A Review of Microplastics in Freshwater Environments: Locations, Methods, and Pollution Loads

Investigations into microplastics in water environments have clearly established the worldwide presence of these particle pollutants. Microplastics can enter land and water environments as tiny plastic pieces or can be created from the fragmentation of larger plastics. Although most of the published studies on microplastics have been carried out in marine environments, an increasing number of freshwater studies have been reported in the past several years, and most are cited in this chapter. Because many freshwater bodies are sources of drinking water, knowledge of the types and quantities of microplastics is imperative. Sources of microplastics into the marine and freshwater environments are similar, but freshwaters experience the constant influx of these nonbiodegradable microparticles in much lower water volumes. Freshwater bodies are usually closer to the sources of microplastics, such as wastewater treatment and industrial plants, which contribute heavier loads to the water and sediment. Research has established the most abundant microplastic is synthetic microfiber in both marine and freshwaters. These are tiny fibers that are released from textiles, carpeting, and many other products made from polyester, nylon, and other synthetic materials. From this literature review, it is clear that microplastics are more completely retrieved and quantified when a combination of the two major sampling techniques is employed. When water is directly sampled in containers and filtered with small-pore filter disks, greater loads of microplastics are reported because most are smaller than the typical net pore size. However, many larger microplastics are not captured using direct bottle sampling (grab samples) because of the heterogeneous distribution of microplastics. When large volumes of water are sampled using plankton nets, the larger, less concentrated microplastics are more effectively collected. Therefore, this review focuses on sampling methods, which are a critical component in the quantification of microplastics in water environments. This chapter also offers a geographical perspective on the published studies and reported quantities of microplastics in freshwater around the globe. Data from freshwater sediment is presented from the studies that sampled both water and sediment, and a few additional sediment investigations are reviewed. Several studies show heavier loads of microplastics in sediments near urban and industrial areas. Similar to marine studies, the most common processing steps, performed to exclude natural constituents of samples, are density separations and oxidative reactions, the latter mostly promoted by hydrogen peroxide. Finally, analysis techniques used in microplastic studies and mitigation methods are briefly discussed.