Fly ash as a proxy for anthropogenic industrial pollution in upper Sangamon River alluvium, central Illinois

Fly ash particles have been produced from coal burning in the central USA since approximately 1850, in power plants, stream locomotives, farm equipment, and household sources. Because of their distinctive spheroidal shape and ease of separation, magnetic fly ash can readily be used to identify to contact between pre- and post-settlement alluvium. Magnetic fly ash was extracted and microscopically identified ( approximately 150x) in vertically sampled alluvial sediments from 12 sites in the Upper Sangamon River Basin, Illinois. The proportion of spheroidal grains, in 10 to 60 mu m silt fractions, ranged from approximately 3-45% in post-settlement alluvium (PSA) and typically 0-2% in pre-settlement alluvium. Scanning electron microscope observations (1000x) confirm binocular microscope observations and particle compositions. Notable increases in proportions of magnetic fly ash enable delineation of PSA thickness, which ranges from 20-70 cm across the basin area. Spatial variations in PSA thickness are mainly a reflection of valley width (r (super 2) = 0.85), with thicker PSA in the main Sangamon Valley (50-70 cm) and thinner PSA in tributaries and upper valley reaches (20-40 cm). Thicker PSA corresponds to a lesser degree with relief in the watershed upstream of each site (r (super 2) = 0.47). At some sites, the proportion of fly ash decreases in the upper 20 cm of PSA, likely coincident with the mid-late 20th century conversion from coal-burning to diesel locomotives and the implementation of pollution controls in power plants. PSA average sedimentation rates (1- 5 mm/yr) were about 10x pre-settlement average rates ( approximately 0.2 - 0.5 mm/yr) due to accelerated erosion from agriculture and other development. Overall, the occurrence of magnetic fly ash in alluvial records is a relatively simple, inexpensive, and accurate method for identifying both initial anthropogenic industrialization (late 19th century) and post-industrial particulate pollution reductions.