Ecological risk assessment of rare earth elements in agricultural soils adjacent to an abandoned coal gangue pile in Chongqing, Southwest China

Rare earth elements plus yttrium (REYs) are increasingly recognized as emerging contaminants, yet their ecological risks in agricultural soils remain are poorly understood. This study assessed REY contamination and ecological risk in agricultural soils adjacent to an abandoned coal gangue pile in a post-coal-mining area of Chongqing, Southwest China. Geochemical characteristics, together with stratigraphic and paleogeographic context and multivariate statistical analysis, suggest that coal gangue likely serve as a conduit for REYs sourced from volcanic materials associated with the Emeishan Large Igneous Province (ELIP) and their weathering products into nearby soils. Pollution indices indicate site-wide REY contamination. The pollution load index (PLI) values of 1.72–3.30 (mean 2.19) classified all soils as polluted. Element-specific geoaccumulation index (Igeo) values were predominantly within 0–1 in 96% of cases, indicating uncontaminated to moderately contaminated conditions for individual REYs, with a few subsoil cases reaching moderate contamination. Despite a generally consistent contamination level across the 0–30 cm profile, the 0–20 cm horizon exhibited slightly higher ΣREY concentrations and elevated PLI and RI values relative to the 20–30 cm layer. The potential ecological risk index (RI) is dominated by moderate risk conditions, with strength risk cases occurring mainly in the upper 0–20 cm layer. Elemental contributions to RI ranked, from highest to lowest, as Lu, Tb, Eu, Ho, and Tm, which together accounted for about 63% of the index. The HREYs (Lu, Tb, Ho, Tm) contributed about 51% of the RI, underscoring their dominant role in shaping ecological risk. Future studies should extend toward integrated, region-wide assessments of REY contamination, ecological risk, and potential human exposure in agricultural soils affected by coal gangue piles derived from Late Permian Longtan Formation coal mining, in order to support risk-based environmental management in post-coal-mining regions.