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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.

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. (Watch a dramatic time-lapse video of the tidal range at the study site: “Puget Sound Groundwater Sampling Time Lapse.”)

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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.

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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.

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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.

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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 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.

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, or contact Peter Swarzenski (pswarzen@usgs.gov), Kevin Kroeger (kkroeger@usgs.gov), or Christopher G. Smith (cgsmith@usgs.gov).