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Levels of elements in the surficial estuarine sediments of the Hugli River, northeast India and their environmental implications

The present paper is the first document of a detailed geochemical and mineralogical study of estuarine sediments (0-5 cm, of <63 mm) of the Hugli River collected from five different sites along its course in the coastal areas of West Bengal, northeast India. The present work attempts to establish the status of distribution and environmental implications of 52 elements in the surficial estuarine sediments and their possible sources of derivation. The elemental composition was determined using aqua regia extracts and inductive coupled plasma-mass spectroscopy (ICP-MS) and ultra trace 2 program. The physicochemical variables (temperature, pH, organic carbon and total nitrogen) and granulometric properties of sediments were determined for better understanding of the environment of deposition. The level of both metallic and non-metallic elements shows a wide range of variation all along the course of the estuary and can be attributed to their differential derivation from the source rocks and differential discharge of untreated effluents originating from industrial, agricultural, aquacultural as well as domestic sewage. In the downstream stretch of the Hugli estuary, near its mouth at Gangasagar, almost all the elements showed elevated values. This might be related to the weak hydrodynamic setting of this site represented by fine sandy to muddy texture of the sediments. The element contents, particularly the heavy metal content in the sediments are the lowest in the upstream part of the estuary at Diamond Harbor, whereas, the heavy metal contents in sediments are slightly higher in the intermediate stretch of the estuary at Haldia. This reflects the fundamental principles of lowering competence and capacity in the more and more downstream stretch of a river. The metals that are carried from upstream find their ultimate depositional sink at the delta mouth near Gangasagar. The metal contents at two other stations, one in the upstream stretch at Canning by the side of Matla River and the other at downstream stretch at Patharpratima by the side of Saptamukhi River, both off from the direct influence of the Hugli River but within the tidal channel network systems of the Hugli-Matla drainage basin have been estimated to test the behaviour of metal concentration. It is noticed that values of metal content at Patharpratima are very similar to that of Haldia. So it is assumed that the hydrodynamics of Saptakukhi tidal inlet and its associated creeks are similar that of Haldia point. Interestingly, majority of the elements have their highest concentrations at Canning. This site is severely contaminated with huge organic load from domestic sewage, aquaculture, intensive trawling activities and agricultural runoff. Moreover, the site suffers from heavy siltation load causing an almost moribund condition of Matla River at this point.Two-way analysis of variance (ANOVA) revealed highly significant variation within the month as well as within the metals. Minerlogically, quartz being the most available mineral is present consistently at all the stations (>30% as reflected from XRD patterns) whereas, the labile sodic and potassic feldspar species (albite and microcline) are quite inconsistent at different stations. Albite is present in the range <5% to >30%. Microcline was either not detected or present from about 5-10%. The flaky mica group of minerals is also inconsistent in patterns but the content is higher than found for albite. Chlorite group of minerals is present in all sediments from <5% to 10-30%. Hornblende was detected as trace mineral only in sediment of Haldia station. These flaky minerals remain in suspension for a long time and do not follow the usual law of settling of particles depending on the hydrodynamic regime prevailing in the area. Presented data will serve as a baseline against which future anthropogenic effects may be assessed. A comprehensive account of heavy metal content in sediments from different coastal regions of peninsular India has also been presented for purpose of comparison.

Marine sediment; Trace elements; Heavy metals; Texture class; Coastal pollution; Sundarban; Hugli River estuary

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