POLLUTION STATUS AND CHEMICAL SPECIATION IN SURFACE WATER AND SEDIMENTS OF TWO RIVERS IN AKWA IBOM STATE, NIGERIA

Abstract

Cross River (CR) andQua Iboe River (QIR) in AkwaIbom State are prone to pollution from diverse sources including urbanisation and agricultural activities. The state and national agencies are exploiting the river banks for establishing industrial and recreational parks. Due toscarcity of data on the pollution and human exposure risks of these rivers, there is need to establish their pollution status. This work was aimed at determining the physicochemical characteristics, metal speciation in surface water and sediments and saturation indices on these rivers. Surface water and sediment samples were collected monthly between June 2009 and May 2010 from six stations with high human activities along CR (Okopedi, Ayadehe, Nwaniba, Oron, James-Town, Ibaka) and QIR (Iwuokpom1, Iwuokpom2, Iwuochang, Eketai, Atabong, Marina). Sampling was done by compositing three grab samples from each station giving a total of 144 each of water and sediment samples. Selected pollution parameters and trace metal concentrations were determined using APHA methods. A thermodynamic model, pH-Redox-Equilibrium-Computer programming language-Interactive (PHREEQCI), 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 sediment in terms of bioavailability was evaluated using the European Union Binding Corporate Rules reference method. Data were analysed using coefficient of variation and Pearson correlation at p = 0.05. The concentrations (mgL±1) of alkalinity (36.1±19.2, 61.3±35.1), dissolved oxygen (7.6±1.2, 6.4±0.9), nitrate (12.3±9.6, 4.6±2.9), chloride (880±520, 860±140), phosphate (0.05±0.02, 0.10±0.02) and sulphate (30.9±9.0, 36.2±6.6) in the rivers (Cross, Qua Iboe) respectively were obtained. The concentrations (mgL±1) of Cd(0.02±0.01, 0.04±0.05), Pb(0.02±0.01, 0.03±0.03), and Ni(0.05±0.05, 0.06±0.08) in water exceeded the WHO limits while those of Cr(0.02±0.02, 0.04±0.03), Cu(0.04±0.02, 0.07±0.07) and Zn(0.11±0.07, 0.20±0.09) were below the limits. Metal variability in water forCross River was higher for Cd (81.3%) and Ni (94.0%). There were strong correlations for Cr/Co (r=0.9), Cu/Co (r=0.9) and Ni/Co (r=0.8) indicating point source of metal pollution. The dominant metal species for the rivers were the free hydrated species of Co2+(97.0, 97.7%); Cu2+(82.8, 70.2%); Ni2+(98.1, 97.6%); Pb2+(60.0, 62.0%); CdCl+(82.2, 82.3%) and CrOH2+(53.1, 58.7%). The concentrations (�gg�1) of Cd, Pb, Ni, Cr, Cu, Zn and Co in sediments of Cross and Qua Iboe Rivers correspondingly were 0.50±0.24, 1.59±0.87; 59.3±14.2, 13.4±6.2; 6.60±2.3, 4.9±3.0; 15.9±5.3, 20.9±14.0; 71.8±17.3, 45.3±16; 129±12, 122±18; 7.60±2.3, 5.5±3.0. The non-residual sediment fraction of Cd, Pb and Cu in Cross and Qua Iboe Rivers were 89.3 and 88.7%, 62.4 and 64.0%, and 83.1 and 83.3% respectively. Saturation indices showed supersaturation for CdCr2O4, CuCr2O4, PbHPO4 and Pb-pyromorphite minerals. There was a dominance of bioavailable fractions bound to Fe/Mn-oxide and organic/sulphide components. Comparison with PHREEQCI data showed higher metal bioavailability in water than in the sediments. The metal pollution levels in the two rivers may be due to anthropogenic activities in the rivers. The occurrence of bioavailable metals suggests potential health risk from metal toxicities.