Evaluation of water quality pollution indices for heavy metal contamination monitoring. A study case from Akpabuyo-Odukpani area, Lower Cross River Basin (southeastern Nigeria)

Two documented methods, Contamination index (Cd) and the Heavy metal potential index (HPI) were evaluated for their suitability for contamination monitoring of surface water in parts of southeastern Nigeria. Results show that despite the significant correlation between the data generated from these indices, the final classification gave two extreme results. Thus a third model (Heavy metal evaluation index, HEI) was developed to accommodate this deficiency. The HEI values show significant correlation with Cd and HPI but categorised into semi quantitative classes for proper assessment of pollution level. The HEI values show that 65.5% of the sites are classed as low with values lower than the mean and the remaining 34.5% with values greater than the mean are classed as medium. In contrast, water samples from all the 32 locations were classified as high and low using the Cd and the HPI respectively. However, a reclassification of the Cd and HPI categories using the mean values as a criteria show good comparison with the HEI. The differences in the former can be attributed to the variations in the concentrations of the heavy metals used for the different evaluation schemes.