Spatial and Seasonal Variations of Microplastic Concentrations in Portland's Freshwater Ecosystems

Abstract

Microplastics are a pollutant of growing concern and are ubiquitous in a variety of environmental compartments. The majority of microplastics research to date has been conducted in marine waters, and less is known regarding the sources and delivery pathways of microplastics in urban rivers. The first chapter is comprised of a review of the scientific literature regarding the spatial and temporal factors affecting global freshwater microplastic distributions and abundances. Microplastic spatial distributions are heavily influenced by anthropogenic factors, with higher concentrations reported in regions characterized by urban land cover, high population density, and wastewater treatment plant effluent. Temporal variables of influence include precipitation and stormwater runoff and water flow/discharge. Despite these overarching trends, variations in study results may be due to differing scales or contributing area delineations. In the second chapter, two watersheds in the Portland metropolitan area representing an urban-rural gradient were selected to assess microplastic concentrations and potential links with a variety of spatiotemporal factors (e.g., land use, arterial road length, water velocity, precipitation). Samples were collected from four sites in the Clackamas River watershed and from six sites in the Johnson Creek watershed, with one sampling event in the dry season, one in the early wet season, and a third in the mid-wet season. Samples were analyzed for total microplastic count and type, and nonparametric statistics were run to evaluate potential relationships with the explanatory variables, with spatial analyses conducted at both the subwatershed and nearstream scale. Microplastic concentrations in August (dry season) were significantly higher than in February (mid-wet season). August concentrations also negatively correlated with flow rate, suggesting that lower flow rates present in the dry season may have facilitated the accumulation of microplastics. Only one correlation was noted regarding antecedent precipitation amount and microplastics, and included a positive correlation between microplastic concentrations and 24-hour antecedent precipitation in February. Additionally, negative correlations were found between wet season microplastic concentrations and agricultural lands at the nearstream level. While additional research is needed, results indicate that the presence and abundance of microplastics in Portland's waterways may be more strongly influenced by nearstream variables as opposed to subwatershed-scale variables. Fragments were the most commonly observed microplastic morphology, with a dominance of gray particles and the polymer polyethylene. The findings of this research can be used to inform management decisions regarding microplastic waste and identify hotspots of microplastic pollution that may benefit from remediation.