Filling a Pool in Houston: Your Tap Water’s Starting Numbers
Filling a pool in Houston means starting from Houston Public Works (Houston Water), Main System TX1010013’s water — which is moderately hard at roughly 110 mg/L total hardness (about 6.4 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
Reading scale-versus-etch direction needs hardness, alkalinity and pH together, and Houston Public Works (Houston Water), Main System TX1010013 does not publish alkalinity. 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 Houston hardness every time. Replacing one full pool volume through top-off adds roughly another 110 ppm; starting from a fresh fill at about 110 mg/L, that reaches the 400 ppm ceiling after about 2.6 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 22nd 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
Houston Public Works (Houston Water), Main System TX1010013 reports hardness spanning 43 to 137 mg/L — a 3.2x 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 Houston water is like this
VERIFIED: Houston’s fill water is dominated by East Texas surface runoff, not aquifer-fed limestone water. About 86 percent of it is river water impounded in reservoirs (Trinity River into Lake Livingston; San Jacinto River into Conroe and Houston). Surface reservoir supply of this type is what sets the moderate baseline mineral load, and it is why Houston’s numbers do not look like Central or West Texas limestone-aquifer cities.
The remaining 13.5 percent comes from a completely different rock: the Gulf Coast aquifer sands (Evangeline and Chicot) tapped below 750 feet, which is a chemically distinct water blended into the same distribution system. Disinfection is with chloramines (chlorine plus ammonia), not free chlorine, which matters for pool chemistry because chloramine-treated fill water carries combined chlorine and ammonia into the pool rather than free available chlorine.
INFERRED, NOT VERIFIED: the specific claim that Gulf Coast aquifer water here is a sodium-bicarbonate type (low hardness but high alkalinity and elevated pH) is a reasonable expectation for these formations but I did not open a source confirming it for Houston’s wells, so it should not be published as fact.
Also relevant to the trend: groundwater quantities in this region are set by Harris-Galveston Subsidence District regulation rather than by water rights, which is the regulatory reason Houston keeps converting from wells to surface water – so the groundwater share, and the areas still on it, keep shrinking over time.
Chloramines are the disinfectant residual (listed as the disinfectant in the 2023 water quality report). Three surface water purification plants treat the surface supply; groundwater is pumped to one of seven re-pressurization plants before distribution. No lime softening found in the report.
VERIFIED AND LARGE – this is the single most useful Houston fact for a pool owner. The City of Houston does not run one water system, it runs SIX separate community public water systems, and they are not chemically the same. The Main System (TX1010013) is 86.5 percent surface / 13.5 percent ground.
But Kingwood (TX1010348), Willow Chase (TX1011902), District 73 (TX1011585) and District 82 (TX1011593) are each reported as 100 percent GROUND WATER, drawn from the Evangeline and Chicot aquifers; Belleau Woods is the sixth system. So a pool owner in Kingwood is filling from pure deep-aquifer well water while a pool owner in inner Houston is filling from blended surface water – same city, same utility, different source and different numbers.
On top of that, Houston Public Works states groundwater is distributed ‘predominately in the west side of Houston,’ so even within the Main System the ground/surface ratio is not uniform across the service area. A pool owner should identify which of the six systems serves their address before trusting any single citywide hardness figure.
Background sources: houstonpublicworks.org, library.municode.com, waterdatafortexas.org.
Before you drain or refill in Houston
VERIFIED AND UNUSUALLY SPECIFIC. Houston Code of Ordinances Chapter 47, Article VII (Water Shortages), Ord. No. 2012-997. The key move is in the definitions: Sec. 47-249 defines ‘Outdoor use’ to expressly include ‘filling or adding to any swimming pool, spa, or whirlpool.’ That means pool filling AND routine topping off are regulated as outdoor use at every drought stage. STAGE ONE (voluntary, 5 percent reduction goal): voluntary measures only; irrigation requested 7:00 p.m. to 5:00 a.m., no more than two days per week. STAGE TWO (mandatory, 10 percent goal): outdoor use – including pool filling and top-off – is UNLAWFUL except during the declared window, which is 7:00 p.m. to 5:00 a.m. on no more than two days per week (Sundays/Thursdays for even-numbered single-family addresses, Saturdays/Wednesdays for odd-numbered, Tuesdays/Fridays for all others), or a one-day-per-week schedule if that is what the declaration specifies. CRITICAL NUANCE: the hand-held-hose/bucket carve-out written into the ‘outdoor use’ definition applies only to IRRIGATION and only for purposes of Sec. 47-252 – it does NOT exempt pool filling, so there is no ‘I was just holding a hose’ defense for a pool. STAGE THREE (20 percent goal): ALL outdoor use is unlawful, so no pool filling and no topping off at all; the only carve-out is for customers continuing production and protecting inventory of their primary business products. STAGE FOUR (35 percent goal): all outdoor use unlawful PLUS a hard cap of 4,000 gallons per month for single-family residential customers. VARIANCES: Sec. 47-256 allows a written temporary variance from the utility official, including where the applicant cannot comply for technical reasons or will implement alternative measures achieving equal or greater savings; a variance is valid only for the shortage period in effect when issued. PENALTIES: Sec. 47-257, $100 to $2,000 per violation, each act and each day a separate offense; a first Stage Two violation may draw a written warning, Stage Three subsequent violations run $500-$2,000 and repeat offenders can have water shut off. NEGATIVE FINDINGS WORTH STATING:
- Houston’s separate Drought Contingency Plan PDF contains ZERO occurrences of the word ‘pool’ – the pool rule lives entirely in the ordinance’s ‘outdoor use’ definition, not the plan.
- A claim circulating in search results that ‘filling a new pool requires a permit through Houston Public Works’ does NOT appear in either the ordinance or the drought contingency plan; I could not verify any pool-FILL permit and it should not be repeated. (Pool CONSTRUCTION building permits are a separate matter I did not research.)
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 San Antonio — Same state, different water: San Antonio fills at about 300 mg/L against 110 here.
- Filling a pool in Phoenix — The other end of the scale: Phoenix gets about 0.7 pool volumes of top-off against 2.6 pool volumes here.
Source: Houston Water Quality Report, Jan-Dec 2024 (Houston Public Works, Main System TX1010013) (2024 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.