Heavy metals in soil and vegetables of allotment gardens in the Cape Town environment

Abstract

This study investigated the concentration of selected heavy metals in soil, water and vegetable crops from selected allotment gardens in Cape Town. Heavy metals occur naturally in the earth’s crust, but due to human activities, their biochemical balance and geochemical cycles have been altered. Heavy metals are abundant in air, soil and water due to environmental pollution. It was therefore of interest to conduct this study to determine the levels of heavy metals (Pb, Cd, Mn, Zn, Cr, Cu, Ni, Fe and Co) in soil, water and vegetables such as spinach, cabbages, green peppers, brinjals and leek onions. Soil, water and vegetables were sampled during winter and summer from the allotment gardens of Cape Town environment and were analyzed using Inductively Coupled Plasma (ICP). The physicochemical parameters of soil and water were determined during both seasons. The average pH of water in winter was 6.53 ± 0.6, while the summer pH was 6.71 ± 0.7. The average soil pH was 6.58 ± 0.2 in winter, while in summer the soil pH was 6.60 ± 0.2. The soil organic matter ranged from 1.7 % to 13.5 % in both seasons. The heavy metals in water and soil showed seasonal fluctuation (p < 0.05). In summer the concentrations in water ranged from 0.062 mg/L to 0.947 mg/L, while in winter it ranged from 0.002 mg/L to 2.347 mg/L. The soil heavy metal concentrations in summer ranged from 0.52 mg/kg to 1127.41 mg/kg, while in winter it ranged from 0.59 mg/kg to 1209.95 mg/kg, Fe having the highest concentrations for both seasons. The heavy metal concentration in vegetables was generally higher in summer than in winter, although Fe was still the highest in both winter and summer. Fe was particularly high in spinach with a concentration of 144.28 mg/kg in summer, while in winter the concentration was 116.56 mg/kg, followed by leek onion and cabbage. The results for water and soil showed a decrease and weak correlation with a decreasing order Fe > Cu > Cr > Ni > Pb > Co > Zn > Mn > Cd > and Fe > Zn > Mn > Cu > Pb > Cr > Ni > Co > Cd, respectively. The distribution sequence of the heavy metals in vegetables during winter is in the order; spinach (Fe > Zn > Cr > Mn > Cu > Co> Cd ≥ Pb ≥ Ni); cabbage (Fe > Mn > Cr > Zn > Cu > Co > Cd ≥ Pb ≥ Ni) and leek onion (Fe > Zn > Mn > Cr ≥ Co > Cu > Cd ≥ Pb ≥ Ni). The corresponding sequence during summer was; spinach (Fe > Zn > Mn > Co > Cr > Cu > Cd ≥ Pb ≥ Ni); cabbage (Fe > Mn > Cr > Zn > Pb > Co > Cu > Cd ≥ Ni) and leek onion (Zn > Fe > Pb ≥ Mn > Cr > Co > Cu > Cd ≥ Ni); brinjal ( Fe > Mn > Zn > Cr > Cu > Co > Cd ≥ Pb ≥ Ni) and green peppers ( Fe > Zn > Mn > Co > Cu > Cr > Cd ≥ Pb ≥ Ni). Results showed that concentration levels of all the selected heavy metals were below the permissible limits in soil, water and vegetables set by WHO.