Abstract
A study was carried out in March 2010 at Firle Sewage Works in Harare, Zimbabwe to determine the effects of long term wastewater irrigation on the concentrations of heavy metals (Zn, Cu, Mn, Cd, Pb, Ni, Fe and Cr) in soil, and their subsequent accumulation in maize plants. The study revealed that long term wastewater use for irrigation results in heavy metal accumulation in soils and bioaccumulation in plants beyond maximum permissible limits (MPL) for both humans and livestock consumption. Lead had highest transfer factor and iron had the least transfer factor. The soil pH was found to be less acidic (pH = 5.6) in soils exposed to waste water than in soils where no wastewater had been applied (pH = 5). As a recommendation there is need for phytoextraction of heavy metals by intercropping maize plants with local agro forestry shrubs to reduce amount of heavy metals in the soil.
Highlights
Wastewater disposal is becoming a problem in developing countries as large quantities of municipal waste and industrial effluent are being produced due to increased urbanization and industrialization respectively (Alloway & Ayres, 1995)
The mean concentrations of Cd, Ni, Pb and Zn in maize plants harvested from soils exposed to wastewater were found to be above the World Health Organization (WHO) maximum permissible limits for adult livestock feeding (Table 2)
The long term use of wastewater in irrigation resulted in increased heavy metal concentrations namely Cu, Zn, Mn, Pb, Cd, Ni, Fe and Cr in soil
Summary
Wastewater disposal is becoming a problem in developing countries as large quantities of municipal waste and industrial effluent are being produced due to increased urbanization and industrialization respectively (Alloway & Ayres, 1995). The major ones are batteries, consumer electronics, ceramics, light bulbs, house - dust and paint chips, lead foils such as wine bottle closures, used motor oils, plastics, and some inks and glass (Woodbury, 1993). This is a typical example of how the increase in anthropogenic activities has led to the emission of various pollutants into the environment and how different types of hazardous substances are introduced in the soil and land environments (Kho et al, 2007)
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