A National-Scale Framework for Visualizing Riverine Concentrations of Microplastics Released from Municipal...

Down-the-drain exposure models provide a valuable tool for estimating environmental exposure to substances which are treated and discharged by municipal wastewater-treatment plants (WWTPs). Microplastics may enter WWTPs from consumer activities and disposal. An exposure framework was developed using the iSTREEM® model, which estimates spatially explicit concentrations of substances in riverine systems across the United States and portions of Ontario, Canada. One hundred simulations covering a range of WWTP removal and instream loss rates (proxy for net sedimentation) were incorporated into a Web-based visualization tool for user exploration of relative concentrations across simulations. Surface water concentrations specific to user-supplied tonnage were examined via interactive heat maps and cumulative distributions. Exploring the spatial aspect of iSTREEM results showed that modeling 90% WWTP removal and no instream loss resulted in 8.5% of the mass entering WWTPs discharged to marine estuaries (7.4%) or Great Lakes (1.1%) environments, with the remainder of the mass discharged (1.5%) in inland sinks or exiting the United States via rivers. Modeling an example instream loss of k = 0.1 d^-1 (i.e., half-life = 7 d), terminal river segments contained 3.3% of influent mass (2.3% marine estuaries, 1.0% Great Lakes). Varying instream loss rates had substantial impacts on the total mass exported. The Web-based tool provided a user-based mechanism to visualize relative freshwater concentrations of microplastics across a large geographic area by varying simplified particle fate assumptions. Environ Toxicol Chem 2019;39:210-219. © 2019 SETAC.