In this section
Apopka Spring aerial view
Apopka Spring is a second-magnitude spring that discharges from a bowl-shaped depression about 5 to 6 feet in diameter below the central part of the pool. The vent opening is about 40 to 45 feet below the water surface. An underground cave system has been recognized for Apopka Spring. The vent opening narrows vertically downward into the limestone for 16 feet, where it then slopes northward at about 45 degrees to a depth of 90 feet, making for dangerous diving conditions.
Apopka Spring boil
The circular spring pool above the vent is about 180 feet in diameter. When spring flow is high, the spring discharge pool is clean and clear, but when flow is low, murky lake water may cloud the pool. When the lake is still, a gentle boil can be seen at the pool surface over the vent. Emergent vegetation and an organic, mucky bottom surround the pool perimeter in the cove area. Gourd Neck is buffered by marsh and lowland swamp forests toward the west; the east side of Gourd Neck is bordered by sandhill pine trees.
Apopka Spring flow diver
Apopka Spring is a submerged spring, requiring divers to perform discharge measurements at the vent opening. The U.S. Geological Survey measured the spring discharge seven times from 1971 to 1992. In 1997, the St. Johns River Water Management District began measuring the discharge at least four times per year and currently measures discharge six times per year. The difference between the minimum and maximum discharges is 48.9 cubic feet per second (cfs) (see the table below). The maximum discharge of 70.0 cfs was measured in November 1988; the minimum discharge of 21.1 cfs was measured in September 2001. The mean and median discharges for the period from 1971 to 2010 are 27.8 cfs and 26.5 cfs, respectively (see the table below).
Apopka Spring has been sampled by the district from 1986 to 2010, with a gap of sample collection from 1991 to 1994. Since 1995, water quality samples have been collected by divers four times per year, usually at the same time as discharge measurements. The spring was also sampled once by the U.S. Geological Survey in 1972. Summary statistics of the water quality data for selected variables are shown in the table below.
Summary statistics of water quality and discharge at Apopka Spring
|Alkalinity, total, mg/L as CaCO3||53.0||76.9||75.8||109.0||117||1972−2010|
|Calcium, total, mg/L as Ca||25.0||30.8||30.9||34.7||48||1995−2010|
|Chloride, total, mg/L as Cl||7.0||14.0||13.0||27.0||120||1972−2010|
|Fluoride, total, mg/L as F||0.07||0.07||0.07||0.08||22||1995−2009|
|Magnesium, total, mg/L as Mg||7.2||8.2||8.2||9.2||48||1995−2010|
|Nitrate + nitrite, total, mg/L as N||2.54||4.25||4.39||5.50||104||1986−2010|
|Orthophosphate, total, mg/L as P||0.01||0.03||0.03||0.18||83||1986−2010|
|Phosphorus, total, mg/L as P||0.01||0.05||0.04||0.19||109||1986−2010|
|Potassium, total, mg/L as K||1.0||1.3||1.4||1.7||47||1995−2010|
|Sodium, total, mg/L as Na||5.0||6.0||6.1||7.1||48||1995−2010|
|Specific conductance, field, µmhos/cm at 25°C||210||249||249||300||106||1986−2010|
|Specific conductance, lab, µmhos/cm at 25°C||174||250||254||273||39||1972−2010|
|Sulfate, total, mg/L as SO4||2.0||10.2||10.0||17.0||118||1972−2010|
|Total dissolved solids, mg/L||108||147||144||233||115||1972−2010|
|Water temperature, °C||17.0||23.7||24.1||27.5||110||1972−2010|
mg/L = milligrams per liter
cfs = cubic feet per second
Age of discharge water
The age of water discharging from Apopka Spring was determined by measuring the concentration of tritium, helium-3, helium-4, and neon in the spring discharge, which was measured in August 1999. Apopka Spring had a tritium concentration of 2.9 tritium units, which suggests that the water is less than 46 years old. The tritium and helium-3 age of water discharging from Apopka Spring was 24.5 years.