Transport Processes along Riverine Environments and their Impact on Microplastics

Microplastics (MP), small plastic particles less than 5 millimeters in diameter, are persistent environmental contaminants originating from the breakdown of larger plastic waste or their use in products like cosmetics and industrial processes. Found in diverse ecosystems such as oceans, rivers, and soils, these particles resist biodegradation and are readily ingested by wildlife, posing potential ecological and health risks. In river systems, MP act as markers of local anthropogenic activities, as these waterways serve as conduits between land and sea. This dissertation introduces a comprehensive model for predicting the fate and transport of MP in river networks, integrating hydrodynamic principles, environmental factors, and MP properties. The first part proposes an advection-dispersion (ADE) model tailored for large-scale MP transport in river networks, incorporating river dynamics and the impact of human activities. Utilizing open source MERIT-Hydro network database and estimating MP load based on publicly available anthropogenic information (such as population density and solid waste generation), the developed model offers a simple yet comprehensive approach to understand the fate of this important emerging contaminant when in-stream removal processes (i.e. sedimentation and burial into the stream bed) balance the resuspension. The transport scenario analyzed in the first part focuses on floating MP. The second part extends the advection-dispersion model by exploring a different scenario in which the complex dynamics of sedimentation, burial, resuspension, and bank removal processes affect the MP fate. To account for all these processes and their dynamics along the river network, the advection-dispersion-reaction equation (ADRE) is solved by characterizing the different processes by means of proper removal/production rates according to the different hydromorphological conditions of the river reaches. The model's efficacy for the different scenarios (i.e. solved using ADE or ADRE) were tested by using available literature data from different river networks around the world. Among the different processes that may affect the MP fate, those that control the burial and resuspension are the subject of current research since they are controlled (under some hydrodynamics conditions) by the coupled effect of turnover (induced by bed form migration) and pumping (induced by the near-bed pressure gradient). With the aim to better characterize these processes, the third part of the thesis focuses on the local reach scale where an analytical solution is proposed to study the effects of turnover and pumping in sand-bedded streams characterized by two-dimensional bed forms (i.e. dunes). In particular, the developed solution allows to characterize and quantify the penetration depth and the release (i.e. the resuspension process) of MP due to the trapping-releases successions as hyporheic pathways connecting the downwelling (i.e. the stream bed entering locations) and upwelling zones (i.e. the stream bed exiting locations) change in time. A key outcome of this research is the development of predictive tools for assessing MP pollution in river systems, significantly advancing MP research. Notably, the model facilitates the assessment of pollution using the Pollution Load Index (PLI), offering a rapid method for evaluating environmental health. This work provides a novel perspective on the environmental fate and transport mechanisms of MP in freshwater ecosystems, contributing valuable insights to the field. Finally, the proposed methodology is flexible enough to be adapted to other emerging contaminants (i.e. removal and production of guanylurea).