Heavy metal pollution of vegetable crops irrigated with wastewater 41 Heavy Metal Pollution of Vegetable Crops Irrigated with Wastewater in Accra, Ghana

Abstract

Heavy metal concentrations in irrigation water (samples =120), soil (samples =144) and edible parts of both exotic and traditional vegetables (samples = 240) irrigated with wastewater from some parts of Accra were studied. The concentrations of heavy metals in mg/l were quantified in wastewater from Accra and groundwater at Mampong as Fe (0.67; 1.00), Mn (0.78; 0.31), Cu (0.06; 0.07), Zn (0.14; 0.13), Pb (0.08; 0.12), Ni (0.06; 0.13), Cr (< 0.006), Cd (< 0.002) and Co (< 0.005), soil Fe (164.38; 162.92), Mn (39.39; 20.09), Cu (7.21; 6.13), Zn (6.03; 7.45), Pb (9.31; 7.63), Ni (5.00; 2.97), Cr (0.51;0.85), Cd (0.07;0.09) and Co (0.73;0.87), and vegetables from Accra and Mampong. The wastewater used for irrigation had the highest concentration (mg/l) of Mn (0.78),  followed by Fe (0.67), Zn (0.14), Pb (0.08), Cu (0.06), Ni (0.06) while Cr, Cd and Co were below detection limits. Fe (164.38; 162.92), Mn (39.39; 20.09), Cu (7.21; 6.13), Pb (9.31; 7.63) and Ni (5.00; 2.97) levels were higher in wastewater irrigated soils than groundwater irrigated soils, respectively. However, average values were all below the FAO/WHO  recommended mean levels in mg/l for wastewater and soil as Fe (5.00; 50,000), Mn (0.20; 2,000), Cu (0.20; 100), Zn (2.00; 300), Pb (5.00; 100), Ni (0.20; 50.00), Cr (0.10;50.00), Cd (0.01;3.00) and Co (0.05;100), respectively. Concentration levels of heavy metals (mg/kg) in vegetables crops analysed from all sites were not elevated except for Pb in cabbage, (10.51), lettuce (10.19), green pepper (9.44), hot pepper (7.61) and ayoyo (9.05) compared to the FAO/WHO maximum recommended limit of 0.30 mg/kg for Pb. Health risk assessments showed that hazard indexes for the crops were below 1 (USEPA), indicating that normal  consumption of vegetables analysed pose no risk from heavy metal toxicities. However, to prevent any chronic health risk and extent of heavy metal contamination, steps must be taken to reduce human activities at the sites. Regular monitoring of heavy metals in the vegetables grown in wastewater irrigated areas is also necessary.