Evidence of soil particle-induced ecotoxicity in old abandoned mining area.

Ecotoxicity of heavy metals in soil is primarily associated with their bioaccessibility and bioavailability in the soil media. However, in some exceptional cases, soil ecotoxicity has been observed despite high total metal concentrations and low extractable metal concentrations in contaminated field sites; therefore, other exposure pathways must be considered. Therefore, the aim of this study was to evaluate the soil-particle induced ecotoxicity in an old mining area. We hypothesized that heavy metals, strongly adsorbed onto soil particles of consumable size for soil organisms, exhibit ecotoxicity, especially on soil particles ∼1 µm to 300 µm in size. A plant seedling assay, in vivo cytotoxicity assay using earthworm immune cells, and a metal bioconcentration assessment were performed. The results of soil particle toxicity revealed that the soil from the study area (A1-A4) had a low contribution to the soil ecotoxicity of extractable metals. For instance, the concentration of extractable arsenic was only 1.9 mg/kg soil, despite the total arsenic concentration reaching 36,982 mg/kg soil at the A1 site. The qualitative and quantitative analyses using SEM-EDX and ICP-OES, as well as principal component analyses, supported the hypothesis of the present study. Overall, the study results emphasize the importance of soil particle-induced ecotoxicity in long-term contaminated field soils. Our study results can inform on effective site-specific soil ecological risk assessment as they suggest the inclusion of soil particle-induced ecotoxicity as an important criterion in old, contaminated field sites, even when the extractable metal fraction in the field soil is low. ENVIRONMENTAL IMPLICATION: Bioaccessibility and bioavailability are primary factors contributing to the soil ecotoxicity of heavy metals. However, in some cases, such as long-term contaminated field sites, soil ecotoxicity has been confirmed even when low extractable metal concentrations were detected alongside high total metal concentrations. The findings of this study reveal that soil particles of edible size could be sources of soil ecotoxicity in the case of long-term contaminated fields with low extractable metal concentrations. The results of this study would contribute to the area of site-specific soil ecological risk assessment.