Interpretation of aged sorption studies for pesticides and their use in European Union regulatory leaching assessments

First-tier regulatory exposure assessments for pesticides assume that pesticide sorption is instantaneous and fully reversible. In European Union (EU) regulatory guidance, an increase in sorption over time (“aged sorption”) can be considered at the higher tier to refine predicted environmental concentrations in groundwater. Research commissioned by the UK Chemicals Regulation Directorate (CRD), funded by the Department for Environment Food & Rural Affairs (Defra), formed the basis of a draft regulatory guidance document proposing 1) a protocol on how to measure aged sorption of parent compounds in laboratory studies, 2) procedures to fit kinetic models to the experimental data, 3) criteria to test the reliability of the parameters, and 4) procedures for use of the parameters in the groundwater exposure assessment. The draft guidance was revised after feedback from stakeholders and testing of the guidance was performed against real data sets by an independent consultancy. The Chemicals Regulation Directorate submitted the revised document to the European Food Safety Authority (EFSA) for scrutiny. This article gives an overview of the draft guidance and explains the reasoning behind the recommendations made. Integr Environ Assess Manag 2015;11:276–286. © 2015 SETAC Key Points This work bridges the gap between scientific research and regulatory assessments for pesticides. For the first time, a methodology was developed to implement aged sorption of pesticides in soil into EU regulatory leaching assessments in a rigorous and reproducible manner. Experimental methods to assess aged sorption in the laboratory were compared. Incubation of pesticide-treated soil and determination of aqueous extractable and total residues of the pesticide at several time points was recommended. Data quality requirements were developed (e.g. eliminate day 0 and 1 samples, ?2 replicates). Based on a review, it is recommended to describe the experimental data with a two-site one-rate aged sorption model. Fitting techniques, acceptance criteria (goodness of fit, evidence of aged sorption, parameter uncertainty) and the use of the results in leaching assessments are described. The initiative was welcomed by industry, academia and regulators. Engagement with these stakehoders led to the current version of the draft guidance document. Although commissioned by a UK authority, the draft procedures are now under scrutiny at EU level with the aim to develop EU wide guidance.