Effects of using industrial wastewater for irrigation on heavy metals in soils and crops: a case of kano metropolis, nigeria

Heavy metals can now be considered as natural constituents of the environment, but indiscriminate use for human purposes has altered their geochemical cycles and biochemical balance. This study was conducted in Kano metropolis to assess the heavy metals contamination in soils and crops where wastewater is used for irrigation. Soil and crop (Cucumber, Maize, Onion and Spinach) samples were randomly collected from 3 sites, digested and analyzed for Cd, Cr, Ni and Pb using Atomic Absorption Spectrophotometer (AAS). The results were compared with the standard guidelines set by World Health Organization (WHO) and were used to estimate the health risks of these heavy metals via consumption of the crops. The concentrations of heavy metals found in the soil (4.76 – 52.38 mg/kg, 2.45 – 3.43 mg/kg and 7.65 – 14. 21 mg/kg for Cr, Ni and Pb respectively) were within the acceptable threshold of 100, 50 and 100 (mg/kg) for Cr, Ni and Pb respectively. Cd was recorded in the range of 3.67 to 4.28mg/kg, which is above the acceptable limit of 3.00mg/kg in all the soil samples. The pH values were below the normal range while the Electrical Conductivity (EC) were all above the normal range. For the plant samples, the mean concentrations of the heavy metals studied were above the maximum acceptable threshold recommended by WHO/FAO except Ni. The Estimated Daily Intake (EDI) concentrations of Cd and Cr in all plant samples were above the acceptable tolerable daily intake (PTDI) limit as recommended by European Food and Safety Agency (EFSA). Hazard Quotient (HQ) value of Cd and Pb and the Hazard Index (HI) values for all the samples were greater than 1, which indicates that there is high potential of health risk associated with the consumption of the contaminated crops. The study highlights that irrigation with untreated wastewater is, likely, the major source of heavy metals contamination to the soils and plants examined. It is, therefore, recommended that use of wastewater for irrigation farming should be discouraged.