Project Description:

Microplastics (MPs) are a class of emerging contaminants of concern that result from the degradation products of plastic-based fibers, particles, and films. Transportation related pollution contributes notably to MP pollution in the environment, particularly in urban areas where pollutants such as anthropogenic debris and tire wear particles are exported to rivers and streams through urban stormwater and roadway runoff. Moreover, MPs are increasingly found in drinking water and drinking water sources, which may contribute to disproportionate impacts on communities of concern located near urban areas. Despite the abundance of MPs and their impacts to health and the environment, there are currently notable knowledge gaps related to MP fate and transport processes. Addressing these knowledge gaps is needed to support the design of approaches for mitigating transportation related MP pollution through improved process-based understanding. 

The focus of this project is specifically on the process of MP flocculation, or aggregation, with other MPs and/or other nearby materials occurring in the surface water environment. Figure 1 conceptually illustrates the transport of MP pollution from transportation related sources and through surface waters. MPs will settle to the sediment bed at different rates depending on their settling velocity, which can be notably impacted by the flocculation of MPs with other material (e.g., sediment, organic material) into larger composite particles known as “flocs”. The flocculation process can increase settling velocities and even cause otherwise buoyant MPs to settle. While the process of MP flocculation has been highlighted in recent studies, quantitative understanding of the impacts of environmental parameters (e.g., suspended sediment, salinity) on MP flocculation are lacking. In addition, the potential impacts of chemicals of concern, such as per- and polyfluoroalkyl substances (PFAS), on the MP flocculation process has not been investigated. Hydrophobicity and surface chemistry affect flocculation processes as MPs interact with nearby material and may be further impacted by PFAS sorption to MP surfaces. This project will experimentally examine the effects of environmental conditions PFAS on MP floc sizes and settling velocities by using an imaging-based MP floc measurement system. These results will provide new quantitative information about MP fate and transport that can be used in future modeling efforts and to inform design of pollution mitigation strategies.
 

_{Figure 1. The effects of microplastic (MP) flocculation on the fate and transport of transportation related MP pollution.}

US DOT Priorities: 

This project is an investment in fundamental research that advances our basic understanding of microplastics transport from transportation related pollution sources, through rivers and streams, and to potential accumulation zones. This work addresses critical needs for improved quantitative understanding of MP transport mechanics and supports future modeling efforts for MP fate. Settling velocity is a critical input parameter in these models and uncertainty concerning MP floc formation notably hinders current prediction accuracy. Through the impacts of this research on future modeling of MP pollution in the environment and of the effectiveness of mitigation strategies, this work supports the USDOT Strategic Goals for “Transformation” and “Climate and Sustainability.” Addressing MP pollution in surface water and stormwater is also important for community health since MPs are increasingly found in drinking water and drinking water sources. Since MP pollution tends to be highest near urban areas, addressing these issues is a matter of concern for equity since nearby communities of concern in densely populated urban areas may be disproportionately affected by MPs. The improved understanding of MP pollutant transport provided here will thus support the strategic goal for “Safety” and “Equity” by informing pollution mitigation strategies.

Outputs:

This project will result in new quantitative data that provides insight about fate and transport mechanics of MPs that are input to the environment via transportation related pollution. The datasets will quantify reference settling velocities for isolated MPs. The datasets will also quantify the effects of environmental conditions and the presence of PFAS on MP flocculation and settling velocity. Quantification of these effects is limited in the literature. This dataset will enable future modeling of MP transport with evidence-based consideration of processes and environmental factors that may drive MP settling and accumulation in the sediment beds of inland, estuarine, and marine waters. Additionally, a white paper will be produced to present correlations between PFAS adsorption and MP transport based on the study results.

Outcomes/Impacts:

By advancing the quantitative accuracy of MP transport modeling efforts, this work will support informed design of microplastic mitigation strategies that can preserve environmental resources and improve the sustainability of transportation infrastructure designs. Moreover, this work can enable evidence-based cost-benefit assessment of various pollution mitigation designs used to mitigate transportation related microplastic pollution. This type of MP transport modeling can be used to assess the risks associated with a continuation of “business as usual” practice and support future regulatory and policy decisions concerning the mitigation of MP pollution, particularly in urban environments. This project will also inform regulations focused on multiple environmental contaminants, namely MPs and PFAS, which are both emerging contaminants of concern that may affect both environmental and public health.