Heavy Metal Speciation and Bioaccessibility in Soils of the Zamfara Gold Mining Region: Implications for Human Health Risk Assessment

The Zamfara lead poisoning crisis is one of the most severe environmental health emergencies in Nigerian history, yet detailed metal speciation data essential for accurate risk assessment and remediation design remain incomplete. This study investigates the chemical speciation and in vitro bioaccessibility of lead, cadmium, arsenic, and zinc in soils from residential, agricultural, and mining processing areas across three local government areas in Zamfara State, North West Nigeria. Sequential extraction procedures following the modified Tessier and BCR methods were applied to apportion metals across exchangeable, carbonate-bound, iron-manganese oxide-bound, organic matter-bound, and residual fractions. In vitro bioaccessibility was measured using the simplified bioaccessibility extraction test simulating gastric and intestinal conditions relevant to child hand-to-mouth exposure. Lead speciation reveals predominance of exchangeable and carbonate-bound fractions in processing area soils, indicating high mobility and bioaccessibility. Bioaccessible lead fractions in children's play areas reached 73 percent of total lead concentration, substantially elevating exposure estimates compared to those based solely on total concentrations. Correlation analysis between speciation parameters and pH, organic matter, and mineralogy provides mechanistic insights relevant to immobilisation strategies. Revised health risk estimates incorporating bioaccessibility data yield child blood lead prediction values consistent with documented clinical cases. The study provides the most detailed chemical speciation dataset for Zamfara contaminated soils and directly informs remediation target-setting and residual risk management. Keywords: lead speciation, bioaccessibility, Zamfara, health risk assessment, sequential extraction