Filling a Pool in Fresno: Your Tap Water’s Starting Numbers
Filling a pool in Fresno means starting from City of Fresno Department of Public Utilities’s water — which is hard at roughly 129 mg/L total hardness (about 7.5 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 City of Fresno Department of Public Utilities 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 Fresno hardness every time. Replacing one full pool volume through top-off adds roughly another 129 ppm; starting from a fresh fill at about 129 mg/L, that reaches the 400 ppm ceiling after about 2.1 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 19th 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
City of Fresno Department of Public Utilities reports hardness spanning 87 to 150 mg/L — a 1.7x 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 Fresno water is like this
Two mechanisms: a mild aquifer, then deliberate dilution. First the geology – DWR Bulletin 118 for the Kings Subbasin states ‘The groundwater is predominantly of bicarbonate type. The major cations are calcium, magnesium, and sodium’, with sodium and some chloride higher in the western part of the subbasin.
On mineral load it reports that TDS in the Fresno area ‘seldom exceeds 600 mg/L’, with a typical basin range of 200-700 mg/L, and DHS Title 22 supply-well data averaging just 240 mg/L TDS across 414 samples (range 40-570 mg/L). That is roughly HALF the dissolved-mineral load of Bakersfield’s Kern County Subbasin (400-450 mg/L average) – the single clearest reason Fresno fill water is only moderately hard while Bakersfield’s is genuinely hard, despite both cities sitting in the same San Joaquin Valley.
The City’s own 2024 Water Quality Annual Report puts total hardness at 89.3 mg/L (5.2 grains per gallon) with a range of 65-190 mg/L. IMPORTANT CORRECTION FOR ANY WRITEUP: water-softener marketing pages describe Fresno as ’18 grains per gallon’ and ‘extreme’; the utility’s own report does not support that – 5.2 grains average, and even the top of the published range (190 mg/L) is about 11 grains.
Because the water is calcium/magnesium BICARBONATE, hardness and alkalinity travel together, so the scale-vs-etch balance leans mildly toward scale, but far less aggressively than Bakersfield.
Second, the dilution: since 2004 Fresno has been substituting Sierra Nevada surface water for pumped groundwater, and snowmelt out of Millerton and Pine Flat carries very little dissolved mineral – the Director’s 2024 letter notes the conjunctive-use program ‘has reduced its utilization of groundwater to quantities last seen in 1955’. Surface-served neighborhoods therefore get noticeably softer, lower-alkalinity fill water than well-served ones.
Disinfection is sodium hypochlorite (free chlorine), reported at 1.33 mg/L – so fill water carries a chlorine residual but no ammonia/chloramine to complicate pool chemistry.
Surface water treated at three plants (NESWTF, T-3, SESWTF); wells treated individually where needed, and a well that violates standards ‘will be removed’ from service. Disinfection with sodium hypochlorite (listed in the report as ‘Chlorine (NAOCL)’). NO lime softening or hardness-removal process is described – the softening effect across the system comes from BLENDING low-mineral Sierra surface water into groundwater, not from treatment chemistry.
SEASONAL/ANNUAL: the Water Division states ‘In wet or very wet years, the City will utilize more surface water than groundwater’ and ‘In dry or very dry years, the City will rely on the banked groundwater reservoir for most of the drinking water production.’ The same address can therefore be on soft Sierra surface water in a wet year and harder banked groundwater in a dry one.
ZONAL: the three surface plants are geographically named and specific – NESWTF serves northeast Fresno, T-3 east Fresno, SESWTF southeast Fresno – so the east/northeast/southeast of the city are the areas most likely to see surface water, while the remainder leans on the roughly 260 wells. The published hardness range of 65-190 mg/L is close to a 3x spread across one city.
LIMITATION: the CCR reports one citywide average plus a range and does NOT break hardness out by plant or by well, so the citywide average is a poor predictor for any individual pool.
Background sources: fresno.gov, water.ca.gov.
Before you drain or refill in Fresno
Fresno Municipal Code Sec. 6-520 ‘WASTAGE OF WATER’ – in the list of measures the Council may implement – contains two provisions aimed directly at pools, quoted verbatim from the City-hosted code text.
- DRAIN FREQUENCY PLUS A LAB-TEST PERMIT THRESHOLD: ‘Prohibition of draining of swimming pools more than once every three years, except for structural repairs or to comply with public health standards determined by the County Health Officer… Residents with private swimming pools shall file a written application for a permit prior to draining their pools with the Water Division Manager. The application shall include the results of a pool water test conducted by an independent testing organization which shows a cyanuric acid level above 100 parts per million, total dissolved solids over 2,500 parts per million, or calcium over 450 parts per million, or stating the nature and duration of repairs to be made and the date on which the pool will be drained.’ This is remarkable and highly usable: actual pool-chemistry thresholds (CYA above 100 ppm, TDS above 2,500 ppm, or calcium above 450 ppm) written into municipal law as the evidentiary standard for earning a drain permit – and the test must come from an INDEPENDENT testing organization, not the homeowner’s test kit.
- PEAK-HOUR FILL BAN: ‘Prohibition of the filling or refilling of swimming pools during peak hours of 5:00 a.m. to 8:00 a.m. and 5:00 p.m. to 8:00 p.m., except that a standard hose up to 3/4 inch may be used to fill the pool and keep the tile and plaster wet during these hours.’ OPERATIONAL LAYER from the City’s own Water Conservation pages: ‘Topping off of established swimming pools to replenish water lost to evaporation or use must be completed during the appropriate outdoor water use hours’; ‘draining established swimming pools more than once every three years is prohibited, except as necessary to complete structural repairs or to comply with public health standards’; ‘Supporting documentation must be provided if a pool drain is required sooner than three years from the last drain.’ To drain-and-refill, or to completely fill a new or refurbished pool, a customer applies for a SHORT-TERM EXEMPTION – ‘Customers may be allowed to use water outdoors on a different day than allowed by the current Outdoor Water Use Schedule to drain and fill an established swimming pool’ – submitted at https://waterweb.fresno.gov/short-term.php?single=true, and ‘Applications must be submitted no less than 2 business days before the proposed non-compliant water usage.’ Contact: 559-621-CITY (2489) or 311. GOVERNING OUTDOOR SCHEDULE (April 1 – October 31): odd-ending addresses Tuesday/Thursday/Saturday, even-ending addresses Wednesday/Friday/Sunday, ‘Watering is not allowed on Mondays and NEVER between 10:00 a.m. and 6:00 p.m., on any day.’ TWO HONEST CAVEATS: (a) some search results describe a separately-named ‘pool fill permit’ from the Water Division for new/refurbished pools – I could not confirm that exact term on any City page, only the Short-Term Exemption route, so treat ‘pool fill permit’ as unverified terminology; (b) the pool provisions sit in Sec. 6-520(c), the measures the Council MAY implement under declared conditions, yet the City’s live Water Conservation pages present the three-year drain rule as currently operative – so the rule is being administered as current even though the code frames it as a conditional measure.
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 Anaheim — Same state, different water: Anaheim fills at about 296 mg/L against 129 here.
- Filling a pool in Scottsdale — The other end of the scale: Scottsdale gets about 0.6 pool volumes of top-off against 2.1 pool volumes here.
Source: City of Fresno – 2025 Annual Water Quality Report (CCR) (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.