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Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

Subsurface Characterization of Selected Water Bodies in the St Johns River Water Management District, Northeast Florida

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Crescent Beach Spring
near St. Johns County, Florida

Introduction | Subsurface Characterization

Introduction

Crescent Beach Spring Track Map.
Seismic Survey Track Map for Crescent Beach Spring showing profile location and karst features. Click on the image to see a larger version.
The submarine spring near Crescent Beach, St. Johns County, is approximately 4 km (2.5 mi) offshore in 18 m (59 ft) of water in the Atlantic Ocean. The spring is a major discharge point for water from the Floridan aquifer and is evident on seismic profiles to be a spring vent rather than a collapse-type sinkhole. The profiles show a vent area of approximately 90 to 150 m (300-500 ft) in diameter (A-A' and B-B') and a depth of over 35 m (115 ft) below sea level. NOAA/NOS navigation chart number 11486 lists a water depth of 43 m (140 ft) at the base of the vent. Brooks (1961), conducted a detailed survey of the ³sink² using SCUBA, with a maximum recorded depth by diving of 40 m (132 ft). Brooks described the base of the sink to be comprised of secondary craters up to 4 m (12 ft) across. He noted that spring water discharge is from the bottom of these secondary craters. The spring water rises to the sea surface as density driven boils, at the surface these boils can be seen for some distance from the spring. In the seismic profiles, the velocity contrast between the fresher water discharging into the seawater produces reflections that can be used to define the discharge plume.

In 1923, the U.S. Coast and Geodetic Survey obtained water samples from three areas in the spring that indicate the source water has a chloride content between 7090 mg/l and 7680 mg/l. These values are similar to those of the Floridan aquifer obtained from a well about six kilometers onshore to the west (Brooks, 1981). In 1995 water samples were collected from the spring by USGS staff to determine chlorides and age of the water using isotopes. The Chloride value from samples that isolated the discharge from the seawater was 3630 mg/l and the age based on Carbon-14 techniques was 10’500 years (Toth, 1999).

Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

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