The Fate of Microplastics in Rural Headwater Lake Catchments

In this study, the fluxes of microplastics (mp) were quantified during a 12-month period for three rural headwater lake catchments in Muskoka-Haliburton, south-central Ontario, Canada. A novel catchment particle balance approach was used, incorporating inputs from atmospheric deposition and stream inflows against lake outflow and sedimentation. This approach provides the first reported observation-based estimates of microplastic residence time in freshwater lakes. Atmospheric deposition had the highest daily microplastic flux (3.95–8.09 mp/m2/day), compared to the inflow streams (2.21–2.34 mp/m2/day), suggesting that it is the dominant source of microplastics to rural regions. Approximately 44–71% of the deposited microplastics were retained in the terrestrial catchments and 30–49% of the microplastics in the stream inflows were retained in the study lakes. Given that output fluxes ranged from 0.72–3.76 mp/m2/day in the sediment and 1.18–1.66 mp/m2/day in the lake outflows, the microplastic residence time was estimated to be between 3 and 12 years, suggesting that lakes are an important reservoir for microplastics. Fibers were the dominant shape in atmospheric deposition, streamwater, and lake water; however, in lake sediment, there was a higher proportion of fragments. Across all media, poly­(ethylene terephthalate) was the dominant polymer identified (23%).