The journey of microplastics: from land to sea

Microplastics (plastic particles <5 mm in size) are now recognised as a widespread environmental contaminant. They can be found in locations worldwide, including those with little human influence such as remote islands and even Antarctica, but how are they getting there?

In addition to being introduced to the environment as microplastics, for example via cosmetics or plastic pre-production pellets (‘nurdles’), they can also be formed from large litter items which fragment into smaller and smaller pieces over time. All plastics items will ultimately break down over time as the plasticiser chemicals that give them their flexible and durable nature leach out, leaving them brittle and susceptible to degradation. However, their very limited or non-existent biodegradability means that even if these items form small particles, these particles may not ever disappear.

Microplastics can be transported throughout the environment by environmental processes such as water currents or wind, or by human activities. While plastic may enter the environment to land or water in many places due to human activity, it is likely that many of the microplastic transport routes will lead them to travel from their source to ultimately reach the oceans in the following ways:

  • Due to surface runoff (e.g. from roads/agricultural land) into rivers which transport these particles to the sea
  • Direct littering to rivers where these items fragment or the whole piece is transported to the sea
  • Direct input as microplastics into rivers (i.e. from sewage effluent)
  • Direct input to the oceans from land by wind action
  • Littering on beaches
  • Maritime activities, especially fishing activities which can lead to the loss of fishing gear such as netting and lines
  • Industrial shipping, which can lead to loss of litter or cargo overboard
  • Dumping at sea

However, not all microplastics on land or in freshwater systems will reach the oceans. Many studies have found plastics within freshwater environments, including sediments, and it is possible that sedimentary and hydrological processes (for example sediment deposition or low water flows) will allow for the retention of significant portions of these particles within the sediment. This may especially be the case in lakes where there is little to no flow.

There have been very few studies reporting on microplastic presence within soils, however given the prevalence of microplastics across aquatic environments, it is highly likely that microplastics will also be present on land. These would have similar potential to become incorporated into soils and retained long-term, with as-yet unknown consequences.

Plastic litter in an agricultural environment
Plastic litter in an agricultural area. Photo credit: Alex Walton

Alice Horton

Alice Horton is an ecotoxicologist at the Centre for Ecology & Hydrology.