Recent USGS Field Studies of Nearshore Hydrogeologic Exchange and Submarine Groundwater Discharge on U.S. West Coast and Hawai‘i


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Recent USGS Field Studies of Nearshore Hydrogeologic Exchange and Submarine Groundwater Discharge on U.S. West Coast and Hawai‘i

By Peter W. Swarzenski and Helen Gibbons
November 2009

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Above: Low tide at a coastal spring near the Nisqually River delta. Continuous measurements of ²²²Rn, a tracer for submarine groundwater discharge, were being made by instruments on the boat and in the spring vent (foreground) to quantify discharge rates. [larger version]

One is hard-pressed to find three study sites whose groundwater systems are more diverse than a tropical bay next to Honolulu, Oahu, Hawai‘i (Maunalua Bay), a famous beach just up the coast from Los Angeles, California (Surfrider Beach, Malibu), and a high-energy delta system in southern Puget Sound that is poised for large-scale ecological restoration (Nisqually River delta, Washington). In each of these three contrasting sites, the exchange of surface water and groundwater and the associated transport of chemical constituents (for example, nutrients, metals, organic compounds) in nearshore coastal waters can play an important, even defining role in the health of the sites’ ecosystems.

Scientists with the U.S. Geological Survey (USGS) Geology Discipline—Peter Swarzenski, Eric Grossman, and Leticia Diaz (Santa Cruz, California); Robert Rosenbauer (Menlo Park, California); and Chris Reich (St. Petersburg, Florida)—joined forces with scientists with the USGS Water Resources Discipline—Gordon Tribble and Sarah Rosa (Honolulu, Hawai‘i); John Izbicki, Carmen Burton, Nick Teague, Dave O’Leary, and Dara Goldrath (San Diego, California); Jacob Fleck (Sacramento, California); and Rick Dinicola and Steve Cox (Tacoma, Washington)—to investigate the exchange of water and associated constituents between groundwater and coastal surface water using a suite of geochemical (radionuclides, trace elements, and nutrients) and geophysical (electromagnetic seepmeters, electrical resistivity) tools.

Ample evidence now exists to confirm that the persistent discharge of coastal groundwater can significantly affect the chemistry of nearshore waters. For example, groundwater commonly contains land-derived nutrients, such as nitrogen and phosphorus, in higher concentrations than surface water. When elevated amounts of these nutrients are delivered to coastal waters by submarine groundwater discharge, they can fuel algal blooms and contribute to coastal eutrophication—a process in which decomposition of excessive algae depletes oxygen dissolved in the water, leading to increased stress and even death for fish and shellfish. Submarine groundwater discharge may also transport microorganisms into nearshore waters.

Among the objectives of our studies were to systematically characterize the dynamics of submarine groundwater discharge and the associated flux of terrestrial material and to evaluate how the three ecosystems are responding to natural and man-made environmental stressors. Such results contribute to ongoing USGS ecosystem studies, including the Coastal Habitats in Puget Sound (CHIPS) Project and the Coral Reefs Project, which aim to improve our understanding of ecosystem responses to climate change and land use, including restoration.

Above: Study sites in Maunalua Bay, Oahu, Hawai‘i. A, Niu study site. On rock in foreground are (right to left) a battery, a pump, and a water-quality probe for measuring water temperature, salinity, pH, and dissolved oxygen. The boat contained additional water-quality probes, plus instruments for making continuous measurements of ²²²Rn, a tracer for submarine groundwater discharge. [larger version] B, A time series of ²²²Rn (half-life = 3.8 days) over 4-plus days at the Black Point study site shows clear maxima just after each low tide and can be used to calculate a submarine-groundwater-discharge rate at this site (approximately 33 cm per day). dpm, disintegrations per minute. [larger version]

Maunalua Bay is on the south side of the island of Oahu and forms part of the eastern shoreline of Honolulu. Over the past 50 years, increased sedimentation and pollution from the land have affected the bay ecosystems. Nonnative plants and animals have invaded the bay, the diversity and abundance of coral-reef species have diminished, and populations of reef fishes have declined. We began an investigation to determine whether submarine groundwater discharge might be affecting ecosystem health. In parts of Maunalua Bay, we identified and quantified widespread groundwater discharge of nearly fresh water using time-series measurements of radon (²²²Rn), a naturally occurring noble gas. As a radioisotope, ²²²Rn has a short half-life (3.8 days) and is much more concentrated in groundwater than in surface water, making it an excellent tracer for groundwater discharge. Analysis of these and additional data will help us identify the impacts of submarine groundwater discharge on the bay.

Above: The beach at Malibu Lagoon becomes a temporary lab from which to study how the tides move water and microorganisms through the permeable berm sand. A, early morning, low tide [larger version]; lagoon to left, Pacific Ocean to right. B, midnight, high tide; lagoon to right. [larger version]

Surfrider Beach in Malibu, California, about 15 km up the coast from Los Angeles, is one of several Southern California beaches where bacterial contamination has caused frequent beach closures. We have begun an investigation here to study the relation between submarine groundwater discharge and nearshore water quality. Preliminary results indicate that the sandy, permeable beach berm at Surfrider Beach in Malibu provides a leaky but effective filter for many microorganisms as the water from Malibu Lagoon is tidally pumped through the permeable sand. Additional data and analyses will help us better understand the role of submarine groundwater discharge in the delivery of land-derived contaminants to coastal waters. Interestingly, a great white shark about as long as our research boat was observed near the Malibu Pier while we were conducting offshore surveys. To view video footage of the shark and get a general look at our study area, visit http://www.myfoxla.com/dpp/news/local/Great_White_Shark_Sighting_in_Malibu_20090723.

Dikes constructed to drain wetlands for agricultural use more than a century ago are gradually being removed in the Nisqually River delta as part of a project to restore some of Puget Sound's richest estuarine habitat (see Nisqually Delta Restoration). At Nisqually, we focused our nearshore-exchange study on a large coastal spring, where tides with ranges greater than 5 m surge into and out of the hydrologically transmissive glacial terrain, thereby modulating spring discharge and associated material transport. Further studies will help us understand how the changes associated with restoration affect the patterns of submarine groundwater discharge.

Preliminary results confirm the utility of the techniques we used to better understand land/sea exchange processes, including submarine groundwater discharge and the flux of associated constituents. This research benefited tremendously from the help of local citizens, including Alyssa Miller and Carol Wilcox (Malama Maunalua), Barbara Cameron (City of Malibu), and Daniel Hull (Nisqually Reach Nature Center).

For more details about submarine groundwater discharge, visit the USGS Submarine Groundwater Discharge Web site. For more information about the projects described in this article, please contact Peter Swarzenski, U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, CA 95060, phone 831-427-4729, e-mail pswarzen@usgs.gov.