RISK ASSESSMENT, LEVELS, SPECIATION AND SEASONAL VARIATION OF HEAVY METALS (Cd and Cu) IN WATER AND PARTICLE SIZES OF SEDIMENTS FROM HUNKI OX-BOW LAKE, AWE LGA, NASARAWA STATE-NIGERIA

This research aimed at assessment of the levels, speciation, risk assessment indices and seasonal variation of heavy metals (Cd and Cu) in water and sediments from Hunki Ox-bow Lake in Awe L.G.A of Nasarawa State, using Atomic Absorption Spectrophotometer (AAS), during the dry and wet seasons. The mean concentration of Cd in water samples of both dry and wet seasons was higher than that of Cu, but Cu was higher than Cd on the whole (raw) sediment samples. The baseline levels of Cd and Cu in water samples (mg/L) were as follows: Cd (0.354) and Cu (0.014), which revealed that Cd level in water was higher than the WHO Guidelines for Drinking Water Quality. The baseline levels of Cd and Cu in sediment samples (mg/kg) were as follows: Cd (7.149) and Cu (8.363) which showed that Cd and Cu were lower than Interim Sediment Quality Guidelines – High of (Cd: 10 and Cu: 270 mg/kg). However, Cd level from sediment was higher than Interim Sediment Quality Guidelines – Low of 1.5 mg/kg but that of Cu was lower (65 mg/kg). Results of the Tessier five-step sequential extraction procedure indicated that Cd was more in the first four steps of the sequential extraction (non-residual) which accounted for 84.64 - 94.52 % in both seasons. Copper dominated more of the organic matter fraction of the sediments (48.17 – 85.54 %) for both dry and wet seasons. The mean of Bioavailability Index (BI) for wet season was relatively higher than that of dry season. The mean values of Mobility Factor (MF) revealed that Cd had higher values than Cu irrespective of seasons. This indicated that Cd was more mobilizable and bioavailable to this aquatic ecosystem compared to Cu. The geo-accumulation Index (I-geo) values of Cd showed practically unpolluted (sites 1, 2 and 5 of dry season), moderately polluted (sites 3 and 4 of dry season), heavily polluted (sites 1, 2, 4 and 5 of wet season) to very heavily polluted (site 3of wet season). The I-geo values of Cu indicated practically unpolluted. In dry season generally, Contamination Factor (Cf) values for Cd were all moderate in dry season. The Cf values of Cd indicated very high contamination factor for wet season, while that of Cu was low in both seasons. In both seasons, the ecological risk index (Er) of Cd in dry season had low to moderate ecological risk, and high to serious ecological risk in wet season. The Er values of Cu were low. This study revealed that: sediments and water samples of the dry season had lower concentration of Cd and Cu than in wet season; concentrations of Cd and Cu from sediments were more of manmade sources rather than natural sources and most ecological risk indices evaluated revealed that Cd had greater ecological risk.