Plastics are one of the most widely used materials in the world. Degradation of plastic in the environment generates microplastics (MP), highly persistent polymer particles < 5 mm in size. It is estimated that 80% of MP entering the marine environment, up to 13 metric tonnes, originate from land-based sources. Studies of ocean surface MP have found much lower concentrations than expected given the estimated terrestrial inputs. Assessing MP accumulation in transitional environments—between freshwater and marine environments—is critical to accurately estimate the flux of MP entering the marine environment.
Recent research suggests that estuarine wetlands may be a significant reservoir for MP. There has been limited study of the ability of such wetlands to filter and remove MP from transport. Studies in estuarine environments in the eastern U.S. show that MP occur in similar or greater concentrations than those observed in the open ocean. MP pollution of estuarine environments is of concern as tidal wetlands provide essential habitats for many commercially and recreationally important species, including fish and shellfish. Assessing MP across environmental matrices is essential to determine the level of hazard these pollutants pose to aquatic organisms and, by extension, to human health through the consumption of seafood.
Estuaries receiving water from inland rivers draining densely populated, urban watersheds may serve as a reservoir for MP debris and act as potential export hotspots of MP pollution during flood/storm conditions. Our study site in the John Heinz National Wildlife Refuge (JHNWR), the largest urban refuge of its kind in the U.S., is located in southwest Philadelphia 20 km upstream of the Delaware River salt line. With a tidal range of almost 2 m, this wetland receives water and sediment inputs from the Delaware River, making this site an ideal location for investigating historic MP accumulation from riverine sources. We hypothesize that in this highly urbanized environment, influent river water and storm water outfall contribute significant MP to the estuary, which are in turn trapped in riparian vegetation and accumulated as benthic sediments that will record variations in mass and MP type with time.