Vertical distribution of heavy metals in wastewater-irrigated vegetable garden soils of three West African cities

Abstract

Application of untreated wastewater to irrigate urban vegetable gardens is raising serious concern about possible health risks associated with the consumption of these vegetables particularly with regard to the concentrations of heavy metals (HM) in their edible portions. The soil concentrations of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn), were investigated in seven vegetable gardens from the three West African cities of Kano (Nigeria), Bobo Dioulasso (Burkina Faso) and Sikasso (Mali). Also determined were input–output balances of Cd and Zn from five vegetable gardens under 30 years of wastewater irrigation in Kano. In these gardens Cd (2.3–4.8 mg kg−1) and Zn (13–285 mg kg−1) concentrations throughout the profile attained unsafe levels. The concentrations of Cu (0.8–18 mg kg−1), Cr (1.8–72 mg kg−1), Ni (0–17 mg kg−1) and Pb (0.6–46 mg kg−1) were below the safety thresholds for arable soils. Overall, concentrations of Zn, Cd, Pb and Ni were higher in Kano than in Bobo-Dioulasso and Sikasso. Input–output analyses in Kano indicated that irrigation wastewater contributed annually 400–3,700 g Cd ha−1 and 7,200–22,300 g Zn ha−1, fertilizer 30–2,100 g Cd ha−1 50–17,600 g Zn ha−1, harmattan dust 0.02–0.4 g Cd ha−1 and 40–200 g Zn ha−1 while 300–500 g Cd ha−1 and 2,700–4,700 g Zn ha−1 came from rainwater inputs. Input–output calculations subtracting the amounts of HM taken out in vegetable biomass and that lost to leaching from total inputs yielded an annual net positive balance of 700–4,160 g Cd ha−1 and 9,350–39,700 g Zn ha−1. If such balances remain unchanged for another 10–20 years vegetables raised in these garden fields are likely to be unsuitable for human consumption.