Spatial Distribution And Transport Patterns Of Microplastics In A Typical Small Watershed Of The Loess Plateau

The small size and lightweight of microplastics often make them very susceptible to degradation and migration,widely distributing microplastics all over the world.As an important form of water and soil loss,soil erosion can directly affect the migration and spatial redistribution of microplastics in the environment.The Loess Plateau was a typical area with a large amount of plastic in Western China and it is also a serious area of soil erosion.However,the spatial distribution characteristics and factors affecting distribution of microplastics in the soil in this area were still unclear.Understanding the spatial distribution and influencing factors of microplastics can help provide a theoretical basis for the problem of microplastic pollution.This study collected soils from a typical watershed（Wangdonggou small watershed）on the Loess Plateau,based on field investigation,combined with multiple density flotation extraction and infrared optical analysis,multiple density flotation extractions,and infrared optical analysis,the abundance,composition and morphological characteristics of the microplastics were compared and analysed,the abundance,composition and morphological characteristics of the microplastics were compared and analysed,with the aims to identify the spatial distribution characteristics of microplastics in the watershed.Finally,sediment disturbances and abrasion were simulated by rotating wet soils at different concentrations,and the microplastic abundances,compositions and diameters in different treatments were compared to reveal how rotating and abrasion break microplastics.Our main findings include:（1）The abundance and morphological characteristics of microplastics at the typically small watershed.The average abundance of microplastics followed the pattern:tableland(6900 n·kg^-1)>slope(4475 n·kg^-1)>valley head(3413 n·kg^-1)>valley end(2250 n kg^-1).The microplastics collected from the slope soils（39.29μm）and valley sediments（41.8μm）were much smaller than that from the tableland soils（62.6μm）,indicating that the microplastics may have experienced more aging and breaking in-transit from the tableland to slope or the valley.Furthermore,the abundance and composition of microplastics in sediments at the valley bottom had a better synchronization pattern with that on the side slopes(3450 n·kg^-1 vs.2000 n·kg^-1,5350 n·kg^-1 vs.10000 n·kg^-1,4250 n·kg^-1 vs.3300 n kg^-1),suggesting that the microplastics in the sediments at the end of valley were more likely come from the adjacent side slopes rather than the upstream runoff and sediment.（2）The abundance and morphological characteristics of microplastics in soils differed significantly among land use types.Among them,the average abundance of microplastics in the cropland soil(7550 n·kg^-1)was significantly higher（P<0.05）than that in the landfill soil(2283 n·kg^-1).The average area of microplastics in apple orchard soil（8045.3μm²）was significantly larger（P<0.05）than in cropland soils（4203.4μm²）and landfill soils（3635.8μm²）;and the morphological characteristics such as height and width were also significantly greater（P<0.05）than in cropland soils and landfill soils.This was mainly because microplastics in cropland soils were more affected by human activities（multi-year mulching and tillage measures）and thus more fragmented,whereas the microplastics in the apple orchard soils were relatively less disturbed and thus less degraded.（3）The abundance of microplastics differed among the three simulated disturbance treatments:wetsoil rotating(14400 n·kg^-1)>dry soil stand-still(3767 n·kg^-1)>wetsoil stand-still(2067 n·kg^-1),while the average area of microplastics followed a different pattern:dry soil stand-still（5233.8μm²）>wetsoil stand-still（4382.5μm²）>wetsoil rotating（2867.6μm²）.This implies that wet soil disturbance（such as erosion and sediment transport）could encourage microplastics to fragment and thus promote their mobility.In addition,after rotated under different sediment concentrations(560 kg·m^-3,800 kg·m^-3,930 kg·m^-3),the abundance,average area,and morphological characteristics of microplastics also differed.The average abundance of microplastics followed the pattern:medium sediment concentration(18300n·kg^-1)>low sediment concentration(13733 n·kg^-1)>high sediment concentration(8667n·kg^-1),whilst the average area of microplastics followed the pattern:low sediment concentration（3932.0μm²）>medium sediment concentration（2471.55μm²）>high sediment concentration（2099.3μm²）.These findings suggest that during the transport with sediment,microplastics were more vulnerable to be fragmented and degraded when the sediment concentration was moderate,as too low or too dense sediment tended to have limited abrasion.This study links the microplastic pollution and soil erosion,and explores the spatial distribution characteristics and potential migration behavior of microplastics in different locations of the watershed from the perspective of erosion and migration.The impact of soil microplastics on the Loess Plateau provides data support for the study of the background value and composition characteristics of soil microplastics;and the simulation study of sediment erosion provides new ideas and a new basis for the aging and fragmentation characteristics of microplastics in the process of migration in the watershed.