Non-extractable residues in persistence assessment

The formation of non-extractable residues (NER) is a frequently observed phenomenon during the degradation of substances in soil and sediment. So far, there is no accepted approach to assess their hazard potential for the environment. At a workshop in February 2021, UBA presented two proposals for evaluating NERs in the persistence assessment.

Table of Contents


What are non-extractable residues?

An essential part of the assessment of environmental hazards caused by the release of a chemical is the question of how long this substance remains in water, soil, and sediment. Laboratory studies that examine the degradation of the substance of interest by simulating the conditions of the particular environmental compartment play an important role to answer this question. Such studies provide information on the half-life of a substance, which is derived from the extractable amount at a series of consecutive sampling dates. By using radiolabelled test material, it became clear that in many cases the entire amount of the radioactive marker cannot be extracted after application and that the remaining residues sometimes even increase in the course of incubation. These so-called non-extractable residues are formed by various processes, e.g. sorption to organic matter or the soil matrix, entrapment in soil pores or incorporation of the label into the biomass of the microbial degraders. The relevance of NER formation depends on substance properties and characteristics of the solid matrix extraction methods impact the amount of NER formed but are difficult to standardise. The boundary between an extractable fraction and NERs  is currently not precisely specified. NER comprise parent and transformation products of unknown extent and unknown composition


Hidden hazard or a safe sink?

NERs have many facets – experimental data indicate the following processes: Transformation into biomass which also can represent necromass, that itself will re-enter the metabolic cycle (biogenic NER type III), is a safe sink. NER may be covalently bound to organic matter (NER type II) and thus should not be able to be mobilised again and can also be seen as a sink. On the other hand adsorption or physical entrapment of NER (type I) in the soil matrix are hidden hazards as it is possible that parent substance or transformation products are able to be mobilised again.

For regulatory use, classification of different NER types according to their binding status is recommended/required. Up to now, however, there is no generally accepted, standardised approach to separate NER into a hidden hazard for the environment or safe sink. Extraction schemes for experimental determination of these NER types in laboratory testing were proposed by BfG/UBA and ECHA ECHA Discussion paper on NER.

A research and development project funded by UBA (Umweltbundesamt, German environment agency) developed a refined proposal for routine testing by comparing these two different approaches to determine total NER and differentiating types of NER.

UBA and the research partners Fraunhofer IME Institute Schmallenberg, RWTH Aachen University, The Helmholtz-Centre for Environmental Research GmbH (UFZ) Leipzig and the DTU University Lyngby (DK) organised a virtual EU-wide workshop in February 2021 to present the experimental results and to discuss the proposals. The aim of the workshop was to discuss and agree on a harmonised concept to be used in environmental persistence assessment of pesticides, biocides, pharmaceuticals and ⁠REACH⁠ chemicals.

More than 80 participants from science, industry, consultants and environmental agencies attended the workshop and enabled a lively and fruitful discussion. We thank all participants for their valuable input!
An overview of the questions raised during the workshop and answered by UBA and the project partners can be found in our FAQ.


New approaches for consideration of NER in persistence assessment

Beside the proposal of the research partners, which is mainly suitable for the decision ‘persistent (P)/’very persistent (vP)’ or ‘not persistent (not P)’ in the framework of pure hazard assessment, the UBA developed a second approach for more realistic half-lives in combination with risk and hazard assessment.

The two concept proposals (Proposal 1Proposal 2) were further discussed by scientists, regulators and industry in a public consultation, which ended in September 2021. UBA thanks all those who participated in the consultation! The comments received and the responses of UBA and the project partners can be viewed here (commenting table public consultation)

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 Non Extractable Residues  Extraction procedures  Type I NER  Type II NER  biogenic NER  MTB