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Submarine Ground-Water Discharge and Its Influence on Coastal Processes and Ecosystems
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.
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:
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in this issue:
North Carolina Submarine Groundwater
Gulf of Maine Mapping Initiative Forensic Geology Assists Investigation Submarine Groundwater Discharge
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U. S. Department of the Interior | U.S. Geological Survey
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