Impacts of Treated Wastewater on the Physico-Chemical Properties, Microbial Community and Heavy Metals Distribution in the Soils

This study aims to study the soils adjoining the WUPA sewage treatment Plant in Abuja so as to assess the influence of treated wastewater on the physical, chemical properties, microbial community, and the accumulation of some heavy metals in the soils. Soil samples were collected from five locations in the entire area (four locations along the main canal conveying the treated wastewater into the Wupa River and a location 50 m away from the canal to serve as a control site). Samples were collected and analysed for particle size distribution and some selected chemical properties, as well as some heavy metals, following standard laboratory procedures. The soils were generally sandy loam in texture with a high proportion of sand and a low clay fraction. The sand content ranged from 642 to 772 g/kg, while the clay ranged from 47 to 61 g/kg. Soil pH ranged from 5.58 to 6.29, placing the soil into the range of moderately acidic to slightly acidic. Electrical conductivity ranged from 9.63 to 43.00 µScm-1. Exchangeable cations were generally low; however, exchangeable acidity was influenced by the soil’s contact with treated wastewater since the lowest value of 9.63cmolkg-1 was recorded from the control site. The distribution of trace and heavy metals showed that there was an increase in the concentration of the trace and heavy metals in the soils, as the lowest values were only observed in the control site. However, the range of values observed for the trace and heavy metals was within permissible environmental safe limits except for Ni, which was higher than the permissible safe limit for plants. Fe ranged from 23.22 to 415.40 mg/kg, Mn ranged from 19.36 to 157.97 mg/kg, Cu ranged from 0.59 to 2.92 mg/kg, while Zn ranged from 2.21 to 8.87 mg/kg. For the heavy metals, Cr ranged from 12.69 to 29.77 mg/kg, while Ni ranged from 16.94 to 25.53 mg/kg. Cadmium in all the samples was below detectable limits. The infiltration rate of the soils ranged from 7.6 to 21.4 cmhr-1, rated as slow (at the control site) and rapid and very rapid (at the sites which had contact with treated wastewater). Soil microbial population showed that live bacterial counts were lower in the soils that had contact with treated wastewater, while in the control site, live bacterial count (32 %) was higher in the soils of the control site. It was concluded that there is a need for the adoption of improved sewage treatment processes that will eliminate toxic metals such as Ni, Cr, Zn, and Mn, among others. There is a need for further study on the quality of treated wastewater discharged into the Wupa River, where it eventually ends up as part of irrigation water in the area. Fishes from the Wupa River and vegetables grown in the area should be studied for possible bioaccumulation of heavy metals.