Submarine groundwater discharge to the Gulf of Mexico and the Bay of Bengal: Elemental flux estimates and sediment-groundwater interactions

In constructing water budgets and mass flux estimates for the coastal margins, submarine groundwater discharge is often overlooked because it represents a non-point source, occurs over a broad subterranean area, and is therefore more difficult to quantify. However, our investigations in Alabama and the Bengal Basin have shown that groundwater discharge can be substantial and can influence oceanic chemistry through the discharge of nutrients from fertilizers and trace metals from the weathering of the aquifer protoliths.; We estimate that the mature siliceous aquifer of Baldwin County, AL has a groundwater flux to the Gulf of Mexico of 1.9 ± 0.8 × 10 9 m3/yr and its submarine nitrate discharge is 1.1 ± 0.01 × 108 mol/yr or 0.8% of the annual Mississippi River input. If other coastal zones have similar submarine fluxes, then groundwater discharge would effectively double the nitrate discharge to the Gulf. In the Bengal Basin, the groundwater fluxes of Sr and Ba to the global ocean are estimated to be 3.9% and 1.5%, respectively, of the global continental flux. This strongly suggests that groundwater discharge into the coastal oceans can be a significant source of trace metals such as Sr and Ba and influences the global Sr isotopic balance and its evolution through time. In coastal floodplains with high precipitation rates and rapid accumulation of immature sediment, such as the Bengal Basin, submarine discharge can make significant contributions to global oceanic budgets equal to or greater than the surface river flux.; Within the Bengal Basin, a significant portion of the population suffers from arsenic poisoning. The sediments appear capable of supplying the dissolved arsenic to the groundwater. The microbial mediated reductive dissolution of iron oxy-hydroxides liberates arsenic from the sediment into the moderately reducing groundwater used by over 40 million people. The strong correlations between high levels of dissolved arsenic with iron, methane, and ammonia links the bacterial breakdown of As-bearing FeOOH to the Holocene deposits of the Ganges-Brahmaputra Rivers floodplain.