MAGNETIC CHARACTERIZATION OF COAL ASH AND TRACE METAL CONCENTRATIONS IN RIVER SEDIMENTS CONTAMINATED BY THE KINGSTON SPILL

In December 2008, an unprecedented spill of coal ash occurred at the Kingston, Tennessee Fossil Plant. Despite dredging efforts by the Tennessee Valley Authority, approximately 229,000 m³ of ash remains in the rivers of the Watts Bar Reservoir system and is available for transport during high discharge events. Grab samples of bottom sediment were collected in 2010-11 adjacent to the spill and as far as 24 miles downstream. The morphology of coal ash is heterogeneous but several of its magnetic properties are remarkably consistent between samples. Point counting of magnetic separates and Curie point analysis indicate that anthropogenically formed magnetite resides primarily in black ash spheres. The magnetic grain size found in all samples is uniformly pseudo-single domain (PSD). Low field magnetic susceptibility (χ_LF) varies depending primarily on the magnetite content and to a lesser extent the natural hematite and maghemite contributed by background sediment. A bivariate plot of anhysteretic susceptibility (χ_ARM) versus χ_LF identifies two distinct populations of samples upstream and downstream of the spill. Overlap of these populations occurs at χ_LF < 1.0 x 10^-6 m³/kg and χ_ARM < 2.8 x 10^-6 kg/m³ supporting morphologic evidence that there are sources of ash in the Tennessee River that did not originate from the Kingston spill. The mixture of sediment and ash from the spill in a sample is defined by a mixing line (R²=0.85). The position of each sample on this line is proportional to its concentration of magnetic material.

Acid digestion of the samples yielded total trace metal concentrations. The concentrations of As and Se are positively correlated with χ_ARM, indicating that these metals are associated with ferrimagnetic minerals such as magnetite. However, Cd and Pb are independent of magnetic properties. Analysis of the magnetic properties of coal ash contaminated sediments can effectively be used to monitor changes in ash content and trace metal concentrations resulting from downstream transport.