Hidden Cobalt Hazards in Children: A Soil Risk Assessment at the Sokolov-Sarbai Complex

Iron ore mining is often assumed to pose a low soil-contamination risk because magnetite is dense and metal-poor. We tested this assumption around the Sokolov-Sarbai magnetite-skarn complex (Kostanay Region, Kazakhstan) by sampling soils at four settlements and a nearby background site, analyzing sixteen elements in pseudo-total and exchangeable forms, and comparing classical pollution indices with disaggregated US EPA health risk metrics on the same dataset. The two frameworks diverge sharply. Integrated indices (PLI, PERI) classify the area as low-risk, yet the child hazard index (US EPA cobalt reference dose) exceeds one at all four settlements, driven almost entirely by cobalt-a single, highly mobile element released from the late-stage sulfide paragenesis of the host skarn against an already cobalt-rich regional background. This relative result-cobalt as the dominant contributor and Rudny City as the most affected settlement-is robust to exposure-parameter variation and, at Rudny City and Konstantinovka, to unfavorable bioaccessibility assumptions. Carcinogenic risk stays within the acceptable range and reflects regional-background nickel and chromium rather than a mining increment. For single-element, regionally elevated contaminants, integrated indices can therefore understate a genuine child health hazard; adequate assessment requires mobile-fraction characterization and bioaccessibility-explicit risk computation.