch site is characterised by a sandy soil. PEARL predictions were very satisfactory for both soil moisture content and groundwater level. Predictions for the tracer, bentazone and ethoprophos concentrations in the soil profile were also successful. The Swedish site is characterised by a structured clay soil. PEARL was successful in predicting soil moisture profiles and the draining water. PEARL performed well in predicting the soil concentration of bentazone at different depths, however, it did not predict accurately solute concentration in the drainage water. A sensitivity analysis of the PEARL model performed on the Swedish site showed the greater importance of the pesticide degradation properties over the soil hydraulic properties. [Copyright &y& Elsevier]

The article presents a study that discusses a screening tool for vulnerability assessment of groundwater to pesticide leaching in the islands of Hawaii. The study uses first-order uncertainty analysis to identify uncertainties in pesticide and soil properties. Pesticides are classified as "likely", "unlikely", or "uncertain" based on the uncertainty and a comparison of the retooled attenuation factor with uncertainties and values of two reference chemicals. The study suggests a conservative approach to selection of pesticide parameters, and interpretation of the results because the screening tool is connected with uncertainties that are not identified.

Summary: Preferential flow through soil macropores in tile drained soils can significantly increase the risk of pollution of surface water bodies by agricultural chemicals such as pesticides. While many field studies have shown the importance of preferential flow in tile-drained fields, few have included detailed numerical modelling of the processes involved. The objective of this study was to compare four conceptually different preferential flow and/or transport approaches for their ability to simulate drainage and pesticide leaching to tile drains. The different approaches included an equilibrium approach using modified hydraulic properties near saturation, and three non-equilibrium approaches: a mobile–immobile solute transport model, a dual-porosity approach, and a dual-permeability formulation. They were implemented into the HYDRUS-2D software package. The model predictions were compared against measurements of drainage and pesticide concentrations made at an undulating, tile-drained field in southern Sweden (Näsbygård) during a period of 6 weeks following spray application of the herbicide MCPA. The dual-permeability approach most accurately simulated preferential drainage flow, even though this approach somewhat overestimated the drainage rates. The equilibrium and mobile–immobile approaches largely failed to capture the preferential flow process. The dual-porosity approach predicted much more distinct and higher drainage flow events as compared to the dual-permeability approach. Differences between measured and simulated tile drainage rates using the dual-permeability approach could be partly explained by water by-passing the tile drains and recharging the deeper aquifer. The dual-permeability and dual-porosity approaches closely captured the dynamics in measured pesticide concentrations. Both the equilibrium and mobile–immobile approaches completely failed to match measured MCPA leaching by underestimating the peak concentrations by more than two orders of magnitude. We conclude that two-dimensional models are suitable tools for studying pesticide leaching from undulating fields with large spatial variability in soil properties. [Copyright &y& Elsevier]

Abstract: A field experiment was conducted on a Calcaric Cambisol soil to study the consequences of the penetration depth and properties of pesticides on the risk of subsequent leaching. Three pesticides with different mobility characteristics and bromide were injected at 30cm (where soil organic matter (OM) was 2%) and 80cm (soil OM 0.5%) on irrigated plots without a crop. The migration of injected solutes was assessed for two years by sampling the soil solution using six porous cups installed at 50 and 150cm depth and by relating solute contents to drainage water flux estimated by the STICS model (Simulateur mulTIdisciplinaire pour les Cultures Standard). Pesticides injected at 30cm were strongly retained so that no metolachlor or diuron was detected at 50 and 150cm. The ratio of atrazine peak concentration in the soil solution to concentration in the injected solution (C/C 0) was 1×10−3 and 0.2×10−3, respectively, at 50 and 150cm. When injected at 80cm, (C/C 0) of atrazine, metolachlor and diuron were 10×10−3, 1×10−3 and 0.3×10−3 at 150cm, respectively; 1/(C/C 0) was correlated with K oc values reported from databases. The ratio of drainage volume to the amount of water at field capacity in the soil layer between the injection point at 30cm and the water sampling level (V/V 0) at 50 and 150cm was 0.6 and 0.9, respectively, for bromide and 1.6 and 1.0 for atrazine. V/V 0 of the injected solutes at 80cm was for bromide, atrazine, metolachlor and diuron 0.6, 0.9, 1.2 and 1.7, respectively; pesticide V/V 0 was correlated with K oc. The retardation factor was a good indicator of migration risk, but tended to overestimate retardation of molecules with high K oc. Atrazine desorption represented an additional leaching risk as a source of prolonged low contamination. The large variability in soil solution of bromide and pesticide concentrations in the horizontal plane was attributed to flow paths and clods in the tilled soil layer. This heterogeneity was assumed to channel water fluxes into restricted areas and thereby increase the risk of groundwater contamination. The methodology used in the field proves to provide consistent results. [Copyright &y& Elsevier]

