Comparative analysis of heavy metal concentration in secondary treated wastewater irrigated soils cultivated by different crops|
Dikinya, O. & Areola, O.
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.
Heavy metals; Maize; Olive; Spinach; Tomato; Wastewater