Occurrence and Characteristics of Microplastic Pollution in Typical Inland Waters in China

中國典型內陸水體的微塑料污染研究

Student thesis: Doctoral Thesis

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Award date15 Feb 2018

Abstract

Plastics are light in weight, strong in structure, durable in use, resistant to corrosion, and non-conductive to electricity. Due to these unique properties, plastics have become widely used in our daily life. However, their advantages for human use have become disadvantages for environmental sustainability. Since substantial amounts of plastics have been produced without proper disposal after use, large volumes of plastics have entered the environment, causing multiple adverse problems. Worse still, small plastic debris can be generated during the breakdown of larger plastics. Plastic debris smaller than 5 mm in diameter is commonly known as microplastic, and have been demonstrated to have potential negative effects on aquatic organisms, and are considered as “contaminants of emerging concern” (CECs).

Microplastics have been found ingested by aquatic animals due to their small sizes and similarities in size and appearance to sediment particles and certain types of plankton. The ingested microplastics, including fibers and fragments, might clump and knot, thus causing choking or pseudo-satiation in marine biota following microplastic ingestion. The leaching of additives added to plastics may pose another threat to marine biota. Due to the large surface-area-to-volume ratio, additives and monomers are most readily leached out from microplastics which can be ingested by organisms. In particular, plastics favor the accumulation of hydrophobic organic compounds, thus acting similarly to lipids in organisms and organic carbon in sediments. Once ingested, a possible secondary release of these chemicals may occur, which could further cause adverse impacts on the exposed organisms.

The occurrence of microplastics in the marine environment has been well studied. Investigation areas covered nearshore waters and remote regions, and even the deep seas and polar regions have been examined. However, there are relatively few studies in the terrestrial environment. It is therefore important to investigate the abundance of microplastics in the terrestrial environment to better understand the pollution status as well as the terrestrial environment as potential pollution sources of microplastics in the marine environment. China, a country with many rivers and lakes, is the world’s largest producer and consumer of plastics. Poor waste management in rural areas leads to a common scene of untended dumping of plastic waste in fields. These wastes can easily enter the surrounding waters, and thus microplastics may be present in high concentrations in some water bodies. However, there were no studies on microplastics in China before 2014.

In this study, two types of water bodies were chosen — rivers in the Three Gorges Reservoir and lakes in the Tibetan Plateau. The occurrence and distribution of microplastics in the surface water of the Yangtze River from the Three Gorges Reservoir (TGR) were investigated. The abundances of microplastics ranged from 3.4077 × 106 to 1.36175 × 107 particles/km2 in the main stretch of the Yangtze River and from 1.925 × 105 to 1.18897 × 107 particles/km2 in the estuarine areas of its four tributaries. The abundance of microplastics in the main stretch of the Yangtze River generally increased in locations closer to the Three Gorges Dam. The microplastics collected were exclusively polyethylene (PE), polypropylene (PP), and polystyrene (PS). Apart from microplastics, a high abundance of coal/fly ash was also observed in the surface water samples. Compared with previously reported data, the abundance of microplastics in the TGR was approximately one to three orders of magnitude greater, suggesting TGR as a potential hot spot for microplastic pollution.

In addition to the study in the Yangtze River from the TGR, the occurrence and distribution of microplastics in the surface water of the Xiangxi River’s back water areas were investigated seasonally. Sediment and fish from the river were also examined. Microplastics were detected in both surface water and sediment with concentrations ranging from 5.5 × 104 to 3.42 × 107 particles/km2 and 80 to 864 particles/m2, respectively. Polyethylene, polypropylene, and polystyrene plastics were identified in surface water, whereas polyethylene, polypropylene, and polyethylene terephthalate particles, as well as pigments, were observed in sediment. In addition, microplastics were also detected in the digestive tracts of 25.7% of fish samples, where polyethylene plastics and nylon were identified. Redundancy analysis indicates a weak correlation between microplastic abundance and water quality variables but a negative correlation with water level of the reservoir and Secchi depth. Results from this study confirm the presence of a high abundance of microplastics in reservoir-impacted tributaries, and suggest that water level regulated hydrodynamic conditions and nonpoint-source input are important factors for microplastic accumulation and distribution in the backwater areas of reservoir tributaries.

