Magnitudes of submarine groundwater discharge from marine and terrestrial sources: Indian River Lagoon, Florida

[1] Magnitudes of terrestrial (fresh) and marine (saline) sources of submarine groundwater discharge (SGD) are estimated for a transect across Indian River Lagoon, Florida. Two independent techniques (seepage meters and pore water Cl concentrations) show terrestrial SGD decreases linearly to around 22 m offshore, and these techniques, together with a model based on the width of the outflow face, indicate a cumulative discharge of between 0.02 and 0.9 m/d per meter of shoreline. Seepage meters and models of the deficiencies in Rn activity in shallow sediments indicate marine SGD discharges of roughly 117 m/d per meter of shoreline across the entire 1800-m-wide transect. Two surface streams nearest the transect have an average discharge of about 28 m/d per meter of shoreline. Marine SGD is thus 4 times greater then surface water discharge and more than 2 orders of magnitude greater than terrestrial SGD. The magnitude of the terrestrial SGD is limited by the amount of regional precipitation, evaporation, recharge, and groundwater usage, while marine SGD is limited only by processes circulating marine water into and out of the sediments. The large magnitude of marine SGD means that it could be important for estuarine cycling of reactive components such as nutrients and metals with only slight modification from estuarine water compositions. The small magnitude of terrestrial SGD means that large differences from estuarine water composition would be required to affect chemical cycling.