| Microplastics,which are plastic particles or fibers with a diameter of less than 5mm,are becoming a spotlight of global attention as a type of emerging contaminants.Currently,research on microplastic pollution in the marine environment is relatively rich,while less attention has been paid to microplastic pollution in the inland environment,especially in lakes,rivers,and other environments closely related to human activities.Dongting Lake,with strong flood storage capacity,plays an important role in regulating and storing the Yangtze River Basin.The inland river network in Dongting Lake Basin is densely covered with villages and towns,accompanied by frequent agricultural activities,which gives microplastic pollution regional characteristics;Meanwhile,the characteristics of regulation and storage lakes lead to complex relationships between rivers and lakes,strong connectivity between land and rivers,and significant temporal and spatial migration characteristics of microplastics.It is of great significance to study the occurrence and spatiotemporal distribution of microplastics in main ambient mediums for such objects.Our research takes the Dongting Lake Basin,a typical regulation and storage lake as the object.Samples were collected during September 1-9,2020(end of the high-water period,abbrev.HPE),December 4-12,2020(beginning of the low-water period,abbrev.LPB),and April 2-14,2021(beginning of the rising-water period,abbrev.RPB).Through these,the abundance,occurrence characteristics,and temporal and spatial distribution rules of microplastics in main ambient mediums(surface water,sediment,and soil)during different water periods in the basin are systematically investigated.The main results are as follows:(1)The abundance of microplastics in the surface water of the Dongting Lake Basin ranges from 1000 to 38500 items/m~3,which shows significant different between HPE and RPB(12000 items/m~3 on average)and LPB(7000 items/m~3 on average).The overall level of microplastic pollution in surface water is at a medium level in China(higher than North Lake and Dianshan Lake,lower than Poyang Lake),while some areas with serious pollution in the basin are at a high level.The particle size of microplastics is mainly below 1.5 mm in diameter,presumably due to their strong buoyancy ability to aggregation in surface water.Fiber-shaped microplastics account for the highest proportion of samples from all seasons and regions,while blue,black,and transparent microplastics account for a large proportion in color,reflecting the contribution of plastic fibers that fall off from washing clothes to microplastics pollution.The most common microplastic component in surface water samples is polyethylene terephthalate(PET),followed by polypropylene(PP)and polystyrene(PS),indicating that it may come from plastic products such as chemical fiber clothing or disposable lunch boxes.The abundance is higher during the high-water period in spring and summer,while the abundance is lower during the low water period in winter.From a spatial perspective,the abundance of microplastics in the upper reaches of the rivers entering the lake in the west of the Dongting Lake basin that receive the input of the Yangtze River is relatively large,while the abundance of microplastics in the surface water at the lower reaches of the rivers is relatively small;the pollution level of microplastics in east Dongting Lake is the highest,followed by west Dongting Lake,and south Dongting Lake is the lowest;The abundance of microplastics at the exit of Dongting Lake to the Yangtze River(Chenglingji section)is relatively high,indicating that the urban sources around east Dongting Lake(especially Yueyang City)have a significant impact on it.The pollution load of microplastics in the surface water of the Dongting Lake basin is at a moderate level,with a low potential pollution risk,by using pollution load index(PLI)and potential risk index(PRI)for assessing.(2)The abundance of microplastics in the surface water of the Dongting Lake Basin ranges from 1000 to 38500 items/m~3,with the highest of HPE(260 items/kg on average),following by LPB(220 items/kg on average),and the lowest of RPB(160items/kg on average).The overall level of microplastic pollution in sediment is at a lower medium level in China(higher than Taihu Lake and Yangtze River Estuary,lower than Poyang Lake).The proportion of small particle size microplastics below 1.5 mm and fiber-shaped microplastics is the highest,same as that of surface water samples,but the proportion of large particle size and flake-shaped microplastics is higher than that of surface water samples,possibly due to the larger weight of these microplastics or their ability to adsorb other substances,which increases their weight capacity,making it easier to settle down into the sediment.The most common microplastic components in sediment samples are polypropylene(PP),polyethylene(PE),polyethylene terephthalate(PET)and cellophane,and the distribution of chemical components varies slightly with season,indicating that the occurrence of microplastics in sediment is relatively stable.The pollution load of microplastics in the sediment of the Dongting Lake Basin is at a low level,with a low potential pollution risk,by using PLI and PRI for assessing.(3)The abundance of microplastics in the topsoil of farmlands in Dongting Lake Basin ranges 100-260 items/m~3 in Sep.(160 items/m~3 on average),120-320 items/m~3in Dec.(200 items/m~3 on average),and 80-200 items/m~3 in Apr.(120 items/m~3 on average),which is at a lower medium level in the world.At the soil depth of 5-10 cm in farmlands,the abundance of microplastics in the three periods ranged 0-140 items/kg,0-100 items/kg,and 0-160 items/kg,with an average decrease of 69.62%,75.30%,and58.69%compared to the surface soil.The abundance of microplastics in the topsoil of hydro-fluctuation belts in Dongting Lake Basin ranges 0-420 items/m~3(180 items/m~3on average).At the soil depth of 5-10 cm in hydro-fluctuation belts,the abundance of microplastics ranged 0-260 items/kg,with an average decrease of 67.31%compared to the surface soil.Very little or no microplastic was detected in all soil samples with a depth of 10-15 cm.The large amount of transparent,black,and white flake PE microplastics detected in the soil of farmlands using plastic films reflects the contribution of the use of films by farmers in the Dongting Lake Basin to the soil microplastics,while the components of microplastics in the soil of non-filmed farmlands and hydro-fluctuation belts are relatively diverse,with PP,PE,and PET as the main components.The pollution load of microplastics in the soil of the Dongting Lake basin is at a low level,with a low potential pollution risk,by using PLI and PRI for assessing. |
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