Uranium removal during low discharge in the Ganges-Brahmaputra mixing zone

Abstract

The Ganges-Brahmaputra river system supplies more dissolved uranium to the ocean than any other system in the world (S arin et al., 1990; S ackett et al. 1973). However, there have been no investigations to determine whether riverine supplies of uranium are altered by geochemical reactions in the river-ocean mixing zone. In this study, uranium and salinity data were collected in the Ganges-Brahmaputra mixing zone during a period of low river discharge. The uranium distribution with salinity shows that in waters < 12 ppt salinity, uranium activities are significantly lower than predicted from conservative mixing of river and seawater. This suggests that uranium is being removed within the mixing zone. The behavior of uranium in the Ganges-Brahmaputra is in sharp contrast to its in teh Amazon mixing zone where MCKee et at. (1978) found uranium activities significantly higher than predicted from conservative micxing. The contrasting behaviours for uranium in theses systems are due to the different locatons where mixing between river and seawater occurs. For the Amzon, mixing takes place on the continental shelf whereas for the Ganges-Brahamputra, mixing occurs within shoreline sedimentary environments. The physiochimical processes controlling uranium removal to sediment deposits in the Amazon are partly known. We discuss mechanisms which may be removing uranium to suspended and mangrove sediments in the Ganges-Brahmaputra.