The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
Data from the Tampa Bay Water HBMP, which are not included in Figure 3-9, are
available for salinity, temperature, pH, and dissolved oxygen at a large number of sites,
and for chlorophyll a, color, total suspended solids, and total and dissolved organic
carbon at a subset of these. Because salinity and dissolved oxygen concentrations are
most closely affected by Sulphur Springs flow, the discussion on the effects of Sulphur
Springs on the water quality of the Hillsborough River is limited to those parameters.
3.9.2.1 Empirical salinity data plots
Flows from Sulphur Springs have their greatest effect on the water quality of the Lower
Hillsborough River during times when there is no flow at the dam. Box and whisker plots
of salinity in 2-kilometer intervals along the lower river are presented in Figure 3-13
using data from the Tampa Bay Water HBMP and the EPHC/HIMP programs during
2000-2002. This period is chosen for analysis as it represents the most spacially
extensive data for the lower river. Statistical distributions of surface and bottom water
salinity values are graphed separately for flow and no-flow conditions from the
Hillsborough River reservoir. Since the USGS frequently reports flow values up to 3 cfs
to represent leakage from the dam, no-flow conditions were classified as less than 3 cfs
to denote no flow from the reservoir spillway.
Brackish waters can extend to the base of the Hillsborough River dam during times of
no flow. The median salinity values for surface and bottom waters near the base of the
dam (km 16) were approximately 10 ppt for no flow conditions (Figure 3-13). Compared
to other long-term data for the river, these values are particularly high, as the
HBMP/HIMP data are heavily influenced by the prolonged 2000-2001 drought when
salinity in the lower river reached record levels. Surface salinity values do not show a
strong gradient between kilometers 10 and 16 during no-flow conditions. In fact, the
median surface salinity value near the dam (km 16) is greater than the median value
near Sulphur Springs (km 12). This reverse salinity gradient, in which salinity increases
upstream, is due to the effect of Sulphur Springs, which reduces surface salinity in the
river near the spring outfall. Salinity values in the lower river are substantially lower for
flow conditions. Median salinity values at or near fresh water extend downstream to km
10 for surface waters and to km 12 for bottom waters. Under many flow conditions,
fresh waters extend from the dam downstream past Sulphur Springs.
Box and whisker plots of salinity data from the four EPCHC fixed location sites show
similar relationships (Figure 3-14), but reflect a longer period of record. The median
surface salinity value at Rowlett Park (km 15.6) under no-flow conditions is 6 ppt, which
is substantially lower than the median value in the HBMP/HIMP data set. Surface
salinity values are typically fresh down to km 10.6 for flow conditions, but the median
bottom salinity at this location during flows is near 8 ppt. It should be noted the period of
record at this site (Sligh Avenue) is much shorter than for the other EPCHC stations
(Figure 3-12.). In general, the box and whisker plots show there are marked differences
in the salinity characteristics of the lower river between flow and no-flow conditions,
particularly upstream of Sulphur Springs.
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