Table_3_Heavy metals and trace elements contamination risks in peri-urban agricultural soils in Nairobi city catchment, Kenya.docx

IntroductionWith urbanization and industrialization in the developing world, urban and peri-urban agriculture is increasingly contributing to urban food systems, employment, and income generation opportunities. However, urbanization and industrialization may release harmful pollutants, including heavy metals and trace elements into agricultural soils, posing ecological, environmental and public health concerns. This paper assessed the potential risks of soil contamination with heavy metals and trace elements in peri-urban farmlands in Nairobi city catchment in Kenya.
MethodsA total of 60 soil samples were collected from ten vegetable farming zones (S1-S10) and processed following standard protocols. The concentrations of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn were analyzed in the samples using inductively coupled plasma-mass spectrometry (ICP-MS).
Results and discussionResults revealed that the soil samples have elevated concentrations of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn, with some elements including As (1.7%), Cd (13.3%), Mn (80%), Pb (1.7%) and Zn (11.7%) exceeding the permissible thresholds for agricultural soils. The spatial distribution of the elements exhibited three similar distribution patterns with slight variations between the hotspot sites of the different elements: (i) Co and Mn, (ii) Cu and Cr and (iii) As, Cd, Fe, Hg, Pb and Zn, while Ni and V exhibited singular spatial distributions compared to other elements. Elements such as As, Cd, Co, Fe, Hg, Mn, Pb and Zn appeared to be of anthropogenic enrichment, while Cr, Cu, Ni and V appeared to originate mainly from lithogenic and natural sources. The elements As, Cd, Hg and Pb exhibited high ecological risks, with Cd contributing about 46-66% of the overall ecological risk. The sampling sites S1, S5, S7, S9 and S10 depicted the highest ecological risks of 145, 103, 146, 121 and 146, respectively. The findings call for proper zoning of suitable agricultural areas and sound waste management protocols in urban and peri-urban landscapes. Further, remediation of contaminated soils and farmers’ sensitization are recommended for ecological and public health risk alleviation.

引言：随着发展中国家的城市化与工业化进程推进，城市及城郊农业对城市食物系统、就业及创收机会的贡献日益凸显。然而，城市化与工业化过程可能向农业土壤中释放重金属及微量元素等有害污染物，引发生态、环境与公共健康层面的诸多隐患。本研究针对肯尼亚内罗毕集水区城郊农田的土壤重金属与微量元素污染潜在风险展开评估。
方法：本研究从10个蔬菜种植区（S1-S10）共采集60份土壤样品，并依照标准操作规程进行样品前处理。采用电感耦合等离子体质谱法（inductively coupled plasma-mass spectrometry, ICP-MS）对样品中砷（As）、镉（Cd）、钴（Co）、铬（Cr）、铜（Cu）、铁（Fe）、汞（Hg）、锰（Mn）、镍（Ni）、铅（Pb）、钒（V）及锌（Zn）的浓度进行测定。
结果与讨论：分析结果显示，本次采集的土壤样品中砷、镉、钴、铬、铜、铁、汞、锰、镍、铅、钒及锌的浓度均出现升高，其中砷、镉、锰、铅及锌的超标样品占比分别为1.7%、13.3%、80%、1.7%及11.7%，超出农业土壤的容许限值。各元素的空间分布呈现三类相似的分布模式，不同元素的热点区域仅存在细微差异：（1）钴与锰；（2）铜与铬；（3）砷、镉、铁、汞、铅及锌；而镍与钒的空间分布模式与其他元素相比呈现单一性特征。砷、镉、钴、铁、汞、锰、铅及锌等元素表现出人为富集特征，而铬、铜、镍及钒则主要源自岩石成因与天然来源。砷、镉、汞及铅元素表现出较高的生态风险，其中镉对整体生态风险的贡献率约为46%~66%。采样点S1、S5、S7、S9及S10的生态风险指数最高，分别为145、103、146、121及146。本研究结果呼吁在城市及城郊区域合理划定适宜农业用地，并建立完善的废物管理规程。此外，为缓解生态与公共健康风险，建议对受污染土壤开展修复工作，并加强对农户的宣传教育。