Heavy Metals Pollution Source Identification and Probabilistic Health Risk Assessment of Shallow Groundwater in Onitsha, Nigeria

This paper assessed the degree of heavy metals pollution, the possible pollution sources, and the human health risks of groundwater from shallow Eocene Aquifer in selected urban centers of Onitsha, Nigeria. The nitrate pollution was low in all of the groundwater samples. However, most of the heavy metals exceeded their maximum allowable limits. The heavy metal pollution index and contamination index, respectively, revealed that 45.83 and 70.83% of the total samples are unsuitable for drinking. However, both the water quality index and synthetic pollution index revealed that 91.67% of the samples are unfit for consumption. Based on the ecological risk index, 58.33% of the samples pose a very high ecological risk. The order of the heavy metals' impacts as revealed by the calculated health hazard quotients was Pb > Cd > Ni > Cu > Mn > Fe > Cr (for children) and Pb > Cd > Ni > Mn > Cu > Fe > Cr (for adults). Hazard index values ranged from 0.438 to 46.367 for children and 0.127-20.294 for adults. In this study, 91.67% of the samples pose a medium to high chronic risk due to ingestion by both children and adult populations. However, it is predicted that 62.5, 66.67, 20.83 and 66.67% of the samples, respectively, pose high cancer risks due to Ni, Pb, Cr, and Cd ingestion by children. Meanwhile, 62.5, 37.5, 33.33 and 66.67% of the samples respectively pose high cancer risks due to Ni, Pb, Cr, and Cd ingestion by adults. Multivariate statistical analyses revealed that the release of these heavy metals into the groundwater system is primarily influenced by anthropogenic inputs rather than natural processes.