Filling a Pool in Tucson: Your Tap Water’s Starting Numbers
Filling a pool in Tucson means starting from Tucson Water (City of Tucson Water Department)’s water — which is very hard at roughly 209 mg/L total hardness (about 12.2 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
Reading scale-versus-etch direction needs hardness, alkalinity and pH together, and Tucson Water (City of Tucson Water Department) 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 Tucson hardness every time. Replacing one full pool volume through top-off adds roughly another 209 ppm; starting from a fresh fill at about 209 mg/L, that reaches the 400 ppm ceiling after about 0.9 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 12th 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
Tucson Water (City of Tucson Water Department) reports hardness spanning 78 to 340 mg/L — a 4.3x 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 Tucson water is like this
Tucson Water says it directly on its own Hard Water page: ‘In Tucson, our groundwater is naturally hard. The increased hardness levels come primarily from mixing our area’s hard groundwater with Central Arizona Project (CAP) water. As the percentage of CAP water increases, the hardness level will increase.’ That single sentence is the whole story.
Tucson’s tap is a hybrid: Colorado River water that already carries a heavy dissolved-mineral load from the basin’s sedimentary rock, then deliberately infiltrated through Avra Valley alluvium where it mixes with naturally hard native basin-fill groundwater, then pumped back up. The aquifer is the filter, not a softener – nothing in the process strips calcium, magnesium, or bicarbonate.
Treatment is chlorine disinfection only (about 1 part per million added at well sites, reservoirs, and other facilities), so hardness, alkalinity, and pH arrive at the hose bib essentially as the blend produced them. The practical consequence for a pool owner: fill water is calcium- and alkalinity-loaded from the moment it enters the pool, and every gallon of top-off water in a 70-plus-inch-evaporation climate leaves its minerals behind and takes none away.
Note the direction of Tucson’s hardness lever is the opposite of what people assume – more Colorado River water in the blend means HARDER water, not softer.
Chlorine disinfection only, approximately 1 ppm added at well sites, reservoirs, and other facilities, with residual maintained throughout distribution and 248 routine chlorine samples collected per month. No lime softening, no membrane or desalination step, and no chloramination is described in the annual report – meaning the fill water’s hardness, alkalinity, and pH are set by geology and blending rather than by treatment. Soil-aquifer passage during recharge is effectively the polishing step.
PFAS-affected areas of the local groundwater are avoided rather than treated in-line, and wells with detectable PFOA or PFOS are removed from service; PFAS has not been detected in the Colorado River supply.
Background sources: tucsonaz.gov, azmet.arizona.edu.
Before you drain or refill in Tucson
HIGH VALUE, and Tucson’s rules are unusually specific about draining rather than filling.
- FILLING – no permit or utility notification is required under current conditions. Tucson Water is currently in Tier 2 of its Drought Preparedness and Response Plan (four tiers, 0 through 3, triggered by Lake Mead elevations of 1090-1075 ft, below 1075 ft, below 1045 ft, and below 1025 ft respectively). None of Tiers 0 through 3 restrict pool filling; the measures are audits, water use guidelines, targeted conservation outreach, possible drought surcharge, and suspension of Pre-Annexation Development Agreements.
- THE DORMANT KILL SWITCH – separately from the drought tiers, Tucson City Code Sec. 27-95 (Emergency Water Conservation Ordinance, Ord. No. 8461, Chapter 27 Article VII), reproduced in the current drought plan, provides that upon a declared water emergency by mayor and council, ‘The filling of swimming pools, fountains, spas or other exterior water features is prohibited.’ No such emergency is currently declared. Violation of that article is a civil infraction of $250 to $1,000 per violation and can lead to service disconnection.
- DRAINING – the preferred and Pima County Regional Wastewater Reclamation Department-approved route is discharge into the private property sewer cleanout on your own lot: plan it for low-flow times such as afternoon or late night, use a small-volume pump, and DO NOT use public manholes or cleanouts.
- SURFACE DRAINING is allowed only when no other disposal option exists and ALL of these are met: you do not have access to a private sewer cleanout; the pool sits untreated at least 72 hours after the last chlorine addition (this waiting period is the dechlorination requirement); the owner measures pH before discharge and adjusts it into the range 7-8 and monitors it throughout; acid-washed water may NOT be discharged off-site at all; the water is clear, not cloudy or discolored, and free of algae; discharge goes by temporary flexible hose to a stormdrain, drainage channel, or paved-street curbline gutter, never into unpaved or strip-paved alleys or streets; it must not transport sediment or erode channel banks or bottoms; and it must not run onto other private property, across a sidewalk, or impair street access. Under no circumstances may a pool or spa be permanently connected to a stormdrain or wash. This is allowed under the ADEQ De Minimis General Permit.
- FILTER BACKWASH – roughly 75 gallons per backwash; surface flow of backwash off your property is NOT allowed under state and local discharge permit requirements. It should be absorbed on your own lot, used only on salt-tolerant plants (the flyer lists oleander, rosemary, bougainvillea, natal plum, Texas ranger, olive, native mesquite, desert broom, saltbush, aloe, deer grass, bear grass, ice plant), and the hose moved frequently to avoid salt buildup and mosquito-breeding stagnant water.
- WATER WASTE – Tucson Code 27-15 prohibits allowing water to escape onto public property such as alleys or streets or onto another person’s property, and prohibits ponding deeper than one quarter inch or over more than 150 cumulative square feet in a street or parking lot. Fines are a minimum of $250 first offense and $500 for a second or subsequent offense within three years.
- BILLING QUIRK WORTH KNOWING – Pima County calculates the monthly sewer charge from the average of your December, January, and February water billing periods, so a summer pool fill or refill raises the water bill but does NOT raise the sewer bill. Pima County offers an appeal of the winter averaging.
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 Mesa — Same state, different water: Mesa fills at about 290 mg/L against 209 here.
- Filling a pool in Austin — The other end of the scale: Austin gets about 3.3 pool volumes of top-off against 0.9 pool volumes here.
Source: Tucson Water Quality Dashboard (utility’s WQPUBLISH ArcGIS feature service: LATESTWQQUARTERLY hardness table and LATESTWQMONTHLY pH/TDS/chlorine table) + Tucson Water 2023 Annual Water Quality Report, main system, for disinfectant practice (2025-2026 report data), accessed 2026-08-11. 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.