The Tibetan Plateau is known as the Third Pole of the world, characterized by a low population density with very limited anthropogenic impact. The Tibetan Plateau possesses the greatest number of high-altitude inland lakes in the world. The pollution status of several kinds of emerging contaminants have been investigated in this remote region. However, no information is currently available on the characteristics of microplastic pollution in the lakes within the Tibetan Plateau. In this work, lakeshore sediments from four lakes within the Siling Co Basin in northern Tibet were sampled and examined for microplastic occurrence. Microplastics were detected in six out of seven sampling sites with abundances ranging from 8 ± 14 to 563 ± 1219 particles/m2. Riverine input might have contributed to the relatively high abundance of microplastics observed in this remote area. The morphological features suggest that the microplastics are derived from the breakdown of daily-used plastic products. Polyethylene, polypropylene, polystyrene, polyethylene terephthalate, and polyvinyl chloride were identified in the microplastic samples using laser Raman spectroscopy, and oxidative and mechanical weathering textures were observed on the surface of microplastics using a scanning electron microscope. These results demonstrate the presence of microplastics in inland lakes from remote areas with very limited anthropogenic impact, indicating that microplastic pollution is a global issue.

Different from the lakes in Siling Co Basin, Qinghai Lake is the largest lake in China attracting millions of visitors annually. Though located in a remote area of the Tibetan Plateau with less dense population, the nonnegligible impact of tourism should be of concern. The abundances of microplastics in the surface water of Qinghai Lake ranged from 5 × 103 to 7.58 × 105 particles/km2, which were higher in the central and southern area of the lake and lower in the north and the estuary. The size range of 0.112 - 0.5 mm and shape of sheets were dominant in the samples collected from Qinghai Lake and PP and PE were the dominant materials. The distribution of microplastics in the surface water and lake shore sediments indicated the tourism around Qinghai Lake could be the main source of plastic litter for Qinghai Lake, and the lake current contributes to the transportation of microplastics to the central area, resulting in the highest abundance found in the central area. The high intensity of ultraviolet light and the wind could facilitate the generation of microplastics, which can occur both on the land and in the lake. These results reveal that this remote lake cannot avoid the threat of microplastic pollution without effective management. The enclosed water body can accumulate microplastics even though the catchment area is relatively clean. The fragile ecosystem in plateau lakes deserves more attention concerning the threat of microplastics.

A strong affinity for pollutants such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) has been found in plastic debris. However, the investigated debris was relatively large. Smaller microplastics having a high surface-to-volume ratio exhibit the capacity to sorb more contaminants. In this study, the sorption behavior of pharmaceuticals and personal care products (PPCPs), including carbamazepine (CBZ), 4-methylbenzylidene camphor (4MBC), triclosan (TCS), and 17α-ethinyl estradiol (EE2), to PE debris (250 to 280 μm) was investigated. The sorption coefficients (Kd) under linear estimation were 191.4, 311.5, 5,140, and 53,225 L/kg for CBZ, EE2, TCS, and 4MBC, which were related to their hydrophobicity. The increase of salinity from 0.05% to 3.5 % did not affect the sorption of 4MBC, CBZ, and EE2, but enhanced the sorption of TCS, which is likely due to the salting-out effect. The increase of dissolved organic matter (DOM) content, using Aldrich humic acid (HA) as a proxy, reduced the sorption of 4MBC, EE2, and TCS, all of which show a relatively strong affiliation to HA. Results from this work suggest that microplastics may play an important role in the fate and transport of PPCPs, especially for hydrophobic compounds.

In conclusion, two types of water bodies in China were investigated for microplastic pollution. The sorption capacity and behavior of PPCPs in relation to polyethylene debris were evaluated. A high abundance of microplastics was observed in both the TGR region and lakes on the Tibetan Plateau. An increasing trend of microplastic abundance from upstream towards the Three Gorges Dam was observed. Tributaries contributed a significant amount of microplastics to the main river. Distinctive hydrodynamic characteristics in the backwater areas of reservoir tributaries also contribute to the accumulation and distribution of microplastics. Microplastics can settle to the river bottom and accumulate in sediment, especially in areas of low surface water flow and high nutrient levels, which may be related to the density of microplastics and biofouling. Due to the closed water system, all plastic waste within the watershed of the plateau lakes enters the lakes eventually, which poses a serious problem of microplastic pollution of the Tibetan Plateau. Our study further found that microplastics might act as an important carrier for the transport of PPCPs, especially hydrophobic compounds. To our knowledge, this is the first study to reveal the presence of microplastics in inland waters in China.

    Research areas

  • Microplastics, TGR, Tibetan Plateau, Sorption, PPCPs