Risk assessment of selected trace metals Cr, Cd, Pb, Zn and Cu at selected growth stages (15 days – germination or seedling stage, 30 days – tillering stage, 45 days – booting/shooting stage, 60 days – heading/earing stage, 75 days – flowering stage and 90 days – ripening stage) of two varieties each of wheat – Triticum aestivum L. (Pavon-76 and Siettecerros) and yellow maize-Zea mays L. (TZEE-Y) and Popcorn (Zea mays everta L.) were investigated in Kano State, a cereal growing Sudan savanna zone of Nigeria. Doruwa Salau at a distance of 345.79metres perpendicular to the Kano-Zaria Highway with average daily traffic density of 19,288 is the experimental site. The control site was the Irrigation Research Station (IRS), Kadawa at a distance of 1934.61metres from the roadside with an average daily traffic density of 3. A preliminary analysis of the trace metals and physico-chemical parameters of the soils was determined. Samples of leaf, stem and root at the selected growth stages were collected in triplicates from the two cultivars of wheat and maize grown at the experimental and control sites. Corresponding soil samples were collected in duplicates. Pb, Cd and Cu, Cr, Zn of the plant and soil samples were assessed using atomic absorption spectrophotometer and energy dispersive X-ray fluorescent respectively. Selected risk assessment techniques like soil-plant transfer coefficient (TC), plant uptake factor (PUF) and translocation factor (TF) were worked out. Trace metal levels were highest in the crops than in the soils at both sites indicating atmospheric origin. Plant parts trace metals levels were higher at Doruwa Salau than at the IRS, Kadawa, indicating proximity to the source of pollution. The total trace metal concentration was generally higher at the 15-days and 30-days growth stages reflecting the growth dilution of chemicals as the plant biomass increases. ANOVA for individual trace metals was highly significant (P = 0.01) for Zn (Pavon-76), Cr, Cu and Zn (yellow maize) and Cr (popcorn) indicate a strong selective ability of the crops to accumulate certain metals. ANOVA for leaf trace metal level was highly significant in Pavon-76 indicating influence of the plant parts to metals in crops. ANOVA for soil trace metals among the four sampling units was non-significant suggesting that the crop trace metal levels were of atmospheric origin and not from the soil. Correlation analysis identified a weak and non-significant relationship between soil and cereal crops indicating atmospheric deposition. Cu (15.70 – 107.17mg/kg), Zn(29.75 – 903.36mg/kg), Cr (3.25 – 20.65mg/kg), Pb (200 – 2520mg/kg) and Cd (115 – 370mg/kg) exceeded the permissible limits of the Joint FAO/WHO Food Standards (2006) (Pb = 0.1mg/kg; Cd = 0.01mg/kg; Cr = 1.30mg/kg; Cu = 10.00mg/kg; Zn = 5.00mg/kg). Health risk assessment of metals in roadside grown crops consumed by humans is a very good technique for predicting threats to life. Pavon-76 and Popcorn were identified with high phyto-remediation potentials and can be classified as hyper-accumulators. They can be employed in bioremediation approach for clean-up of polluted lands. It could be concluded that metals in the cereal crops originated from polluted atmosphere by motor vehicles and not from the soils.
TRACE METAL CONTENTS AT SELECTED GROWTH STAGES OF WHEAT AND MAIZE GROWN ON ROADSIDE SOIL AT KADAWA KANO STATE, NIGERIA