[A-HW18] 流域圏生態系における物質輸送と循環：源流から沿岸海域まで

[AHW18-09] Plastic fluxes and composition change in transportation and accumulation process of the urban river, Mongolia

*Dolgormaa Munkhbat¹、Batdulam Battulga²、Bolormaa Oyuntsetseg³、Masayuki Kawahigashi¹ (1.Department of Geography, Tokyo Metropolitan University、2.Nuclear Science and Engineering Center, Japan Atomic Energy Agency、3.Department of Chemistry, National University of Mongolia)

キーワード：River, Plastic debris, Microplastic, Plastic flux

Rivers flow through highly concentrated cities termed “Urban rivers”, resulting in wide distribution of anthropogenic debris in natural aquatic environment, which is gradually elucidated in recent microplastic studies. Plastic waste is one of the high amounts of pollutants released from urban areas to the river. We had a field survey from urban rivers in Ulaanbaatar city, Mongolia. Ulaanbaatar occupies 0.03% of the Mongolian territory, but 51% of the total population lives in the city. Ulaanbaatar city is bordered by a mountainous area, and the Tuul river runs through the city from the east (upper stream) to the western province border (downstream). We collected plastic samples from the 9 sites at the river shore of Tuul and its tributaries. Distribution of plastic occurrence was determined in the river shore sediment and in the river flow using a plankton net. The plastic flux in the river flow was estimated by river discharge at each sampling site. River shore plastic items ranged between 0.82±0.34 and 13.72±10.53 item/m². Micro, meso, macro, and mega-sized plastics with the composition of fiber, film, foam, and fragments distributed more diversely in all sampling sites in the river shore. This may relate to various waste approaches to the river shore by water level changes or its transportation from inland. River shore plastic samples dominantly contained foam type of plastic followed by film, fiber, and fragment types, while any foam plastics were not detected in the sediment and only a single sampling site contained less than 10% of foam in surface water samples. Plastic items found in the sediment samples ranged between 4x10² and 26.3x10² items/kg. The number of the sedimentary plastics increased during low river discharge or flowrate. Therefore, the plastic waste deposition process to the river bottom may be affected by flow rate of river discharge. Sedimentary plastics mainly consisted of fiber and fragments accompanied by less amount of film plastic and the size were micro and mesoplastic. This result showed that plastic items accumulated in the sediment notably reduced their own sizes in comparison with those of the river shore plastics. Plastic items in the surface water consisted of a high proportion of microplastic with less than 10% of mesoplastics, indicating that small-sized plastic items were easily transported through river flow because of their buoyancy. Plastic fluxes ranged between 2.16x10³ and 9.23x10⁵ items/day from the upper stream to downstream of the river. In conclusion, plastic types, size, and shapes are different between plastics in surface water, at the bottom sediment and on the river shore in the river system, which indicated that plastic transportation or accumulation process differs according to hydrological factors such as river flowrate and waste discharge due to anthropogenic activity.