Heavy metals concentration in cabbage {brassica oleracea), lettuce [lactuca sativa)t and spring onion {allium cepa) grown along some urban rivers in the Kumasi Metropolis

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This study assessed heavy metals (lead, cadmium, arsenic, nickel, copper, iron and zinc) concentration in lettuce, cabbage and spring onions grown along River Subin and its tributaries; Rivers Wiwi, Oda and Sisa which are extensively used by urban vegetable farmers in Kumasi to irrigate their vegetables. Atomic Absorption Spectrophotometry was used to evaluate the levels of heavy metals in the vegetable samples at four sitps along the river banks. There were no well defined trends in heavy metals concentration in all the vegetable samples. However, heavy metals in vegetables at the KNUST and Ayeduase sites were relatively higher compared to the Sepetimpon and Ofoase sites. Levels of nickel, zinc, copper and iron in all three vegetable samples (lettuce, cabbage and spring onion) at the four sites were below the WHO/FAO recommended guideline levels of 67.90 mg/kg for nickel, 99.40 mg/kg for zinc, 73.3 mg/kg for copper and 425 mg/kg for iron. However, comparing the three vegetables in terms of levels of heavy metals accumulation, averagely-nickel was highest (0.192mg/kg) in spring onion, zinc- (2.85mg/kg) in spring onions, copper (0.355mg/kg) in spring onion and iron (13.91 mg/kg) in cabbage. Levels of cadmium, lead and arsenic in all three vegetable samples were above the WHO/FAO recommended guideline levels of 0.2 mg/kg, 0.3 mg/kg and 0.43 mg/kg, respectively. Cadmium was highest (1.06mg/kg) in spring onion, lead (1.57mg/kg) in spring onion and arsenic (4.14mg/kg) in lettuce. The presence of cadmium, lead and arsenic above the recommended WHO/FAO guidelines levels presents a potential hazard to the health of consumers.
A Thesis submitted to the Department of Theoretical and Applied Biology, College of Science, Kwame Nkrumah University of Science and Technology, in partial fulfilment of the requirements for the Degree of Master of Science (Environmental Science), 2008