Source–Sink Relationships and Environmental Risks of Surface Soil Heavy Metals and Metalloids: Multi-Media Monitoring in a Southwest China County

Abstract

The pollution of soil by potentially toxic elements in industrial–agricultural transition zones threatens global food security and public health owing to their persistence and bioaccumulation. This study focused on Miyi County, Sichuan (China), a typical region with intensive vanadium–titanium magnetite mining and modern agriculture, and systematically analyzed eight heavy metals and metalloids (Cd, Hg, As, Pb, Cr, Cu, Zn, and Ni) across the categories of atmospheric deposition, irrigation water, agricultural inputs, and soil–crop systems. A rigorous four-stage full-chain diagnosis (concentration–load–ecology–health) was executed to evaluate pollution levels and pathways. The single-factor pollution index identified cadmium (Cd) as the primary pollutant, exhibiting a maximum index of 32.63. The Håkanson potential ecological risk index (RI) demonstrated that Cd was the absolute dominant contributor, reaching a catastrophic single-element risk factor (Ei) of 2191.8 and contributing over 70% to the comprehensive ecological risk. Spatially, soils displayed a distinct point-source cluster diffusion pattern: the northern metallurgical zone was dominated by a Cr-Zn-Cu-Ni industrial assemblage, while the southern zone was enriched in Cd, Pb, As, and Hg. Positive matrix factorization (PMF) source apportionment quantitatively demonstrated that industrial emissions via atmospheric deposition were the primary driver, contributing 55–75% of the total soil exogenous inputs, while agricultural sources (livestock manure and legacy arsenic pesticides) exacerbated localized accumulation. While overlying irrigation water remained safe, channel sediments acted as historical pollution sinks. The human health risk model revealed that children in industrial core areas faced unacceptable carcinogenic hazards, with a lifetime carcinogenic risk (LCR) reaching 5.6 × 10−4. These highly specific multi-media findings support a macro spatial risk zoning and source interception strategy to decouple economic growth from regional food safety degradation in global transition economies.