Physicochemical Characteristics and Heavy Metals Contents in Soils and Cassava Plants from Farmlands along A Major Highway in Delta State, Nigeria

Soil samples and cassava tubers and leaves collected from farmlands along Agbor-Asaba expressway were analysed for their heavy metal levels in order to assess their levels of contamination on the environment as a result of traffic activities. Physicochemical properties of the soil samples were also determined. The soil pH had a mean value of 5.15± 0.48 indicating that the soils were moderately acidic. Total Organic Carbon and Total Nitrogen mean values were 1.20 ± 0.13% and 0.09 ± 0.80% respectively showing presence of some organic matters. Electrical conductivity of the soil samples had the mean value of 5.94 ± 1.32μscm^-1 indicating significant presence of ionisable materials in the soils. Particle size was dominated by sand size fractions followed by clay and then silt which revealed that the soils were coarse and have low supply of nutrients and moisture. The mean levels of heavy metals in the soil samples were (mgkg^-1) 142.93± 42.16 for Fe, 59.34 ± 25.21 for Zn, 14.27 ± 5.39 for Cr, 13.63 ± 5.41 for Pb and 24.98 ± 15.57 for Ni. These metal levels were in the abundance trend of Fe> Zn> Ni> Cr > Pb. The mean metal concentrations obtained in the cassava leaves and tubers respectively were (mgkg^-1) 21.70 ± 3.45 and 9.62 ± 3.53 for Fe, 4.15 ± 1.01 and 1.15 ± 0.44 for Zn, 5.12 ± 2.75 and 0.37 ± 0.63 for Cr and 3.46 ± 1.58 for Pb only in leaves. For both the soil and plant samples, the heavy metal levels were significantly higher than the levels obtained in the control sites confirming some heavy metal enrichments in the soils studied. The plant concentration factor values were in the order Cr > Pb > Fe > Ni showing that chromium was the easiest to migrate among all the metals studied and that all the metals fall into the category showing medium accumulation except for Nickel which fell into the category of elements lacking accumulation. The overall results show that there is some metal enrichments on the soils and cassava plants as a result of automobile emission on the highway.

Keywords
Heavy metals; pollution; busy expressway; farmland soils; cassava plants