Spatial Differentiation and Integrated Risk Assessment of Heavy Metals in Paddy Soils Affected by Lead–Zinc Mining and Smelting

Subsurface contamination in mining-affected paddy soils raises questions about how concentration changes with depth relate to contamination persistence and metal partitioning. We sampled 46 paddy field sites in Guangdong, China, at 0–20, 20–40 and 40–60 cm. Total concentrations of eight elements were measured by inductively coupled plasma mass spectrometry (ICP-MS) after microwave-assisted acid digestion. Non-residual concentrations were measured by ICP-MS using pooled extracts from the first three modified BCR extraction steps. Pollution indices, potential ecological risk assessment, spatial statistics and multivariate analyses were integrated. Cd, Pb and Zn were the dominant contaminants. Mean potential ecological risk index (RI) values decreased from 413.23 at the surface to 234.41 in the deep layer, with Cd contributing 49.3–61.1% of layer-specific mean RI values. Despite declining mean concentrations, Cd, Pb and Zn exceeded agricultural risk screening values in all three layers at 91.3%, 100% and 100% of sites, respectively. Their non-residual proportions did not differ significantly among layers, while Pb showed stronger surface-to-deep spatial correspondence than Cd or Zn. These findings distinguish concentration decline from persistent contamination and differences in metal partitioning, supporting depth-specific monitoring and assessment of index-based potential ecological risk across the sampled profile.