The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
There is evidence that flows have been gradually decreasing over time from Sulphur
Springs. Daily flow records recorded by the United States Geological Survey (USGS)
begin in 1956. Withdrawals from the spring pool began in the mid-1960s, but records for
the withdrawals are unavailable for the period prior to 1984. For the period of 1984 to
present, it is possible to correct the recorded flows on a monthly basis by adding back
the quantities withdrawn. Consequently, the flow data prior to 1984 flow show much
more scatter due to occasional low flows that are not corrected for withdrawals.
Figure 2-19 shows time series plots of flows for both uncorrected pre-1984 data and
1984 to present data that are corrected for withdrawals. In both cases a Kendall-Theil
regression line has been fitted to the data to illustrate the overall average trend. The
Kendall-Theil method is a non-parametric form of regression that is independent of the
data distribution and more robust in the presence of extreme values. The results of the
regression for the two time periods are fairly similar and indicate a statistically significant
(α = 0.05) slight decrease in flow over time. The slopes of the regression lines are –0.60
cfs/yr for the pre-1984 period and -0.36 cfs/yr for 1984 to present.
2.5 Water Quality of Sulphur Springs
The water quality characteristics of the Sulphur Springs pool are described below.
Seasonal and long-term trends in specific conductance are described first, since this
parameter is a good indicator of the overall mineralization of the spring water and
changes in the spring's ground-water sources. Periodic long-term specific conductance
data from the USGS and the City of Tampa are evaluated with recent data from a
continuous specific conductance recorder located in the spring pool. This is followed by
a discussion of other water quality characteristics of the spring (e.g., nutrients, color),
which are related to the ecological characteristics of the spring run and the adjoining
reaches of the Lower Hillsborough River.
2.5.1 Data sources
Water quality data for the Sulphur Springs pool examined in this report are taken from
two sources. Since 1990, the City of Tampa has taken samples from Sulphur Springs
pool for water quality analysis on a monthly basis. These data are submitted to the
SWFWMD as part of the City’s water use permit for withdrawals from the spring.
The USGS has also collected periodic water quality samples from the Sulphur Springs
pool, with records dating back to the 1945 for some parameters. The frequency of
sampling has varied considerably, ranging from a few measurements every several
years prior to 1966 to roughly bi-monthly sampling during much of the 1970s and 1980s.
Temperature, pH, specific conductance and dissolved oxygen have been the most
frequently measured parameters. Water quality sampling by the USGS has decreased
since the early 1990s, largely because regular water quality sampling was instituted by
the City of Tampa. Selected variables in the USGS database are described as they
compare to more recent data collected by the City.
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