Geochemical Fractionation of Stream Sediments Impacted by Pb-Zn Mining Wastes: Missouri, USA

Stream sediments affected by historic mining in Aurora, Missouri, were digested via sequential extraction to determine the amount of Zn, Pb, and Cd bound to each of five geochemical phases: exchangeable, carbonate, Fe-Mn oxides, organic matter, and residual. The metal bound to the exchangeable and carbonate phases represents an approximation of the metal that is bioavailable, and thus a source of toxicity to aquatic biota and a contamination risk to the aquifer underneath. The bioavailable fraction was found to increase with total metal content, comprising an average of 19% of the total Zn, 13% of the Pb, and 14% of the Cd. In general, Pb was preferentially bound to the residual fraction, Zn to the Fe-Mn oxides fraction, and Cd to the organic fraction. As a result of metals binding to different chemical fractions, each of these metals will mobilize differently if/as environmental conditions change and chemical weathering progresses. The most mobile of the three metals was Cd and the least mobile Pb. The concentration pattern for each metal downstream reflects their relative mobility.