Ecological and health risk assessment of heavy metals in sediments, surface waters and oysters (Crassostrea gasar) from Eastern Obolo marine ecosystems, Akwa Ibom State, Nigeria

Abstract

The contamination of the environment by heavy metals is significant because of the non-biodegradable and persistent nature of these sets of metals. Reports on the contamination of water by heavy metals can be regarded as uncertain if the levels of these metals in sediment are not taken into consideration. Sediments, surface waters and oysters (Crassostrea gasar) from the Atlantic coastline, Iko river, Obolo river, Amadaka river and Emeremen river were analyzed for the levels of the concentrations of Co, Tl, Th, U, Se, Tl and Ag using inductively coupled plasma-optical emission spectroscopy method. Seasonal and spatial distribution of the metals were determined as the bases for ecological and health risk assessment of the environment. The distribution of the analyzed metals in the sediment followed te following order ((mg/kg dw) Th (38.05) > U (22.70) > As (11.06) > Tl (0.56) > Se (0.45) > Co and Ag (below detection limit). In the oysters were Th (46.67) > U (9.20) > As (7.11) > Se (5.05) > Tl (0.63) > Co and Ag (below detection limits). The water had far lower concentrations of the metals (0.01 – 0.19 mg/l). The results obtained indicate no statistically significant variation between the seasonal sediment data sets at P ≤ 0.05. A strong correlation (0.50 – 0.99) between the metals was revealed indicating a common source (mainly anthropogenic) for the metals. Of the five study sites, only the Atlantic coastline and Iko river were contaminated by some of the metals studied; Se, Th, U, As and Tl, with Iko river found to be more polluted. Co and Ag were below the detection limit in all the study sites in sediments, water and oysters in both seasons. All the metals were below the detection limit along Obolo River, Amadaka River and EmeremeEastern River during both seasons, thus indicating the sites were relatively uncontaminated by the heavy metals studied. The metal contamination status was significantly bio-indicated by the oysters at all the study sites in both seasons. The ecological risk indices revealed no contamination status for Obolo river, Amadaka river and Emeremen river, but low to moderate pollution status for the Atlantic coastline and Iko river, especially for As, Th and U. Based on bioconcentration data of the oysters, the health risk indices (daily intake of metal, health index, target hazard quotient and total target hazard quotient, etc) have revealed that As in the oysters from Iko river and the Atlantic coastline of Eastern Obolo may pose a significant health risk to consumers. Adequate regulation and remediation of the ecosystems to safeguard ecological and human health is therefore recommended.