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BACKGROUND: As part of the Dutch authorisation procedure for pesticides, an assessment of the effects on aquatic organisms in surface waters adjacent to agricultural fields is required. The peak concentration is considered to be the most important exposure endpoint for the ecotoxicological effect assessment. Macropore flow is an important driver for the peak concentration, so the leaching model PEARL was extended with a macropore module. The new model has two macropore domains: a bypass domain and an internal catchment domain. The model was tested against data from a field leaching study on a cracking clay soil in the Netherlands. RESULTS: Most parameters of the model could be obtained from site-specific measurements, pedotransfer functions and general soil structural knowledge; only three macropore-flow-related parameters needed calibration. The flow-related macropore parameters could not be calibrated without using the concentration in drain water. Sequential calibration strategies, in which firstly the water flow model and then the pesticide fate model are calibrated, may therefore be less suitable for preferential flow models. CONCLUSION: After calibration, PEARL could simulate well the observed rapid movement towards drains of two pesticides with contrasting sorption and degradation rate properties. The calibrated value for the fraction of the internal catchment domain was high (90%). This means that a large fraction of water entering the macropores infiltrates into the soil matrix, thus reducing the fraction of rapid flow. Copyright © 2011 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

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]

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]

Harmonisation of the assessment of pesticide leaching to groundwater for EU registration is desirable to minimise confusion in the decision-making process at EU level. Recently, the FOCUS groundwater scenarios have been developed for three chromatographic models (PEARL, PELMO and PRZM) to increase this harmonisation. This study investigates the role of dispersion parameterisation in explaining the cause of the differences in pesticide leaching calculated by these models. PEARL describes dispersion via a physical parameter, ie the dispersion length. PELMO and PRZM simulate dispersion via a numerical procedure which generates an effective dispersion length equal to 0.5 times the thickness of the numerical compartments. The hypothesis was tested that the difference in the dispersion length input parameter (ie 5 cm for PEARL and about 2.5 cm for PELMO and PRZM) is a major cause of the difference in calculated leaching. It was tested whether results of PEARL calculations with a dispersion length of 2.5 cm corresponded much better to results of PELMO or PRZM than results of PEARL calculations with a dispersion length of 5 cm. This was done by calculations for one substance and all nine FOCUS scenarios and by calculations for a range of substances and two FOCUS scenarios (Châteaudun and Sevilla). All calculations were for winter wheat and an application at 1 day before emergence. Both tests showed that reduction of the dispersion length from 5 to 2.5 cm in PEARL led to a much better correspondence between PEARL and either PELMO or PRZM. Hence the hypothesis was supported. It is likely that harmonisation of the dispersion length in the FOCUS groundwater scenarios would reduce the differences in calculated leaching between PEARL and PELMO or PRZM considerably for part of these scenarios. [ABSTRACT FROM AUTHOR]