The Li isotope ratio (Li-7) is expected to be a useful tracer of silicate weathering in river and groundwater systems, which is an important contributor to the seawater compositional changes that accompany the evolution of the Earth's surface environment. To obtain accurate estimates of continental Li fluxes to the ocean, we determined Li-7 values of dissolved Li in the lower Ganges-Brahmaputra river system in both the dry and rainy seasons, and in deep groundwater in the Bengal basin. Dissolved Li and Li-7 values in the lower reaches of the rivers (0.04-0.66 mu mol kg(-1) and +19.1 to +34.2, respectively) were predominantly derived from silicate weathering, as is the case in the upper parts of these rivers. We observed large changes in Li-7 over a distance of more than 1000 km downstream that were due mainly to Rayleigh-type removal of Li from river water. Extremely high Li concentrations (1.15-1.67 mu mol kg(-1)) and low Li-7 values (+5.1 to +11.6) in groundwater samples indicate congruent isotope leaching and dissolution of silicate minerals in the deep aquifer, where the water residence time is long. In the rainy season, Li concentrations and Li-7 values were lower than in the dry season, owing to the shorter residence time of river water and the substantial input of local subsurface flow through lowland alluvium. These results suggest that accurate estimation of continental Li fluxes to the ocean should take account of downstream and seasonal changes, as well as aquifer depth variations, in Li-7 values.