Comparative analysis of heavy metal concentration in secondary treated wastewater irrigated soils cultivated by different crops

Abstract

ABSTRACT:The use of treated urban wastewater for irrigation is a relatively recent innovation in Botswana and knowledge is still limited on its impact on soil heavy metal levels. The aim of this study is to analyze and compare heavy metal concentration in secondary wastewater irrigated soils being cultivated to different crops: olive, maize, spinach and tomato in the Glen Valley near Gaborone City, Botswana. The studied crop plots have been cultivated continuously under treated wastewater irrigation for at least 3 years. Most crop farms have sandy loam, loamy sand soils. Based on food and agriculture organization, heavy metal threshold values for crop production have been studied. Results showed that the wastewater irrigated soils in the Glen Valley have higher cadmium, nickel and copper than desirable levels, while the levels of mercury, lead and zinc are lower than the maximum threshold values recommended for crop production. The control sites show that the soils are naturally high in some of these heavy metals (e.g copper, zinc, nickel) and that crop cultivation under wastewater irrigation has actually lowered the heavy metal content. Comparing between the crops, mercury and cadmium levels are highest in soils under maize and decline linearly from maize to spinach to olive to tomato and control site. By contrast, concentrations of the other metals are at their lowest in maize and then increase from maize to spinach to olive to tomato and to control site.

Concentrations of Some Trace Elements in Vegetables Sold at Maun Market, Botswana

“…Results of this study show that the concentration of cadmium in vegetables collected from street vendors was in the range 0.18 -0.56 mg/Kg while it was 0.19 -1.00 mg/Kg for vegetable samples from supermarkets. Other studies (Dikinya & Areola, 2010;Sharma et al, 2009) obtained concentrations in the ranges 0.04 -0.14 mg/L, 0.10 -4.30 mg/Kg in Botswana and India respectively. The permissible limit of cadmium in plants recommended by FAO/WHO is 0.02 mg/Kg.…”

“…On the other hand, concentrations of copper in vegetables obtained from supermarkets ranged between 1.28 -61.93 mg/Kg with the least concentration in tomatoes and the highest still in tomatoes. Concentrations of copper were found in the range of 0.75 -2.00 mg/Kg in tomatoes grown in Glen Valley farms in Botswana (Dikinya & Areola, 2010). Harmanescu et.al, (2011) found mean concentrations of 1.77 ± 0.23, 1.37 ± 0.14 mg/Kg of copper in cabbage and onions.…”

Sorption of Heavy Metals by Ferralic-Arenosol and Vertic-Luvisol: A Comparative Isothermal Study

“…The soil samples were analyzed for physico-chemical characteristics, including particle size distribution (sand %, silt %, clay %), soil colour, pH and cation exchange capacity (CEC) using laboratory standard methods (Breitbart, 1988;van Reeuwijk, 1993in Dikinya & Areola, 2010. Clay extracts were further analyzed/scanned for morphological and chemical characteristics (Figure 1 and 2) using the Philips XL30 Environmental Scanning Electron Microscope/EDAX.…”

Efficiency of chelating agents in retaining sludge-borne heavy metals in intensively applied agricultural soils

“…Hence, the choice of the three heavy metals (Cu, Co and Zn). In particular, Cu and Zn are reported to build-up on subsurface soils following additions or irrigation with consequent accumulation on vegetable crops grown in that media (Dikinya and Areola 2010).…”