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Scientists from Four USGS Science Centers Collaborate in Study of Coastal Groundwater Exchange in Hood Canal, Washington

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Hood Canal is a fjord forming the western arm of Puget Sound in Washington State. Lynch Cove, located at the head of Hood Canal, was the site of a recent interdisciplinary study of complex processes related to coastal groundwater exchange. In July and August 2012, U.S. Geological Survey (USGS) scientists from across the country, representing three USGS Coastal and Marine Science Centers and the USGS Washington Water Science Center, conducted a field study in Lynch Cove addressing a suite of physical and biogeochemical processes associated with enhanced submarine groundwater discharge (SGD). This interdisciplinary approach, drawing on experts in geochemistry, hydrology, geology, and oceanography, is the central theme of the USGS Coastal Aquifer Project (CAPII), which was recently restructured by Peter Swarzenski (USGS Pacific Coastal and Marine Science Center), Kevin Kroeger (USGS Woods Hole Coastal and Marine Science Center), and Christopher G. Smith (USGS St. Petersburg Coastal and Marine Science Center) to align with current USGS science strategies and opportunities. CAPII currently seeks to address the response of coastal ecosystems to a host of environmental "stressors," including projected sea-level rise and the global trend toward increasing density of population and associated infrastructure in the coastal zone. These stressors negatively impact coastal ecosystems, both in the short and long terms, and can increase their vulnerability to future geohazards. (To learn more about CAPII, see USGS Coastal Aquifer Project II [CAPII] .)

Map of northwestern Washington state
Above: Northwestern Washington, showing locations of Puget Sound, Hood Canal, and Lynch Cove. Boundaries of Olympic National Park are outlined in red. [larger version]

Lynch Cove is a perfect setting in which to study the dramatic interplay between the terrestrial and marine processes that affect coastal groundwater exchange. The high annual rainfall—200 centimeters (80 inches) per year at the study site and 360 centimeters (140 inches) per year in neighboring Olympic National Park—maintains a shallow water table in the permeable glacial sediments surrounding the cove, and this shallow water table drives the discharge of fresh groundwater into the cove. In detail, however, submarine groundwater discharge at the study site is complicated by the extreme tidal range, typically 4 to 5 meters (13–16 feet). At low tide, the discharge of fresh groundwater is so vigorous that small geysers commonly erupt from the exposed beach face. At high tide, fresh groundwater moves seaward more slowly and mixes with saltwater that seeps through the now-submerged beach face. The battle between these terrestrial and marine "forcings" thus plays out dramatically at Lynch Cove, changing hour by hour but with submarine groundwater discharge generally prevailing. (To see a dramatic time-lapse video of the tidal range at the study site, please visit Puget Sound Groundwater Sampling Time Lapse.)

High and low tides in Lynch Cove, WA
Above: High tide (left) and low tide (right) at study site in Lynch Cove, at the head of Hood Canal, Washington. Summer 2012 USGS fieldwork was made possible by Dr. Bill Portuese and his family, who graciously offered the use of their dock and beach for the USGS experiments. Photographs by Peter Swarzenski, USGS. [larger version]

Submarine groundwater discharge benefits coastal ecosystems by conveying fresh water, nutrients, and other vital constituents into nearshore waters, where they contribute to ecosystem health, sustainability, and even resilience to external stressors. (SGD can also have adverse impacts if the groundwater contains contaminants or an excess of nutrients such as nitrogen.) A complete understanding of this physical control on coastal ecosystems, both today and into the future, is the overarching goal of CAPII.

For the summer 2012 fieldwork, nine USGS Coastal and Marine Geology Program (CMGP) employees and contractors converged on Lynch Cove from the three CMGP science centers: Peter Swarzenski, Leticia Diaz, Cordell Johnson, and Jeremy Merckling from the Pacific Coastal and Marine Science Center (PCMSC) in Santa Cruz, California; Kevin Kroeger, Sandy Baldwin, and Wally Brooks from the Woods Hole Coastal and Marine Science Center (WHCMSC) in Woods Hole, Massachusetts; and Christopher G. Smith and Marci Marot from the St. Petersburg Coastal and Marine Science Center (SPCMSC) in St. Petersburg, Florida. Three scientists from the USGS Washington Water Science Center in Tacoma, Washington (Matt Bachmann, Steve Cox, and Rich Sheibley), provided hydrologic expertise and scuba-diving assistance.

USGS research team members at the study site in Lynch Cove, WA
Above: Some members of the USGS research team at the study site in Lynch Cove (left to right): Peter Swarzenski, Wally Brooks, Christopher G. Smith, Sandy Baldwin, Marci Marot, Cordell Johnson, and Kevin Kroeger. Photograph by Leticia Diaz, USGS. [larger version]

The group designed a suite of complementary experiments to examine large-scale, tidally driven sea-level fluctuations in relation to:

  • the physics of groundwater and seawater mixing,

  • geologic controls on the physical exchange of groundwater,

  • biogeochemical cycles of nutrients, trace metals, and uranium-thorium series radionuclides (naturally occurring geochemical tracers), and

  • submarine groundwater discharge and associated material fluxes (such as nutrients and contaminants) into coastal waters.

