Nonextractable Residues, an Analytical Iceberg, Contribute Significantly to the Pool of Halogenated Organic Chemicals in Soils across Various Regions

The nonextractable residues (NER) of halogenated organic chemicals (HOC), a crucial yet underexplored component in field soil/sediment monitoring, have traditionally been overshadowed by analyses focusing solely on the solvent extractable fraction (EF). This has led to NER-HOC becoming an invisible “analytical iceberg”, with their compositions, levels, and potential risks obscure. Building on our prior research, which revealed distinct distributions of EF-HOC and NER-HOC in industrialized urban and remote permafrost soils, we expanded our study to include suburban soils from both a megacity and a smaller city. Employing solvent extraction and alkaline hydrolysis, we were able to analyze both EF-HOC and NER-HOC. Our findings showed an increased NER-HOC to EF-HOC ratio and a higher prevalence of metabolite HOC compared to parent HOC in areas with reduced anthropogenic impacts. Notably, except for the soils from a point source site, all other locations exhibited NER to EF ratios exceeding 1, indicating a dominance of NER in these HOC pools. This highlights the potential importance for incorporating NER-HOC analysis in soil/sediment monitoring, especially in nonpoint source and remote locations. Our findings contribute to the understanding of the HOC distribution from diverse sources in varying environmental settings and suggest broader NER-HOC analyses in environmental assessments.