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Research

Submarine Ground-Water Discharge and Its Influence on Coastal Processes and Ecosystems


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Two coastal submarine-ground-water-discharge sites.
Above: Two coastal submarine-ground-water-discharge sites (arrows) in Tampa Bay, FL, directly affecting shoreline geomorphology. Aerial photograph courtesy of C. Kovach, Florida Department of Environmental Protection. [larger version]

U.S. east coast, showing some of the sites where the USGS is studying submarine ground-water discharge.
Above: U.S. east coast, showing some of the sites where the USGS is studying submarine ground-water discharge. [larger version]

Submarine ground-water discharge has recently been recognized as a ubiquitous phenomenon that can strongly influence coastal-water and geochemical budgets and drive ecosystem change. For example, the discharge of nutrient-enriched ground water into coastal waters may contribute to eutrophication, the process by which a body of water becomes enriched in dissolved nutrients that stimulate the growth of aquatic plant life, commonly resulting in excessive algal blooms and the depletion of dissolved oxygen. Similarly, submarine ground-water discharge can also directly affect the availability of fresh water to coastal communities, impact fragile coastal ecosystems, such as estuaries and coral reefs, and influence shoreline geomorphology.

A 1996 paper by W.S. Moore in Nature (v. 380, p. 612-614) raised awareness of the global importance of submarine ground-water discharge, and much effort has subsequently been devoted to developing new tracer techniques and methods for identifying and quantifying submarine ground-water discharge. Because the discharge of coastal ground water commonly occurs as diffuse seepage rather than focused discharge, assessing submarine ground-water discharge has remained difficult for both oceanographers and hydrologists. Through national and international research programs, scientists have developed a variety of complementary approaches for quantifying submarine ground-water discharge, using a wide assortment of tracers and methods. Intercalibration experiments, such as those conducted in coastal waters off Australia, Brazil, and Long Island, NY, demonstrate that careful measurements can accurately quantify submarine ground-water discharge, confirm some of the driving mechanisms (such as climatic and tidal forcing), and constrain the spatial and temporal scales at which these mechanisms operate. Now that approaches for rigorously quantifying submarine ground-water discharge are becoming better established, scientists can begin to investigate the wide variety of coastal processes affected by submarine ground-water discharge.

The U.S. Geological Survey (USGS) is uniquely poised to conduct comprehensive investigations of submarine-ground-water-discharge processes in coastal ecosystems because of its broad scientific expertise. For example, USGS scientists representing the Water Resources Discipline (WRD) have well-established expertise in ground-water sampling and variable-density-modeling techniques. USGS scientists representing the Coastal and Marine Geology Program are developing and applying a host of new, complementary geophysical and geochemical tools. For example, sites of submarine ground-water discharge can be inferred by using streaming electrical-resistivity instrumentation, which detects pore-water conductivities based on variations in electrical resistance. This technique complements more traditional methods (such as sidescan sonar, multibeam sonar, and subbottom acoustics) for mapping subsurface geology. New instruments capable of in situ analyses of radon-222 (a naturally occurring radioactive gas with a half-life of 3.8 days that is more concentrated in ground water than in surface water) can also help pinpoint locations of submarine ground-water discharge. This technique can also be used to infer regionally averaged discharge rates by contrasting ground-water and surface-water activities. A new complementary tool is the autonomous seepage meter, which allows direct quantification of bidirectional submarine ground-water discharge over areas of approximately 1 m2. Multiport piezometers and other equipment, including floating drilling platforms, allow samples of submarine ground water to be collected at different depths. These samples can then be analyzed for a suite of constituents, such as salinity, chlorine ions (Cl-), chlorofluorocarbons (CFCs), and nutrients. Residence times of coastal waters and submarine-ground-water-discharge rates can be derived by measuring four natural isotopes of radium whose half-lives range from 3.7 days to 1,600 years. The USGS science centers in St. Petersburg, FL, and Woods Hole, MA, have recently acquired many of these capabilities, and additional analyses are available through collaborations with other USGS and academic scientists.

dealized hydrogeologic cross section showing submarine ground-water discharge.
Above: Idealized hydrogeologic cross section showing submarine ground-water discharge. [larger version]

The study of submarine ground-water discharge is valuable for understanding the availability and quality of coastal ground water and its control on many coastal processes that span the disciplines of geology, geomorphology, geochemistry, biology, hydrology, and ecology. Specific examples of research areas where USGS submarine-ground-water-discharge studies can help to solve interdisciplinary problems include:

  • assessment of the redox-controlled and microbially controlled delivery of nutrients and trace elements in submarine ground-water discharge (linking the fields of geochemistry and coastal hydrogeology)
  • evaluation of coastal-ecosystem change in response to variations in submarine ground-water discharge (linking the fields of biology, ecology, and biogeochemistry to coastal hydrogeology and meteorology)
  • examination of shorefaces breached by submarine ground-water discharge and the influence that submarine ground-water discharge associated with paleochannels has on erosional hotspots (linking the fields of coastal geology and geomorphology to coastal hydrology)
  • developing the capability to forecast processes and events associated with submarine ground-water discharge (for example, can we predict lagtimes between the initiation of wastewater discharge into coastal aquifers and the onset of impacts on coastal ecosystems by nutrients derived from submarine ground-water discharge?)
Current USGS projects on submarine ground-water discharge are primarily focused on such first-order questions as "What are the locations and rates of discharge?", as well as studies of nutrient delivery from submarine ground-water discharge. Some of the areas on the U.S. east coast where the USGS is studying submarine ground-water discharge are shown on the map accompanying this article. Additional studies are underway in other parts of the United States and on Israel's Mediterranean Sea coast.


Related Sound Waves Stories
Integrated Science Team Deploys New Tools to Study Submarine Ground Water in North Carolina
June 2004
Studying Submarine Ground Water in Rhode Island Under Arctic Conditions
March 2004
Studying Underwater Water in the Land of Misty—Chincoteague Bay, Maryland
October 2003
Progress in Delineating Submarine Ground-Water Discharge in Delmarva Coastal Bays
June 2002

Related Web Sites
St. Petersburg Science Center
U.S. Geological Survey (USGS)
Woods Hole Field Center
U.S. Geological Survey (USGS), Woods Hole, MA
Nature (Vol. 380 No. 6575)
Nature Publishing Group

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in this issue: Fieldwork
cover story:
Benthic Habitats in Glacier Bay

North Carolina Submarine Groundwater

Research Mucus-Hosted Microbial Communities

Gulf of Maine Mapping Initiative

Forensic Geology Assists Investigation

Submarine Groundwater Discharge

Outreach
USGS Participates in Marine Quest X

Meetings
Caribbean Tsunami Hazard Workshop

Awards
USGS wins Blue Pencil, Gold Screen Awards

Publications
June Publications List


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