These individual research components included high-resolution, two-dimensional water sampling down to 10 meters (33 feet) below the sediment-water interface, concurrent time-series sampling at strategic sites throughout a tidal cycle and across the beach face, high-resolution temperature profiling, sediment coring, and geophysical surveys using ground-penetrating radar (to image subsurface earth materials) and multichannel electrical-resistivity methods (to characterize salinity).

Pete Dal Ferro, Cordell Johnson, Jeremy Merckling, and Peter Swarzenski (all at the PCMSC), along with Christopher G. Smith (SPCMSC), went to the field area early—in June 2012—to install temporary wells that would enable the team to characterize different groundwater masses and their response to tidal forcing. Four sets of "nested" wells (designed to sample different depth intervals) were installed between the high- and the low-tide lines and instrumented with pressure, electrical-resistivity, and temperature probes.

The most striking observation during the July and August fieldwork was that groundwater in wells at the low-tide line was fresher and had more stable salinity values than groundwater in wells at the high-tide line. Subsequent groundwater sampling across the entire intertidal zone and offshore using a piezometer (a hollow probe pushed down to the water table) confirmed that fresh groundwater persisted at the low-tide line regardless of the tidal stage. At the high-tide line, in contrast, encroaching seawater mixed with fresh groundwater twice a day during the high tides and then drained from the beach face during the intervening low tides.

Photo of well cluster
Above: Well cluster installed in June 2012 to a depth of 10 meters (33 feet) below the beach face. Photograph taken at low tide; the white PVC extensions prevented the wells from being flooded during high tide (compare with photograph of research team standing on same dock, above). Photograph by Peter Swarzenski, USGS. [larger version]

How this dynamic exchange at the high-tide line and the persistent fresh-water outwelling at the low-tide line affect the transport and transformation of dissolved nutrients, metals, and major cations and anions will be addressed by CMGP scientists. The CMGP team will also model the production, transport, and decay of dissolved and solid phases of radium and radon, two uranium-thorium series radionuclides that are useful for tracing groundwater movement. (To learn more about these naturally occurring geochemical tracers, see Uranium-Thorium Series Geochemical Tracers—Radium and Uranium-Thorium Series Geochemical Tracers—Radon.)

To further study shallow exchange processes in the intertidal zone, custom-designed temperature rods with multiple thermistors (temperature sensors) were installed during different tidal stages. The temperature rods allowed for detailed examination of the vertical temperature structure and the influence of tides on groundwater temperature. These temperature data can provide an independent indicator of complex fluid exchange to corroborate the rates of submarine groundwater discharge derived from radon tracer studies.

Temperature rods installed on the beach
Above: Two six-thermistor temperature rods installed on the beach face to a depth of 1.3 meters (4.3 feet). Photograph by Peter Swarzenski, USGS. [larger version]

Geophysical surveys using ground-penetrating radar and multichannel electrical-resistivity methods were also conducted across the beach face to evaluate possible geologic controls on submarine groundwater discharge to the sea and to better understand how tides affect the fresh water/saltwater interface.

Look for updates to this and related coastal-groundwater projects at http://walrus.wr.usgs.gov/sgd/, or contact Peter Swarzenski (pswarzen@usgs.gov), Kevin Kroeger (kkroeger@usgs.gov), or Christopher G. Smith (cgsmith@usgs.gov).

Related articles from the Sound Waves archives include:

Related Sound Waves Stories
Recent USGS Field Studies of Nearshore Hydrogeologic Exchange and Submarine Groundwater Discharge on U.S. West Coast and Hawaii
November 2009
Submarine Groundwater Discharge Along the West Florida Shelf: Is Groundwater an Important Nutrient Source for Florida's Red Tides?
June/July 2009
Scientists Go Deep to Track Algae-Feeding Nitrogen in Washington State's Hood Canal
July 2006

Related Web Sites
Coastal Aquifer Project II [CAPII]
Puget Sound Groundwater Sampling Time Lapse
Uranium-Thorium Series Geochemical Tracers—Radium
Uranium-Thorium Series Geochemical Tracers—Radon

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cover story:
Water-Quality Dynamics in Barnegat Bay-Little Egg Harbor Estuary

Hurricane Sandy Disrupts Estuary Study, Provides Additional Research Opportunities

USGS Scientists Collaborate in Coastal Groundwater Study

Tracking Pacific Walrus: Expedition to the Shrinking Chukchi Sea Ice

USGS Contributes to Success of St. Petersburg Science Festival

South Korean Geoscientists Visit the USGS in California

Strategic IODP Planning Workshop for Ultra-Deep Drilling into Arc Crust

Training to Use New Lidar Scanner

Staff Remembering Asbury "Abby" Sallenger

Research Vessel Named for Retired USGS Scientist

A Passion for Educational Outreach—Profile of Carol Reiss


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