Abstract: The catabolic activity of incumbent microorganisms in soil samples of eleven dissimilar soil series was investigated, with respect to the herbicide isoproturon. Soils were collected from a 30×37 km area of river catchment to the north-west of London, England. Catabolic activity in each soil type during a 500 h assay was determined by 14C-radiorespirometry. Results showed four soils that exhibited high levels of catabolic activity (33–44% mineralisation) while the remaining seven soils showed lower levels of catabolic activity (12–16% mineralisation). There was evidence to suggest that soils exhibiting high catabolic activity had low (<22%) clay content and tended towards lower organic carbon content (<2.7%), but that these higher levels of catabolic activity were also related to pre-exposure to isoproturon. The 14C-radiorespirometric results were used to produce a GIS layer representing levels of catabolic activity for the dissimilar soils across the study area. This layer was combined with other GIS layers relating to pesticide attenuation, including soil organic carbon content, depth to groundwater and hydrogeology, to produce a map showing risk of groundwater contamination by isoproturon. The output from this approach was compared with output from an attenuation-only approach and differences appraised. Inclusion of the catabolism layer resulted in a lowering of risk in the model in 15% of the study area. Although there appears to be limited benefit in including pesticide catabolic activity in this regional-scale groundwater risk model, this type of addition could be useful in a site-specific risk assessment. [Copyright &y& Elsevier]

Abstract: GIMMI [2] GIMMI IST-2001-34245 receives funding under the EU IST program. 2 project, started in April 2002, aims at bridging the gap of communication in the pesticide impact assessment domain between data providers (soil, meteorology, agronomy, pesticide experts), scientists (chemists, geologists, modellers and academic institutions), service providers (local and central governments, public administration bodies, private chemical industries manufacturing pesticides) and final users (agronomists, consultants and even citizens). This can be achieved by allowing the inter-operability via Web of Geographic Information Systems (GIS), environmental protection data and service (e.g. mathematical models) providers physically distributed, by providing the proper IT structures to represent and manage temporal knowledge, and by integrating state-of-the-art legacy systems for document management and report generation. The main results of GIMMI will be an open Web brokerage system supporting different web-services such as: On-line Data Access to seek and drill down into huge amount of distributed Geographic Information; On/Off-line Simulations allowing inter-relation of distributed databases to run mathematical models. “Open systems” means that new data and service providers can easily be integrated in the running system while remaining autonomous. The validation scenario chosen for GIMMI is the field of pesticides impact assessment in agriculture practices and Land Protection, by the adoption of four alternative EC-validated pesticide leaching models. [Copyright &y& Elsevier]

Abstract: The one-dimensional pesticide fate model MACRO was loose-linked to the three-dimensional discrete fracture/matrix diffusion model FRAC3DVS to describe transport of the pesticide mecoprop in a fractured moraine till and local sand aquifer (5–5.5 m depth) overlying a regional limestone aquifer (16 m depth) at Havdrup, Denmark. Alternative approaches to describe the upper boundary in the groundwater model were examined. Field-scale simulations were run to compare a uniform upper boundary condition with a spatially variable upper boundary derived from Monte-Carlo simulations with MACRO. Plot-scale simulations were run to investigate the influence of the temporal resolution of the upper boundary conditions for fluxes in the groundwater model and the effects of different assumptions concerning the macropore/fracture connectivity between the two models. The influence of within-field variability of leaching on simulated mecoprop concentrations in the local aquifer was relatively small. A fully transient simulation with FRAC3DVS gave 20 times larger leaching to the regional aquifer compared to the case with steady-state water flow, assuming full connectivity with respect to macropores/fractures across the boundary between the two models. For fully transient simulations ‘disconnecting’ the macropores/fractures at the interface between the two models reduced leaching by a factor 24. A fully connected, transient simulation with FRAC3DVS, with spatially uniform upper boundary fluxes derived from a MACRO simulation with ‘effective’ parameters is therefore recommended for assessing leaching risks to the regional aquifer, at this, and similar sites. [Copyright &y& Elsevier]

