Geochemical cycling in the Hooghly estuary, India

Abstract

U-Th decay series isotopes, δ 18 O and Si measurements in the river estuarine waters and sediments of the polluted Hooghly estuary as well as the surface waters of the Bay of Bengal, its high salinity end member, are reported. Dissolved Si indicates that there are probably two mixing regimes, dissolved U behaviour is nonconservative and δ 18 O behaves conservatively in the overall estuarine region. Isotopes of reactive elements, viz. 234 Th and 210 Po, are removed from the estuarine waters in < 2 days and < 1 month, respectively, which is due to high suspended matter (30-301 mg 1 -1 ), 228 Ra and 226 Ra are profusely released into the estuarine waters in the low to mid-salinity regions. As expected, the opposite trend is observed in the case of estuarine sediments and suspended matter. Reactive isotopes of Th, 210 Pb and 210 Po are enriched, whereas Ra isotopes are depleted with respect to their parent nuclides in the estuarine sediments and suspended matter. 232 Th/Al ratio appears well suited to study the distribution and mixing of the bed load sediments of the Ganga-Brahmaputra (G-B) and the Hooghly rivers with those from other rivers on the Bay of Bengal floor.