The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
The experimental withdrawals ended on July 5, 2002 when high river flows returned to
the spillway at the Hillsborough River dam. Due to an unusually wet year, there was
nearly continuous river flow at the Hillsborough River through the remainder of 2002
and all of 2003. Consequently, diversions of spring water to the base of the dam were
not necessary to meet minimum flows at the dam. Additional experimental withdrawals
to evaluate minimum flows for Sulfur Springs were also not conducted because the river
was usually fresh near the spring mouth. Experiments that involved diverting various
amounts of water to the base of the dam were conducted during the spring of 2004,
during a two-month period when there was no flow from the reservoir. The findings of
these flow experiments will be included in the re-evaluation of the minimum flow for the
Lower Hillsborough River scheduled for 2005.
2.5.3 Concentrations and trends of other constituents
Summary statistics for water quality parameters measured monthly in the spring pool by
the City of Tampa are presented in Table 2-2 for the years 2001-2002. As is
characteristic of Florida springs, the water in the spring pool typically has very low levels
of color, turbidity, and total suspended solids. Total organic carbon and biochemical
oxygen demand (BOD) concentrations are also low. Although water clarity is not
measured in the pool, the water generally appears to be very clear, again typical of
spring discharge.
The average temperature of the spring water is 25.1 degrees Celsius (77.2 degrees
Fahrenheit), with only minor seasonal variation. There are typically low levels of
dissolved oxygen in the spring pool; with a median concentration of 1.78 mg/l. Ground
water discharges are frequently low in dissolved oxygen, and the occurrence of
dissolved oxygen in Sulphur Springs may be partly due to circulation and aeration that
occurs within the spring pool. Photosynthesis by attached algae, which can become
dense on the sides and bottom of the pool, may also contribute to high dissolved
oxygen concentrations that are periodically observed.
Nutrients in the spring pool are generally low, but do indicate some nutrient enrichment.
The median nitrate nitrogen concentrations in the pool were 0.25 mg/l during 2001-
2002, with an inter-quartile range (25 th to 75 th percentile) of 0.19 to 0.73 mg/l. The
median total nitrogen concentration was 0.72 mg/l, with inter-quartile range of 0.53 to
1.45 mg/l. By comparison, Friedman and Hand (1989) reported a median concentration
of 1.2 for total nitrogen in Florida streams, with values of 0.70 and 1.9 mg/l for the 20 th
and 80 th percentiles, respectively. The median total phosphorus concentration for
Sulphur Springs (0.1 mg/l) was very similar to the median values reported for Florida
streams (0.11 mg/l).
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