Fishing for plastic in Florida: Field monitoring of plastic loads in an urban river

Urbanization and population density have a direct impact of plastic pollution in river systems, yet there is a lack of data and knowledge on sources, transport, and fate of plastics from urban catchments. The objective of this study is to investigate tempo-spatial variations in plastic loads through an urban river system with the ultimate goal to enhance the development of mitigation and prevention strategies. Monthly sampling is conducted along the Hillsborough River, an excellent case study as it meanders through the 2^nd largest city of Florida (Tampa), before discharging in Tampa Bay and the Gulf of Mexico. Data collection began in May 2018 at three strategic sites, representing distinct degrees of flow accumulation and increasing urbanization. Stationary sampling is conducted using a neuston net with 500 μm mesh size, collecting three samples at the surface, one in the water column, and one at the bottom of the river. An Acoustic Doppler Current Profiler (ADCP) is used to create a corresponding flow and discharge profiles, and combined with plastic samples to assess plastic loads moving through the river. Samples are processed in the laboratory and potential plastics detected by visual and microscopic examination. Fourier-transform infrared (FTIR) spectroscopy analysis confirms and identifies polymers. Plastics are categorized and quantified by mass, count, size, and polymer type. Six months of data of the ongoing collection and analysis are available. Sizes range typically between 500 μm and 10 cm. Plastic concentrations vary between 0 and 2 counts/m3, with a majority found at the surface. In high flow conditions, sediment resuspension from the riverbed shows high plastic loads as well. Loads are noticeable increasing as the river passes through the city, particularly small sizes below 1 mm. This research contributes to the understanding of sources, transport mechanisms, and seasonal and timely variation of plastic pollution, particularly in the Hillsborough river watershed but also for rivers in general. This is one of the first studies conducted in a middle size river looking at changes and variation of loads over length and water column and identifying the direct impact of a city; it will serve to close some of the current data gaps and provide input for the calibration of a transport and fate model.