Plastic Pollution in Urban Rivers: Spatial and Temporal Patterns of Plastic Release and Transport

This dissertation investigates plastic release and transport in urban rivers with a focus on the driving spatial and temporal factors. Plastic is a key pollutant of this century that affects humans and the environment around the world. Rivers are a focal point of release and management, as they concentrate and accumulate plastics from urban watersheds and convey them to the world’s oceans. This research assessed cross-sectional, longitudinal, and seasonal patterns of plastics in rivers, and evaluated the role of urban pollution and watershed hydrology. A detailed background is provided in the literature review. This work is built on two case studies: an 18-month seasonal monitoring in the Hillsborough River in Florida, USA and a wet season field campaign in the Mekong floodplain in Cambodia, Southeast Asia. The first section explores the effects of flow conditions on cross-sectional distribution and transport of plastic during the wet season in Florida. This includes a detailed analysis of polymer characteristics and abundance linked to flow profiles as well as the determination of advective, vertical, and lateral transport fluxes of plastics. Advective fluxes were two orders of magnitude higher than lateral and vertical fluxes under calm flow conditions. With increased turbulence, enhanced particle exchange in the cross-section resulted in a three to tenfold increase in lateral and vertical plastic fluxes. Plastics are differently affected by turbulence depending on their properties: Large, irregular, and rougher particles tend to mix better than smaller, rounder and smoother ones. The second section evaluates the effects of urban pollution and rainfall seasonality on river plastic transport in the Hillsborough River. Annual plastic loads are highly uncertain, with median estimates at the river mouth of 3.33 billion particles (coefficient of variation 89.3%) or 2.04 tons of plastic (coefficient of variation 201%). My work provided clear evidence on the impact of urban pollution and seasonal (rainfall) dynamics through the year. A multi-variable statistical analysis was conducted to assess the role of plastics, flow components, and rainfall in the context of urbanization. Plastic loads are concentration-limited, whereas increases in rain, flow and runoff promote dilution and drive mobilization. This finding provides new insight into human and environmental factors governing urban river plastic pollution. The last section looks at the extremes of urban river plastic pollution. I present the findings of what may be the peaks of plastic release and transport in the large Mekong-Tonle Sap-Bassac river confluence in Phnom Penh, the capital of Cambodia. Driven by seasonal high flows, large amounts of plastic are released; an increase of plastic count loads by one and a half times and of mass loads by over 20 times was measured within the city boundaries. The plastic mass is mostly transported at the surface, directing plastics downstream towards the ocean as well as to Cambodia’s largest freshwater fisheries. A portion of the plastic load is mixed into the water column to be potentially retained in the rivers, where it could break down and/or be released over long periods of time. This chapter synthesizes the findings of an understudied area in Southeast Asia with a massive environmental impact on Cambodia’s largest water bodies and plastic pollution in the Mekong and the ocean. This research advances our understanding of spatial and temporal drivers of plastic pollution in urban rivers. It highlights and explains the impact of urban areas, watershed hydrology, and seasonal characteristics on plastic transport and distribution. With stormwater runoff as a main source of plastic in rivers, these findings emphasize the necessity to address plastic as a pollutant in stormwater management practices.