Ecological risk of selenium, cadmium, and arsenic in high-background sedimentary terranes: Insights from element enrichment and speciation in rocks

The main geological factors causing high levels of heavy metals (HMs) in soils in areas with sedimentary rocks are the exposure of black rock series (BRS) and the development of karst. The combined relationships, background concentrations, and speciation of Se and HMs in clastic rocks, carbonate rocks, and BRS remain unclear, which restricts HM traceability and remediation in high background soils. This study focuses on Ediacaran to Jurassic sedimentary rocks in South China and determines the total concentrations of Se, Cu, Zn, Pb, Hg, Cd, Cr, As, Ni, and total organic carbon (TOC), as well as their speciation. The results indicate that the TOC in sedimentary rocks is significantly positively correlated with Se, Hg, Cd, Cr, and Ni, followed by Cu and As, but not Pb and Zn. Cluster analysis reveals that Se is strongly associated with Cd and As. Compared with those in the upper continental crust (UCC), the enrichment levels of elements in BRS are ordered Se > Cd > Hg > As > Cr > Ni > Cu > Pb > Zn. The mean concentrations of Se, Cd, Hg, and As in BRS are 957.8, 62.9, 28.4, and 8.5 times those in the UCC, respectively. These elements are also relatively enriched in carbonate rocks. A risk assessment based on speciation indicates that Cd, Se and As in BRS have the greatest ecological risks, as well as Cd and As in carbonate rocks. Active speciation of Hg in all three rock types is less than 0.1% of the soil risk screening, indicating low risk. Therefore, the elevated risks of Cd, Se, and As in high-background areas may fundamentally stem from parent rocks. With a limited budget, the immediate focus should be on remediating Cd, As, and Se in the BRS areas, while also stepping up monitoring of Cd and As in the karst zones.