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Green Cove Spring main view
Green Cove Spring
Green Cove Spring is a third-magnitude spring on the west bank of the St. Johns River. A concrete railing measuring 2 feet high with a diameter of about 15 feet encircles the main vent of the spring pool. The pool tapers downward as an irregularly shaped funnel into soft marl to a 2-foot-diameter cavern opening in its bottom, approximately 30 feet below the pool surface. The 2-foot vent then opens into a cavern 25 feet wide, trending in a northeast direction toward the St. Johns River. The roof of the cavern descends to a depth of 50 feet, and the bottom of the cavern falls to 98 feet (Rosenau et al. 1977). In August and September 2010, divers from Karst Environmental Services reported that the deepest penetrable point in the cave system was at 98 feet. The divers reported that all of the flow was coming vertically, from deeper depths.
Green Cove Spring vent
Discharge from the spring pool is through a 4-foot-wide weir opening on its northeast side to a 50-foot by 100-foot swimming pool. The swimming pool overflows through a weir on its east end to a spring run that flows about 450 feet eastward to the St. Johns River. Some or all of the flow from the spring can be directed from a flume to bypass the swimming pool. The spring is part of a city-owned park and recreation area, with picnic tables, benches, walkways, and shade trees.
Green Cove Spring run
Discharge at Green Cove Spring was measured by the U.S. Geological Survey (USGS) seven times from 1929 to 1972. St. Johns River Water Management District measured discharge 54 times from 2000 to 2010 and currently measures discharge six times per year. From 1929 to 2010, the difference between the minimum and maximum discharges is 3.8 cubic feet per second (cfs). The maximum measured discharge of 5.4 cfs occurred in February 1929; the minimum discharge of 1.6 cfs occurred in March 2000. The mean and median discharges for the period from 1929 to 2010 are 2.94 cfs and 2.82 cfs, respectively (see the table below).
Green Cove Spring was sampled by USGS seven times from 1956 to 1989. The district sampled Green Cove Spring 40 times from 2000 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 Green Cove Spring
|Green Cove Spring||Min||Mean||Median||Max||Count||Period|
|Alkalinity, total, mg/L as CaCO3||77.7||84.8||82.0||170.0||43||1956−2010|
|Calcium, total, mg/L as Ca||25.0||27.9||28.0||29.3||40||2000−2010|
|Chloride, total, mg/L as Cl||4.8||6.0||6.0||8.7||45||1956−2010|
|Fluoride, total, mg/L as F||0.20||0.26||0.27||0.30||13||2000−2003|
|Magnesium, total, mg/L as Mg||13.0||14.8||15.0||16.0||39||2000−2010|
|Nitrate + nitrite, total, mg/L as N||0.00||0.02||0.01||0.04||9||2001−2009|
|Orthophosphate, total, mg/L as P||0.00||0.02||0.01||0.04||7||2001−2008|
|Phosphorus, total, mg/L as P||0.00||0.01||0.01||0.03||7||1972−2010|
|Potassium, total, mg/L as K||1.2||1.5||1.5||1.9||40||2000−2010|
|Sodium, total, mg/L as Na||4.1||4.5||4.5||5.6||40||2000−2010|
|Specific conductance, field, µmhos/cm at 25°C||223||282||290||332||39||2000−2010|
|Specific conductance, lab, µmhos/cm at 25°C||261||285||284||326||34||1956−2010|
|Sulfate, total, mg/L as SO4||44.5||52.5||53.1||58.0||45||1956−2010|
|Total dissolved solids, mg/L||152||181||178||223||42||1957−2010|
|Water temperature, °C||23.5||24.7||24.7||25.9||43||1960−2010|
mg/L = milligrams per liter
cfs = cubic feet per second
Age of discharge water
The age of water discharging from Green Cove Spring was determined by measuring the concentration of tritium, delta carbon-13, and carbon-14 in the spring discharge, which was measured in March 2001. Green Cove Spring did not have any tritium, which suggests that the water is old. Green Cove Spring had a delta carbon-13 value of −7.32 parts per thousand and a carbon-14 concentration of 5% modern carbon. The Fontes and Garnier (1979) carbon-14 age is 14,397 years old.