Mineralogy and Geochemistry of the Sediments in Rivers and Estuaries Along the East Coast of India

Abstract

Bottom sediments from 15 rivers and estuaries and 2 lakes along the east coast of India are investigated for their mineralogy and geochemistry to better understand the influence of tropical weathering, lithology and controls of low saline water on the chemistry of sediments. Among the rivers, the Cauvery, Ponnaiyar, Palar, Pennar, Nagavali and Vamsadhara rivers predominantly drain the Archean-Precambrian Terrain (APT), whereas the Krishna, Vasishta Godavari and Gautami Godavari Rivers dominantly drain the Deccan Trap Volcanic Terrain (DVT). The Mahanadi, Brahmani, Baitarani, Subarnarekha, Haldia and Hoogly Rivers drain Mixed-Lithology Terrain (MLT) consisting of metamorphic and igneous rocks and soils. The Kolleru Lake receives sediments largely from DVT, while the Chilka - brackish water lake receives sediments from MLT. The lt 2 µm fraction and lt 4 µm fraction of sediments were used for their mineralogy and geochemistry, respectively. The sediments showed high kaolinite followed by illite and smectite for the rivers draining the APT, high smectite followed by illite and kaolinite for those draining the DVT and, high illite followed by kaolinite, smectite or chlorite for those draining through MLT. The Post-Archaean average Australian Shale (PAAS)-normalized major element distribution showed depletion of Ca, K and Na and enrichment of Al, Fe and Ti. The high values of Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) and, high Rb/Sr and Th/U ratios compared to Upper Continental Crust (UCC) suggest moderate to intense chemical weathering on source rocks. The PAAS-normalized elemental distribution showed slight enrichment of Fe, Mg, Sc, V, Co, Cr and Ni and depletion of Sr, Nb, Zr, Hf, Y and Ta. The mean enrichment factor was high for Cu, Zn and Pb and in the range significantly polluted to moderately polluted in APT- and DVT-sediments. The plots of TiO2 vs. Fe2O3+MgO, Th/Sc vs. Sc and La Th Sc showed sediment composition intermediate between granite and basalt,