Microplastic Sources, Abundance, and Transport Dynamics in Water and Sediments of a Representative Appalachian Headwater Basin

Microplastics (plastic particles < 5 mm in size) represent a contaminant of emerging concern on a global scale. The primary objectives of this 2-year study were to (1) further our understanding of the short-term spatiotemporal variations in the concentrations of MPs in both water and sediments of Richland Creek and similar headwater basins in the southern Appalachians, (2) investigate the abundance of MPs in caddisfly (Hydropsychidae) larvae and cases as well as the acute toxicity of MPs to caddisfly larva using laboratory exposure experiments, and (3) assess the source(s) of MPs to the streams, including atmospheric inputs. MP concentrations and characteristics were determined, in part, by analyzing 458 water samples collected primarily from 8 sites within the Richland Creek Basin and an additional 156 samples from two sites within the Cullasaja Watershed over a range of flow conditions including baseflows, stormflows, and the Hurricane Helene event. Mean MP concentrations in water were unexpectedly high for predominantly forested, lightly developed headwater basins, falling within the upper quartile of globally reported values (ranging from 0 to 70.21 MPs/L). Most particles were fibers (>90 %), black and blue in color, and predominantly composed of PET (15.9 %), styrenes (15.0%), PP (12.2%), and PA (9.4%). At a site, MP concentrations were highly variably, exceeding an order of magnitude, and were observed to be higher, lower, or exhibit no difference between base- and stormflows, depending on location and upstream basin characteristics. During an event, MP concentrations increased, decreased, or showed no pattern with increasing discharge depending on the site and event. Variations in concentration are attributed to temporal changes in rainfall-runoff characteristics and MP source and source inputs which included runoff from urban/suburban areas and the decomposition of larger plastic debris, among others. Similarities in the morphotype, color, and composition of MPs in water and atmospheric deposition samples suggest that atmospheric deposition is an important, if not the predominant, source of MPs in these headwater basins. Atmospheric deposition at three sites located across the region ranged from 0 to 670 MPs/m2/day. Atmospheric deposition during Hurricane Helene was 8.1 times, 3.4 times, and 22.7 times higher than pre-Helene rates at our three monitoring sites. Mean MP concentrations in channel bed sediments ranged between 500 and 1,000 MPs/kg at most sites along Richland Creek prior to Helene, indicating that they act as an important sink and secondary source of MPs that influence downstream particle transport and fate. Both caddisfly larva and casings contained MPs; mean concentrations ranged between 0 and 2.4 MPs/caddisfly and 0 to 3.79 MPs/case, respectively. Their occurrence in benthic biota suggests that MPs may be transferred to higher tropic organisms. Acute toxicity tests conducted on caddisflies indicate that microfibers were consistently the most toxic particles, followed by fragments and tire wear particles. Denser MPs posed less of a threat than more buoyant particles. Our data indicated that that the full range of variations in MP concentrations need to be determined to assess potential risks to biota, and the analysis of atmospheric deposition should be included in future studies of riverine ecosystems.