Heavy Metals in the Hooghly River Estuary Sediment Core: Geochemical Cycling and Anthropogenic Enrichment

Karayakath, Amrutha (2016) Heavy Metals in the Hooghly River Estuary Sediment Core: Geochemical Cycling and Anthropogenic Enrichment. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Distribution of metals in estuarine sediments is controlled by different biogeochemical processes. This study is focused on geochemical cycling and anthropogenic enrichment of heavy metals in a 1.6 m long sediment core taken from the Hooghly estuary. Major and trace metals (Co, Cr, Ni, Cu, Zn and Pb) were measured in acid digested samples using ICP-MS. Organic carbon and nitrogen were also measured using an element analyzer attached to an IRMS. Principal Component Analysis (PCA) was also carried out to evaluate the factors that control the elemental variation.Results of PCA show that three principal components account for most of the variability of the dataset. PCA biplots define two clusters for samples, one comprising the shallow depth samples whereas samples from deeper sections of the core represent the second cluster. This suggests that controlling mechanisms for metals distributions are different for shallow and deep samples. Down core distribution of metal concentrations showed an increase in concentration in the top 50 cm core depth. Investigating whether this increase is related to increased supply of metals require an understanding of the metal cycling. Study of heavy metal cycling provide understanding on how the metals get redistributed between different phases. Correlation analysis of concentrations of metals was performed to infer about the phase association.Trace metals are strongly correlated with Fe and Mn. This, together with the absence of significant correlations with Al and organic carbon, suggests that metals are mainly associated with Fe- Mn oxyhydroxide phases. High value of organic C/N ratio and their significant negative correlation with Fe and trace metals indicate the importance of post depositional processes in the sediment core and redistribution of metals to Fe-Mn oxy hydroxide phase. Thus to understand the temporal variation in metal concentration, their redistribution was taken into account by normalizing the heavy metal concentrations with Fe concentration. In Fe normalized concentration depth profile, the overall increase in the top 50cm was absent. This indicates the importance of understanding metal cycling in evaluation of metal pollution. EF (Enrichment Factor) calculated shows an anthropogenic enrichment in the metal concentrations of Cu, Ni and Pb. Nearby industries were inferred to be one of the sources of high amount of this heavy metals in the sediment column.

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