Heavy metals in the Ganga (Hooghly) River estuary sediment column: evaluation of association, geochemical cycling and anthropogenic enrichment

In this study, the distribution of heavy metals (Co, Cr, Cu, Ni, Zn and Pb) in a sediment core from the Ganga (Hooghly) River estuary has been investigated. The data on concentrations of heavy metals, together with the concentrations of Al, Fe, Mn, carbonate, organic carbon (C_org) and nitrogen (N_org), have been used to understand (1) the phase association of metals, (2) their geochemical cycling in the sediment column and (3) if metals are enriched by anthropogenic contributions. The metals show significant positive correlations with Fe and Mn, indicating that Fe–Mn oxyhydroxide phases are the major host of metals in the sediment core. This observation together with significant negative correlations of Fe, Mn and the heavy metals with (C/N)_org provides the evidence for diagenetic redistribution of metals in the sediment column and their partitioning into the Fe–Mn oxyhydroxides. The increasing upward exchangeable Mn concentrations and association of metals with Fe and/or Mn in the exchangeable phases provide the supporting evidence for digenetic remobilization of the metals and their cycling with the Fe–Mn oxyhydroxides. Down-core variations of the metal concentrations generally match with that of Mn and Fe. The results of calculations for enrichment factors (EF) show that EF > 1.5 for Pb in the sediment core, suggestive of non-crustal sources. The elevated concentrations of Pb with EF > 1.5 in the sediment samples of the effluent water that drain into the estuary suggest that industries are an important source of Pb to the Hooghly River estuary sediments. The results and observations of this study suggest that an assessment of temporal variation of metal pollution in estuaries will require understanding of phase association and cycling of metals in the sediment column.