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Studying Submarine Ground-Water Discharge at Santa Barbara, California, and Hood Canal, Washington
What do contaminants in waters around Santa Barbara, Calif., have in common with fish kills in Hood Canal, Washington? Both problems are likely influenced by submarine ground-water discharge, the movement of ground water into the ocean through underwater springs and seeps. This largely invisible process has recently been recognized as an important way that contaminants and excess nutrients, such as nitrogen, enter the ocean. U.S. Geological Survey (USGS) specialists in oceanography, geology, and hydrology are collaborating in studies of submarine ground-water discharge in Santa Barbara and Hood Canal, where they conducted fieldwork from May 29 to June 12, 2006. Peter Swarzenski, Christopher Reich, and Jason Greenwood of the USGS Florida Integrated Science Center office in St. Petersburg, Fla., traveled to Santa Barbara to work with John Izbicki of the USGS California Water Science Center's San Diego field office. They gathered chemical and physical data to better understand the mechanisms that control ground-water movement beneath the beach near Stearns Wharf. This study of submarine ground-water discharge is part of a much larger project to better understand and evaluate the sources of fecal contamination of streams and ocean beaches in the Santa Barbara area (see Sources of Microbial Contamination, Santa Barbara). The recent fieldwork marked the second time that the USGS team from St. Petersburg has worked with John Izbicki in the Santa Barbara area.
Multiple techniques were implemented to quantify ground-water flow beneath the beach and flux across the seabed. Equipment for making time-series measurements was set up on a 1920s salmon-fishing boat (the merchant vessel Pieface) anchored just offshore to continuously monitor surface-water radon activity (a tracer of ground water, which commonly has much higher radon activity than seawater), seepage flux, and surface- and bottom-water temperature, salinity, pH, and dissolved oxygen. In addition, an hourly sampling regimen was established during a falling tide (over an 11-hour period) to collect water samples from the sea surface and from a monitoring well on the beach. To get a better sense of how the ground water behaves in response to tidal forces, a time-series resistivity system (by Advanced Geosciences, Inc.) was deployed at various sites along the beach. Electrical resistivity is a geophysical method of detecting changes in both lithology and pore-water salinity: the saltier the water, the lower its resistivity, and vice versa. Since lithology doesn't change, the variation measured by the resistivity system is due solely to pore-water salinity changes. The resistivity system took measurements every 30 minutes to produce a series of two-dimensional profiles of the surficial aquifer's salinity down to 30 m below the sea floor. These profiles show how ground water moves as a result of tidal forces.
The second leg of the trip took Chris Reich and Jason Greenwood on a 2-day drive from Santa Barbara to Hood Canal in Washington State, where they met up with Bill Simonds of the USGS Washington Water Science Center in Tacoma and Don Rosenberry of the USGS Water Resources Discipline's National Research Program office near Denver, Colo. Bill and Don have been working in Hood Canal for more than 2 years. The plan for the recent fieldwork was to survey submarine ground-water discharge in much the same fashion as for Santa Barbara. With the kind generosity and assistance of local residents Al Adams and Bill Portuese (both from near Union) and Bob Hagar (from Sunset Beach), the scientists were able to:
Hood Canal is a fiord off Puget Sound formed by glacial scouring and outwash from glacial meltwater some 13,500 to 15,000 years ago. The ice sheet was 0.6 to 1.3 km thick over the Puget Sound region during the latest glacial maximum. Because the elevation of the surrounding terrain is moderately high (approx 170 m) and the surficial geology consists of glacial tills, which are moderately permeable, the area has a steep hydraulic gradient, resulting in springs onshore and submarine ground-water discharge at the coastline. Numerous low-salinity (less than 5 ppt) seeps were observed along the beach face at low tide, indicating discharge of ground water into the overlying marine waters of Hood Canal. Radon and seepage data corroborate the greatly increased discharge of ground water visually observed at low tide. Developing better constraints on the quantity, quality, and distribution of submarine ground-water discharge into Hood Canal will help identify problems, such as nutrient inputs, that can result in low-dissolved-oxygen (hypoxic) bottom waters that have historically caused fish kills and other problems in Hood Canal (see Sound Waves article, "Scientists Go Deep to Track Algae-Feeding Nitrogen in Washington State's Hood Canal).
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in this issue:
Submarine Ground-Water Discharge in Santa Barbara and Hood Canal California Sea-Otter Numbers Dip in 2006, Overall Population Trent Positive
Parasites, the Thread of Food Webs?
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U. S. Department of the Interior | U.S. Geological Survey
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