Understanding Microplastics in Sediments

In the absence of regulatory standards, how can the risk of microplastics in dredged sediment best be managed? 

Microplastic (MP) Sources

The accumulation of plastic pollution in the environment has rapidly increased in recent decades. Microplastics (MP) are very small pieces of plastic (typically considered to be 1 nanometer to 5 millimeters) that come from a variety of sources. MP sources can be classified into two groups: primary and secondary. Primary MPs are intentionally created within that small size range – such as exfoliating beads in cosmetics, glitter, or plastic nurdles (tiny plastic pieces that are the building blocks of plastic products). Secondary MP are formed when larger plastic objects degrade into smaller and smaller pieces. Some common sources of secondary MP include synthetic clothing fibers, tire wear particles, micro-films from plastic bags and micro-fragments from packaging materials. The widespread use of plastics in packaging, consumer products and industry has resulted in the spread and accumulation of plastic throughout nearly all corners of the ecosystem.

Transport Pathways in Surface Water and Sediment

Starting at source areas such as landfills or littered urban areas, both anthropogenic and natural mechanisms such as vehicles, wind and rain can transport plastics into the wider environment.  Plastics—either in the form of larger macroplastic objects or as MP—can then be swept into a nearby river and transported to a downstream lake, estuary, or ocean.  Alternatively, these particles can be deposited into the sediment. However, MP in sediment can also later be resuspended into the surface water from currents or bioturbation, making surface water flow to downgradient receptors a key pathway for MP transport. If the MP continue on through surface water transport, they can move through the water as bed load, suspended load, or floating in the stream current.

MP Degradation Mechanisms

Along these flow paths, macroplastic objects undergo fragmentation and degradation, breaking down into smaller sizes down to the macro- or even nanoscale. Common natural processes that cause this breakdown include abrasion, freeze/thaw, ultraviolet radiation, biofouling (breakdown by algae), or ingestion by organisms. The fragmentation can create MP in a variety of irregular shapes, ranging from long thin fibers to small, jagged fragments.

MP density and shape strongly affect environmental distribution. Because there are many different polymer types, differences in size, shape and density in MP makes it difficult to predict their fate and transport in the ecosystem. Lighter plastics (e.g., films) tend to float in the water column, while the denser plastics (e.g., tire wear particles) tend to settle into sediment. Studies have also found that MP are more concentrated in clay-dominated as opposed to sand-dominated sediments and in sediments with higher total organic carbon content (Environmental distribution, fate, and transport – Microplastics (itrcweb.org)). Improving our understanding of how MP move throughout the environment and how they degrade during their lifetime in the ecosystem are essential components in assessing the risks posed by MP in sediment.

USACE Found MP in All Federal Navigable Waterways Sampled

The United States Army Corps of Engineers (USACE) led an investigation into the occurrence, abundance and polymer composition of MP in sediments collected from nine dredged waterways and two non-dredged reference areas.  The locations were spread out throughout the country, ranging from the Great Lakes, the East Coast, the Mississippi River and the Gulf Coast and included both freshwater and marine sites. MP were detected in all sediments examined, with concentrations averaging 1,611 particles per kilogram. Five MP shapes were detected; the most common shape was the fragment group, which occurred in 100% of the samples, followed by fibers, spheres, foams and films.

USACE’s Upcoming Dredging Guidance

In response to their discoveries in the study outlined above, in 2023 USACE is developing deliverables with the goal of addressing the uncertainty presented by the ubiquitous MP observed during dredging operations with the lack of any regulatory standards. These 2023 deliverables were proposed in the USACE fact sheet “Guidance for Communicating Risks of Microplastics and Nanoplastics in Dredged Sediments” and will include:

1. A literature review of MP occurrence and abundance relevant to USACE dredging operations
2. A database to store MP information on fate, effects, and risks
3. A MP decision support framework to help dredging operation managers communicate with stakeholders

With the lack of any state or federal regulatory standards regarding surface water or sediment, these resources will fill a much-needed gap by providing direction to best mitigate MP risk.

What Can be Done Upstream?

The best way to reduce the spread of MP in the environment is to start at the source: think of the old adage of turning off the faucet before cleaning up the spill. The furthest upstream solutions begin with actions such as substituting plastic materials with more sustainable alternatives and reducing the consumption of plastics through education and awareness. Slightly further downstream, solutions can include actions such as improving MP removal rates from wastewater discharges or installing structures in waterways to remove macroplastic debris.  Further downstream once MP are released into the environment, they are much more difficult to control.  However, large amounts of research and federal funding are being invested into mitigation and remediation strategies to reduce MP concentrations in water, soil, sediment and air.

TRC is Engaged in Finding Solutions

TRC is actively tracking and researching engineering technologies to overcome the obstacles posed by MP management during dredging, dewatering and treatment of impacted sediments.  Watch for an upcoming blog that will provide a summary of both established and exciting emerging remedial solutions to reduce and even clean up MP from our waterways and sediment. We are prepared to assist you with your microplastics-related questions. For more information, please contact our experts below.