Study of seasonal variations and risk assessment of selected metals in sediments from Mangla Lake, Pakistan

Composite sediment samples were collected from Mangla Lake, Pakistan, in order to determine the seasonal variations in the concentrations of selected metals (Ca, Cd, Cr, Cu, Fe, Mg, Mn, Pb and Zn) and to evaluate the associated risk assessment. Average metal levels in acid-extract of the sediments revealed similar decreasing trend during summer and winter: Ca > Fe > Mg > Mn > Zn > Cr > Pb > Cu > Cd. Generally, elevated metal levels were observed during winter. Metal pollution index was evaluated using enrichment factor (EF) and geoaccumulation index (Igeo), which showed significantly higher contamination by Cd and Pb. Association with adverse biological effects to aquatic biota was also assessed using the classification of the sediments and sediment quality guidelines. Pb exceeded the threshold effect level (TEL) and Cd even exceeded the probable effect levels (PEL) during the both seasons, whereas, Cu and Cr exceeded the TEL levels during winter and Zn exceeded the limits during summer. However, potential acute toxicity and PELq assessment for the sediments demonstrated that the risk for the benthic biota was insignificant. Multivariate principal component analysis and cluster analysis evidenced considerable anthropogenic contribution of the metals in sediments during both seasons. The sediments were also extracted with a weak electrolyte solution (0.1 M Ca(NO3)2) to predict the bioavailability of the metals which revealed that Cd and Pb were the most bioavailable, while Fe and Mn were the least bioavailable.

► Cd and Pb were the most bioavailable metals in sediments, followed by Cr and Cu.
► Frequent/occasional adverse biological effects were associated with Cd and Pb.
► PCA and CA revealed significant anthropogenic contamination of the sediments.
► Severe enrichment of Cd, Pb, Ca and Zn in sediments was noted during both seasons.
► PELq assessment demonstrated insignificant risk for the benthic biota.