Assessment of concentrations of heavy metals in three leafy vegetables irrigated with wastewater in Hadnet district, Mekelle, Ethiopia

Abstract

Mekelle city is one of the Ethiopian cities suitable for urban and peri-urban agriculture for cultivating leafy vegetables using wastewater. The consumption of unprocessed and processed leafy vegetables is also very high in the city. Wastewater samples collected from four experimental sites (ESs) in the district were found to have higher concentrations in 4 (i.e., Cd, Cr, Cu, and Mn) of the 10 heavy metals tested than the permissible limit established by the FAO (2004). Interestingly, spring water samples collected from another district called Kallamino, designated as a comparison site, also have higher concentrations in 4 (i.e., Al, As, Cd, and Cu) of the 10 heavy metals tested. However, the leafy vegetables grown in the least contaminated ES were found to have higher concentrations in 6–7 of the 10 heavy metals tested. The lettuce and spinach had elevated concentrations of As and Al, respectively. The wastewater used to irrigate vegetable farms in the Hadnet district of Mekelle is not safe enough. More importantly, the soils of the farms might have accumulated far more amount of heavy metals. The cultivated lettuce, spinach, and cauliflower are highly contaminated. Thus, the use of wastewater for irrigating urban and peri-urban farms needs to be regulated.