3. Results and discussion
3.1. Metal concentration in water
The results of heavy metal concentrations in surface waters of the river Korotoa are shown in Table 2 revealed a significant variation (p < 0.05) among the sites. The average concentration of studied metals in water followed a decreasing order of Cr > Cu > As > Ni > Pb > Cd. The mean concentration of Cr in water was observed 83 and 73 g/L during winter and summer season, respectively which was much higher than the WHO standard level for drinking water (5 g/L). The average concentration of Cu was observed 73 and 161 g/L during winter and summer season, respectively. Interestingly the highest value of Cu was observed at S8 site (119 g/L in winter) which might be attributed to the domestic sewage and runoff from extensively farmed areas (Koukal et al., 2004; Wu et al., 2008; Islam et al., 2014). Arsenic forms a variety of inorganic and organic compounds of different toxicity reflecting the physicochemical properties of arsenic at different valencies. The average concentration of As was higher in winter (46 g/L) than that in summer (37 g/L) which exceeded the WHO standard (10 g/L). In river water, considerable amount of