Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Cizdziel J., Scircle A.

Microplastic (MP) concentrations along the northern Gulf of Mexico are among the highest levels reported globally. The most likely source of the plastic pollution is the Mississippi River (MR) which drains much of the central portion of the USA. Yet, surprisingly little is known about the concentrations, types, sizes, and loadings of MPs in the MR and its major tributaries. This lack of data is hindering our understanding of the magnitude and sources of the problem. Because the MR is an intricate system of waterways, tributaries, and commercial routes, an in-depth spatial study is needed to fully assess MP pollution in the system. Our research aims to systematically quantify the concentrations and loads of MPs in the MR system, and characterize their shapes, size distribution, and chemical composition in the MR system - a source of drinking water to over 18 million people. We used Nile Red dye to stain the MPs and fluorescence microscopy to count them, as well as vibrational spectroscopy to the identity the plastics. The morphology of the MPs was dominated by fibers (~75%), followed by fragments (~23%) and beads (~2%), with the proportion of fragments increasing slightly moving down the river. The concentration of MPs is relatively low for smaller tributary rivers (Tennessee and Yazoo) and higher in larger tributaries rivers (Ohio and Missouri), with the later having higher concentrations than the MR itself. Counts and loads of MPs generally increased down the main stem of the MR until past New Orleans, where loads declined, possibly due to deposition with slowing water. Sites near population centers (e.g. Memphis) had higher MP concentrations. Overall, this work is an important first step to assess possible relations between MPs levels and characteristics with sources and different watershed attributes.