Concentration levels of metals in vegetables grown in soils irrigated with river water in Addis Ababa, Ethiopia

Abstract

Samples of vegetables, water and soil were collected from four vegetable farms in Addis Ababa to evaluate the extent and trend of metal accumulation in these systems and health risk concerns to consumers. Vegetable samples were digested in HNO 3 and HClO 4, soil samples in Aqua Regia and water samples were pre-concentrated with methyl isobutyl ketone (MIBK) using the chelating agent ammonium pyrrolidine dithiocarbamate (APDC). All the samples were analyzed for Co, Cr, Cu, Mn, Ni, Zn, Cd and Pb with flame atomic absorption spectrophotometer. The concentrations of Cd (0.12–1.13 mg kg −1) and Pb (0.11–0.89 mg kg −1) in the vegetables surpassed the maximum recommended levels. The total metal concentrations in soils were (mg kg −1): Cr, 9.9–22.8; Co, 28.0–47.3; Cu, 25.1–51.4, Mn, 1000–1054; Ni, 16.4–55.8; Zn, 146–149; Cd, 1.4–1.8 and Pb, 22.0–50.7. The trace metals Cd, Co, Cu, Mn and Ni in most of the water samples collected from Goffa, Kera and Akaki farms also surpassed irrigation water guideline limits, which might be a case for high accumulation of metals in the soils. However, the soil pH (6.5–7.6) and high cation exchange capacity (CEC), 38.41–50.18, coupled with high clay content, 37–51%, of the soil seemed to limit metal uptake by the vegetables. The physical parameters, pH (7.43–7.89) and electrical conductivity (0.33–1.54 dS/m) of irrigation waters measured at 25 °C were found within the acceptable range.