An integrated method for assessing pollution risk of heavy metals specified or unspecified in China's national standard for agricultural soils: Illustration on a case study.

Heavy metal pollution in soil has become an environmental challenge attracting global concern. In China, the national standard GB15618-2018 is widely adopted for classifying heavy metal pollution risk in agricultural soils into three levels, including the background, screening, and intervention levels. However, it only specifies risk screening and intervention values for five heavy metals (Hg, Cd, As, Pb, Cr), which make systematic risk classification for other metals unfeasible. To address this limitation, an integrated method combining GB15618-2018 with the geo-accumulation index (Igeo) is proposed. Risk levels are determined preferentially through risk values specified in GB15618-2018, and otherwise by Igeo values (with new thresholds of 1.4 and 3.7). Analysis of 493 soil samples from the Yuanjiang area confirmed Igeo as a valid supplementary tool, enabling unified three-level risk classification for all heavy metals. Application in the Yuanjiang area showed most areas at the background level, with small screening-level patches requiring supervision and rare intervention-level spots requiring control. Source apportionment using multivariate statistical analyses mainly identified three distinct sources: ultramafic rocks (Cr, Ni, Co, Hg, As), Pb-Zn mineralization (Cd, Pb, Zn), and Cu mineralization (Cu and V). Although developed within China's regulatory framework, the proposed method addresses a challenge common to many countries, namely incomplete regulatory coverage for certain heavy metals, and thus offers a template adaptable to other regions worldwide.