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 (∼2 years) records of seawater 222Rn 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 ∼0.9 cm d−1 at low tide, which is