The Soil, Sand, and Sea: The Journey of Microplastics
As we approach the start of the UN Decade of Ocean Science for Sustainable Development in 2021, it is time we face our unseen but ubiquitous problem: microplastics. Chances are by now you have heard of the extensive plastic pollution in the ocean such as in the Great Pacific Garbage Patch, but what we are quickly finding is that the plastic bottles and bags you see floating in these patches are only the tip of the iceberg—ocean waters, freshwater, soil, and air are being infiltrated by tiny pieces of plastic that are difficult to detect. Microplastics are particles of plastic usually less than 5mm, and the concept was first introduced by a team of UK researchers in the 2004 article Lost at Sea: Where is All the Plastic? Since the production of plastic ramped up over the past 60 years, most of plastic is not ending up in landfills or ocean garbage patches, but “disappearing” as they break down into microplastics that we cannot see but remain ever-present. To understand what to do about this problem, we first need to understand from where microplastics predominantly originate, what happens to them as they move around ecosystems, and in turn what this microplastic pollution does to its environments.
From Sources to Different Sinks
In a literature review, researchers Jian Gong and Pei Xie identified several sources of microplastics that cause pollution in our land, atmosphere, and water. Microplastics can be categorized as primary or secondary microplastics; primary microplastics are particles designed and produced to be small, such as in cosmetics or resin particles used as industrial raw materials (the microbeads found in face scrubs and other cosmetics are now banned in the US by the Microbead-Free Waters Act of 2015, though New Zealand researchers found them in locally-available facial cleansers). Secondary microplastics began their life as bigger pieces of plastic but became microplastic-sized through the physical, chemical, and biological breakdown of large plastic waste.
Most of the microplastics we hear about or study are the freshwater and marine microplastics. Despite the sewage treatment process, wastewater can contain microplastics and be a major source of microplastic pollution in water. Personal care products and other rinse-off products containing plastic wash into sewage treatment systems where most of the microplastics get removed by settling into the sludge. However, the remaining particles that make it through can flush around 65 million microplastic particles into water every day.
While the sludge from wastewater treatment captures around 90% of the microplastics in sewage, the microplastics do not stay there. Sludge gets applied to agricultural soils, and sludge treatment methods do not remove these plastic particles. Sludge farming adds more microplastics to our land annual than the amount of microplastic enters the ocean every year. In addition to sludge, many farms make extensive use of plastic films when growing crops—a practice known as plasticulture. These plastic films are usually not recovered and remain in the soil, degrading and breaking down, becoming a primary source of soil microplastic pollution.
Synthetic textiles and fibers are constantly shedding little bits of plastic—especially during laundry—and these light plastic fibers can get whipped up by a breeze and become part of our ambient air and even the atmosphere. These plastic fibers, along with ultrafine particles generated by car tire wear and industrial production, create an “urban dust” that we and other organisms can inhale.
The Pollution in Microplastics Pollution
We can safely say microplastics are everywhere now— in the air we breathe, the water we drink, the soil we farm on, and even in organisms. To add to the problem, once these microplastics find their way into these environments, they are incredibly difficult to remove. Separation of microplastics from soil, much like most soil pollution, is incredibly difficult to do. In water, microplastics get swept up by currents and particles with higher densities sink and accumulate in marine sediments. Much like the unpredictable travel of microplastics on ocean currents, airborne microplastics floating around with the winds and air currents are incredibly hard to track. If these microplastics did not interact with the environment or affect organisms’ health, their ubiquitous existence would be no problem. However, because of the properties of these small plastic particles with large surface areas, organisms and pollutants can readily attach to the plastic and get transported long distances with them. Certain types of microplastics allow for easy adsorption of organic pollutants; for example, polypropylene and polyethylene, two common materials used in a variety of products (like in binders and storage bins), can adsorb the harmful compounds polycyclic aromatic hydrocarbons and polychlorinated biphenyls. In addition, the plasticizers and additives used in the plastic-making process can be pollutants themselves and harm organisms, such as by disrupting their endocrine systems or directly poisoning them. As organisms inadvertently eat microplastics, some of the particles can accumulate in their tissue, and the microplastics can bioaccumulate up the food chain, creating dangerously high levels of these pollutants in organisms high in the food chain, like us.
A Brave New (Micro-) World
Though we know of the existence of microplastics and a little about where they go and what they do, much has yet to be investigated to assess the extent of harm these little particles do. While lab studies on the effects of microplastics on human cells and tissues are promising, they do not show the long-term impact on actual humans. Most of the attention on microplastics is on the ocean, where it forms a “plastic soup” of seawater, organisms, and plastic. However, microplastics are just as persistent in the soil and can be toxic to the plants we grow, and we know even less about the airborne microplastics that we may be breathing in or are circulating the world on air currents and getting deposited in remote places. Much like climate change, microplastics pollution is a problem without borders and requires international cooperation to address, and the first step to addressing that is getting a better picture of what these particles are doing and what they are affecting. Immense progress has been made, but informed action requires even more research across many disciplines. In millennia to come, future geologists will find plastic particles in the layers of rock—our human contribution to the sedimentary record—and the least we can do now is more fully understand where this plastic is ending up, how it affects us and the environment, and how to prevent further pollution.
Video showing modeled microplastic movement in oceans, from research by Victor Onink, David Wichmann, Philippe Delandmeter and Erik van Sebille (2019). The role of Ekman currents, geostrophy and Stokes drift in the accumulation of floating microplastic. Journal of Geographical Research: Oceans, 124. https://doi.org/10.1029/2018JC014547
Reference: Gong, Jian, and Pei Xie. 2020. “Research Progress in Sources, Analytical Methods, Eco-Environmental Effects, and Control Measures of Microplastics.” Chemosphere 254: 126790. https://doi.org/10.1016/j.chemosphere.2020.126790.
