Pesticide leaching to ground water at 1 m depth and pesticide persistence in the plow layer were calculated with a mathematical model for a sandy soil continuously cropped with maize (Zea mays L.) and exposed to weather conditions in a temperate climate. The pesticide was applied in spring. In the model, water flow was described by Darcy's law and water uptake by the crop was included. Dally averages of meteorological conditions (rainfall, evapotranspiration, soil temperature) were used as input. The model assumes first‐order transformation, equilibrium sorption (Freundlich equation), and passive plant uptake. Pesticide leaching and persistence were calculated as a function of pesticide sorption (characterized by the organic‐matter/water distribution coefficient, Kom) and of transformation rate. It was found that pesticide leaching is very sensitive to both Kom and the transformation rate: changing Kom or the transformation rate by a factor of 2 changes the fraction of the dose leached typically by about a factor of 10. Pesticide persistence in the plow layer was found to be sensitive to Kom at low transformation rates and sensitive to the transformation rate at high Kom values. Additional calculations showed that autumn application results in much higher leaching of nonsorbing pesticides with short half‐lives than spring application (difference of two orders of magnitude).

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In the Netherlands, many of the fresh groundwater resources are vulnerable to pollution. Owing to high population densities and intensive farming practices, pesticide residues are found in groundwater at many places. Hence a number of drinking water abstraction wells contain pesticides residues, causing considerable costs for purification. The Water Framework Directive (WFD) requires countries to assess the chemical status of groundwater bodies and set up monitoring plans for groundwater quality, including pesticides. 771 groundwater samples were taken from monitoring wells in 2006 and analysed for a broad list of pesticides in order to fulfil these requirements. Pesticide were detected in 27% of samples, while in 11% the WFD limit of 0.1 microg/l was exceeded. In this paper, these and earlier measurements are evaluated further, considering also measurements in drinking water wells, information about the origin of measured pesticides and calculated trends in use and emissions. The measurements in the monitoring wells showed that where pesticides are used, 15-55% (minimal and maximal estimation) of the wells in shallow groundwater (1 to 20 m below soil surface) contain pesticides residues at concentrations above 0.1 microg/l. When the metabolites BAM and AMPA are excluded (as not relevant in human toxicological terms), the estimation range is 7-37%. These patterns observed in shallow groundwater are reflected by the occurrence of pesticides in vulnerable abstraction wells that are used for the production of drinking water. The WFD requires the determination of both status and trends. The design of current monitoring network is evaluated from this perspective. Several recommendations are made for more adequate and efficient monitoring.

To support EU policy, indicators of pesticide leaching at the European level are required. For this reason, a metamodel of the spatially distributed European pesticide leaching model EuroPEARL was developed. EuroPEARL considers transient flow and solute transport and assumes Freundlich adsorption, first-order degradation and passive plant uptake of pesticides. Physical parameters are depth dependent while (bio)-chemical parameters are depth, temperature, and moisture dependent. The metamodel is based on an analytical expression that describes the mass fraction of pesticide leached. The metamodel ignores vertical parameter variations and assumes steady flow. The calibration dataset was generated with EuroPEARL and consisted of approximately 60,000 simulations done for 56 pesticides with different half-lives and partitioning coefficients. The target variable was the 80th percentile of the annual average leaching concentration at 1-m depth from a time series of 20 yr. The metamodel explains over 90% of the variation of the original model with only four independent spatial attributes. These parameters are available in European soil and climate databases, so that the calibrated metamodel could be applied to generate maps of the predicted leaching concentration in the European Union. Maps generated with the metamodel showed a good similarity with the maps obtained with EuroPEARL, which was confirmed by means of quantitative performance indicators.

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GeoPEARL_NL is used as a higher tier instrument in the leaching assessment of plant protection products in the Netherlands. Because the soil organic matter contents in arable soils in the current version were too high, a new soil organic matter for the Netherlands was needed. The new 3D organic matter map has been generated in two steps; first a trend model was developed based on data at 1210 locations selected using a well-defined stratified random sampling scheme followed by interpolation of the residuals using about 770 000 determinations of organic matter content from the Dutch Soil Information System (BIS-Nederland). In general, the predicted soil organic matter contents in the top layer of arable soils correspond well with those measured. Because preliminary calculations showed higher leaching concentrations using the new GeoPEARL version, the consequences of this new version in combination with the Dutch Decision Tree for leaching to groundwater was investigated. The results of the computations revealed that the PEC90 calculated using GeoPEARL in tier 2 is higher than the PEC80 calculated using FOCUSPEARL in many cases. This inconsistency between the first and second tiers could be remedied by the introduction of a calibration factor.Calibration factors of 5 and 10 are necessary to ensure consistency between these tiers for spring and autumn applications, respectively. Suggestions are given for improvement and justification of the calibration factors in the Decision Tree. It is recommended to systematically compare the predicted leaching concentration in groundwater abstraction areas with that in the Dutch agricultural area as a whole. To improve the reliability of model predictions for these smaller areas, development a more flexible schematisation is needed.Keywords: decision tree, GeoPEARL, groundwater, leaching, national authorisation, plant protection product, pesticide, soil organic matter • Acquisition, duplication and transmission of this publication is permitted with clear acknowledgement of the source.• Acquisition, duplication and transmission is not permitted for commercial purposes and/or monetary gain.• Acquisition, duplication and transmission is not permitted of any parts of this publication for which the copyrights clearly rest with other parties and/or are reserved. Analysis of the data showed that the organic matter content in the top 0.3 m of arable soil profiles in GeoPEARL was 1.5. to 2 times higher than that measured in these soils. This difference was introduced in the schematisation at the time of the development of the schematisation, because the aim was to estimate the average organic matter content in a grid cell, which results from a mixture of grassland and arable soil profiles. The consequence of the error in the organic matter content of arable soils is that the leaching concentrations to groundwater are underestimated. To remedy this, the Working Group on the Dutch Decision Tree for Leaching to Groundwater started to work on the improvement of the soil schematisation for G...