Heavy metals in waste water, soil and vegetables around Chunga gardens in Lusaka and their potential health risks

Abstract

Dietary exposure to several heavy metals at high concentrations including Cadmium (Cd), Copper (Cu), Lead (Pb) and Zinc (Zn), have been recognized as a risk to human health. This is through the consumption of vegetables. Hence, the study aimed at determining the concentration and bioaccumulation factors of heavy metals in wastewater, soil and vegetables around Chunga gardens in Lusaka District of Zambia and their potential human health risks. A cross sectional study design was conducted, with a total of 45 samples collected, which included: 9 wastewater samples, 8 soil samples and 28 vegetable samples namely Brassica rapa (Chinese Cabbage), Brassica napus (Rape), Brassica juncea (Mustard Green:Mpilu) and Cucurbita moschata (Pumpkin Leaves) to ascertain heavy metal concentrations. Heavy metal analysis was determined using Atomic Absorption Spectrophotometer (AAS) and Stata version 14 was used for data analysis. The study revealed that the concentrations of Cd, Cu, Pb and Zn were beyond the maximum permissible standards in wastewater, soil and vegetables based on the World Health Organisation standards. The concentrations of heavy metals found in the wastewater ranged in the order of Cu (2.59 mg/l - 0.01 mg/l) > Pb (0.66 mg/l - 0.01 mg/l) > Zn (0.60 mg/l - 0.01 mg/l) > Cd (0.03 mg/l - 0.01 mg/l).The concentrations of heavy metals found in the agricultural soil were in the sequence of Cu (133.00 mg/kg – 2.59 mg/kg) >Zn (73.90 mg/kg - 0.87 mg/kg) > Pb (42.20 mg/kg -8.20 mg/kg) > Cd(19.00 mg/kg – 0.10 mg/kg). The concentrations of heavy metals in edible parts of selected vegetables were in the order of Zn (418.26 mg/kg - 23.15 mg/kg) > Cu (416.185 mg/kg -0.94 mg/kg) > Pb (254.515 mg/kg - 0.01 mg/kg) > Cd (5.08 mg/kg- 0.01 mg/kg).The median concentrations of Cd, Cu, Pb and Zn were not different from the vegetables irrigated with wastewater and the freshwater (control group). Median Concentrations of heavy metals in wastewater, soil and vegetables were statistically different (p<0.05).Based on the observed bio-accumulation factors, Brassica napus showed more hyper-accumulation potential as compared to the other vegetable samples. The potential non-carcinogenic health risk assessment showed that Cd, Cu, Pb and Zn are heavy metals which are likely to produce non-carcinogenic adverse health effects, while average carcinogenic risk values obtained for Pb and Cd using Cancer Slope Factors (CSF) in this study indicated a lifetime (70 years) probability of contracting cancer from the ingestion of contaminated vegetables to both adults and children. Based on the findings of the study repetitive consumption of the contaminated vegetables might pose undesirable health effects inclusive cancer for the consumers. Therefore, the study emphasizes the need for systematic monitoring of the environment contaminated with heavy metals to avoid health risks as well as an assessment of pollution source distribution is highly recommended.

Keywords: Heavy metals, Wastewater; Irrigation; Soil; Vegetables; Concentration; Permissible limits; Health Risk; Bio-accumulation Factor: