Towards global quantification of seawater circulation in coastal aquifers

Abstract

Submarine groundwater discharge (SGD) is significant to coastal water chemistry and ecology. Nonetheless, the majority of SGD flux to the ocean comprises circulated seawater. This study deals with seawater circulation in coastal aquifers on a global scale in order to assess solute fluxes through SGD into the ocean. While the circulated seawater does not affect the water budget, it has a much higher impact on the ocean solutes budget due to water-rock interactions. We present a global assessment of saline SGD mechanisms' role using numerical simulations and analytical calculations. The numerical model simulates three main circulation mechanisms in coastal aquifers: density-driven circulation (long-term), tidal-driven nearshore circulation, and tidal pumping (short-term), while we calculate the wave-driven benthic exchange flux analytically using the same settings of the numerical model. The model tests the typical range of geohydrological parameters such as hydraulic conductivity, hydraulic gradient, tidal amplitude, and more. Our results revealed that: (1) increasing hydraulic conductivity increases the density-driven and decreases the tidal-driven nearshore circulations; (2) increasing the hydraulic gradient (or freshwater recharge) has no significant effect on the density-driven circulation while it slightly decreases the short-term nearshore circulation; (3) tidal pumping fluxes are a relatively large fraction of the overall SGD flux (30%-60%). Together with global hydraulic parameter distributions, the model results enable assessing the global SGD component of seawater circulation. Preliminary results reveal that the total density-driven SGD is about 0.5-1% of the river fluxes to the oceans. Based on the enrichment of calcium in the long-term SGD component, our global assessment of the calcium flux through density-driven flow may reach the same calcium flux through rivers into the ocean.