A soil column experiment under outdoor conditions was performed to monitor the fate of 14C-ring-labelled sulcotrione, 2- (2 -chloro-4-mesylbenzoyl) cyclohexane- 1, 3-dione and atrazine, 6-chloro- N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine, in water leachates and in the ploughed horizon of a sandy loam soil. Two months after treatment, the cumulative amounts of herbicide residues leached from the soil were 14.5% and 7% of the applied radioactivity for sulcotrione and atrazine, respectively. Maximum leachate concentrations for each herbicide were observed during the first month following application: 120 and 95 μg litrc-1 for sulcotrione and atrazine respectively. After 2 weeks, 78% of the sulcotrione and atrazine was extractable from the soil, whereas after two months only 10 and 4%, respectively, could be extracted. The maximum sulcotrione content in the first 10 cm of soil was identical with that of atrazine. For both molecules, the content of non-extractable residues was low, being around 15%. Sulcotrione seems to be more mobile than atrazine but the consequences for water contamination are similar since lower doses are used. [ABSTRACT FROM AUTHOR]

Testing of pesticide leaching models is important to increase confidence in their use in pesticide registration procedures world-wide. The chromatographic PEARL model was tested against the results of a field leaching study on a cracking clay soil with a tracer (bromide), a mobile pesticide (bentazone) and a moderately sorbing, persistent pesticide (imidacloprid). Input parameters for water flow and solute transport were obtained from site-specific measurements and from literature. The model was tested using a stepwise approach in which each sub-model was sequentially and separately tested. Uncalibrated simulations for the water flow resulted in moisture profiles that were too wet. Calibration of the hydraulic relationships resulted in a good description of the moisture profiles. Calibration of the dispersion length was necessary to obtain a good description of bromide leaching. The calibrated dispersion length was 61 cm, which is very long and indicates a large non-uniformity of solute transport. The half-life of bentazone had to be calibrated to obtain a good description of its field persistence. The calibrated half-life was 2.5 times shorter than the half-life derived from the laboratory studies. Concentrations of bentazone in drain water and groundwater were described reasonably well by PEARL. Although measured and simulated persistence of imidacloprid in soil corresponded well, the bulk of the imidacloprid movement was overestimated by PEARL. However, imidacloprid concentrations in drain water were underestimated. In spite of the extensive calibration of water flow and tracer movement, the behaviour of the moderately sorbing pesticide imidacloprid could not be simulated. This indicates that the convection-dispersion equation cannot be used for accurate simulation of pesticide transport in cracking clay soils (even if extremely long dispersion length is used). Comparison of the model results from a poorly sorbed chemical (bentazone) and a moderately sorbed... [ABSTRACT FROM AUTHOR]

Contamination of the groundwater is an important side-effect of the usage of plant protection products in agriculture. Today, the use of plant protection products that potentially contaminate the groundwater is banned by registration procedures at both the European level (Council Directive 91/414/EEC), and the level of individual member states. The Directive places great importance on the use of models to calculate Predicted Environmental Concentrations (PECs) as a basis for assessing the environmental risks. In the first tier of the current procedure, point scale leaching models are combined with a limited number of worst-case scenarios to assess PEC groundwater in Europe. An alternative procedure would be to use spatially distributed leaching models. Such models provide policy makers with a wealth of additional information, allowing identification of high and low risk areas in terms of spatially varying environmental and land use properties. In this study, such a spatially distributed leaching model, the EuroPEARL model, was implemented to assess the leaching risk of plant protection products at the Pan-European scale. This model is one of the products that has been delivered within the framework of the APECOP project (FP5-QLK-1999-01238), which is a European project supporting the harmonised registration of plant protection products in Europe. Simulations were performed for 1062 unique combinations of Soil Mapping Unit, Climate Zone and Country. Soil properties, including soil horizon designations, were obtained from the Soil Profile Analytical Database of Europe. Daily weather data were obtained from the MARS database. Other data like irrigation data, crop data and product properties have been compiled from various sources, such as inventories, field-studies and the literature. The 1062 unique combinations together represent 75% of the total agricultural area of the European Union. Austria, Sweden and Finland could not be included in the simulations, because there was insufficient soil profile information for these countries. Results are presented with a resolution of 10×10 km2, which is the highest justifiable resolution based on the EU soil map 1:1,000,000. The Pan-European results confirm that the predicted leaching concentration generally increases with precipitation and irrigation and decreases with increasing organic matter content. Because of the strong sensitivity of the leaching concentration to soil properties, there is a strong variability of the calculated leaching concentration at relatively short distances. Results further indicate that due to large irrigation amounts combined with large temporal variation of rainfall in the Southern European countries, the trend in the calculated leaching risks from North to South was less than expected. This implies that areas of high leaching risk (‘hotspots’) as assessed by means of the EuroPEARL model occur in all countries of the European Union, including the Southern European countries. [Copyright &y& Elsevier]

