The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
pumping events resulted in salinity increases of 4 to 5 ppt. Vertical profile
measurements in the spring run conducted as part of these tests also showed that
some pumping events caused salinity increases while others did not (Table 3-1). The
findings of these intermediate flow tests are discussed in more detail in Chapter 5
(Results of the Minimum Flow Analysis).
3.5 Dissolved oxygen concentrations in the spring run and response to
withdrawals from the spring pool.
Dissolved Oxygen (DO) data for the spring run are limited to the vertical profile
measurements taken by UF and District staff. As described in Section 2.5.3, the groundwater
discharge to the spring pool is typically not well oxygenated. The median DO
value for grab samples from the pool is 1.78 mg/l (Table 2-2). During periods of normal
flow, however, waters in the spring run are well oxygenated, indicating that the fall over
the outlet structure and turbulence in the plunge pool is effective at aerating the spring
water. Summary statistics for DO profiles measured by UF are included in Table 3-1.
During periods of normal spring flow, mean DO values for surface and bottom waters
ranged between 4.4 and 6.1 mg/l. Out of a combined total of 240 salinity readings on
these dates, only 6 values were slightly below 4 mg/l.
Low DO values were observed by UF during withdrawal events in May and July 2000,
when discharge from the pool was reduced to zero or very low rates of flow (1 cfs).
Sixty-three percent of the DO values were below 4 mg/l during the May 26 sampling, but
none were below 2 mg/l. DO values were considerably lower during the July 14
sampling, as 81% of the DO values were less than 2 mg/l, with a mean bottom DO
value of 0.9 mg/l. These bottom DO values may have been related to density
stratification in the run, as evidenced by the difference between mean surface and
bottom salinity values (Table 3-1).
DO concentrations in vertical profiles measured by the District in 2001 and 2002 are
also summarized in Table 3-1. For the first nine sampling dates, when flows ranged
between 16 and 32 cfs, average DO values for all depths ranged between 5.3 and 9.2
mg/l and there were no DO readings below 4 mg/l. On June 12, 2002, when flows were
13 cfs, nine percent of the DO readings were below 4 mg/l. There was significant
density stratification on this date, as mean surface and bottom water salinity values
differed by 10.2 ppt at the time of sampling. Vertical profiles were also taken by the
District when withdrawals were taken from the spring at a rate of 31 cfs (June 20, 2002).
Six percent of the DO values were below 2 mg/l, while two percent were between 2 and
4 mg/l.
In general, these data indicate the DO concentrations in the spring run are well
oxygenated during times of normal flow from the spring pool. The occurrence of hypoxia
(low oxygen concentrations) differed between periods when withdrawals reduced the
rate of flow to zero or very low values. The occurrence of hypoxia may be related to the
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