Aged Sorption in Pesticide Risk Assessment: FAQs
It is well known that pesticides adsorb to soil particles in a non-linear process with sorption increasing over time. Time-dependent sorption or “aged sorption”, has not been accounted for in standard pesticide environmental risk assessments until recently. The European Commission published guidance in 2021 on how aged sorption studies should be conducted and implemented in regulatory assessments.
Enviresearch and Fera have teamed up to provide aged sorption studies and modelling as a joint service. We reflect on our experience of successfully performing several aged sorption studies, following the new guidelines and answer frequently asked questions from our clients:
FAQs
Q: How are the aged sorption parameters used in groundwater modelling?
A: Once you have aged sorption parameters, it is easy to implement them in existing groundwater models. The FOCUS PEARL v5.5.5 model user interface already contains a section for ‘non-equilibrium sorption’ to include the parameters: Kdes (desorption rate constant) and fNE (the ratio between equilibrium and non-equilibrium Freundlich coefficients). The DegT50 needs to be converted to a DegT50EQ to complement the aged sorption parameters.
Q: Will it be worthwhile performing these studies? Will the outcome result in passing the risk assessment?
A: Enviresearch offers preliminary scoping to test the potential reduction of the PEC groundwater values by aged sorption parameters. Our colleagues at Fera offer a preliminary lab test to check the increase of sorption over a short period (e.g. 28 days) showing whether aged sorption can be measured. The outcome of the pre-test will also help with selecting the optimum sampling times for the real study and improve the chances of success.
Q: Do we need that many timepoints?
A: An aged sorption experiment is set up very similarly to the OECD 307 aerobic soil degradation study. A minimum of 6 timepoints are required for fitting, however we recommend that 10 are included to allow for loss of unreliable timepoints. In our experience, we have found that often the timepoints are spaced out too far for aged sorption studies. Also, the study should not be extended much further than when 10% residues are reached, as this may lead to unreliable sorption measurements. This is why we strongly recommend the pre-test. Enviresearch and Fera laboratories work closely to carefully consider the design of aged sorption studies.
Q: Can we use the existing OECD 106 batch equilibrium adsorption study? How quickly do the aged sorption studies need to be performed after the batch equilibrium studies?
A: The first-tier sorption study and the aged sorption study must be performed using the same soil samples. Therefore, a batch equilibrium study must be performed around the same time as the aged sorption study. Careful planning of the study is essential, as the aged sorption study needs to be performed on fresh soil, and the equilibrium sorption study needs to be performed on the same soil batch. We therefore recommend collecting a first batch of soils for preliminary investigations and method development and then to collect a second batch of fresh soils for performing the equilibrium and aged sorption studies. Effective method development is crucial and requires ample preparation time before initiating the main incubation study itself. Then the sorption isotherm measurements can be performed whilst the incubation study is underway.
Q: Can the aged sorption guidance be used for metabolites?
A: Yes, you can apply the guidance to refine parameters for a soil metabolite leaching to groundwater using a metabolite-dosed study.
Q: How does field data come into this?
A: Aged sorption studies cannot be performed in the field as the results are not sufficiently reliable for deriving the modelling parameters. Field degradation data may show shorter DT50 values and therefore you may wish to combine laboratory aged sorption data with field DegT50 values. There is a method to do this, however at time of writing EFSA are still uncertain. It is a conservative method that doesn’t always offer benefit over lower tiers. We are part of the Working Group drafting a proposal for updating the guidance to allow combining field DegT50 with aged sorption, that would replace the current conservative approach.
Q: Have you used aged sorption parameters to successfully refine PECgw values?
A: Yes, most of the studies that we have performed to date, the PECgw was improved either through more favourable sorption parameters or degradation parameters.
Q: Have the authorities evaluated any aged sorption studies yet? And if so, have they accepted the refined parameters?
A: To the best of our knowledge, RMS evaluations are still ongoing and we do not know the outcome yet. Watch this space!
Enviresearch partners with Fera laboratories to perform bespoke, high quality aged sorption studies. Continuous communication with the study directors throughout the study results in optimum study design and performance. Follow this link for information on our joint services.