IDENTIFICATION OF COAL ASH IN THE DAN RIVER USING MAGNETIC SUSCEPTIBILITY

A spill from the Duke Energy Plant at Eden, NC introduced a slurry of approximately 40,000 tons of coal ash into the Dan River in February 2014. This ash was transported some 70 miles downstream to the Kerr Reservoir in Virginia. Ash deposited within the riverbed is difficult to identify once it is mixed with the native sediment, including minerals and organic matter. At the TVA coal ash spill within the Watts Bar Reservoir system in Tennessee we successfully used magnetic susceptibility to quantify ash within the riverbed. The spill at Eden provides an opportunity to apply the method in another river system, with different ash composition and complex bedrock geology.

We collected bottom grabs and sediment cores at access points along the Dan River from the spill to Kerr Reservoir in VA. Mass-normalized low field magnetic susceptibility (X_LF) was measured on these samples as well as samples from the coal ash storage pond. The percent ash and mineral content of each sample was measured by point counting 300 grains with polarized light microscopy (PLM). X_LF is positively correlated with total ash. Samples containing >15% coal ash have X_LF between 4.66 x 10^-7 m³/kg and 3.40 x 10^-6 m³/kg (R² = 0.7926). Samples with the highest ash content were collected between Eden and Schoolfield Dam in Danville, VA, and X_LF predicts the ash content fairly well. Further downstream, lower concentrations of ash were sampled and mafic minerals eroded from diabase dikes within the Triassic basin add to X_LF of the background sediment.

Magnetic separation yielded 17% magnetic fraction (MF) in samples from the storage pond and 0.13 to 3.4% MF in river samples. Magnetic minerals in coal ash originate from burning pyrite within the coal. Using PLM and SEM we identified black, glassy aluminosilicate spheres coated with ferromagnetic magnetite and maghemite as the magnetic carrier. This compositional difference may be used to further identify coal ash from weakly magnetic watershed sediment in the Dan River Basin.