In this section
Croaker Hole Spring main view
Croaker Hole Spring
Croaker Hole Spring is a second-magnitude spring. Croaker Hole is a relatively deep, small-diameter hole in the bottom of the St. Johns River. The river is generally about 5 feet to 9 feet deep in this area, except in the immediate vicinity of Croaker Hole, where it plunges to a depth of about 48 feet.
Discharge at Croaker Hole Spring was measured by the U.S. Geological Survey (USGS) six times from 1981 to 2000. St. Johns River Water Management District measured discharge 38 times from 2000 to 2010. The difference between the minimum and maximum discharges is 28.48 cubic feet per second (cfs) over the period. The maximum measured discharge of 90.5 cfs occurred in November 1998; the minimum discharge of 62.02 cfs occurred in September 2001. The mean and median discharges for the period are 74.52 cfs and 73.73 cfs, respectively (see the table below).
Croaker Hole Spring was sampled by USGS in 1981, 1991, and 1998. The district sampled Croaker Hole Spring 28 times from 1998 to 2010. Summary statistics of the water quality data for selected variables are shown in the table below.
Summary statistics of water quality and discharge at Croaker Hole Spring
|Croaker Hole Spring||Min||Mean||Median||Max||Count||Period|
|Alkalinity, total, mg/L as CaCO3||61.6||65.7||64.0||74.8||27||1995−2010|
|Calcium, total, mg/L as Ca||69.0||73.6||72.9||86.0||27||2001−2010|
|Chloride, total, mg/L as Cl||322.9||631.3||640.3||736.0||31||1981−2010|
|Fluoride, total, mg/L as F||0.10||0.12||0.12||0.14||11||1995−2003|
|Magnesium, total, mg/L as Mg||34.0||39.4||39.0||47.0||27||2001−2010|
|Nitrate + nitrite, total, mg/L as N||0.06||0.09||0.08||0.22||23||1995−2010|
|Orthophosphate, total, mg/L as P||0.01||0.02||0.02||0.03||24||1995−2010|
|Phosphorus, total, mg/L as P||0.01||0.02||0.02||0.04||16||2006−2010|
|Potassium, total, mg/L as K||7.7||8.7||8.7||10.5||27||2001−2010|
|Sodium, total, mg/L as Na||308.0||342.6||342.4||394.2||27||2001−2010|
|Specific conductance, field, µmhos/cm at 25°C||2089||2486||2520||2887||9||1995−2003|
|Specific conductance, lab, µmhos/cm at 25°C||2270||2427||2400||2600||15||2002−2010|
|Sulfate, total, mg/L as SO4||118.0||147.9||147.0||170.0||30||1981−2010|
|Total dissolved solids, mg/L||1190||1383||1367||1600||30||1981−2010|
|Water temperature, °C||21.6||22.9||22.5||25.7||9||1995−2002|
mg/L = milligrams per liter
cfs = cubic feet per second
Age of discharge water
The age of water discharging from Croaker Hole Spring was determined by measuring the concentration of tritium, delta carbon-13, and carbon-14 in the spring discharge, which was measured in July 1995. Croaker Hole Spring had a tritium concentration of 4.5 tritium units, which suggests that the water is less than 43 years old. Croaker Hole Spring had a delta carbon-13 value of −8.47 parts per thousand and a carbon-14 concentration of 31% modern carbon, which results from the mixing of Upper and Lower Floridan aquifer water. The adjusted carbon-14 age is recent.