5:15 PM - 6:45 PM
[MIS09-P07] Characteristics and deposition environment of microplastics in downstream and estuarine areas of rivers discharging into Tokyo Bay
Keywords:Microplastics, Downstream and estuarine areas of rivers, Sedimentary environment, Brackish water, Sediment mixing
Plastics are used for various purposes because they are lightweight, easy to process, and chemically stable, but at the same time, the amount of plastics released into the environment is increasing. These plastics are transported to the ocean, where they become microplastics (Micro Plastics; MPs) due to physical and chemical effects during transport. It has been pointed out that MPs are a cause of ecosystem destruction and environmental pollution, and countermeasures against them are urgently needed. Recent studies have shown that rivers are the major transport pathway of MPs from land to sea, and that river-bed sediments are the reservoirs of MPs. Understanding the conditions and environments under which MPs migrating in river water settle and become deposited is key to understand the dynamics of MPs in estuarine ecosystem.
In this study, sediments were collected from the estuaries of the Tama, Edo, and Hanami Rivers and the lower reaches of the Arakawa River, which flow into Tokyo Bay, to determine the characteristics of MPs in the sediments and to analyze the sediments in order to clarify the factors controlling the distribution of MPs in the sediments.
Sediments collected from the Tamagawa River, the Edogawa River, the Hanamigawa River estuary, and the downstream of the Arakawa River all contained MPs, especially in a debris-like form. In addition to high-density polyethylene terephthalate and acrylic rubber, low-density polyethylene and polypropylene were also observed, indicating that even low-density MPs settle and are deposited. The depth distribution of MPs was not dependent on the sediment depth, but MPs were contained ubiquitously in the sediments, and the concentration of MPs tended to be particularly high at the mouth of the river. This is considered to be because the area around the mouth of the river is brackish, and MPs are difficult to be resuspended and re-transported once deposited due to the cohesive action of debris particles, and the settling rate is fast due to cohesion, and the pore space is small due to the consolidation of the sediment, resulting in the preservation of MPs in the sediments. In addition, MPs were not observed in the near-surface layers as we moved offshore from the mouth of the Edo River, suggesting that those deposited near the surface are agitated by tidal action and biological disturbance, and are discharged to the ocean or transported and decomposed to deeper depths. These MPs are suggested to be immobilized and miniaturized in the sediments. In particular, it is clear that pore filling and the depositional environment have a significant influence on the deposition of MPs. It is necessary to understand the downstream and estuarine areas of rivers and estuaries not as a connection area between land and sea, but as a reservoir of MPs, integrated with the ocean.
In this study, sediments were collected from the estuaries of the Tama, Edo, and Hanami Rivers and the lower reaches of the Arakawa River, which flow into Tokyo Bay, to determine the characteristics of MPs in the sediments and to analyze the sediments in order to clarify the factors controlling the distribution of MPs in the sediments.
Sediments collected from the Tamagawa River, the Edogawa River, the Hanamigawa River estuary, and the downstream of the Arakawa River all contained MPs, especially in a debris-like form. In addition to high-density polyethylene terephthalate and acrylic rubber, low-density polyethylene and polypropylene were also observed, indicating that even low-density MPs settle and are deposited. The depth distribution of MPs was not dependent on the sediment depth, but MPs were contained ubiquitously in the sediments, and the concentration of MPs tended to be particularly high at the mouth of the river. This is considered to be because the area around the mouth of the river is brackish, and MPs are difficult to be resuspended and re-transported once deposited due to the cohesive action of debris particles, and the settling rate is fast due to cohesion, and the pore space is small due to the consolidation of the sediment, resulting in the preservation of MPs in the sediments. In addition, MPs were not observed in the near-surface layers as we moved offshore from the mouth of the Edo River, suggesting that those deposited near the surface are agitated by tidal action and biological disturbance, and are discharged to the ocean or transported and decomposed to deeper depths. These MPs are suggested to be immobilized and miniaturized in the sediments. In particular, it is clear that pore filling and the depositional environment have a significant influence on the deposition of MPs. It is necessary to understand the downstream and estuarine areas of rivers and estuaries not as a connection area between land and sea, but as a reservoir of MPs, integrated with the ocean.