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

Abstract

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.

Emerging environmental contaminants: Sources, effects on biodiversity and humans, remediation, and conservation implications

“…These pollutants are released into the environment through agricultural runoff, direct discharge, or wastewater treatment facilities. 23 , 25 – 27 Some EECCs are released into the environment by the application of untreated human excreta and animal waste on farms. 5 , 25 These excreta and wastes contain a significant quantity of hazardous EECCs as well as other materials such as polycyclic aromatic hydrocarbons, trace metals, and other contaminants.…”

“… 25 Some farmers in developing countries, for instance in Kenya, India, Bangladesh, Zambia, Zimbabwe, and Tanzania, to mention a few, utilize agrochemicals and human and animal waste extensively in their fields, which increase the accumulation and risks of EECCs. 23 , 25 , 28 , 29 …”

“…Besides, this can affect the interactions between organisms (e.g. plant–pollinator interactions) in the ecosystem, leading to the collapse of primary productivity 7 , 23 and ecosystem services, for instance, airflow to the soil and the cycling of nutrients. 3 , 7 , 12 , 13 This can further have an impact on the species behavior, e.g.…”

“…Previous studies highlight that the most deprived local communities in the developing world, particularly those residing near mining or industrial areas and cultivating their crops on contaminated soils, are the most vulnerable, as is wildlife that forages or drinks in EECC-contaminated water bodies. 9 , 15 , 22 , 23 However, people in developing countries still lack knowledge about EECCs, their problems (i.e. threats to human well-being, ecosystem safety, and the environment), and remediation technologies.…”

Effect of Atmospheric Deposition of Pollutants on the Concentration of Trace Metals on Some Sun-Dried Food Items Dried at a Non-Vehicular Corridor in Ile-Ife Nigeria

“…The farm products cassava (Manihot esculenta), plantain (Musa x paradisiaca) white yam (Dioscorea rotundata), yellow yam (Dioscorea cayenensis), sweet potato (Ipomoea batatas), and cocoyam (x sagittifolium), are widely consumed in Nigeria as sources of carbohydrates and essential nutrients for human health. Furthermore, farm products have been shown to contain heavy metals: lead (Pb) and cadmium (Cd), as well as essential trace elements copper (Cu), zinc (Zn), manganese (Mn), and nickel (Ni) (Pilarczyk et al, 2013;Corguinha et al, 2015;Kumar & Seema, 2016;Malinidevi & Inbanila, 2016;Orisakwe et al, 2017;Hitler et al, 2018;Rai et al, 2019;Motas et al, 2021;Ahogle et al, 2023). While the essential elements zinc, manganese, copper, and nickel are widely recognized for their nutritional and beneficial functions in the human body (Olivares & Uauy, 1996;Brown et al, 2001;Soetan et al, 2010;Bhowmik et al, 2010;Temple & Masta, 2004;Devi et al, 2014;Prashanth et al, 2015;Kelkitli et al, 2016;Karim, 2018;Fallah et al, 2018;Vinha et al, 2019), lead and cadmium have no known benefits to man, however, their adverse health effects on man are well documented (Godt et al, 2006;Nordberg et al, 2018;Satarug, 2018;Andjelkovic et al, 2019).…”