The challenges of validating and implementing methods for characterising non-extractable residues in soils

SETAC Europe 2024 -- In the course of conducting OECD 307 and OECD 308 studies, the amount of test substance and its metabolites extracted from the soil or sediment can decline over time, leading to an increase in the amount of residues remaining unextracted. This can only be quantified where radiolabeled test item has been applied. According to IUPAC, the definition of non-extractable residues (NER) in soils or sediments is species originating from chemicals that remain unextracted by methods which do not significantly change the chemical nature of the parent substance or its degradates or the structure of the matrix. Regulatory opinion on NER formation differs considerably. Pesticide regulations have historically considered NER to be irreversibly bound degraded residues of no environmental concern. However, ECHA considers NERs as potentially bioavailable, non-degraded residues (parent substance) unless it has been demonstrated that the chemical is completely degraded or irreversibly bound.