Land or ocean?: Assessing the driving forces of submarine groundwater discharge at a coastal site in the Gulf of Mexico

Submarine groundwater discharge (SGD) is an important pathway of trace element and nutrient cycling in the coastal ocean. Since terrestrial and marine SGD driving forces are usually superimposed, it is difficult to separate their relative contributions. This work uses continuous, long-term (similar to 2 years) records of seawater Rn-222 to estimate total SGD rates and hydrological observations to examine the driving forces of SGD at a coastal site on the Gulf of Mexico. While seasonal cycles cannot be clearly identified, short-term fluctuations were ubiquitous. Radon-derived SGD was strongly associated with the neap spring tidal cycle, at least during a period of extreme drought when minimum external disturbances were present. We applied multiple independent approaches to separate the relative contribution of fresh and saline SGD. A salinity mixing model indicated that the contribution of fresh SGD ranged between 3 and 12%. The rate of freshening of water within a seepage meter required vertical freshwater advection of similar to 0.9 cm d(-1) at low tide, which is < 10% of the total SGD estimated from Rn-222 (similar to 11 cm d(-1)). Darcy's law calculations and water table decline rate also suggested a fresh SGD contribution < 5%. The temporal trends in total SGD and the reasonable agreement of our multiple approaches indicate that transient marine forces, likely tidal pumping, dominate benthic advective exchange in these permeable sediments. Compared to regional river inputs, our conservatively extrapolated saline SGD is on the same order of magnitude, but fresh groundwater only represents a small fraction. Future investigations should refine techniques for scaling up local observations.