Distribution And Ecological Risks Of Microplastics In Water Of Typical Agricultural Areas In Qinghai Province

As a novel synthetic organic pollutant,microplastics are discharged through human activities into the atmosphere,water,soil,sediment and other environmental media,then enter the biosphere in a variety of ways,causing lasting harm to the ecological environment and human health.The freshwater environment is not only a major vehicle for human activities,but also a primary source of marine microplastics.However,research area is still concentrated in developed cities along the Yangtze,Pearl and some inland rivers.For the agricultural areas of the eastern Tibetan plateau,the occurrence characteristics of microplastics in river water remain unclear.Therefore,this paper took the Huangshui River basin in Hehuang Valley of Qinghai Province as the research area,and investigated the occurrence characteristics of microplastics in Huangyuan County,Mutual Tujia Autonomous County and Minhe Hui Tujia Autonomous County during the wet and normal season.The distribution characteristics of microplastics in water bodies were investigated by means of metallographic microscopy,Fourier infrared spectroscopy,field investigation and image data analysis.Combined with human social economic activities,hydrological and water quality indexes,physical and chemical properties of sediments and meteorological factors,the main factors affecting the distribution of microplastics were analyzed to further explore the source of microplastics in water.Two risk assessment models are used to assess potential ecological risks.The main research results are as follows:（1）The abundance of microplastics in surface water ranges from 415 to 8780 n·m^-3,which is at a medium and low level.The abundance of microplastics in normal period(3553±1159 n·m^-3)was higher than that in wet period(2900±962 n·m^-3),and the abundance of microplastics in Huangyuan county(4170±1435 n·m^-3)was the highest.Microplastics mainly gather in the middle and lower reaches of rivers,and are mainly small size（0.45～50μm）,transparent（66%）and thin films（35%）.From upstream to downstream,the proportion of microplastics with<20μm particle size increasesed,and the degree of breakage of microplastics increasesed.The average abundance of microplastics in riparian sediments was（1012±296）n·kg^-1,also at a moderate level.The abundance in normal period(1060±335n·kg^-1)was higher than that in wet period(964±256 n·kg^-1),and the abundance decreased with increasing depth.Microplastics are mainly small sizes（20～50μm）,transparent（59%）,thin films（38%）and particles（30%）.With the increase of depth,the proportion of<20μm microplastics increased by 10.5%,and the proportion of granular microplastics also increased by 6%.（2）The distribution of microplastics is influenced by a number of factors.The abundance of microplastics showed a significant positive correlation not only with the fraction of primary industries,per capita disposable income,and gross domestic product,but also with DO,TN,TP and ammonia.This suggests that the discharge of domestic sewage,agricultural and livestock waste affects the distribution of microplastics.High UV radiation accelerates microplastic degradation,while surface runoff from short periods of high rainfall enhances microplastic input,and wind action can either be a source of microplastics in remote areas or remove some of the less dense plastic.Fine-grained sediment can absorb more microplastics.The increase in sediment pH enhanced the electrostatic repulsion between sediment and microplastics,slowing down the migration of microplastics.（3）The chemical composition of surface water microplastics is dominated by PE,PP,PS,and other low-density polymers,accounting for 80%of the total.High-density polymer polyester,acrylic fibers,synthetic fibers and other important chemical fiber textiles as well as PVDF,PAI and other industrial raw materials were detected in the sediment.Combined with the various morphological features of microplastics,major sources of microplastics included:agricultural sources-agricultural films,fertilizer bags and pesticide bottles are imported into rivers by farm water or wind;Sources of life-packaging bags and disposable lunch boxes from everyday life are discarded,fibers from clothing in laundry water and plastic particles added to hygiene products such as toothpaste and face wash;Production sources-Construction pipe industry,fertilizer production,wire and cable and various paint coating production drainage.（4）The PLI distribution of microplastics in water was in the range of 1 to 2,and the ecological risk was at a moderate level.The RI of microplastics in the 2021 normal water cycle met the Class Ⅲ standard,indicating a strong risk of contamination.The RI in Huzhu and Minhe counties exceeded 100 in some periods,reaching the standard of Class Ⅲ,while the RI in Huangyuan county was always below Class Ⅰ.Sediments from both periods also have PLI levels that are Class Ⅱ,indicating moderate ecological risk.During the wet season,RI reaches the Class Ⅲ standard and the pollution is strong,which is closely related to the highly toxic PVC and acrylic in the water body and sediment.In the future,the impact of microplastics on the ecological environment of river basins should be fully considered,the management of plastic waste and the removal of microplastics should be strengthened.