ABSTRACT

The study area, Akwa Ibom State is located in the coastal Eastern Niger Delta where the vast majority of the populace depend largely on the rivers and streams for their agricultural, recreational and sometimes domestic water supplies. Cross River (CR) and Qua Iboe River (QIR) in Akwa Ibom State are prone to pollution from diverse sources including urbanisation, population growth and agricultural activities. The area is characterised by different forms of anthropogenic activities with heaps of domestic, industrial and institutional wastes in the environment. The state and national agencies are exploiting the river banks for establishing industrial and recreational parks especially the Ibaka seaport for commercial use. Due to scarcity of data on the pollution and human exposure risks of these rivers, there is need to establish their pollution status. This study therefore, assessed the physicochemical characteristics, trace metal levels, metal speciation in surface water and sediments and saturation indices on these rivers. Surface water and sediment samples were collected bimonthly between April 2018 and March 2019 from six stations downstream and upstream with high human activities in triplicate along Cross River (CR) (Ibaka, Oron, Nwaniba, Ayadehe, Okopedi and Enwang) and Qua Iboe River (QIR) (Iwuochang, Mkpanak, Ukpenekang, Atabong, Eketai and Marina). A composite sampling technique was adopted. Selected pollution parameters and trace metal levels were determined using APHA and AAS methods. A thermodynamic model, PH-Redox-Equilibrium-Computer-Programming-Language-Interactive (PHREEQCL) was employed to determine some metal species and saturation indices of associated mineral phases in water samples. The relationship between metal speciation in water with sediments in terms of bioavailability was evaluated using the European Union Binding Corporate Rules (BCR) reference method. Data were analysed using coefficients of variation and Pearson Correlation at 0.05.The concentration (mgl^-1) of alkalinity (36.26±14.67, 43.58±18), dissolved oxygen (DO) (6.91±0.65, 6.07±0.98), nitrate (11.30±8.24, 5.62±2.91), chloride (998.31±481.27, 0.10±0.04) and sulphate (30.33±5.85, 37.76±9.78) in Cross River  and Qua Iboe Rivers respectively were obtained. The levels (mgl^-1) of Cd (0.01±0.004, 0.04±0.04), Pb (0.02±0.004, 0.03±0.02) and Ni (0.03±0.01, 0.06±0.07) in water exceeded the WHO limits while those of Cr (0.02±0.006, 0.04±0.03), Co (0.03±0.005, 0.05±0.04), Cu (0.03±0.003, 0.06±0.06) and Se (0.001±0.0001, 0.001±0.0004) were below the limits. Metal variability in water for Cross River was higher for Cd (65%) and Ni (80.1%). There were strong correlations for metals in QIR Cu/Ni (r=0.9), Co/Cu(r=0.9), Cr/Cu (r=0.9), Pb/Cu(r=1) and Pb/Ni(r=0.8) indicating point source of metal pollution. The dominant metal species for the rivers were the free hydrated species of Co²⁺ (97.4, 85.1), Cu²⁺ (86.0, 72.1%), Ni²⁺ (98.3, 98.0%), Pb²⁺ (65.4, 68.9%), CdCl⁺ (80.3, 77.5%) and Cr(OH)⁺₂ (71.5, 70%). The level (µgg^-1) of Cd, Pb, Cr, Co, Cu, Se and Ni in sediments of Cross River and Qua Iboe River were 0.52±0.24; 44.08±49.36; 57.78±15.60; 25.75±22.92; 16.22±5.48; 44.61±37.35; 7.52±2.28; 48.28±46.40; 69.81±16.90; 64.44±64.56; 0.75±0.44; 0.7±0.52 and 6.87±2.26, 62.97±75.85 respectively. The non-residual sediment fractions: Cd, Pb and Cu in Cross River and Qua Iboe River were 87.1 and 87.5%, 63.5 and 65.5% and 83.4 and 83.1% respectively. Saturation indices showed supersaturation for CdCr₂O₄, CuCr₂O₄, PbHPO₄ and Pyromorphite – OH minerals. Comparison with PHREEQCI data showed higher metal bioavailability in water than in the sediments. The metal pollution levels in the two river systems may be due to anthropogenic activities in the rivers. The occurrence of bioavailable metals suggests potential risk from metal toxicities.