The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
greatest (see Section 3.3.7.4). The potential effects of such manipulations combined
with salinity changes is discussed in the context of significant harm to the upper spring
run at the end of this chapter.
5.2.6 Salinity response in the lower spring run
Much of the minimum flow analysis focused on the upper spring run, as this represents
over 85 percent of the total area of the entire spring run. Secondly, due to the presence
of the weir, it is more isolated from the effects of the lower Hillsborough River and likely
to maintain low salinity fauna that are scarce within the Lower Hillsborough River
system in the dry season. With these considerations, the response of salinity in the
lower spring run was determined not to be a critical indicator for evaluating minimum
flows from the spring pool. In other words, the transition to a more saline fauna in the
lower spring run could be allowed if the low salinity fauna in the upper spring run is
maintained. However, the response of salinity in the lower run to flows from the spring
pool provides useful information on general salinity relationships in the spring run
system.
As discussed in Section 3.4, a continuous recorder was operated at the mouth of the
spring that collected 15-minute data for temperature and specific conductance, which
was converted by calculation to salinity. Stage was not measured at this recorder. A box
and whisker plot of salinity at this recorder versus flow in 1 cfs increments is shown in
Figure 5-11. In general, the distributions of salinity values for the flow classes showed
breaks at similar flow rates as for the recorder in the upper run, but the salinity values at
the mouth were higher. For example, median salinity values at the mouth show a break
between 13 and 14 cfs, dropping from about 5-6 ppt to about 2-3 ppt. Seventy-fifth
percentile values show a distinct break between 17 and 18 cfs, dropping from 8-9 ppt to
values less than 3 ppt at higher flows.
To some extent, the breaks may represent the effect of lower salinity in the river at
higher flows. However, the list of maximum salinity values for each day of flow record in
Appendix I indicates that salinity remained high in the river at 18 cfs flow. Thus, the
break at the mouth recorder may truly represent the effect of increased flushing by flow
from the spring. Unlike the upper run, there were no conspicuous breaks in the 95 th
percentile and greater salinity values at 18 cfs, indicating brackish river water can
periodically inundate the recorder at the mouth of the run at higher flows depending on
tide and salinity conditions in the river. In summary, although the lower run may not be a
critical criterion for minimum flow establishment, breaks in the salinity-flow relationship
show some clear similarities to relationships in the upper run, supporting the validity of
those findings.
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