Understanding sedimentary processes and degree of pollution through geochemistry and magnetic properties of mudflat sediments from a tropical micro tidal estuary, Central West Coast of India

Abstract

Two shallow cores (Core GN and GH) collccted from two tidal mudflats in the Aghanashini or Tadri River Estuary were analyzed for sediment components viz sand, silt, clay and Organic Carbon (OC); geochemistry and magnetic properties to assess the variations in distribution, sedimentary processes and historical pollution records. The core GH is of mature mudflat consisted of slightly sandy mud with higher concentrations of Ca, Mg and Mn together with higher silt, clay and OC percentage, which is likely because of more marine influences in this mudflat and by grain size effect whereas core GN is characterized by comparatively coarser sediments of sandy mud of a mudflat environment and shows higher concentrations of Pb and Co together with higher value of sand, which are likely to be caused by comparatively nearer to continental source reflecting a pollution origin. The unsupported 210Pb activity in core GH exhibit enhanced sedimentation rates for last 13 years, which is likely to be a response to the human impact in the form of differential erosion and accelerated sedimentation in combination with relative sea level rise. The geochemical data indicates that both Mn and Fe have been remobilized and that these diagenetic processes have slightly modified the vertical distributions of Zn, Cu, Cr and Co. The magnetic data reveal evidence for changes in magnetic mineralogy and grain size within the cores. In both the cores, there is substantial decrease in total magnetic concentration and coarsening of the magnetic grain size, but at different depths, which is in agreement with Fe and Mn profiles, suggesting dissolution of fine grained magnetite under reducing conditions. However, general trends in pollutant loading, as indicated by F.F of Cu, Cr, Co and Fe can still be identified in Core GH with maximum inputs (EF greater than 1.5) occurring around 1900 increasing towards the present day. Both cores show closely similar traces of gradual increase of hematite and magnetite deposition with a steeper rise of hematite in the upper part of the respective cores which may reflect the increased erosion of more weathered (magnetically harder) materials with increase in anthropogenic activities. Though, the construction of pollution chronology in the core GN is limited by different processes, unlike in core GH, Pb shows gradually increasing trend towards the surface with enrichment values above 5 which may signify input of higher degree of contamination upstream side of the estuary.