REGULATORY NEWS

FOCUS degradation kinetics guidance – Working group for improvement

Date: 15 December 2020

In 2015, a project group started to optimize the FOCUS degradation kinetics guidance with the aim to reduce the room of interpretation and subjective decision making in the field of degradation kinetic assessment. While headed by HSE (UK) during the last years, AGES (AT) agreed to lead on the development of the guidance from 2020 on. The recommendations by the working group were open for Member states’ commenting until end of October 2020.

The revised points in the new guidance were summarized during the 22nd international Academy Fresenius conference on behaviour of pesticides in air, soil, and water:

The flow charts on derivation of persistence endpoints will be simplified and be more linked to the text. The draft guidance on metabolite modelling endpoints includes a check if the sum of formation fraction in ≥1 will be included as well as additional checks on metabolites increasing throughout the study period. Additional flow charts include default options for the case that no acceptable fit could be found.

Generally, DFOP kinetics are recommended for derivation of modelling endpoints in case of biphasic degradation behavior. A further new point is an averaging procedure to find a weighted geomean of databases containing a mix of SFO (single first order) and DFOP fits as a unweighted geomean value would underestimate the degradation in the second phase.

A new similarity check comparing the pseudo SFO curve (derived from DFOP DT₉₀/3.32) with the average DFOP curve can help for the decision if the average degradation behavior is “sufficiently” bi-phasic to justify the use of bi-phasic degradation model. The check is based on the fact that the SFO curve overestimates residues compared to average DFOP curves up to the DT₉₀ and underestimates the residues compared to average DFOP curve after the DT₉₀.

The working group tested the new approaches with a beta version of FOCUS PRZM groundwater model which is capable of modelling DFOP kinetics. This allowed the comparison of using SFO vs. DFOP and pseudo SFO approaches. The test showed that DFOP endpoints may result in PECgw values that are higher by an order of magnitude than those based on SFO endpoints. It is assumed that this effect is mostly due to the final few percent of assumed substance mass in soil, which tend to become increasingly higher for DFOP kinetics compared to SFO kinetics even when kinetic fits are similar. However, most kinetic data are based on studies covering ca. 120 d when only a few percent of the substance remain, making the description of the behavior more uncertain, while groundwater modelling is based on much longer periods. On the other side a move to more “best fit” kinetics for modeling would bring much more complexity and conservatism.

Generally, the working group on the kinetic guidance development tries to maintain more pragmatic approaches as

• Using the SFO for modelling when fits are “good enough”
• Retain pseudo SFO approaches
• Using a similarity check to justify use of overall SFO even when some soils show biphasic degradation behavior.

The working group for the improvement of FOCUS degradation kinetics tried balancing pragmatism in decision making with critical selection of modelling endpoints. We are looking to hearing about the Member states ‘opinion on the proposed pragmatism.

SCC will keep you informed on the further progress on updating the FOCUS kinetic guidance.

Please get in contact to make use of our experiences with kinetic modelling and skills for developing robust strategies on the kinetic behaviour of your substance.