Filling a Pool in St. Petersburg: Your Tap Water’s Starting Numbers
Filling a pool in St. Petersburg means starting from City of St. Petersburg Water Resources (supplied by Tampa Bay Water regional blend)’s water — which is hard at roughly 158 mg/L total hardness (about 9.2 grains per gallon). Here is what those published numbers actually do to a pool — and what they do not.
Which way this water pushes a pool
At a typical 84 F pool temperature the published City of St. Petersburg Water Resources (supplied by Tampa Bay Water regional blend) numbers index at +0.70 — mildly scale-forming (how this is calculated). That puts St. Petersburg 2nd of the 15 cities here that publish enough to compute an index, counting from the most scale-forming down.
Read it as a ceiling rather than a verdict: utilities publish TOTAL hardness while the index wants the CALCIUM fraction, so the true figure sits at or below +0.70 — close enough to the balanced band that it should not alarm you. What it does settle is which mistake costs money in St. Petersburg. Cloudy water and scale at the tile line are the failure mode here; etched plaster is not.
The top-off arithmetic
Evaporation removes pure water and leaves the minerals behind, so make-up water re-adds St. Petersburg hardness every time. Replacing one full pool volume through top-off adds roughly another 158 ppm; starting from a fresh fill at about 158 mg/L, that reaches the 400 ppm ceiling after about 1.5 pool volumes of make-up water — a couple of pool volumes, which a high-evaporation summer gets through faster than most owners expect. That is the 18th shortest budget of the 27 cities in this lookup.
Evaporation-only and deliberately the fastest case: splash-out, backwash and rain all dilute, and this uses published total hardness rather than the calcium fraction a kit reads (how this is calculated).
Why St. Petersburg water is like this
Three chemically unlike waters are mixed before delivery: Floridan groundwater is hard limestone water; Alafia/Hillsborough/Bypass Canal river water is much lower in mineral content but loaded with natural organics; and RO-desalinated seawater comes off the membranes essentially mineral-free and has to be chemically re-stabilized before it can go into the mains. Tampa Bay Water says explicitly: ‘Our supplies are interconnected, so we can shift sources as needed.
For example, when river water is plentiful, we can use more of that supply and less of others.’ On top of that, the utility states it ‘adds lime, caustic soda and alkalinity to adjust pH and help prevent corrosion in pipes.’ So a St. Petersburg pool owner’s fill-water hardness and alkalinity are a moving weighted average of three sources PLUS deliberate lime and alkalinity dosing – not a fixed aquifer signature the way Jacksonville’s or Orlando’s is.
Practical consequence: a number measured from the hose in the dry season is not necessarily the number you will get in the wet season, and the published annual average understates the swing. Test the fill water rather than trusting a published city average.
Second practical consequence: the whole regional system’s final disinfectant is CHLORAMINE, and Tampa Bay Water states it deliberately holds residuals above regulatory minimums – chloramine does not read or behave like free chlorine on a fill-water test, and it does not dissipate the way free chlorine does.
Source-specific trains, all converging on chloramine. Groundwater: minimal treatment because ‘The Floridan Aquifer serves as a natural filter… eliminating the need to remove particles before we disinfect with chloramines and blend it with our other sources.’ Surface water (Tampa Bay Regional Surface Water Treatment Plant): screening, conventional coagulation/settling, OZONE disinfection, biologically activated carbon filtration, then chloramines.
Seawater (Tampa Bay Seawater Desalination Plant): conventional pretreatment, progressively finer filters, reverse osmosis membranes with pores about 0.001 microns, then chemical stabilization, then chloramines. System-wide: lime, caustic soda and alkalinity added for pH adjustment and corrosion control. Chloramine as the final disinfectant is the single most pool-relevant treatment fact here.
Background sources: cms5.revize.com, tampabaywater.org, swfwmd.state.fl.us, ask.ifas.ufl.edu.
Before you drain or refill in St. Petersburg
St.
- POOL FILL BILLING ADJUSTMENT – the city’s Utility Billing FAQ: ‘Pool fill adjustments are granted to customers who have either purchased or built a new pool, or had an existing pool repaired.’ The city defines the covered structure broadly: ‘any structure, located in a residential area, that is intended for swimming or recreational bathing and contains water over 18 inches deep, including, but not limited to, in-ground and aboveground swimming pools; hot tubs; and nonportable spas.’ Documentation must include the date of the fill (or refill if repaired), the estimated number of gallons, and the service address. LIMIT: one per calendar year. Requested with the Utility Bill Adjustment Request Form from stpete.org/utilities, with supporting documentation. (Separately, leak adjustments are limited to two per calendar year and explicitly exclude voluntary uses like lawn watering.)
- DRAINING – the City of St. Petersburg ‘Good Neighbor Guide: Pool Maintenance’ sets hard numeric limits. Flat prohibition first: ‘Pool and spa water cannot be discharged into the sanitary sewer system.’ Then, for discharging to the environment: ‘The free chlorine residual must be less than 0.01 mg/L and the pH must be between 6.5 and 8.5’ – testable with a standard pool test kit, and the guide suggests sodium thiosulfate if you need to drain quickly. Also required: the water must be clear and free of solids before draining; direct the discharge over a vegetated surface such as a lawn so natural filtration can occur; control the rate of discharge to avoid erosion and nuisance conditions (water ponded longer than 24 hours can create odors, mosquito breeding or flooding); do not discharge onto areas recently treated with herbicides, fertilizers or pesticides; follow manufacturer instructions before discharging water recently dosed with algaecide. And critically for drought timing: ‘Avoid draining/backwashing your pool during periods of drought and during significant rainfall events. Do not drain your pool when more stringent drought watering restrictions are in place.’ Improper pool maintenance is reportable to the St. Pete Service Center (727-893-7111) or via SeeClickFix.
- DROUGHT – all of Pinellas County is inside SWFWMD’s Modified Phase III ‘Extreme’ Water Shortage restrictions, effective April 3, 2026 and EXTENDED THROUGH OCTOBER 1, 2026 (one day per week irrigation by house-number ending: 0/1 Monday, 2/3 Tuesday, 4/5 Wednesday, 6/7 Thursday, 8/9 Friday; watering windows 12:01-4 a.m. or 8-11:59 p.m.; low-volume irrigation any day before 8 a.m. or after 6 p.m.; aesthetic fountains limited to four posted hours a day). The SWFWMD order text I reviewed contains NO provision addressing filling, refilling or topping off swimming pools – but note the St. Petersburg pool guide’s own instruction not to DRAIN while stricter drought restrictions are in place, which are in place now.
Rules change and enforcement varies — confirm with your water provider before draining or refilling. Nothing here is legal advice.
Worth a look next door
- Filling a pool in Jacksonville — Same state, different water: Jacksonville fills at about 246 mg/L against 158 here.
- Filling a pool in Mesa — The other end of the scale: Mesa gets about 0.4 pool volumes of top-off against 1.5 pool volumes here.
Source: City of St. Petersburg 2025 Drinking Water Quality Report (2025 report data), accessed 2026-08-10. Values are the utility’s published system-wide figures; your tap can vary by service zone and season.
Data page generated 2026-08-15. Part of the pool fill-water lookup.