Evaluation of Heavy Metals, Dissolved Organic Phosphorus (DOP) and Nitrogen (DON) in three Wetlands of Northern Nigeria

S Yusuf, A. A. Audu, M. Waziri


The concentrations of some heavy metals, dissolved organic phosphorus (DOP) and dissolved organic nitrogen (DON) were seasonally determined in water samples from 35 sites from three wetlands areas. The water samples were digested with concentrated trioxonitrate (V) acid at 85 0C for 2 hours on a hot plate and the metal levels were determined using atomic absorption spectrophotometer. The following ranges of values were obtained, copper (Cu) (0.00349±0.001 to 0.0750±0.04 mg/l), lead (Pb) (0.194±0.074 to 0.870±0.001 mg/l), cadmium (Cd) (0.00448±0.0003 to 0.0198±0.0001 mg/l), chromium (Cr) (0.000 to 0.0469±0.001 mg/l), zinc (Zn) (0.00849±0.0001 to 0.144±0.003 mg/l) and nickel (Ni) (0.0042±0.0006 to 0.0454±0.0015 mg/l). Generally, the metal concentrations increased in the dry but decreased in wet season. Pearson correlation coefficient, analysis of variance, Tukey’s test, principal component analysis (PCA) and cluster analysis (CA) were used to assess the data and it was observed that Pb has the highest mean concentration. When the metals levels were graduated, they were found to be in the order of Pb>Cr >Zn >Cu >Cd > Ni in the wet season but the order changed in the dry season as Pb>Cu >Zn >Ni >Cd >Cr. The Pb and Cd concentrations in all the sites were higher than the WHO and NAFDAC guidelines, indicating that the wetlands water samples are unsafe for human consumption, thus relying on these wetlands as sources of food would have serious public health implications. Principal Component analysis (PCA) and Cluster analysis (CA), were used to analyse the relevance of different heavy metals and identify their major sources. The results showed three factors for the heavy metals variability (71.4% of the total variance) that Zn and Cu, were dominated by principal component 1 (PC1), Cd and Pb were associated with high values in the second component whereas Cr and Ni are higher in the third component. PCA and (CA) for the sampling sites showed that industrial wastewater, runoff from farmlands and atmospheric depositions were the major sources of heavy metals particularly Pb, Cd, Cr, Zn and Cu in these wetlands. The average values for DON and DOP ranged from 10.7±2.28 to 71.1±14.9 mg/l and 0.464±0.229 to 1.84±1.39 mg/l respectively in the wetlands. There is no significant difference in DON and DOP concentration in both the dry and the wet seasons. High concentrations of DOP and DON in wetlands have been implicated for high yield of agricultural produce of the farmlands around the wetlands. 

Keywords: Wetlands, water samples, heavy metals, DOP, DON. 

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