Assessment of the Relation between Microplastics and Sediment Characteristics in the San Antonio River near Elmendorf, TX

Microplastic (MP) pollution is a growing concern as increased research efforts are identifying health impacts to ecosystems and a broader understanding of MP pollution level is emerging. The research herein seeks to develop knowledge about MP dispersion in the river environment by studying the relation between MP and sediment dispersion and comparing MP pollution between river depositional environments (i.e. riverbed, bank, and floodplain). In total, 20 bed, 24 bank, and 20 floodplain locations were sampled with a Ponar, Eckman dredge sampler or hand trowel. Dry sieving and hydrometer analysis were used to characterize sediment size characteristics for each sample. An existing MP extraction methodology was adapted to process sediment with high levels of organics and clay present. To start, a density separation using a lithium metatungstate solution was completed to remove MPs between 300 µm and 5 mm. Organics were dissolved by exposing samples to 30% H2O2 and heating at 75 °C until adequate dissolution was achieved. Remaining material was then analyzed for MPs under an Olympus SZX9 stereo microscope using previous established criteria for MP identification. MP did not increase with distance from the wetted channel and the concentrations were highest in the floodplain with a mean of 0.61 ± 0.08 (SE) MP g−1, which is four times greater than the 0.15 ± 0.03 (SE) MP g−1 value documented in bed material. Correlation analysis shows a statistically significant increase in MP concentration with decreasing sediment median diameters (D50). Research here increases the understanding of MP deposition within the river environment. Future research should focus on increasing knowledge of MP pollution in floodplains and develop more consistent methodology for MP research.