In this section
Green Springs main view
Green Springs is a third-magnitude spring with a roughly circular pool about 90 feet in diameter. Water in the pool is milky green with poor visibility. Rosenau et al (1977) reported that the pool is about 125 feet deep in its north-central part, with a conical-shaped hole in the limestone that gradually diminishes in diameter with depth. An eastward extension of the pool is bordered on the north and east by a concrete retaining wall about 2 feet high. Spring flow is into a shallow run about 3 to 6 feet wide that flows toward the southeast for about 200 feet to a small creek, which then flows south 0.25 mile to Lake Monroe.
Green Springs run
Discharge at Green Springs was measured sporadically by the U.S. Geological Survey (USGS) from 1932 to 1972. St. Johns River Water Management District measured discharge from 1995 to 2010 and currently measures discharge bimonthly. The difference between the minimum and maximum discharges is 2.92 cubic feet per second (cfs) over the period. The maximum measured discharge of 2.92 cfs occurred in November 2004. In 2000 and 2001, no discharge was measured four times, and in 2008, no discharge was measured two times. The mean and median discharges for the period are 1.45 cfs and 1.34 cfs, respectively (see the table below).
Green Springs was sampled by USGS in 1972. The district sampled Green Springs 44 times from 1995 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 Springs
|Alkalinity, total, mg/L as CaCO3||109.0||160.2||158.3||200.0||45||1972−2010|
|Calcium, total, mg/L as Ca||60.4||86.1||87.8||97.0||44||1996−2010|
|Chloride, total, mg/L as Cl||166.6||625.7||690.0||836.0||45||1972−2010|
|Fluoride, total, mg/L as F||0.08||0.09||0.09||0.10||14||1996−2002|
|Magnesium, total, mg/L as Mg||12.4||39.2||41.8||48.4||44||1996−2010|
|Nitrate + nitrite, total, mg/L as N||0.01||0.04||0.03||0.14||24||1996−2009|
|Orthophosphate, total, mg/L as P||0.03||0.07||0.07||0.26||45||1996−2010|
|Phosphorus, total, mg/L as P||0.06||0.08||0.08||0.15||29||2001−2010|
|Potassium, total, mg/L as K||4.7||10.9||11.8||13.5||44||1996−2010|
|Sodium, total, mg/L as Na||88.1||343.8||370.1||447.0||44||1996−2010|
|Specific conductance, field, µmhos/cm at 25°C||1113||2361||2417||3000||41||1996−2010|
|Specific conductance, lab, µmhos/cm at 25°C||1340||2454||2580||3040||31||1972−2010|
|Sulfate, total, mg/L as SO4||29.1||95.1||102.0||129.0||45||1972−2010|
|Total dissolved solids, mg/L||572||1386||1450||1930||43||1972−2010|
|Water temperature, °C||18.2||23.0||22.8||27.2||46||1972−2010|
mg/L = milligrams per liter
cfs = cubic feet per second
Age of discharge water
The age of water discharging from Green Springs was determined by measuring the concentration of tritium, delta carbon-13, and carbon-14 in the spring discharge, which was measured in January 2001. Green Springs had a tritium concentration of 2.48 tritium units, suggesting that the water is less than 43 years old. Green Springs had a delta carbon-13 value of −9.90 parts per thousand and a carbon-14 concentration of 49% modern carbon, which results from the mixing of Upper and Lower Floridan aquifer water. The adjusted carbon-14 age of Green Springs is recent.