Heavy metals in an urban valley bottom soil in south-western Nigeria

Valley bottom soils were investigated for heavy metal contamination to assess the possible risk for human nutrition and health in the city of Ibadan in southwestern Nigeria. Seventeen soil and seventeen plant samples were taken from valley bottom land of major streams within the city where intensive dry season farming are being practiced. The contents of Pb, Cr, Ni, Co, Cu and Zn were analyzed. Total HNO3 - HCl soluble metals in the soils were determined by Flame atomic absorption spectrophotometer (FAAS). Plant samples were digested with HNO3 and analyzed by FAAS. A number of soils had total heavy metal a content above the threshold values for garden soils applied in many European countries, Canada, Australia and New Zealand. Many of the soils contained sufficient quantities of these metals to pose potential health risks if the contaminated materials entering into the food chain. Valley bottom soils near motorways, automobile workshops and scrap yards had contents larger than soils elsewhere in the city. Heavy metal contents in the soil ranged from 183.6 to 4059.3 mg kg(-1) Pb, 58.9 to 281.4 mg kg(-1) Ni, 56.9 to 250.8 mg kg(-1) Cr, 71.9 to 927.2 mg kg(-1) Cu, 624.3 to 4907.7 mg kg(-1) Zn with C.V. 69.97% and 65.4 to 347.2 mg kg(-1) Co with C.V. 48.14%. These values are 500% higher than the referenced country side valley bottom soils. The range of metal concentration in the edible tissues of vegetables were from < 1(-1) to 34.3 mg kg(-1) Pb, 2.7-5 mg kg(-1) Ni, < 1 to 36 mg kg(-1) Cr, 19 to 73.3 mg kg(-1) Cu, 95.3 to 384.7 mg kg(-1) Zn and 11.3 to 29.3 mg kg-1 Co. The contents of metals especially Pb in some locations within the city are enough to pose health hazard to consumers of the vegetables. It was concluded that the valley bottom soils of Ibadan city and the vegetables grown on them may pose a health hazard to consumers especially young children.