Certainties and uncertainties in accessing toxicity of non-extractable residues (NER) in soil

Background: Discussion concerning the bioavailability and ecotoxicological relevance of non-extractable residues (NER) in soil is still ongoing. Is NER formation a detoxification process or a hidden hazard? The use of radiolabelled chemicals enables detection of NER, but the identity of NER is usually unknown. Regulations require clear measurable parameters and the approach of Ortega-Calvo et al. (Environ Sci Technol 49:10255–10264, 2015) defines these. Results: Following that approach, we studied the fate of three ecotoxic, NER-forming chemicals over a period of 6 months after application to three different soils. Initial ¹⁴C experiments showed formation of NER for all chemicals. For the chemical 2,4,6-trinitrotoluene (TNT), NER-formation was reproducible in all soils. We applied a recently standardized method using Tenax® to remove the bioavailable fraction of the chemical at test start and test end. Removing the bioavailable fractions also removed toxicity. Further experiments without radiolabelled TNT clearly showed that the toxicity measured in applied soils was caused by the bioavailable chemical and not by NER. Conclusions: The tool developed can be used if the fate of the chemical including NER formation is well known and reproducible. The other selected chemicals, cypermethrin and carbendazim, showed unexpected behaviour in ¹⁴C-fate experiments. The degree of biodegradation was not reproducible for cypermethrin and unexpected losses occurred with carbendazim. This indicated a very large uncertainty when using non-radiolabelled compounds in NER experiments and thus the tool is not suitable in non-radiolabelled experiments.