Filling a Pool in Austin: Your Tap Water’s Starting Numbers
Filling a pool in Austin means starting from Austin Water’s water — which is moderately hard at roughly 93 mg/L total hardness (about 5.4 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
Not a question the usual arithmetic can answer here, and that is itself the finding: the saturation index the pool trade uses is only defined for water between pH 6.5 and 9.5 with alkalinity above 40 mg/L. In Austin, the published pH of 9.70 sits outside that range. Running the numbers anyway would produce a confident figure that means nothing.
Practically, Austin fill water is unusual enough that a test kit is not optional here. Fill, let the water circulate, and read it before you decide anything.
The top-off arithmetic
Evaporation removes pure water and leaves the minerals behind, so make-up water re-adds Austin hardness every time. Replacing one full pool volume through top-off adds roughly another 93 ppm; starting from a fresh fill at about 93 mg/L, that reaches the 400 ppm ceiling after about 3.3 pool volumes of make-up water — a comfortable margin you are unlikely to burn through quickly. That is the 25th 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).
How much your own tap may differ
Austin Water reports hardness spanning 70 to 126 mg/L — a 1.8x range end to end. That is not measurement noise. It is what blending different sources into different service zones does over a year, and it means the single number at the top of this page is an average your own tap may not match. A pool a few neighbourhoods away can legitimately fill at a different hardness than yours. Test what comes out of your hose; do not plan off the midpoint.
Why Austin water is like this
THE KEY FACT: Austin LIME-SOFTENS its water, which is why Austin is by far the softest of these four Texas metros despite sitting in the same Hill Country limestone belt that makes San Antonio’s water brutally hard.
Austin Water’s 2025 Annual Water Quality Report states plainly that ‘Austin Water’s lime softening process helps coat pipes and prevent corrosion that could leach lead into drinking water’, and lists daily operational checks on ‘disinfectant (chlorine/chloramine) residual, turbidity, pH, alkalinity and water softening.’ The published 2025 total hardness is 70-126 ppm as CaCO3 (4.1-7.4 gpg), average 93 ppm (5.4 gpg).
So the raw Colorado River water arrives loaded with calcium carbonate dissolved out of Cretaceous limestone across the Highland Lakes watershed, and the plants deliberately knock a large share of it back out before it reaches the tap.
Practical consequence for a pool owner: Austin fill water adds calcium hardness far more slowly than SAWS or Corpus Christi fill water, so Austin pools are much more often fighting LOW calcium hardness (plaster etching, corrosive water) than scale — the opposite of the regional stereotype.
Residual disinfectant is CHLORAMINE, not free chlorine (2025 chloramines 0.06-3.1 ppm, average 2.3 ppm), so fill water carries combined chlorine into the pool and will read as combined chlorine / chloramine on a DPD test right after a big top-off.
Lime softening (confirmed in Austin Water’s own 2025 report) plus coagulation/flocculation, powdered activated carbon for taste and odor, and chloramination as the distribution-system residual. Austin Water also routinely tests Lake Austin and Lake Travis raw water and finished water from all three plants for cyanotoxins from algae blooms, at least weekly for cyanobacteria. Lime softening is the single most pool-relevant treatment step here because it directly sets delivered calcium hardness and alkalinity.
Background sources: austintexas.gov, austin.widen.net, gato-docs.its.txst.edu.
Worth a look next door
- Filling a pool in San Antonio — Same state, different water: San Antonio fills at about 300 mg/L against 93 here.
- Filling a pool in Tampa — The other end of the scale: Tampa gets about 0.9 pool volumes of top-off against 3.3 pool volumes here.
Source: Austin Water – Water Quality Reports (2025 WQR) + 2026 Q2 Water Quality Summary (austin.widen.net/content/qqi8p9jltb/pdf/WQS-Q2-2026.pdf) (2024-2026 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.