Processes Controlling Submarine Groundwater Discharge: Examples from Florida, Brazil, and Norway

Submarine groundwater discharge (SGD) occurs along any coastline where continental aquifers intersect marine environments and hydraulic heads are above sea level. Coupled to this hydrologic connection are physical and biological interactions occurring across the seafloor, including waves, tides, currents, bioirrigation, and bioturbation. These marine SGD processes pump water into and out of sediments on time scales of minutes to hours and generally result in no net transport of water in either direction. Their flow rates are essentially piston velocities across the seabed, but they serve an important role in porewater biogeochemistry and early diagenesis of sediments to depths ranging from 10 to 150 cmbsf (cm below seafloor). We measured SGD in Indian River Lagoon, Florida, USA, in Flamengo Bay, Ubatuba, Brazil, and in Sommarøya, Troms Kommune, Norway, and present a summary of processes that contribute to SGD measurements in these coastal waters. The Florida lagoon is microtidal and the primary mechanisms for SGD are water table fluctuations, wave/currents, and bioirrigation. Terrestrial SGD is between 0.1 and 8 cm/day and represents about 1% of total flow, while marine SGD (the seawater component) was measured as high as 115 cm/day and represented 99% of total SGD. In Flamengo Bay, Brazil, groundwater discharge from the coastal aquifer is strongly coupled to rapid recharge from precipitation events and tidal forcing at the coast. The fresh component of SGD in the Brazil site ranged between 64 to 87% nearshore and decreased quickly to less than 4% fresh; total SGD rates ranged from 130 to 295 cm/day. In Sommarøya, Norway, discharge appears to be driven by snowmelt recharge in the surficial aquifer and tidal forcing at the beach. Preliminary estimates of SGD are less than 14 cm/day at the Norwegian beach during snowmelt season.