The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
Since April 1999, the USGS has also collected water temperature and specific
conductance data in the spring pool every 15 minutes using a continuous recording
device. Water level records at this site (# 02306000) go back to 1959, with periodic
measurements extending back further. Fifteen-minute data for temperature and specific
conductance at this recorder began in April 1999 as part of the District-funded minimum
flow study for Sulphur Springs.
2.5.2 Trends in specific conductance
Specific conductance, or the capability of water to conduct an electrical current, is a
measure of the total dissolved inorganic ions in water. The specific conductance of most
freshwater streams and springs in Florida is less than 500 µmhos/cm. Friedman and
Hand (1986) reported a median value of 366 µmhos/cm for Florida springs, and state
that values over 1,500 µmhos/cm reflect the effects of salt water at some sites.
Based on recent data from the spring pool, Sulphur Springs can be considered a slightly
brackish, mineralized spring. The mean specific conductance value for the monthly data
from the City of Tampa during 2001-2002 was 3,093 µmhos/cm, with a range of 1,971
to 4,040 µmhos/cm. Similarly, the mean specific conductance value for the USGS
recorder for 2001-2002 was 3,218 µmhos/cm, with average daily values ranging from
1,821 to 6,558 µmhos/cm. The mineralization of the spring is also reflected in high total
dissolved solids concentrations (TDS). In the monthly samples for 2001-2002, TDS
ranged between 1,128 and 3,177 mg/l with an average 1,834 mg/l. These
concentrations are well over the Florida potable water standard of 250 mg/l for total
dissolved solids.
Data from the spring pool and a nearby well indicate that ground-water quality in the
immediate vicinity of Sulphur Springs is becoming increasingly mineralized. Specific
conductance values in the spring outflow and the underlying Upper Floridan Aquifer
have been steadily increasing since approximately 1973 to present. The Tourist Club
Floridan monitor well is located approximately 260 feet (79 m) from Sulphur Springs. It
is cased to 80 feet (24 m) and is 318 feet (67 m) deep. Specific conductance measured
in the well has increased from around 5,000 to over 15,000 µmhos/cm at an average
rate of 331 µmhos/cm/yr (Figure 2-20). Salinity, as calculated from the specific
conductance using the equations of Jaeger (1973), has shown an increase of 0.20
ppt/year for the same period.
Specific conductance in the discharge from Sulphur Springs has also shown a
pronounced increase, but at a lesser rate (Figure 2-21). Prior to about 1973, the spring's
specific conductance was fairly constant at about 1,000 µmhos/cm. However, beginning
in 1973, specific conductance values began climbing at an estimated rate of 65
µmhos/cm/yr to the present level of approximately 3,000 µmhos/cm. Some readings
have been as high as 6,000 µmhos/cm. Salinity values have shown an increase of
0.036 ppt/year after 1973.
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