Heavy Metal Contamination and Screening-Level Risks in Soils Affected by Persistent Coal Fires in Northwestern China

Persistent coal fires can mobilize potentially toxic elements and create long-lasting soil contamination. This preliminary screening study evaluated Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb in nine composite surface-soil samples from coal-fire, non-combustion reference, and residential zones at the Rujigou mine in northwestern China. Concentrations were measured by inductively coupled plasma mass spectrometry. Geo-accumulation and potential ecological risk indices were calculated, and non-carcinogenic and carcinogenic risks were estimated for children and adults. Mean Cu, Zn, Cd, and Pb concentrations in the coal-fire zone exceeded regional background values. Cadmium showed the greatest enrichment and ecological-risk contribution, although the combined ecological risk remained low. Hazard indices were below 1 in all zones. Under a hypothetical upper-bound scenario treating all total Cr as Cr(VI), modeled Cr estimates exceeded 1.0 × 10−4; this does not demonstrate actual Cr(VI) exposure or cancer risk. Total-As-based estimates also exceeded this benchmark for children, subject to speciation and bioaccessibility uncertainty. Exploratory multivariate analyses identified possible shared metal patterns, but nine samples cannot establish zone-wide differences or distinguish sources. The results identify Cd as the principal ecological concern and support targeted monitoring of metal speciation, bioaccessibility, dust exposure, and biological responses around persistent coal fires.