[M-IS19] Ocean Plastics, an earth science perspective

[MIS19-09] Spatial distribution of microplastics in the Hasaki beach sediments

*Takashi Ishizawa¹, Kazuhisa Goto², Naofumi Yamaguchi³, Masayuki Banno⁴, Tetsuya Shinozaki⁵, Kai Sato² (1.International Research Institute of Disaster Science, Tohoku University, 2.Department of Earth and Planetary Science, the University of Tokyo, 3.Global and Local Environment Co-creation Institute, Ibaraki University, 4.Port and Airport Research Institute, 5.Research Institute of Earthquake and Volcano Geology, AIST)

Keywords:Microplastic, Beach sediment, Storm surge

Microplastics (MPs) are often defined as small plastic (<5 mm) in various forms. The MPs have characteristics that can adsorb chemical substances and are often mistakenly ingested by organisms as food. Therefore, there is concern about the environmental impact of MPs. The MPs are formed by the decomposition of plastics on the shore by wave action and ultraviolet light and are discharged into the marine environment as fine MPs. It has been reported that MPs accumulate near a high tide line in a beach with other biogenic materials due to their low specific gravity (<1.5 g/cm³) and that MPs are blown by the wind and accumulate in beach berm. In addition, it has also been reported that a large number of MPs are launched on the beach surface immediately after a storm, but whether they are preserved in the beach sediment for a long period is still unclear.
In this study, we investigated the spatial distribution of MPs on the Hasaki coast in Ibaraki Prefecture where topography and wave heights are regularly observed along the observation pier. The survey was conducted on October 15 and November 19, 2021, and beach sediments were collected along the observation pier. The sediments were sampled mainly from the surface layer of the beach, and at one site, a trench was dug to collect sediments in the depth direction. The collected sediment samples were subjected to organic matter treatment and specific gravity separation to separate MPs. As a result, the MPs in the surface sediment were unevenly distributed, and the number of MPs was high near the high tide line in the survey on October 15, but the number of MPs was the same as other sites even at the high tide line in the survey on November 19. In the trench samples, we found an organic layer containing a large number of MPs at a depth of 12-15 cm, which was estimated to be formed by the typhoon in October 2019 based on the survey results along the observation pier. In this presentation, we compare these results with the data obtained from the observation pier and discuss the factor of MPs distribution in the coastal sediment.