Station-Based, Monthly and Seasonal Variations in Heavy Metal Concentrations in Water from Awoye River, Ondo State, Nigeria

The presence and potential toxicity of metals in surface waters represents an environmental concern. This study investigated the concentrations of metals in water samples obtained from three sampling stations (NNPC Platform, Stone Jetty and Awoye Gulf) in Awoye River, Ondo State, Nigeria, monthly between January and June 2015. The levels of nine trace metals (Fe, Zn, Mn, Cu, Ni, Cd, V, Cr and Pb) were determined by atomic absorption spectrophotometry. Significant differences in metal concentrations were reported between sampling stations for some metals. The highest mean concentrations of eight of the nine metals (Fe (9.08 ± 0.86 mg/L), Zn (5.88 ± 0.56 mg/L), Ni (3.74 ± 0.35 mg/L) and V (4.11 ± 0.39 mg/L)) were found at the NNPC Platform while that of Cu was highest at Awoye Gulf (1.46 ± 0.22 mg/L). For the eight metals with complete monthly records, concentrations were generally lowest numerically in March and highest in June, although statistically no significant differences were found among months according to the multiple-comparison results reported. Rainy-season concentrations were higher numerically than those of the dry season for all nine metals at the three sampling stations, while statistically significant seasonal differences were found only for Zn, V and Cr. These results characterize spatial, monthly and seasonal trends in the historical 2015 data set and constitute a baseline record against which future monitoring can be compared. The results should not be taken as representing present-day metal concentrations in Awoye River.