There is an increasing call for tools that provide insight into the complex nature of agricultural systems and that deal with a broad range of sustainability issues related to policy intervention, technological innovations, and changes in environmental conditions (e.g., climate change). Sustainability indicators are useful, but only if their number is limited and the interactions between indicators are taken into consideration. In this context, we propose a methodology for an integrated analysis of tradeoffs between economic and environmental indicators. The analysis to quantify these relationships should be based on a multi-disciplinary approach and as such requires the usage of bio-physical as well as econometric-process simulation models. The communication between these very different models is based on explicit definitions of spatial and temporal scales and model integration software. The methodology is based on spatially explicit econometric simulation models linked to spatially referenced bio-physical simulation models to simulate land use and input use decisions. The methodology has been applied for the potato–pasture production system in the Ecuadorian Andes. Results of the analysis are presented in the form of tradeoff curves between different indicators, but also as maps, and risks diagrams. Besides an analysis of the current status, the approach allows for the analysis of alternative scenarios showing the effect of those scenarios on the position and slope of the tradeoff curve. [Copyright &y& Elsevier]

We describe the theory and current development state of the pesticide process module of the USDA-Agricultural Research Service Root Zone Water Quality Model, or RZWQM. Several processes which are significant in determining the fate of a pesticide application are included together in this module for the first time, including application technique, root uptake, ionic dissociation, soil depth dependence of persistence, volatilization, wicking upward in soil and aging of residues. The pesticide module requires a large number of parameters to run (as does the RZWQM model as a whole) and it is becoming clear that RZWQM will find most interest and use as part of a 'scenario' in which all data requirements are supplied and the predictions of the system compared with a real (usually partial) data set. Such a scenario may then be modified to examine the response of the system to changes in inputs. It also has significant potential as a technology transfer or teaching tool, providing detailed understanding of a specific agronomic system and its potential impacts on the environment. [ABSTRACT FROM AUTHOR]

Groundwater vulnerability is a central concept in pollution risk assessment, yet its estimation has been largely a matter of expert judgment. This work applies a method for the direct calculation of vulnerability from monitoring well observations of pesticide concentrations. The method has two major advantages: it is independent of the compounds being examined, and it has a direct probabilistic interpretation making it ideal for risk assessment. The methodology was applied to data from a groundwater monitoring program in the midwestern United States. The distribution of the vulnerabilities was skewed toward zero. Spatial distribution of the vulnerabilities shows them to be controlled by both regional and local factors. Methods are presented for estimating the necessary sample sizes for vulnerability studies. The further application of the approach developed in this study to understanding groundwater pollution is discussed. [ABSTRACT FROM AUTHOR]

Groundwater vulnerability assessments of nonpoint source agrochemical contamination at regional scales are either qualitative in nature or require prohibitively costly computational efforts. By contrast, the type transfer function (TTF) modeling approach for vadose zone pesticide leaching presented here estimates solute concentrations at a depth of interest, only uses available soil survey, climatic, and irrigation information, and requires minimal computational cost for application. TTFs are soil texture based travel time probability density functions that describe a characteristic leaching behavior for soil profiles with similar soil hydraulic properties. Seven sets of TTFs, representing different levels of upscaling, were developed for six loam soil textural classes with the aid of simulated breakthrough curves from synthetic data sets. For each TTF set, TTFs were determined from a group or subgroup of breakthrough curves for each soil texture by identifying the effective parameters of the function that described the average leaching behavior of the group. The grouping of the breakthrough curves was based on the TTF index, a measure of the magnitude of the peak concentration, the peak arrival time, and the concentration spread. Comparison to process-based simulations show that the TTFs perform well with respect to mass balance, concentration magnitude, and the timing of concentration peaks. Sets of TTFs based on individual soil textures perform better for all the evaluation criteria than sets that span all textures. As prediction accuracy and computational cost increase with the number of TTFs in a set, the selection of a TTF set is determined by a given application. [ABSTRACT FROM AUTHOR]