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Background The regulatory risk assessment of pesticides requires the assessment of exposure of aquatic ecosystems in small surface waters adjacent to agricultural fields. This exposure is predicted using simulation models, for which an important input parameter is the degradation rate in water. In regulatory dossiers, the decline rate in water from outdoor mesocosms is often available, but this rate encompasses more processes than degradation. Therefore, a procedure was designed for estimating the degradation rate in water that was suitable for mesocosm studies with limited datasets, e.g. datasets lacking site-specific sorption coefficients and relevant sediment properties. The procedure, based upon inverse modelling with TOXSWA, has been tested on a dataset for prosulfocarb in stagnant ditches. Results A degradation half-life in the ditch water of 2.9 days (20 °C) was found. This short half-life was to a significant extent accounted for by biodegradation rather than hydrolysis or photolysis. This half-life was considerably shorter than the half-life in the water layer of two laboratory water-sediment system experiments. Conclusion The estimation procedure resulted in a unique half-life for the degradation rate in water. Such outdoor mesocosm studies seem to be better suited to assessing the degradation rate in water in ditches than the conventional water-sediment studies. © 2012 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

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]

Assessment of the risk to aquatic organisms is an important aspect of pesticide registration. This assessment must be based on well-defined exposure assessment goals (EAGs). However, these goals have not yet been defined for the EU authorization procedure. The definition of an aquatic EAG has seven elements, including: type of water body, spatial dimension of this body, spatial population of water bodies, multi-year temporal population of concentrations for a single water body, and the space-time percentile combination to be selected from the spatio-temporal population of concentrations. The seven elements are split into 16 items, three which are within the risk-management domain. The remaining 13 scientific items should preferably be based on consistency with landscape-level approaches. Subdivision of the spatial population of water bodies on the occurrence of exposure routes should be avoided (although this is current practice). The multi-year temporal population of concentrations should be based on all years in rotational crops (including years without applications). Risk managers should be offered a suite of coherent packages of EAGs and effect assessment goals from which they can select the package corresponding to the desired overall level of protection. © 2017 Society of Chemical Industry., (© 2017 Society of Chemical Industry.)