VARIABILITY OF COAL ASH MORPHOLOGY IN RESERVOIR SEDIMENT USING LIGHT MICROSCOPY

Gravity cores were collected from reservoirs adjacent to Roxboro and Riverbend Steam plants in North Carolina to investigate the occurrence of coal ash. Coal ash is known to be enriched in trace metals and thus potentially has negative environmental impacts. Permanent slides were made from sediment at selected depths within different cores. A polarizing light microscope at 500x magnification and a mounted point counter were used to manually point count 300 grains within each slide. The counting enabled us to identify coal ash particles, sand-size mineral grains, clay particles, and biological matter, and to obtain the relative percent of each within the sediment record. Morphologies of coal ash documented within the reservoirs are diverse and comparable with raw fly ash sourced from Appalachian Basin coals. These generally include both spherical and amorphous forms. Spherical ash ranges in diameter from approximately 4 to 92 microns and includes, clear, black, orange, and plerospheres. Amorphous ash was larger and irregular in shape, consisting of both opaque and clear forms. Principle Component Analysis (PCA) was performed on the large multivariate data set composed of the six ash morphologies identified from different core depths and locations within the two reservoirs. PCA of the counts showed systematic changes at different depths in cores and with transport distance. The ash morphologies of clear spherical, clear amorphous, and opaque amorphous account for the systemic variability of coal ash particles with depth in collected cores. Coal ash identified from deeper sections in the cores shows higher loadings of clear and opaque amorphous particles, relative to shallower depths, where clear spherical ash has a higher loading. The ash morphologies of black spherical, clear spherical, and plerospheres account for the systemic variability of transportation distance from near the Roxboro plant ash storage pond downstream to the dam. Changes in transportation distance from source can be observed in the change from high loadings of black spherical and plerospheres to high loadings of clear spherical ash as distance increases over approximately five miles for this reservoir. Light microscopy can be used to quantify the abundance and changes in morphology of fly ash dispersed within fine grained reservoir sediments.