Controls on Microplastics Accumulation in Stormwater Ponds

Abstract

Microplastics (MPs), or plastic particles that are less than 5 mm in diameter, are an emerging threat to aquatic and terrestrial ecosystems because of their potential toxicity and their resistance to degradation. In urban watersheds, stormwater runoff is a major carrier of MPs to downstream water bodies, which often drains into green infrastructure such as stormwater ponds (SWPs). The existing evidence indicates that SWPs may be effective at reducing the export loads of MPs from urban areas. However, the effectiveness of SWPs in retaining MPs and the controls on their accumulation in SWP remain understudied. Hence, it is significant to investigate MP occurrence and factors controlling MP distribution in SWPs. In this thesis, I aimed to (1) assess the variability in MP types, sizes, and abundances within SWP sediment and water samples, (2) determine the influence of sediment properties (e.g., organic carbon concentration, particle size) on the types, shapes and sizes of MPs accumulating in sediments versus within and between SWPs, and (3) investigate the impact of SWP characteristics on MP accumulation, including land use and land cover (LULC). I addressed these research objectives by collecting sediment and water samples from five SWPs with different LULC (industrial, residential, and commercial) in the City of Kitchener, Ontario, Canada, extracting MPs from environmental samples, and characterizing MP particles using laser direct infrared (LDIR) imaging spectroscopy. In Chapter 2, I extracted MPs from sediment cores in triplicates and determined MP counts and morphologies using the LDIR. I also analyzed sediment for grain size, mineralogy, and organic carbon (OC) content. The results revealed that the MPs accumulated in the sediments were predominantly fragments, with concentrations approximately 50 times higher than fibers, implying an important role of particle shape in controlling the accumulation of MP particles in SWP sediments. The highest fragment concentrations (2.3×108 particles kg dw-1) were found in the commercial SWP, while the highest fiber concentrations (4.5×106 particles kg dw-1) were found in the industrial SWP. Surface area-normalized MP accumulation rates in the forebays were generally 2-5 times higher than in the main basins. Sediment grain size and catchment impervious cover were significantly correlated with MP accumulation rates, with increasing MP concentrations observed with finer sediment grain size and higher catchment imperviousness. Polyamide and polyethylene were the two most abundant polymers found in the pond sediment, along with an overwhelming dominance of MP particles less than 50 µm. MP polymer composition and size distribution thus reflected the contribution of urban activities to MP pollution in a watershed. These findings indicate the important role of catchments’ land cover in the build-up and wash-off of sediments and MPs to downstream areas such as SWPs. In Chapter 3, I quantified and characterized MP shapes, types, and sizes in stormwater samples collected bi-monthly from 5 SWPs. In a one-year water sample collections, MPs appeared to fluctuate with significant seasonal variation throughout the year with the highest concentration recorded in a residential pond (up to 20,166 fragment L-1 and 559 fiber L-1). Polyamide and polyethylene accounted for approximately 80% of total MP in the pond water, while small-sized MPs make up 85% of the particles, highlighting the impact of catchment land use on MP occurrence in SWP. Precipitation, wind speed, and pond hydraulic loading were found to wash away surface MPs and dilute MP concentration in the water column. These findings demonstrate the diversity in MP profiles associated with climate factors, implying a need for long-term monitoring to address those spatial and temporal variability. The results from Chapters 2 and 3 overall provided an insight into MPs' preferential partitioning between two different environmental matrices, which can be applied in future research to assess the sources, transport, and fate of MPs in the freshwater ecosystem. Data from this research can be applied to generate MP accumulation rates across the Grand River watershed and eventually the Great Lakes. The outcomes from this study, moreover, actors influencing MP accumulation in urban catchments, and therefore, can support further studies on characterizing MP mass balance and budget for urban watersheds. Since SWPs are an effective indicator of local sources of MP pollution, understanding the MP from urban catchment can inform policymakers in a larger aquatic ecosystem to tailor management strategies accordingly. The research study presented in this thesis therefore contributes to the development of local policies and regulations, which not only address specific sources of MP pollution but also serve as models for larger-scale regulations aimed at protecting the freshwater environment.