U.S. Department of the Interior
U.S. Geological Survey
FS-173-96
Geophysical Mapping of the Freshwater/Saltwater Interface in Everglades National Park, Florida
Water quality in Everglades National Park (ENP) and the discharge of fresh water into Florida Bay are influenced by water use and management policies in South Florida. The flow of fresh water through the Everglades into Florida Bay is critical to the well being of the South Florida Ecosystem (SFE). Restoration activities by Federal agencies are aimed at mitigating the effect of increased demand for water, farming, and flood control practices in South Florida on the SFE. Assessing the effectiveness of restoration efforts is difficult because of inaccessibility of much of this area. Airborne geophysical methods provide a means of rapidly and economically monitoring large areas where ground access is difficult.
The ability of materials to conduct electricity is called
electrical conductivity, which in geologic materials is
controlled primarily by the amount of pore space in the rock and
the salinity of the water in the pore space. Increasing the pore
space or salinity increases the electrical conductivity, while
decreasing these quantities has the opposite effect.
The conductivity of water, which is controlled by the
concentration of dissolved ions, shows how well the water
conducts electricity. For example, fresh water found in the ENP
typically has a chloride ion concentration of 40 mg/L and a
specific conductivity (SC) of 0.450 mS/cm. Saline water found in
Florida Bay has chloride levels of 15,000-35,000 mg/L and an SC
of 20-50 mS/cm (fig. 1). This one-hundred-fold difference in SC
can produce a similar difference in the conductivity of geologic
materials saturated with fresh or saline water.
In our study of the Everglades several EM methods are being used
to map the location of the freshwater/saltwater interface. These
methods include helicopter electromagnetics (HEM), borehole
induction logging, and time-domain electromagnetic soundings
(TEM), which are discussed below.
The apparent resistivity map shows several interesting features
including the prominent transition from higher resistivities in
the landward direction to lower resistivities toward the
shore. This feature is the freshwater/saltwater interface (FWSWI)
and is marked on figure 3. In the region of Shark River Slough
the FWSWI follows the terminus of rivers that have tidal
flow. Taylor Slough, one of the primary sources of water for
Florida Bay, shows up as a resistive feature. The influence of
manmade features is seen where the FWSWI crosses the C-111 canal
where a control structure on the canal produces a distortion in
the interface. A conductive feature is seen along the old
Ingraham Highway where saline water flowed inland in the canal
adjacent to the highway.
To refine the interpretation of the airborne geophysical data, we
rely on borehole and surface geophysical measurements, and water
quality samples. Figure 4 shows an induction log and time-domain
electromagnetic (TEM) sounding from near the FWSWI. The induction
log, which is measured with a tool lowered into a well, provides
detailed information on how resistivity varies with depth near
the well. The TEM sounding, which is a surface measurement, also
gives information on resistivity-depth variations. The transition
from high resistivity surface water to low resistivity saline
water is seen in both measurements at a depth of 10 m, and has
been confirmed by water-quality data. Repeat borehole logs are
being made to monitor seasonal and long-term changes.
Additional airborne surveys are planned. These and other
geophysical data will be used for restoration monitoring and
development of ground-water models.
For more information contact:
Related information:
U.S. Department of the Interior, U.S. Geological Survey
Several geophysical techniques can be used to measure the
resistivity (the reciprocal of conductivity) of the ground. These
techniques make use of a transmitter that induces electrical
current flow in the ground, and a receiver that measures the
electromagnetic (EM) field produced by these induced currents. By
analyzing the electromagnetic fields, information about how the
electrical conductivity varies from location to location and with
depth below the Earth's surface can be determined and resistivity
maps produced.
HEM resistivity mapping makes use of an
instrument pod, called a "bird," which is slung below a
helicopter (fig. 2). The bird contains pairs of transmitter and
receiver coils which measure the electromagnetic response of the
ground at different frequencies to obtain information from
different depths. The bird is flown back and forth over the
survey area along parallel lines about 400 m (1/4 mi) apart. The
raw data are processed to produce apparent resistivity maps such
as the one shown on figure 3. By interpreting the apparent
resistivity maps from different frequencies, information about
how resistivity varies with depth can be determined.
Repeat HEM surveys have been made at the end of the dry and rainy
seasons. Comparison of these results show that there are
significant changes associated with increased fresh-water flows
during the wet season. This result indicates that periodic repeat
HEM surveys can be used to monitor changes in the ground-water
system caused by restoration activity such as increasing flows of
fresh water into Taylor Slough.
David Fitterman
U.S. Geological Survey
Box 25046, MS 964
Denver Federal Center
Denver, CO 80225-0046
(303)236-1382
fax: (303)236-1425
email: fitter@usgs.gov
SOFIA Project: Geophysical Studies of the Southwest Florida Coast
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Last updated: 04 November, 2004 @ 02:13 PM(TJE)