Geochemistry and pollution status of surface sediments of Loktak Lake, Manipur, India

Abstract

Surface sediments (n = 29) from Loktak Lake, the largest freshwater lake in northeastern India, were studied for geochemistry, mineralogy and the heavy metal pollution of the lake sediments was assessed. The XRD spectra of bulk sediments showed the dominance of quartz and clay fractions showed the dominance of illite and chlorite. The major oxides consist dominantly of SiO2, Al2O3 and FeO with low concentrations of K2O, Na2O and CaO. The depletion of CaO, Na2O and K2O compared to upper continental crust, post-Archean average shale and North-American shale composite indicated moderate to strong weathering, recycling of the source rock and their removal during transportation. The chemical index of alteration values (76.86–84.98) also suggested moderate to extreme chemical weathering of the Loktak Lake catchment lithologies. The mean concentrations (in mg/kg) of Zn (117 ± 23.45), Co (34 ± 13.67), Fe (7 ± 1.38 wt%), Cu (67 ± 12.87), Cr (282 ± 45.25) and Ni (194 ± 30.12) are about 2–4 times higher than that of the upper continental crust, except for Mn (425 ± 168.99 mg/kg) and Pb (17 ± 7.47 mg/kg). The heavy metal concentrations follow the trend of Fe > Mn > Cr > Ni > Zn > Cu > Co > Pb. The assessment of heavy metal pollution using enrichment factor, geo-accumulation index and modified degree of contamination suggested non-contamination to moderate contamination of sediments. According to sediment quality guidelines, the sediment-dwelling biota could be affected by the higher concentrations of Ni and Cr. The mean-effect range median-quotient suggests medium–high to high potential of the effects of heavy metal pollution. The uniformly higher concentration of heavy metals in general and Ni and Cr, in particular, indicated the dominance of natural (ophiolitic) sources with no or little, if any, anthropogenic influence.