200-4 Spatiotemporal Variations in Microplastics Concentrations in Alluvial Deposits Prior to and Following Hurricane Helene, Pigeon River Basin, Western North Carolina

Session: Microplastics in the Environment

Presenting Author:

Authors:

Abstract:

Hurricane Helene produced a 1 in 1,000-year, 3-day rainfall event and extensive flooding between September 25-27, 2024 throughout much of the southern Appalachians of North Carolina, including the Pigeon River Basin. Although the impacts of flooding were dramatic and heartbreaking, the event provided a unique opportunity to examine spatiotemporal variations in microplastic (MP) transport and deposition during a rare, high-magnitude flood. Sediment samples, along with geomorphic and sedimentologic data, were collected from channel beds (many of which had been sampled prior to Helene) and Helene flood deposits at 15 locations along the Pigeon River and its tributaries shortly after the event. MP concentrations in channel bed sediment pre-Helene typically ranged from a few hundred to a few thousand MPs/kg, were dominated by fibers (>95 %), and consisted primarily of polyamides, polyethylene terephthalate, and polystyrene. Maximum MP concentrations reached ~7,950 MPs/kg along a depositional reach. The channel bed at most sites was extensively scoured during Helene, and MP concentrations decreased, such that they ranged from 0 to a few hundred MPs/kg post-Helene. Concentrations within the channel have not returned to pre-Helene values nearly a year after the event. Helene also produced extensive floodplain deposits, locally exceeding 100 m in width and 1-2 m in thickness. These deposits primarily consist of sandsheets and bars, but also include finer- and coarse-grained features. MPs within the deposits are dominated by fibers (~90 %) and, to a lesser degree, films, and are composed primarily of polyethylene, polypropylene, and polyethylene terephthalate. Overall, MP concentrations in Helene flood deposits range from ~60 to 900 MPs/kg, and are less than those observed in pre-Helene deposits. Reduced post-flood MP concentrations within the channel bed and floodplain deposits are potentially related to the mixing and dilution of pre-flood sediments with eroded, ‘clean’ sediments deposited along the valley floor prior to the 1950s. MP concentrations between sites and at-a-site were spatially variable. The observed variability cannot be universally explained by differences in sediment size or organic matter content. Rather, variations appear to reflect the complex interplay between geomorphic position (e.g., distance from the channel and sample depth), floodplain topography, floodplain land-use/land-cover (including the occurrence of local anthropogenic features), the degree of upstream floodplain erosion, and the nature of the depositional/erosional environment.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.