MAGNETIC CHARACTERIZATION OF NATURAL AND ANTHROPOGENIC IRON OXIDE ASSEMBLAGES IN NORTH CAROLINA LAKES: TOOLS FOR DETECTION OF COAL COMBUSTION RESIDUALS

Combustion of coal results in oxidation and conversion of iron sulfides into iron oxides, typically melded with aluminosilicates in distinct spherical morphologies. Constituent minerals such as magnetite can be characterized via a range of magnetic behaviors, serving as rapid and sensitive tools for coal ash detection even when distinctive ash layers such as those from large catastrophic spills are not clearly visible. Here we characterize the initial magnetic mineral assemblages of coal ash from coals of the Appalachian, Illinois, and Powder River Basins, and lacustrine sediment recovered from reservoirs and recreational lakes adjacent to coal-fired power plants in North Carolina. Coal ash from all three coal sources is characterized by high magnetic susceptibility (>10^-6 to 10^-5 m³/kg), and pseudo-single domain to multidomain magnetite. Curie temperatures indicate a mixture of magnetite with near stoichiometric composition and magnetite with varying degrees of cation substitution. The lacustrine sediment cores show magnetic zones consisting of hematite and/or goethite-bearing pre-reservoir sediment, a transition to lacustrine sediment with elevated coal ash content, and recent sediment with low and variable coal ash content as determined by optical and electron microscopy. Several lakes display a surface zone, 10-20 cm thick, characterized by superparamagnetic particles and magnetic properties indicative of a stable single domain component whose signal decreases downcore. These signals may represent deposition of detrital magnetic nanoparticles, or a diagenetic signal representing neoformation and dissolution of iron oxides and/or iron sulfides. Identification of the carriers of these fine particle signals is in progress. The evolution of the magnetic mineral assemblage downcore can potentially track the mechanical breakdown and diagenetic alteration of coal ash ferrospheres in subaqueous environments, which can result in the release of harmful trace elements (e.g., As and Pb) previously bound as impurities in iron oxides.