Magnetic susceptibility as a proxy for coal ash pollution within riverbed sediments in a watershed with complex geology (southeastern USA)

Abstract

A study of near surface sediments from the Dan River (southeastern USA) was conducted to assess the use of magnetic properties as proxies of coal ash after a recent spill. The watershed geology is diverse and potentially contributes magnetic minerals to riverbed sediment from diabase dikes in the Dan River Triassic Basin and from granitic gneiss outside the basin. Coal ash is heterogeneous, including aluminosilicate spheres, amorphous particles and carbonaceous rods and lacy particles. The magnetic fraction of ash from the failed storage pond is up to 17 wt% and is mostly composed of black spheres with maghemite and magnetite. Ash was detected in riverbed sediment from quiet water settings such as inside of meander bends, the confluence of tributary streams and near islands between the spill site and 20 miles downstream in the Schoolfield Reservoir, Danville, VA. The strong magnetic signal is detected above background in riverbed samples and is strongly positively correlated with total ash; elevated low field magnetic susceptibility (χ LF) is evident in samples with ≥ 12% ash content. Anhysteretic remanent magnetization and hysteresis parameters delineate native sediment, ash-bearing sediment, and diabase dikes. Between 20 and 70 miles downstream of the spill site, ash concentrations were either buried or too low due to dilution with native sediment to be detected with χ LF in riverbed samples. Cowan, E.A., Epperson, E.E., Seramur, K.C. et al. Magnetic susceptibility as a proxy for coal ash pollution within riverbed sediments in a watershed with complex geology (southeastern USA).

Evaluation and Integration of Geochemical Indicators for Detecting Trace Levels of Coal Fly Ash in Soils

“…Compared to the Lake Norman soil samples, we find systematically higher CAI values in 21 out of the 24 soil samples from Claxton relative to the local background soil collected from the Haw Ridge Park (Figure S1b), with estimated coal ash percent up to 20% (Figure b; Table S5). To further assess the effectiveness of the CAI method, we employed optical point counting to physically identify and quantify fly ash within the soil samples, based on the fact that coal fly ash particles have distinctive spherical morphologies relative to typical mineral grains in soil and sediments (e.g., quartz, calcite, feldspar, and clay minerals). − The counting results confirm our hypothesis that the background soil samples from both Lake Norman and Claxton contain zero coal fly ash, even though the CAI values could suggest ∼4.0 and ∼2.1% of fly ash present, respectively (Tables S4 and S5). Besides the local background soils, seven more Lake Norman soil samples and 13 more Claxton soil samples were selected for optical point counting mostly due to their relatively high estimated ash percent by the CAI method (Tables S4 and S5).…”

“…In this study, we aim to explore the utility of multiple geochemical methods, including trace elements, Ra isotopes, and Pb stable isotopes, as indicators for the presence of trace levels of coal fly ash particles in soils near coal-fired power plants and coal ash disposal sites. We evaluate the sensitivity of these methods by integrating observation and quantification of fly ash particles in soils, using point counting under polarized light microscope (PLM). ,− While some of these methods have been used individually, here, we present the first integration of geochemical methods for the purpose of tracking even trace levels of coal fly ash contamination in the environment. By integrating multiple geochemical tools and microscopic physical observation to investigate surface soil samples collected from areas adjacent to coal-fired power plants in North Carolina (NC) and Tennessee (TN), we demonstrate both their applicability and limitations for the detection of trace levels of coal fly ash presence in the environment.…”

Legacy of Coal Combustion: Widespread Contamination of Lake Sediments and Implications for Chronic Risks to Aquatic Ecosystems

“…Similarly, positive correlations between the coal ash percent and magnetic susceptibility were also observed for surface sediments and sediment cores from the other lakes (SI Dataset S2 and Figure S8). However, watershed and bedrock geology with high rock magnetism (e.g., diabase dikes) could contribute naturally magnetic minerals to lake sediments that may render the magnetic signal of coal ash less sensitive, 11 particularly when the coal ash content is relatively low.…”

“…Micrometer-sized coal ash particles contain elevated concentrations of hazardous metals and metalloids (e.g., Pb, Cr, Cd, Hg, As, Se, and Mo) that are easily leached into aquatic systems. For this reason, the release of coal ash to the environment, whether intentional or incidental, has raised serious public concerns about the potential risks posed to human and ecological health. − Although coal is being replaced by natural gas for power generation, environmental challenges posed by the accumulation of coal ash and its inadequate disposal over decades of coal combustion continue to persist…”

“…For example, previous studies have shown that sediments in Lake Erie (USA), Lake Wabamun and Grand Lake (Canada), , and Lake Macquarie (Australia) contained elevated levels of As, Se, Co, Tl, Sb, Cd, etc., most likely derived from nearby coal ash disposal sites. The southeastern U.S., in particular, has witnessed several catastrophic failures of disposal units resulting in large-scale coal ash releases into aquatic systems in 2008 and 2014 − ,,, and a spill triggered by hurricane flooding in 2018 . Systematic monitoring of coal ash impacts was seriously lacking until the first national regulations on safe disposal of coal ash were finalized by the U.S. Environmental Protection Agency (U.S. EPA) in 2015. , While this “Coal Ash Rule” requires monitoring of water resources specifically, the influences of coal ash particles on the quality of aquatic sediment and the potential ecological risks have been largely overlooked.…”