Proposal for field-based definition of soil bound pesticide residues.

The environmental significance of soil bound pesticide residues (SBPR) is potentially large because approximately one third of the applied mass of the pesticides in agriculture ends up as SBPR. At EU level, there is little regulatory guidance available on the environmental risk assessment of SBPR in spite of some 50 years of SBPR research. This lack of guidance is partially caused by the fact that the current definitions of SBPR are founded on non-extractability in soil in the laboratory whereas for the environmental risk assessment not the soil in the laboratory but the soil in the field is the system of interest. Therefore a definition of SBPR is proposed that is based on the field soil: a molecule (further called 'the mother molecule') is soil bound if a relevant part of this molecule has become part of the solid phase in the soil and if this relevant part will never be released again to the liquid phase in soil under relevant field conditions in the form of this mother molecule or in the form of another molecule that may possibly raise environmental or human toxicological concerns. This mother molecule may be the parent substance that is applied to the soil but it may also be a metabolite of this parent substance. A consequence of the definition is that the SBPR terminology becomes more precise because the mother molecule of the soil bound residue has to be specified. A further consequence is that very strong but reversible sorption of molecules such as paraquat is not considered soil-bound residue anymore (as may be demonstrated by a self-exchange extraction procedure). Furthermore, the definition requires that risk managers have to define what they consider as 'relevant field conditions' (e.g. include also changes of agricultural fields into forests?).

Influence of pH-dependent sorption and transformation on simulated pesticide leaching.

The leaching of a substance is influenced by its physico-chemical characteristics as well as environmental conditions. In spatially distributed modelling the influence of soil properties on the half-life and the sorption constant of the substance might become important and can be taken into account. The GeoPEARL model includes options to account for sorption and transformation being dependent on soil characteristics. Using some of these options in calculations for a herbicide with both sorption and transformation dependent on the pH of the soil, the calculated leaching from an application in spring appeared to be higher than anticipated from calculations according to the so-called paired parameter approach, in which the leaching is assessed for pairs of sorption and transformation parameters at regular pH intervals. The reason for the higher leaching was that the most critical leaching conditions were not covered by the selected pH values. A 'paired approach' might however be useful as a first tier assessment of the leaching potential. The maximum leaching is expected with the highest DegT(50)/K(om) ratio, which might be obtained from plotting this ratio against the characteristic soil property. The leaching potential of the parent was more important for the leaching of the metabolite than the leaching potential of the metabolite itself. This should be accounted for in the evaluation procedure.

Conceptual model for improving the link between exposure and effects in the aquatic risk assessment of pesticides.

Assessment of risks to aquatic organisms is important in the registration procedures for pesticides in industrialised countries. This risk assessment consists of two parts: (i) assessment of effects to these organisms derived from ecotoxicological experiments (=effect assessment), and (ii) assessment of concentration levels in relevant environmental compartments resulting from pesticide application (=exposure assessment). Current procedures lack a clear conceptual basis for the interface between the effect and exposure assessments which may lead to a low overall scientific quality of the risk assessment. This interface is defined here as the type of concentration that gives the best correlation to ecotoxicological effects and is called the ecotoxicologically relevant concentration (ERC). Definition of this ERC allows the design of tiered effect and exposure assessments that can interact flexibly and efficiently. There are two distinctly different exposure estimates required for pesticide risk assessment: that related to exposure in ecotoxicological experiments and that related to exposure in the field. The same type of ERC should be used consistently for both types of exposure estimates. Decisions are made by comparing a regulatory acceptable concentration (=RAC) level or curve (i.e., endpoint of the effect assessment) with predicted environmental concentration (=PEC) levels or curves (endpoint of the exposure assessment). For decision making based on ecotoxicological experiments with time-variable concentrations a tiered approach is proposed that compares (i) in a first step single RAC and PEC levels based on conservative assumptions, (ii) in a second step graphically RAC and PEC curves (describing the time courses of the RAC and PEC), and (iii) in a third step time-weighted average RAC and PEC levels.

Mapping ground water vulnerability to pesticide leaching with a process-based metamodel of EuroPEARL.

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.