Comprehensive geochemical assessment, probable ecological and human health risks of heavy metals in water and sediments from dredged and non-dredged Rivers in Lagos, Nigeria

The study was conducted to demystify the geochemical distributions of heavy metals in water and sediments from dredged and non-dredged riverine systems, with a view to ascertain the source, and the deleterious ecological and human health risks of these metals. Ojo and Badagry Rivers were considered as the dredge-mined sites, while Gbaji River was chosen as the non-mined site. The level of heavy metals in water samples are as follows; [Cd (0.040 - 0.360), Cu (0.010 - 0.130), Cr (0.001 - 0.080), Fe (0.170 - 1.040), Pb (0.001 - 0.050), Mn (0.050 - 0.230-), Zn (0.050 - 0.160), As (0.001 - 0.030), Co (0.003 - 0.040) and Ni (0.001 - 0.050) mg/L]. The Cd and Mn contents in the water from the mined sites were higher than their permissible limits. The heavy metal contents [Cd (0.190 - 1.490), Cu (1.390 - 5.950), Cr (1.180 - 2.630), Fe (144.820 - 261.490), Pb (1.960 - 24.250), Mn (5.170 to 24.110), Zn (5.180 - 35.420), As (0.170 - 0.690), Co (0.190 - 0.840) and Ni (0.740 - 2.140) mg/kg] in sediments are within the background levels, except for Cd from the mined sites. The hazard index was lower than 1 (HI <= 1), inferring no likelihood of non-carcinogenic incidents. Contrarily, the outcome of the cancer risk index revealed that exposure to sediments from the mined sites could lead to cancer risk. The contamination and enrichment factors of the sediments from mined sites showed "high contamination" and "extremely enriched," respectively. Notably, the potential ecological risk index of Cd (high) in sediments from the mined areas contribute significantly to the ultra-high risk of these ecosystems, which might result in ecological collapse of the riverine systems. The calculated Nemerow multifactor index (YN) values of the mined sites implies extreme pollution. The coefficient of variation (CV), Pearson's correlation matrix, and principal component analysis (PCA) clearly indicate that heavy metals in water and sediments are from multiple rather than a single source. Although this study could not ascertain whether the mining activities are solely responsible for the elevated heavy metals in water and sediments from the dredge-mined sites since the study sites frequently experience other anthropogenic activities, the study showed that significant pollution and ecological adversities are associated with water and sediment from the dredged-mined sites.