Filling a Pool in San Antonio: Your Tap Water’s Starting Numbers
Filling a pool in San Antonio means starting from San Antonio Water System (SAWS)’s water — which is very hard at roughly 300 mg/L total hardness (about 17.5 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
Reading scale-versus-etch direction needs hardness, alkalinity and pH together, and San Antonio Water System (SAWS) does not publish alkalinity or pH. That direction cannot be computed from public data for this city and we are not going to guess it — a drop-count test kit reads all of it in about five minutes once the pool is full.
The top-off arithmetic
Evaporation removes pure water and leaves the minerals behind, so make-up water re-adds San Antonio hardness every time. Replacing one full pool volume through top-off adds roughly another 300 ppm; starting from a fresh fill at about 300 mg/L, that reaches the 400 ppm ceiling after about 0.3 pool volumes of make-up water — less than one full pool volume — a hot-summer pool can work through that in a season or two, not decades. That is the 2nd 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
Less than you might think. San Antonio Water System (SAWS) publishes a tight range — 257 to 342 mg/L, only 1.3x end to end — which is unusual on this list and genuinely useful: it means the figures above are stable enough to plan around rather than merely a system-wide average hiding a lot of variation.
Why San Antonio water is like this
THE KEY FACT: San Antonio’s hardness comes from the Edwards — a karst limestone aquifer — and SAWS does NOT soften it. That single difference, not geology, is why San Antonio tap water is roughly three times harder than Austin’s even though both cities sit on the same Cretaceous limestone.
SAWS states on its own FAQ that ‘The typical hardness in the water provided by SAWS ranges from 15 to 20 gpg (grains per gallon)’ (about 256-342 mg/L as CaCO3) and that ‘The calcium carbonate found in Edwards and Trinity Aquifer water is naturally occurring’ — water moving through soil and rock dissolves the minerals that make it hard. Austin takes surface water and lime-softens it; SAWS takes limestone groundwater and delivers it essentially as the aquifer made it.
Practical consequence for a pool owner: every gallon of San Antonio fill water is a calcium-hardness DEPOSIT. Topping off to replace evaporation adds calcium while taking none away (evaporation removes pure water and leaves the minerals behind), so calcium hardness ratchets upward every season and scale/LSI management — not calcium addition — is the standing problem.
YES — and SAWS effectively admits it by publishing MORE THAN ONE water quality report. There is a SAWS Main System report and a separate SAWS Northeast report. The Northeast report states ‘The source of SAWS Northeast drinking water originates as groundwater from the Edwards, Carrizo and Wilcox Aquifers’ and that ‘SAWS Northeast purchases water from Canyon Regional Water Authority’ — a different source mix from the main system, with its own separate results table for the purchased supply.
So a pool owner in the Northeast service area is filling from a measurably different blend than one on the main system, and should pull the report matching their address rather than a citywide average. Note also that the Northeast report does not print hardness/TDS at all; SAWS directs those secondary constituents to Texas Drinking Water Watch. Magnitude of the swing is NOT quantified in SAWS’s published reports.
Background sources: saws.org, apps.saws.org, wateringrules.com, sanantonio-tx.elaws.us.
Before you drain or refill in San Antonio
San Antonio City Code Chapter 34 (Ord. No. 2014-05-29-0376). CURRENT STATUS: SAWS moved DOWN from Stage 3 to STAGE 2 effective Aug. 4, 2026, with the Edwards Aquifer at 650.3 feet — so the Stage I and Stage II pool rules below ARE in effect right now. Stage triggers are Edwards Aquifer level: Year-Round above 660 ft msl, Stage 1 at 660, Stage 2 at 650, Stage 3 at 640, Stage 4 at the City Manager’s discretion.
- STAGE I POOL COVER MANDATE: ‘All swimming pools other than public swimming pools must be covered with an effective evaporation cover or screen or evaporation shields covering at least twenty-five
- percent of the surface of the pool when the pool is not in active use. Active use includes necessary maintenance that requires removal of the cover, screen, or shields.’ Also ‘B. Auto fill feature turned off.’ Inflatable pool toys or floating decorations can count. ‘Evaporative shields’ are defined in Sec. 34-271 as ‘Soft floating pads that have contact with the pool water surface to reduce evaporation, small enough and flexible enough not to present an entrapment or injury hazard to bathers.’
- STAGE II DRAINING RULE: ‘Draining permitted only onto pervious surface, or onto pool deck where the water is transmitted directly to a pervious surface, only if: 1. Draining excess water from pool due to rain in order to lower water to maintenance level; 2. Repairing, maintaining or replacing pool component that has become hazardous; or 3. Repairing pool leak.’ Refilling of a PUBLIC pool is permitted only if it was drained for reasons 2 or 3.
- PERMANENT POOL CONSTRUCTION REQUIREMENTS, in force since May 1, 2013 and NOT tied to drought stage — these are the ones most pool owners and even installers miss: ‘a. Private residential swimming pools shall not be installed with sand media filters.’ ‘d. Automatic pool fill features must be designed so that they may be turned off… e. Automatic pool fill features must include an automatic pool shut-off feature.’ ‘f. Vanishing or negative edge pools must be designed with catch basins large enough to prevent splashing that leads to increased water use.’ ‘g. Backwash systems must be designed so they may be turned off.’ ‘h. Pool skimmers should be managed in such a way as to minimize water consumption. The range of allowable water within the skimmer fill range should allow for several inches of evaporative loss prior to filling.’ ‘i. All residential swimming pools shall have a hose end timer installed at the nearest hose bib location. In addition, a hose bib back-flow prevention device will be connected to the hose bib fixture nearest to the pool.’ ‘j. Pool companies… must provide in writing to every customer specific information on maintenance requirements that include an emphasis on preventative measures for keeping pool water quality high and alternatives to draining pools to correct water quality problems unless draining is needed for physical repair.’
- NO VARIANCE FOR REFILL COST: in October 2024 SAWS confirmed it will NOT grant variances for pool refilling to escape the drought surcharge, which applies to users exceeding 20,000 gallons in a month.
- Residential fountains and water features are allowed at any drought stage; only OUTDOOR COMMERCIAL fountains need a variance in Stages 1-4.
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 Austin — Same state, different water: Austin fills at about 93 mg/L against 300 here.
- Filling a pool in El Paso — The other end of the scale: El Paso gets about 1.3 pool volumes of top-off against 0.3 pool volumes here.
Source: SAWS Water Quality FAQs; 2025 Water Quality Report (PWS TX0150018), saws.org/wp-content/uploads/2025/05/264601.0150018_SAWS_Main.pdf (2024-2025 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.