Heavy metal concentration in amaranthus, cowpea, black nightshade and kale in response to wastewater irrigation in Kitui County, Kenya

Abstract

Low quality water, treated and non-treated wastewater, is used for irrigation and production
of leafy vegetables in developing countries. The water contains crucial plant nutrients but
also toxic substances including heavy metals such as Cadmium, Copper, Zinc, Lead and
Iron. By consuming plants grown with contaminated water containing elevated metal
concentrations puts human life at risk. Field experiments were conducted at South Eastern
Kenya University to evaluate the heavy metal concentration of amaranthus, cowpea,
black nightshade and kale irrigated with wastewater. To test the performance of the leafy
vegetables, treatments were laid in a Randomized Complete Block Design (RCBD) with
three replications. The vegetables were subjected to two irrigation systems, foliar irrigation
(S) and root irrigation (R) with two water treatments, wastewater (WW) and Clean /
fresh water (CW) applied to the vegetables. The vegetables, soil and water samples were
analyzed for heavy metal concentrations using Atomic Absorption Spectroscopy (AAS).
The vegetables were harvested fortnightly upon attaining their horticultural maturity. They
differed in heavy metal accumulation with highest concentration of heavy metals found in
Brassica oleraceae with a concentration of 3.50 mg/kg and 20.33 mg/kg of Cadmiumand
and Lead respectively while Amaranthus, cowpea and black nightshade had relatively lower
concentrations. Some of the vegetables grown with wastewater exceeded the maximum
permissible limits of heavy metals in vegetables. This study recommends consistent
monitoring of heavy metal concentrations in agricultural soil and water used for irrigation
to avoid excessive accumulation in vegetables.