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微塑料通常指粒径<5 mm的塑料颗粒,具有难分解、分布广泛和生态危害大等特点[1]。塑料制品在人们生活中被广泛使用,2021年我国塑料制品产量超过8 000万吨[2],塑料制品废弃后,除部分回收、焚烧等处理外,许多塑料通过不同方式进入自然环境中。由于自身化学性质稳定,塑料废弃物在物理破碎、光化学分解、生物破碎等作用下[3-4],形成数量庞大的微塑料继续保留在自然环境中。在各种自然过程影响下,如水力作用、大气运动和生物作用等,微塑料广布全球。当微塑料累积到一定数量后,可影响水体透明度和热环境,改变沉积物物理、化学性质,吸附重金属和持久性有机污染物,释放有毒有害化学物质等[5-8]。微塑料通过生物摄入等过程进入生物体内,在食物链的累积效应下加剧其生态风险,最终对人类健康产生不可预估的危害[9-12]。
河流是微塑料从陆地输入海洋的重要途径,相关研究显示每年有115万~241万吨微塑料通过河流输入海洋[13]。土地利用和人口密度等人类活动特征[14-16],以及河流的宽度和流速等水文、水力特征[17-18],均可对河流微塑料污染产生重要影响。LI et al[15]发现土地利用可影响微塑料形态,农业区微塑料形态以薄膜状为主,人口密集区则以纤维状为主。然而,人类活动如何影响内陆水体中微塑料的形态、迁移和分布还不清晰。诸多定性分析通常认为,诸如城市土地利用、人口密度和工业生产活动等人类活动是微塑料污染严重的重要原因[14-15, 19-20],同时也有许多定量研究发现,这些人类活动与微塑料污染并非简单线性相关关系[21-23]。SCHELL et al[14]研究发现,河流微塑料污染丰度与城市土地利用相关,位于建成区下游的微塑料污染显著高于其他区域。MAI et al[21]通过定量研究发现,反映人类活动强度的社会、经济指标,如人口密度、城市土地利用、水产品产量和养殖水面面积等,与微塑料污染在部分采样点存在协同变化趋势,但整体并无显著相关性。与氮、磷和重金属等传统污染物相比,微塑料完全来自人类生活、生产活动[3],其在内陆水环境中的迁移过程与分布格局受人类活动影响可能更为复杂。因此,定量研究不同人类活动影响下内陆水体中微塑料污染特性,对理解微塑料迁移转化机理和防治微塑料污染有重要意义。
东江是珠江水系三大主要干流之一,干流全长562 km,流域面积35 340 km2,它是惠州、东莞、深圳和香港等重要城市的关键居民饮用水源,对粤港澳大湾区用水安全有着极其重要的作用。东江干流自博罗县观音阁进入下游,从惠州至东莞其流域城市化程度显著增加,惠州段是东江进入粤港澳大湾区的关键带。本文以东江惠州段为例,研究其干流和主要支流的微塑料污染特征,定量研究土地利用与微塑料污染的关系,初步揭示城市化对东江微塑料污染的影响机制,为城市河流微塑料污染防治提供科学参考。
城市化下东江水体沉积物微塑料分布特征研究
——以惠州段为例Effects of urbanization on the distribution of microplastics in sediments of the lower Dongjiang River
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摘要: 城市是微塑料污染的热点区域,文章以东江下游惠州段为例,采集不同城市化梯度上水体沉积物样品,通过密度分离、目视镜检和地理信息技术等方法,分析微塑料的空间分布特征,探讨城市化对水体微塑料分布的影响。结果显示,东江惠州段水体沉积物微塑料丰度中等偏低,丰度范围为40~1 200个/kg,以纤维状形态为主。分布格局上,微塑料丰度在支流中最高、公园湖泊和干流丰度接近。值得注意的是,各支流中,上游样点微塑料丰度显著高于其下游样点。微塑料丰度与土地利用类型在样点缓冲区和子流域尺度上,均未发现显著相关关系,这可能与城乡间水环境管理措施、塑料废弃物处理方式和点源排放等因素有关。该研究可为城乡水环境微塑料污染治理提供科学依据。Abstract: Cities are hotspots for microplastic pollution, but the effects of urbanization on the microplastic distribution in the freshwaters is still unclear. Sediment samples were collected from the river network of “tributaries - urban lakes - main streams” along an urbanization gradient in the Huizhou section of the Dongjiang River. The spatial distribution characteristics of microplastics were analyzed by density separation, visual microscopy and geoinformation technology. And the influence of urbanization on the microplastic distribution was discussed. The results showed that the abundance of