Processes which control the distribution of radium and uranium in the Ganges-Brahmaputra Delta and t...

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" Processes which control the distribution of radium and uranium in the Ganges-Brahmaputra Delta and their fluxes to the Bay of Bengal "
J. Carroll

Document Type	:	Latin Dissertation
Language of Document	:	English
Record Number	:	1112457
Doc. No	:	TLpq303864776
Main Entry	:	J. Carroll
Title & Author	:	Processes which control the distribution of radium and uranium in the Ganges-Brahmaputra Delta and their fluxes to the Bay of Bengal\ J. Carroll
College	:	University of South Carolina
Date	:	1990
student score	:	1990
Degree	:	Ph.D.
Page No	:	158
Abstract	:	The distribution of uranium and radium isotopes between sediments and seawater was investigated in two major areas of the Ganges-Brahmaputra (G-B) estuarine mixing zone. To the east of the main river channel is a shallow area (<7m)(2.5 usd\timesusd 10 km2) containing several large islands and numerous small ephemeral islands which appear and disappear with tides and storms. To the west of the main river channel is a large mangrove forest (2.8 usd\timesusd 10 km2). In January 1987, freshwater and sediment discharge were low (fw = 7600 m/s and sed = 5.4 usd\timesusd 10 kg/month). Mixing profiles show that radium was released from sediments to seawater. Radium activities in the mixing zone were greater than river and ocean endmember activities by 30-50 dpm/100L for Ra and 50-100 dpm/100L for Ra. These excess activities were supplied from suspended riverine sediment and sediments in the mangrove forest exposed to seawater for the first time, and by erosion of island sediments in the eastern delta. Additional Ra was supplied from nearshore bottom sediments during resuspension by tidal and wave action. Increased river discharge caused the mixing zone to migrate offshore from coastal sediment deposits. Desorption of Ra from suspended sediments discharged by the Rivers increased Ra activities in the 0-10ppt region of the mixing zone. In October 1987, dissolved Ra activities were a factor of 3-5 lower than in January as a result of lower water residence times and limited salt water exposure of nearshore sediment deposits. For the G-B mixing zone, sediment supplies from the Rivers and salt water migration into the western and eastern delta determine dissolved radium fluxes to the Bay of Bengal. The flux of Ra = 8 to 11 usd\timesusd 10 dpm/yr and the flux of Ra = 12 to 16 usd\timesusd 10 dpm/yr. These fluxes are not significantly different from flux estimates determined by the annual sediment discharge multiplied by radium which desorbed from river sediment and island sediment exposed to seawater for the first time. A mixing profile of uranium and salinity during low discharge in January 1987 shows uranium activities in waters <12ppt salinity are 20% lower than the predicted activity based on conservative mixing of river and ocean waters. These observations imply dissolved uranium is removed to sediments in the Ganges-Brahmaputra Delta. Tides and lower river discharge in the Delta prolong the association of nearshore sediments and low salinity water. Possible explanations for removal of uranium include removal of uranium to suspended sediments and adsorption of uranium to organic-rich sediments in the mangrove forest.
Subject	:	Bangladesh
	:	Brahmaputra River
	:	Earth sciences
	:	Geochemistry