Microplastic Contamination of Urban Streams in the Auckland Region, New Zealand

Abstract

Microplastic pollution of aquatic systems is a global environmental concern. Most research has focused on marine systems, but rivers and streams are the main pathways for landbased plastic debris to be delivered into the oceans. Streams located in highly populated areas are particularly vulnerable because all plastic use and disposal is solely associated with human activities. This study examines the level of contamination and potential drivers of variation in the amount and type of microplastics in urban streams in the Auckland region. Eighteen streams along the population density gradient and varying storm water infrastructure (e.g. sanitary vs combined storm water overflow sewers) were tested. Microplastic abundance, size and type were determined for the water column and sediments at 21 sampling sites spanning gradients in catchment land cover (e.g. residential, rural, industrial and native). Laboratory analyses included elutriation of sediments, density separation with Sodium Iodide (NaI), digestion of organic material with peroxide and sulphuric acid, microscopic and spectroscopic analyses. Maximum amount of microplastics was found in streams with relatively high population densities in their catchments and accounted for 303 particles m-3 in the water column and 80 particles kg-1 dry sediment. However, microplastic was found even in streams draining catchments with minimal human land use. The most abundant types were fragments and small particles (63-500μm). Poly(hexadecyl) methacrylate, polyethylene, ethylene/ethyl acrylate copolymer and ethylene/vinyl acetate were the most common polymers found in Auckland streams. Population density and type of storm water infrastructure were weak predictors of microplastic concentrations in urban streams. Among all examined land use types, residential land cover displayed the strongest positive relationship with microplastic abundances in streams. Sediment grain size and downstream transport were found to be potential drivers of spatial variations in microplastic concentrations in sediments. Point-source pollution associated with human activities such as careless litter dumping directly into the stream or on its banks, proximity to construction sites and roadworks, are suggested to be important factors contributing to contamination of freshwater systems with microplastic. The study also highlights the problem of laboratory contamination. Lastly, the results indicate that the freshwater biota ingest small plastics particles.