microplastics was medium to low, ranging from 40 to 1200 n/kg, with an average of 327 n/kg. The microplastics shapes were dominated by fibers, accounting for 60%-87%, followed by fragments and least by particles. The microplastics showed the highest abundance in tributaries and similar abundance in urban lakes and the main streams. It is noteworthy that the microplastics abundance was significantly higher in the upstream sample sites than those in their downstream in all studied tributaries. There was no significant correlation between microplastic abundance and land use types at both buffer and subbasin scales. This may be related to differences in water environment management measures, waste treatment and sewage discharge between urban and rural areas. The results of this study can provide a scientific basis for the microplastic pollution management in urban and rural water environments.
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表 1 不同水体沉积物微塑料丰度分布特征
Table 1. Distribution of microplastic abundance in sediments of different water bodies
个·kg−1 水体类型 分区 样本数/个 平均值 标准差 最小值 最大值 城市
公园
湖泊西湖 10 175 45 110 250 其他 6 398 172 200 690 整体 16 259 153 110 690 西枝
江各
支流城市河流 6 500 101 360 660 郊区河流 2 500 226 340 660 乡村河流 4 495 369 200 1 000 整体 12 498 216 200 1 000 东江
干流
惠州段上段 6 82 38 40 130 下段 6 410 419 80 1 200 整体 12 246 331 40 1 200 研究区整体 40 327 257 40 1 200 表 2 微塑料丰度与土地利用间的Spareman相关系数
Table 2. Sparman correlation coefficients between microplastic abundance and land use
尺度 区域
(样点数/个)土地利用类型 建筑 裸地 绿地 水体 缓冲区
(r=500 m)支流(12) 0.102 −0.241 0.158 −0.295 东江(12) 0.322 −0.622* 0.455 −0.462 支流+东江(24) 0.499* −0.676** 0.589** −0.680** 子流域 支流子流域(6) 0.143 0.371 0.429 −0.657 注:*与**分别表示p<0.05和p<0.01。 表 3 不同城市化河流湖泊沉积物微塑料丰度比较
Table 3. Comparison of microplastic abundance in sediments from different urbanized rivers and lakes
区域 河流 丰度/个·kg−1 主导形态 提取方法 文献来源 湖南 洞庭湖 388.6~501.4 纤维 ZnCl2 [33] 合肥 南淝河 661/330~1 363 纤维57.3% NaCl [34] 深圳 茅洲河 35~560 碎片89.4% ZnCl2 [18] 长沙 城市河流 413/308~581 碎片50.8% ZnCl2 [30] 城市湖泊 561/270~867 珠三角 东江下游 604 碎片 甲酸钾 [35] 西江下游 586 北江下游 132 惠州 支流 498/200~1 000 纤维86.5% NaCl 本研究 湖泊 259/110~690 纤维60.1% 干流 246/40~1 200 纤维66.8% 德国 科布伦茨 城市河流 2562/260~5 220 - ZnCl2 [36] 西班牙 埃纳雷斯堡 城市河流 937/50~2 630 碎片87% NaI [14] 南非 约翰内斯堡 城市河流 166.8/4~1 347 - NaCl [17] 注:“/”前为平均值,后为分布范围。 -
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