Microplastics and polycyclic aromatic hydrocarbons in sediments of Nash Run, urban tributary to the Anacostia River in Washington, DC: Abundance, distribution, and chemical analysis

Microplastics (small plastic particles <5mm) are an emerging pollutant of concern as they are ubiquitous in the environment and are found in the water and sediments of aquatic habitats. Plastic contains complex mixtures of additives which are weakly bound to the core polymer of the plastic and will leach from the plastic over time in the environment. Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants which tend to accumulate in aquatic sediments and are the product of the combustion of fossil fuels. PAHs have been found to adsorb to plastic surfaces as well as bioaccumulate in organisms. Therefore, sediment composition, geochemistry, water quality, and fauna may be affected by microplastics and PAHs, as the sediments serve as a sink for these pollutants. Identifying microplastic sources, fates, and interactions with the environment is imperative in understanding the consequences and management of microplastic pollution. Nash Run, a small tributary in a high-density urban neighborhood in Washington, D.C acts as a model to show how land based-pollution enters the Anacostia River, and subsequently the Chesapeake Bay. This study aims to determine the concentrations and distributions of microplastics and three PAHs in Nash Run sediment samples across eight transects over three seasons. Additive and polymer analysis from microplastic particles found in the samples was performed with Thermal Desorption and Pyrolysis GC/MS. The total microplastic concentrations in Nash Run ranged from 352 MP/Kg (0.35 MP/g) to 4090 MP/Kg (4.1 MP/g) and averaged 1187 ± 392 MP/Kg dried sediment. Microplastic concentrations in the <250 µm size class were higher upstream, closer to urban activity, while the total microplastic concentrations showed no trend. TD/Py revealed additives and contaminants that were adsorbed to the microplastics of in Nash Run. Microbeads were found to be a mixture of both glass and polymers, which may lead to a new and understudied source of microplastics. PAHs were found in all sediment samples and showed no spatial or temporal tends. This study provides useful information for future research in the Anacostia Watershed and add to the existing data of microplastics in the Chesapeake Bay.