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Abstract:Coastal hydrogeologists and oceanographers now recognize the potentially significant contribution that submarine groundwater discharge (SGD) could make to the coastal ocean. SGD may be both volumetrically and chemically important to coastal water and chemical budgets. A worldwide compilation of observed SGD shows that groundwater seepage from the land to the ocean occurs in many environments along the world's continental margins. Further, SGD has a significant influence on the environmental condition of many nearshore marine environments and provides a strong motivation for improved assessments. Our review reveals a critical lack of data from coastal zones of almost all parts of the world, especially in South America, Africa and parts of Asia, making a comprehensive compilation incomplete. SGD should be paid more attention with regard to water and dissolved material budgets at the local and global scales. SGD intercomparison experiments and coastal typologies (classification) may enable evaluation of the accuracy of the SGD estimates and up-scaling of SGD to a global scale.

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[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 À 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 3 /d per meter of shoreline. Seepage meters and models of the deficiencies in 222 Rn activity in shallow sediments indicate marine SGD discharges of roughly 117 m 3 /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 3 /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.

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