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Mud Hole: A Unique Warm-Water Submarine Spring, Located
Offshore Southwestern Florida
Mud Hole Submarine Spring is an unusual warm-water submarine discharge of water offshore of the southwestern Gulf Coast of Florida, located at latitude 2615'48"N longitude 8201'06"W, or about 18.5 km due south of the Sanibel Island lighthouse, Lee County, Florida. This warm-water submarine spring oftentimes has a surface appearance of discolored turbid warm-water which is very attractive to large fish, sharks, turtles, sport fisherman, and scuba divers. The spring discharges warm water from many vents in a sinkhole depression about 61 m in diameter at about 19 m below NGVD. The bottom sediment consists of gray silt and mud atop sand and limestone gravel which, when disturbed, causes clouds of mud to be thrown into suspension, giving the spring the name "Mud Hole". Warm water flowing from several orifices has a constant temperature of 35.31C, pH of 7.25, and a salinity of 34.6 ppt (averaged hourly measurements during the period May 5 through June 17,1998). The plume extends about 1.5 m around the main orifice, about 1 m long by 0.3 m wide, with a velocity estimated at 0.6 m/s and a discharge of about 0.17 m3/s (4 million gallons/day). The total discharge from the whole depression is however considerably much larger. This study mapped the spring and measured physical water quality. The results indicated that the discharge movement is fault-related to the basement Sunniland Fracture Zone, based on the geology of southern Florida. The saline-water temperature of 35.3C indicates that the water comes from a considerable depth, as the surrounding seawater only averages 21C and onshore Upper Floridan aquifer water only ranges from 26.6C to 29.4C. The temperature profile in the oil-exploration well, State Lease #224B-1, located offshore north of Sanibel Island indicates that this temperature exists only at a depth greater than 500 m, that is, in the Boulder Zone of the Lower Floridan aquifer. This study concludes that the source is saline water migrating through faults extending from the basement Sunniland Fracture Zone and moving upwards through the section, mixing with the brackish cooler Floridan Aquifer, and eventually flowing from the spring. As no lithological gradient exists from east to west in southern Florida and higher water temperatures are found near major fault zones, the geothermally-activated convective flow cell hypothesis cannot be valid. Based on the data obtained in this study, the source is not considered the surficial aquifer as reported in the literature by some researchers.
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