The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
3.4 Salinity in the spring run and response to withdrawals from the spring pool.
Average daily salinity values for the recorders in the upper spring run and the spring
mouth are plotted over daily withdrawals from the spring pool for 1999 – 2002 in Figures
3-5 and 3-6. When there were prolonged periods of no pumping from the spring, salinity
values in the spring run were similar to values in the spring pool, generally fluctuating in
a range of 1 to 3 ppt (Figure 3-5). The higher values in this range (2-3 ppt) occurred
during the latter part of 1999 and January 2000, when the pool elevation was lowered
by gate operation. As described in Section 2.2, lowering the pool elevation increased
the specific conductance of the spring discharge.
Withdrawals from the spring pool resulted in large increases in salinity in the spring run.
During a six-month period of extensive withdrawals in the year 2000, average daily
salinity values in the run frequently exceeded 10 ppt, reaching a maximum value near
17 ppt. Salinity values also showed a close relationship with pumpage in 2001, but the
spikes in salinity were not as great as in 2000, typically ranging from 4 to 8 ppt.
These increases in salinity represent the movement of high salinity water from the
Hillsborough River into the spring run. As previously discussed, all withdrawals from the
spring prior to November 2001 were at a rate of 31 cfs (20 mgd), a rate that reduces
flow from the pool to zero or very low values. When this occurs, high salinity waters
from the Hillsborough River migrate upstream of the weir into the upper spring run.
The recorder at the spring mouth showed a similar relationship with pumpage (Figure 3-
6). During periods of no pumpage, salinity values at the spring mouth were similar to the
spring pool, indicating that normal flow from the spring keeps this site flushed by spring
water most of the time. However, large increases in salinity were observed at the spring
mouth during periods of withdrawals. The peaks in salinity at the mouth were higher
than in the spring run, especially in 2001. Also, salinity at the mouth did not drop as
rapidly as in the run when flow from the spring resumed to normal rates of flow.
The results from these two recorders are supported by series of vertical salinity profiles
taken in the spring run. Biologists from the University of Florida (UF) took vertical
profiles of salinity in the spring run on seven dates between May 2000 and November
3003 (Allen et al. 2001, Allen, unpublished data). Two of these dates (May 26 and July
14, 2000) were during periods of prolonged withdrawals that resulted in zero or very low
flows (1 cfs) from the spring pool. The mean salinity values for all stations in the run on
these two dates were 13.2 and 11.6 ppt, respectively. Salinity was high throughout the
spring run during these events, as mean salinity values for stations near the mouth were
only about 1.5 to 2.5 ppt higher than upstream areas (Table 7 in Allen et al. 2001).
Overall, mean surface salinity values were about 4 to 5 ppt lower than mean bottom
salinity values on these dates, indicating there was density stratification in the spring run
(Table 3-1). During the period of no flow (May 26, 2000), this may have resulted from
low rates of springflow seeping through the outlet structure, which layered over more
saline water that entered the run from the lower Hillsborough River.
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