Groundwater quality assessment and human health risks in Ovitoto, Otjozondjupa Region, Namibia

Abstract

Namibia, a dry country relies heavily on groundwater resources which is susceptible to anthropogenic contamination. The study assessed the quality and health risks of borehole water supplied to residents of the Ovitoto community in Otjozondjupa region of Namibia. Water samples were collected from nine boreholes across nine communities over a period of six months and subjected to physicochemical and microbial analyses. Heavy metals were extracted using mineral acid digestion and detected using Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES). Microbial entities were analyzed using standard bacteriological method. Results revealed that the pH, temperature, and electrical conductivity (EC) of samples were within WHO permissible levels for human consumption while Turbidity and total dissolved solids (TDS) were above the limit. Overall mean concentrations of heavy metals were 0.83, 0.01, 0.02, 17.8, and 7.09 mg/L for Zn, Cd, Pb, Fe, and Mn respectively. WHO drinking water permissible levels were obtained for Cd, Pb, Fe and Mn and the water could also be regarded as unsuitable for irrigational use because it is above permissible levels of Cd, Fe, and Mn. Zn and Cd showed a strong correlation (r=0.99) with an average correlation (r=0.55) between Cd and Cu. The human health risk of the metals was assessed using the Target Hazard Quotient (THQ) and Carcinogenic Risk Index (CRI), where HQ >1 and CRI > 1.0×10-4 indicate non-carcinogenic and carcinogenic risks, respectively. THQ values < 1 was obtained for analyzed metals in both children and adults. However, CRI (Mn) values of 4.8 for adults and 18.1 for children indicate potential exposure to carcinogenic risk. Detection of HPC, Tc, and Fc in water samples above permissible levels also gives cause for concern. Pretreatment and monitoring of borehole water samples before distribution for consumption is highly recommended.