We have investigated Os behavior in groundwater sampled at different locations in the Bengal plain. Bangladesh groundwater chemistry is dominated by carbonate weathering, but also includes a variable seawater contribution. Redox conditions are mainly anoxic, and the analyzed samples display highly variable iron and arsenic contents. Due to the reducing conditions and the presence of several Os species, efficient spike-sample equilibration, and thus accurate Os concentrations, are difficult to attain by conventional methods. Therefore, Os contents and 187Os/188Os ratios were determined by a method specifically developed for groundwater samples (heating with H2O2, in a high pressure asher HPA-S at 250°C and ~120 bars). The high temperatures allow complete Os oxidation and thus optimal spike-sample equilibration to be obtained in a few hours for different aqueous matrices (seawater, groundwater and river water).
The analyzed groundwaters have very high Os concentrations (30-250 pg/L) compared to those of seawater and river water (~10pg/L). Surprisingly, Os content variations are not linked with those of iron and redox sensitive elements but instead are correlated with Sr and Ca concentrations. This may indicate that Os contents in groundwater are not controlled by dissolution or precipitation of iron and manganese oxide/hydroxide, but instead behave like most moderately soluble elements. 187Os/188Os isotopic ratios vary greatly (from 0.9 to 2.60) and strongly depend on the sampled area. Aquifers located in the active and inactive Ganges delta are highly radiogenic (187Os/188Os between 1.80 and 2.60) while the Brahmaputra and Meghna flood plains have 187Os/188Os ratios close to those of typical continental crust. This isotopic contrast corresponds to that found in sediments carried by the Ganges and Brahmaputra rivers. Therefore, the Os isotopic ratios observed in Bengal groundwater probably reflect those of the corresponding sediments at depth.
High Os contents underscore the potential influence of Os carried by groundwater on the Os marine budget. The Bengal plain probably has only a limited direct effect on the ocean, because its flat topography suggests a small groundwater flux (~3x108 m3/yr corresponding to an Os contribution of ~1x10-1 mol/yr). However, if our mean Os concentration is typical of other aquifers, the global groundwater contribution could be the second source of Os to the ocean after the river flux. Including the groundwater contribution in the Os marine budget could reduce the calculated Os residence time by ~30% and thus partially reconcile it with the rapid 187Os/188Os glacial-interglacial variations recorded in oceanic sediments.