Geochemical characterisation of soil and road dust from an informal settlement and their possible health implications: a study of Winnie Mandela informal settlement, Gauteng, South Africa

Abstract

In Africa, different types of informal settlements are rising in quantities, capacities and population. The large population in urban informal settlements lead to a huge accumulation of wastes and the release of chemicals such as heavy metals to the surrounding environment. The aim of the study was to give an overview on the current heavy metal content in soil and road dust from the Winnie Mandela informal settlement in Gauteng, South Africa, and to evaluate its possible health risks. The current major and trace elements in soil and road dust samples were analysed by the wavelength dispersive X-ray fluorescence method, whereas the mineralogical compositions of soil and road dust were analysed by the X-ray diffraction method. The pollution status of soil and road dust were assessed by a geo-accumulation index, contamination factor, pollution load index and enrichment factor, while health risks were assessed by a health risk assessment model. The findings of the study exhibited quartz as a dominated mineral in both soil and road dust. The concentrations of major elements in soil were descending as silicone dioxide, aluminium oxide, iron oxide, manganese oxide, potassium oxide, titanium dioxide, calcium oxide, magnesium oxide, phosphorus pent-oxide, sodium oxide, with iron oxide, phosphorus pent-oxide, silicone dioxide and titanium dioxide higher than their average shale values. In road dust, major elements were as silicon dioxide, aluminium oxide, iron oxide, calcium oxide, potassium oxide, sodium oxide, magnesium oxide, manganese oxide, titanium dioxide, phosphorus pent-oxide with silicon dioxide and phosphorus pent-oxide above their average shale values. Moreover, seventeen (17) trace elements (As, Ba, Co, Cr, Cu, Nb, Ni, Pb, Rb, Sc, Sr, Th, U, V, Y, Zn, Zr) were analysed in soil and road dust. Their mean concentrations in soil samples were descending as Cr > Ba > Zr > V > Zn > Ni > Cu > Rb > Sr > Co > Pb > As > Y > Sc > Nb > Th > U, with As, Co, Cr, Cu, Nb, Ni, Pb, Zn, Zr, V and Zr above their average shale values. In road dust, they were Cr > Ba > Zn > Zr > Sr > V > Rb > Cu > Ni > Pb > Co > Y > Nb > As > Sc > Th > U. Trace elements such as Ba, Cr, Cu, Pb, Zn and Zr surpassed their average shale values. The assessment of pollution through the geo-accumulation index revealed that the quality of soil and road dust was moderately to heavily contaminated by Cr, whereas all other elements were categorised as uncontaminated to moderately contaminated. According to the contamination factor, Cr was classified as very high contamination in soil, whereas As, Co, Cu, Nb, Ni, Pb, Sc, V, Zn and Zr were categorised as moderate contamination. Elements such as Ba, Rb, Sr, Th, U and Y were regarded as low contamination. It further exhibited road dust to be very highly contaminated by Cr, moderately contaminated by Ba, Cu, Pb, Zn and Zr, and lowly contaminated by As, Co, Nb, Ni, Rb, Sc, Sr, Th, U, V and Y. Additionally, the pollution load index also affirmed that soil and road dust in this study were very highly polluted by heavy metals, which substantiates the findings that human activities can exacerbated the level of trace elements in urban settings. Moreover, the results of the enrichment factor in soil and road dust categorised Cr as significant enrichment, possibly from the influence of human activities. In soil and road dust, Co and Zn were elucidated as minimal enrichment, respectively, whereas all other trace elements in soil and road dust were of natural origin. The results of the pollution indices also revealed that trace elements in this study had natural and anthropogenic origins. Furthermore, the results of the noncarcinogenic risk assessment in soil revealed a possibility of noncarcinogenic risk to children and adults. In road dust, a possibility of noncarcinogenic risks to children and adults were also found. Children were at higher risk of non-carcinogenic than adults in both soil and road dust. For carcinogenic risk in soil, the total carcinogenic risk values in children and adults were above the acceptable limit signifying a likelihood of carcinogenic risk to the local inhabitants. In road dust, the carcinogenic risk assessment also exhibited a chance of carcinogenic risk to children and adults. The study revealed that children are at higher carcinogenic risk than the adult population. Exposure to As, Ba, Co, Cr, Cu, Nb, Ni, Pb, Zn, V and Zr, which were above their average shale values, may lead to cancer, miscarriages, hearing and visual impairment, asthma, renal failure, high blood pressure, headaches and dizziness, or reproductive system problems and cardiovascular disorders among the local residents. Therefore, this study concluded that activities in urban informal settlements contribute significantly to the rise of heavy metal in soil and road dust. Furthermore, the local residents may suffer non-carcinogenic and carcinogenic risks due to high concentrations of trace elements in soil and road dust. Thus, proper waste management, remediation, cleaning and regular monitoring of heavy metals are recommended for the safety of the population and sustainability of the settlement.