EVALUATING THE IMPORTANCE OF SUB-SURFACE INPUTS OF SOLUTES TO THE COASTAL OCEAN

Since the dawn of oceanography, mass balance calculations for oceanic solutes have relied primarily on surface water analyses of rivers to calculate input fluxes. Prior to 1990, sub-surface inputs to the oceans were difficult to quantify and thought to be of only local importance. The advent of tritium-3He age dating of groundwater provided a means by which the transport rate of water in coastal aquifers could be directly evaluated by the analysis of as few as a dozen discrete well samples. The vertical age gradient of water from a cluster of wells screened at discrete depths yields the recharge rate via a simple evaluation of the Vogel model. At steady state, the vertical flux of water to the coastal aquifer had to balance the discharge to the coastal ocean. The first application of this process was in the Kenyan Rift Valley, Lake Naivasha. Further research took place on the Alabama Coastal Plain aquifer to establish the nutrient flux of agriculturally impacted recharge to the Gulf of Mexico. Application of this technique to the Ganges – Brahmaputra River (GBR) System solved a substantial unresolved question related to Cenozoic Sr-isotope evolution of oceanic Sr. The magnitude of submarine discharge, although only 15% of the surface river flux, input an equal amount of radiogenic ⁸⁷Sr because the concentration of Sr in the groundwater was typically 5x the value of the GBR. Other solutes in the GBR submarine groundwater discharge, in particular Ba and Ra, also played a major role in the mass balance calculation of the global ocean. In addition, other large fluvial deltas such as the Mekong and the Irrawaddy, that have thick deposits of immature Himalayan derived sediments would also impact future oceanic balance calculations.