European guidance for 'aged sorption'

In the current groundwater guidance documents several refinements have been made to assess the leaching of an active substance or metabolite. Adsorption of substances to soil particles can significantly affect their availability for degradation in the soil. The current risk assessment assumes that the sorption process is instantaneous and fully reversible. However, recent studies have shown that the sorption process often increases in proportion to the time of interaction between the substance and the soil. This is known as aged sorption. This results in a possible difference in modelled leaching in the FOCUS models.

As of April 1, a European guidance document (SANTE/12586/2020 – REV 0) has been in force with which we can measure-time dependent sorption and derive model input parameters for the existing FOCUS models. This allows the process of aged sorption – as a refinement – to be included in the FOCUS leaching models.  

The new method means that applicants must include an extra additional step in the standard soil degradation study (OECD 307) and that they perform the standard (batch) sorption study (OECD 106) with the same soils. Once this has been done, the study results can be entered into the software to optimise the relevant parameters for the aged sorption process in the leaching models. The guidance refers to the following relevant software: PEARLNEQ v.5.1 (a modified version of PEARL) and the existing programs Modelmaker and Matlab. Based on this optimisation, it can be determined whether aged sorption is taking place and what the relevant parameters are. These parameters can then be entered into the standard FOCUS leaching models (PEARL, PELMO, MACRO).

Therefore, the Ctgb now accepts assessments in which the process of aged sorption has been included in accordance with the instructions from the guidance, and evaluates this refinement in accordance with the recommendations and indications from the guidance. This refinement can also be implemented in the Netherlands-specific leaching model GeoPEARL v. 3.3.3. The use of this guidance document in the risk assessment for leaching into groundwater is explained in an update of the evaluation manual, chapter 6 Fate and Behaviour in soil: Leaching.

The Ctgb prefers to use the PEARLNEQ v 5.1 model for the optimisation process. This refinement can be presented as a higher tier. If this refinement is used, then a tier 1 calculation based on parameters coordinated in the EU must be provided for Zonal as well as National applications (see also the recommendations in the Working Document on Harmonisation in the Central Zone, Fate and Behaviour).