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Pacific Coastal and Marine Science Center

Submarine Groundwater Discharge

An Introduction to Submarine Groundwater Discharge

Controlling Forces

Idealized hydrogeologic cross section showing submarine groundwater discharge.

Submarine groundwater discharge (SGD) is an ubiquitous coastal process that is driven by a composite of climatologic, hydrogeologic, and oceanographic processes. For example, terrestrial hydraulic gradients that reflect both short and long term climatic conditions almost always transport both surface and ground water toward the coast. In coastal waters, physical oceanographic processes such as wave set-up, tidal pumping, and density-driven circulation impact these hydraulic gradients and thus affect rates of submarine groundwater discharge. Although only fresh groundwater discharge has traditionally been accounted for in numerical simulations of coastal water budgets, the discharge of recirculated saline groundwater may be equally or even more important in terms of material transport (for example, nutrients, metals, organics) across land/sea margins. For the purposes of this site, we therefore define SGD to consist either of fresh ground water, re-circulated seawater, or a composite therefore, and will evaluate and present SGD in terms of a vector for nutrient delivery to coastal waters.

Until the mid-1990s, studies on SGD did not receive widespread attention, because it was generally thought that SGD rates were not large enough to be a direct influence on ocean water budgets. This omission may in part be due to the inherent difficulty in identifying sites and quantifying rates of SGD, because most SGD occurs as diffusive flow, rather than discrete spring flow. This is in sharp contrast to studies of river discharge or river chemistry, which are obviously more easily sampled and quantified. However, there is a growing recognition that the submarine discharge of fresh, brackish, and marine ground water into coastal oceans is just as important as river discharge in some areas of the coastal ocean. This site will thus review the progress made in SGD science (with particular emphasis on new applications of geochemical tracers and novel geophysical tools), provide links to many SGD projects and study sites, and present an inclusive list of relevant publications. The eventual goal of our SGD science is to develop some forecasting or predictive capability based on being able to de-couple climatic and seasonal signatures from SGD rates.

Modernizing our Approach

Increasing population density and changing agricultural practices in coastal areas have led to releases of nutrients (and other contaminants) into the coastal environment from fertilizer use, industrial practices, and wastewater discharge. These increased nutrient releases have led to eutrophication in many coastal waters, which is a widespread concern. Yet, the role that groundwater-derived nutrients has played in coastal eutrophication is not well understood in many areas. The ecological and economic impacts of eutrophication have been substantial in many coastal regions, which demands a better understanding of the contribution of groundwater-derived nutrient fluxes. Management of wastewater treatment practices in coastal regions critically depends on accurate estimates of the flux and quality of ground water in the coastal zone. In addition, informed resource management requires an improved understanding of the geological framework of coastal aquifers, the pathways by which ground water travels to the coastal zone, the specific locations and dimensions of submarine discharge zones, and the geochemical transformations that take place prior to discharge.

Basic science questions related to how fluid flux across ocean margins and fluid recirculation through ocean margin sediments affects elemental cycling at all scales are also scientific priorities of this research effort. Experiments that address more applied aspects of nutrient delivery can also yield information that is valuable for developing a more general understanding of land-ocean aquifer interactions.

While previously, our focus was to examine geologic control on coastal aquifers and groundwater discharge, today it is apparent that knowledge of land–sea exchange must also encompass interplay among ecosystems science including ecosystems health and climate-change-related processes, as well as natural geohazards.

Thumbnail image of cover of Volume 107 Chemical Reviews.Read more

"U/Th series radionuclides as tracers of coastal groundwater," Chemical Reviews, volume 107 issue 2, pages 663-674, doi:10.1021/cr0503761

 

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URL: https://walrus.wr.usgs.gov/sgd/
Science questions to: Peter Swarzenski
Site maintained by: Laura Zink Torresan
Page Last Modified: 29 October 2014 (lzt)