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Fieldwork

In Search of Submarine Ground-Water Discharge Along the Suwannee River Delta, Florida


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In March 2005, researchers from the U.S. Geological Survey (USGS), Florida State University (FSU), and the University of South Florida (USF) traveled to the Suwannee River delta to search for sites of submarine ground-water discharge. Much evidence suggests that submarine ground-water discharge can be important not only for estimating how much water of different types (for example, ground water, river water, seawater) moves across land/sea margins, but also for estimating the amounts of ground-water-borne constituents being delivered to the coastal ocean. Our goal in March was to use a suite of newly developed tools, more or less simultaneously, to identify sites of submarine ground-water discharge and possibly also to quantify discharge rates.

Study area at the Suwannee River delta on Florida' west coast, shown on a map (left) and a satellite image (right).
Above: Study area at the Suwannee River delta on Florida's west coast, shown on a map (left) and a satellite image (right). (The satellite image, a mosaic of several Landsat Thematic Mapper images from the early 1990s, was developed by the Florida Suwannee River Water Management District.)

We have developed the following tools with our collaborators to study submarine ground-water discharge:

  • Airborne thermal imagery, which detects temperature differences between water masses. Because ground-water temperatures remain relatively constant throughout the year, the temperature difference between surface water and ground water is greater during seasons when surface water is warmed or cooled; thermal images taken during these times will show where ground water is flowing into surface water. (USF)
  • Streaming resistivity profiling, which detects pore-water conductivity on the basis of variations in electrical resistance; ground water generally has lower salinity and lower conductivity (higher resistivity) than seawater. (USGS)
  • Analysis of radon-222 (222Rn), a naturally occurring radioactive gas with a short half-life (3.8 days) that is much more concentrated in ground water than in surface water. (FSU and USGS)
  • Analysis of methane (CH4), another naturally occurring gas that is more concentrated in ground water than in surface water. (FSU)
  • Analysis of nutrients, stable isotopes, and noble gases whose concentrations differ in ground water and surface water. (USGS)

Some Highlights of the Fieldwork and Preliminary Observations

Sarah Kruse (USF) deployed a series of temperature loggers randomly throughout the delta and lower river to aid in calibration of the thermal imagery. Kevin Kroeger and Chris Reich (both of USGS) used drive-point piezometers to obtain ground-water samples across a salinity gradient within a marsh site and a riverbank site. Ground-water samples were collected for later analyses of nutrients, trace elements, N2 and Ar gas, and 222Rn. Matt Weiss (USF) operated an electromagnetic-resistivity array to map subsurface salinity anomalies in relation to the dynamic position of the freshwater/saltwater interface. Salinity data from the piezometer profiles were used to ground-truth the electromagnetic-resistivity data.

Jason Greenwood and Brian Blake-Collins (USGS contractors through ETI Professionals, Inc.) worked with Rick Peterson (FSU) to map about 50 km of continuous streaming resistivity and 222Rn data offshore from the mouth of the Suwannee River and upriver. The 222Rn data they collected in March 2005 are compared with 222Rn data collected in June 2004 on the accompanying maps.

Surface-water 222Rn activities in the Suwannee River and delta region measured in June 2004 (top) by the FSU group (Bill Burnett, Rick Peterson) and in March 2005 (bottom).
Above: Surface-water 222Rn activities in the Suwannee River and delta region measured in June 2004 (top) by the FSU group (Bill Burnett, Rick Peterson) and in March 2005 (bottom). Many of the June 2004 data were collected upstream, whereas the March 2005 data were collected mostly offshore, in waters adjacent to the river mouth. Suwannee River discharge (Q) was greater in March 2005 than in June 2004, as reflected in the higher 222Rn activities farther offshore in March 2005. Units of activity are in dpm/L, or disintegrations (number of radon atoms decaying) per minute per liter.

Interestingly, 222Rn and CH4, both of which have been shown to be effective tracers of submarine ground-water discharge, diverge in the upstream section of the survey. Such results suggested that a sinkhole may have developed somewhere upstream in the watershed, producing a slug of ground water rich in 222Rn but low in CH4. Some recent evidence, indeed, confirms the formation of a large sinkhole in the Suwannee River watershed.

Streaming resistivity profiling in the lower reaches of the Suwannee River revealed new details about the position and dynamics of the freshwater/saltwater interface, as shown in the plot of resistivity data that accompanies this article. As expected, the resistivity (inverse of conductivity or salinity) of pore water in the Suwannee riverbed is clearly lower (more saline) at the mouth of the river and becomes higher (that is, fresher) upstream.

Inverted streaming resistivity profile collected on March 2, 2005, along a 5-km-long line moving upstream from the town of Suwannee (left) toward Yellow Jacket (right).
Above: Inverted streaming resistivity profile collected on March 2, 2005, along a 5-km-long line moving upstream from the town of Suwannee (left) toward Yellow Jacket (right). Blue hues denote higher pore-water salinities; orange/yellow hues denote freshened water masses (lower salinities). Surface-water temperature, depth, and salinity are accounted for in this inversion. Water depth is shown as a black line. As expected, the resistivity (inverse of conductivity or salinity) of pore water in the Suwannee riverbed is clearly lower at the mouth of the river (more saline) and becomes higher (that is, fresher) upstream. [larger version]

Preliminary results from this field effort confirm the usefulness of the new techniques and approaches in refining studies of water exchange across land/sea margins and of submarine ground-water discharge. For further information, please contact members of the USGS submarine ground-water discharge team: Peter Swarzenski (pswarzen@usgs.gov), Kevin Kroeger, Chris Reich, Jason Greenwood, and Brian Blake-Collins.


Related Web Sites
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U.S. Geological Survey (USGS)

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