Heavy metal accumulation in untreated wastewater-irrigated soil and lettuce (Lactuca sativa)

Contamination of agricultural soils due to human activities is a major concern globally. The use of urban untreated wastewater for irrigation has been on-going for a long time and the major driving forces of this practice include water scarcity, easy availability, low/no cost and wastewater as a source of plant nutrients. Contamination of agricultural soils with heavy metals as a result of using untreated wastewater for irrigation as well as translocation of the heavy metal into the lettuce was investigated. Soil samples were analysed for pH, electrical conductivity, organic carbon, cation exchange capacity and extractable heavy metals while lettuce samples were analysed for total heavy metal content. The soil samples were found to be sandy clay loam and sandy loamy in texture and alkaline. The organic carbon contents were found to be relatively low with low salt accumulations at both farms. The heavy metal levels in the soil samples from Airport residential farm were in the order Zn > Pb > Cu > Ni > Cd > Cr whilst that of Dzorwulu farm was in the order Zn > Pb > Cu > Ni > Cr > Cd. The soils from the Airport farm were found to be contaminated with Pb, Cu and Zn while the Dzorwulu farm showed contamination for all the metals except Ni. All the heavy metals were observed to have a high degree of transfer from the soil into the lettuce except for Cr at the Airport farm. Heavy metal concentrations in the lettuce samples were all above the FAO/WHO recommended levels. From the observations in this work, irrigation of farm lands with untreated wastewater tends to introduce contaminants into the soils which can be easily transferred into the farm produce. The high levels of these heavy metals pose a health risk to the consumers of such crops.