| Microplastics have become emerging contaminants in recent years.Because of small size,increasing plastic production,extensive usage,poor waste management,and minimal biological degradation,high abundance of microplastics have been identified in ecosystems.An increasing number of studies have implied that microplastics in the environment could pose a potential threat to the ecosystem.Therefore,it is essential to understand the distribution,transport and fate of microplastics in ecosystems.The Yulin River,a typical tributary in the reservoir tail of the Three Gorges Reservoir region,was selected as the research area and the occurrence of microplastics in the environment was revealed by fluorescence treatment and detection method.Additionally,the possible sources of microplastics,the transport and fate of microplastics under different hydrodynamic conditions were also discussed.Microplastics exist in the main stream,bays,tributaries,stratified water body of different depth,sediment and slope soil of Yulin River,which can further demonstrate that not all microplastic float on the surface water and a significant portion of microplastics do not migrate into the sea via rivers but are retained in the river system instead.For microplastic on the surface water,the abundance range of microplastics in the main stream along the Estuary-Shujia section is?0.581.32?×106 item/km2 during the winter impoundment period.The abundance range of microplastics is?0.201.28?item/L in bays and?0.205.40?×10-1 item/L in tributaries.Microplastics abundance shows the order of the bay>tributary>main stream.The abundance range of the Estuary-Xihe section is?0.762.00?×106 item/km2 in winter impoundment period and?1.131.92?×106 item/km2 in summer drainage period.Compared with other studies using trawling method,the abundance range of microplastics on surface water is smaller and basically in the middle pollution level.For microplastic in the sediment of the Estuary-Xihe section,the abundance range is?0.902.54?×103 item/kg dw in winter impoundment period and?0.671.71?×103 item/kg dw in summer drainage period.Compared with other studies using the same unit,the abundance of microplastics in sediment is basically above the average pollution level.It was found that the number and size of microplastics matched basically with power function by analyzing the characteristics of microplastics in different samples.Smaller microplastics have higher numerical abundance,representing the higher degree of fragmentation in the environment.Small-size microplastics were more likely to appear on surface water.The microplastic has different shapes.Microplastics were classified into 3 kinds of shapes according to the 3D spatial dimension:fiber,sheet and fragment.Although the shape ratio varied in different samples,microplastic fibers and fragments were significantly more than sheets.The number of sheets near farmland was higher than other areas and there were more microplastic fibers near the discharge point of sewage treatment plants.The change of impoundment and drainage in different seasons can lead to the change of shape ratio of microplastic in sediment.The distribution of fluorescence intensity of microplastics varied in different samples:there were many microplastics with unusually high fluorescence intensity on surface water of the main stream and the fluorescence intensity near Shujia with intensive anthropogenic activities in the upstream was higher.The environmental conditions in summer drainage period were more beneficial for the lower hydrophobicity,higher density microplastics to transport into the surface water.The success rate of identification of microplastics by fluorescence method reached 100%confirmed by Raman spectroscopy.The proportion of PP,PE,PS in large size microplastics was higher and the proportion of PET and PU in small size microplastics was relatively high.Pollution from anthropogenic activities had a significant impact on the distribution of microplastics in the environment.It was found that microplastics in the main stream of the Yulin River might come from point sources?tributaries,treated or untreated rain/sewage discharge,bays?or non-point sources?agriculture,urban runoff?.Most microplastics had came from secondary sources because of their irregular shape and rough surface.Tributaries contributed differently to the occurrence of microplastics on surface water of the nearby main stream due to their different pollution levels and flow fluxs.The discharge of sewage treatment plant is an important point source of microplastic pollution.Although the bay is relatively closed and difficult to exchange with the main stream,there were still the transport of the microplastic between them.Periodic immersion and light radiation combined with plant puncture,drag and other complex physical and chemical processes have been constantly promoting the accumulation,weathering and fragmentation of large-scale plastic garbage in the bay forming microplastic and then transport into the main stream.The abundance of microplastics,which were likely to come from agricultural non-point source pollution and transfer into nearby water body,in slope soil of Estuary and Paihua was?1.1812.046?×103 item/kg dw.Paihua is a potential hot spot of microplastic pollution in Yulin River on account of many large size microplastic sheets or fragments with strong hydrophobicity and low density.The abundance of microplastic in the upstream of human habitation area was significantly higher than that in the downstream of natural forestland area.The transport and transfer of microplastic in river system is the result of both lateral and vertical movement.The lateral movement of microplastics with the flow direction is greatly affected by the horizontal velocity.Microplastic was easy to accumulate in bays and front of the dam where the velocity is slow.The abundance of microplastic on surface water of the main stream influenced by the backwater of the Yangtze River increased during winter impoundment period.Because of the slow vertical movement in static water,microplastic is easy to be affected by turbulence,therefore external factors such as turbulent mixing have a great influence in natural water body.There were spatial differences in the occurrence of microplastic in water body with different depths.The change of hydrodynamic conditions led to significant differences in the occurrence of microplastic on surface water and sediment of Estuary and Xihe,which are located at the junction of rivers,in different seasons.The hydrodynamic conditions of Estuary located between the main stream of the Yangtze River and the ecological regulating dam across the Yulin River are most special.The seasonal variation of hydrodynamic conditions results in obvious differences between the occurrence of microplastics in front of and behind the dam.The increasing velocity not only transfers the microplastics from sediment to water body,but also makes microplastics more strongly broken.The periodic impoundment and discharge make microplastics deposit and resuspend,transferring constantly among slope soil within the vanishing zone,water and sediment.Results from this study can provide basic data for control of microplastic pollution. |
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