In this section
Gemini Springs main view
Gemini Springs swimming area
Gemini Springs Spring 1 pool
Gemini Springs is a second-magnitude spring that consists of two springs. The springs are about 100 feet apart on an almost straight-line westward projection of the north shoreline of the reservoir. For convenience of identification, the springs are numbered: Spring 1 (Spring 2 of Rosenau et al. 1977) and Spring 2 (Spring 3 of Rosenau) from west to east. Spring 1, as identified by Rosenau, is actually an 8-inch well. The well was initially back-plugged from 340 feet to 141 feet below land surface in September 1991, and the well has been completely abandoned since July 2002.
Spring 1 is about 100 feet west of the reservoir and on higher ground than Spring 2, which is at the northwest edge of the reservoir. Spring 1 has a circular pool about 15 feet in diameter, and flow is from a horizontal cavern opening in the limestone. The cavern opening is about 6 feet high and 8 feet wide, with the cavern appearing to extend laterally underground to the north or northeast. Spring 2 has a circular pool at times inundated by the reservoir. Flow is from a small cavern under a rock ledge about 3 feet below water surface. Spring 1 flows about 150 feet to the east-southeast and converges with the flow from Spring 2 at the west end of the reservoir. The reservoir is impounded on its east end by an earthen dam with a concrete weir outlet. Flow is through the weir, then 1.5 miles east and northeast down a creek and through a marsh area to Lake Monroe on the St. Johns River. The spring area is within a county park recreational area.
Gemini Springs run
Discharge at Gemini Springs was measured by the U.S. Geological Survey (USGS) from 1966 to 1999. St. Johns River Water Management District measured discharge from 1996 to 2010 and currently measures discharge four times per year. The difference between the measured minimum and maximum discharges is 9.1 cubic feet per second (cfs). The maximum measured discharge of 15.3 cfs occurred in November 2005; the minimum discharge of 6.2 cfs occurred in June 1995. The mean and median discharges for the period from 1966 to 2010 are 10.15 cfs and 10.15 cfs, respectively (see the table below).
Gemini Springs was sampled by USGS four times from 1972 to 1999. The district sampled Gemini Springs from 1986 to 2010 and currently samples the spring four times per year. Summary statistics of the water quality data for selected variables are shown in the table below.
Summary statistics of water quality and discharge at Gemini Springs
|Alkalinity, total, mg/L as CaCO3||125.0||137.8||138.3||158.0||55||1972−2010|
|Calcium, total, mg/L as Ca||86.0||100.1||100.8||110.0||54||1995−2010|
|Chloride, total, mg/L as Cl||525.0||622.4||621.5||760.0||56||1972−2010|
|Fluoride, total, mg/L as F||0.08||0.12||0.12||0.14||28||1995−2003|
|Magnesium, total, mg/L as Mg||31.0||38.0||38.0||46.0||53||1995−2010|
|Nitrate + nitrite, total, mg/L as N||0.50||1.04||0.98||2.20||55||1995−2010|
|Orthophosphate, total, mg/L as P||0.02||0.06||0.06||0.12||53||1995−2010|
|Phosphorus, total, mg/L as P||0.06||0.08||0.08||0.13||29||1996−2010|
|Potassium, total, mg/L as K||6.8||8.1||8.1||9.4||53||1995−2010|
|Sodium, total, mg/L as Na||264.9||322.8||323.6||380.0||53||1995−2010|
|Specific conductance, field, µmhos/cm at 25°C||1650||2322||2374||2809||52||1995−2010|
|Specific conductance, lab, µmhos/cm at 25°C||1880||2272||2300||2540||32||1972−2010|
|Sulfate, total, mg/L as SO4||85.0||116.6||118.0||141.0||58||1972−2010|
|Total dissolved solids, mg/L||1240||1408||1400||1900||54||1993−2010|
|Water temperature, °C||21.5||23.1||22.9||25.4||58||1972−2010|
mg/L = milligrams per liter
cfs = cubic feet per second
Age of discharge water
The age of water discharging from Gemini Springs was determined by measuring the concentration of tritium, delta carbon-13, and carbon-14 in the spring discharge, which was measured in May 1996, and tritium/helium-3, as measured in August 1999. Gemini Springs had a tritium concentration of 3.3 tritium units, which suggests that the water is less than 43 years old. The tritium/helium-3 age of the water was 18.6 years. Gemini Springs had a delta carbon-13 value of −10.40 parts per thousand and a carbon-14 concentration of 41% modern carbon, which results from the mixing of Upper and Lower Floridan aquifer water.