Effects of Wastewater Irrigation on Quality of Urban Soils in Kano, Nigeria

Abstract

This study used the soil management assessment framework (SMAF) to evaluate the quality of wastewater irrigated soils in urban Kano, Nigeria in the dry season of 2012. Three sites each on Challawa and Jakara rivers receiving industrial and domestic wastewaters respectively were sampled for water and soil; and compared with three sites at Watari river that receives no wastewater thereby serving as control. Heavy metals pollution and fertility indices were used to establish minimum data set (MDS) because of their effects on yield and safety of crops. Soil fertility and pollution indices used as indicators included pH, EC, bulk density (BD), organic carbon, NPK, total heavy, exchangeable and soluble heavy metals. Physical and chemical properties of irrigation waters were also evaluated and correlated with soil properties. There were variations from low to medium or medium to high for many of the parameters and P was excessively high at Jakara (213.52g/kg). The increased tendency for the wastewater irrigated soils to be saline was also observed due to higher Na content (3.20 and 1.11cmol/kg at Jakara and Challawa respectively). The Jakara sites were lower in BD (1.30) thereby making them easier to till especially against the control (1.60). There was a downstream decreasing pattern in the concentrations of all heavy metals across Challawa and Jakara sites and a reverse phenomenon at the control. The PO43-, NO3-, HCO3-, B and Cl- as well the heavy metals contents in irrigation waters across all sites have all exceeded the increased hazard limit with quality generally in the order Watari>Challawa>Jakara. Many water properties were significantly and positively correlated with soil properties. All the quality indexes were within same marginal range (0.53, 0.498 and 0.485 for Challawa, Jakara and Watari respectively) due to the counterbalancing effect of fertility and pollution indices.