Filling a Pool in San Diego: Your Tap Water’s Starting Numbers
Filling a pool in San Diego means starting from City of San Diego Public Utilities Department (PWS 3710020)’s water — which is hard at roughly 170 mg/L total hardness (about 9.9 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
At a typical 84 F pool temperature the published City of San Diego Public Utilities Department (PWS 3710020) numbers index at +0.63 — mildly scale-forming (how this is calculated). That puts San Diego 3rd of the 15 cities here that publish enough to compute an index, counting from the most scale-forming down.
Read it as a ceiling rather than a verdict: utilities publish TOTAL hardness while the index wants the CALCIUM fraction, so the true figure sits at or below +0.63 — close enough to the balanced band that it should not alarm you. What it does settle is which mistake costs money in San Diego. Cloudy water and scale at the tile line are the failure mode here; etched plaster is not.
The top-off arithmetic
Evaporation removes pure water and leaves the minerals behind, so make-up water re-adds San Diego hardness every time. Replacing one full pool volume through top-off adds roughly another 170 ppm; starting from a fresh fill at about 170 mg/L, that reaches the 400 ppm ceiling after about 1.4 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 17th 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 San Diego Public Utilities Department (PWS 3710020) reports hardness spanning 54 to 285 mg/L — a 5.2x 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 San Diego water is like this
San Diego’s tap is hard for one dominant reason: the Colorado River. Because the city buys mostly RAW Colorado River and State Water Project water and treats it itself, the hardness of the river arrives essentially unchanged – nothing in San Diego’s treatment train removes calcium. Verified 2025 numbers by plant: ALVARADO total hardness 237 mg/L (range 203-270), alkalinity 119 mg/L (108-130), pH 8.29, sodium 87.8 mg/L. MIRAMAR hardness 242 mg/L (200-285), alkalinity 110 mg/L (95.6-126), pH 8.27, sodium 86.2 mg/L.
OTAY hardness 243 mg/L (201-271), alkalinity 121 mg/L (83.3-155), pH 8.07, sodium 111 mg/L. PURCHASED TREATED WATER hardness 145 mg/L (54.4-232), alkalinity 96.0 mg/L (46.0-120), pH 8.40. That last column is the tell: the purchased-treated stream – MWD Skinner plus CWA Twin Oaks plus the Carlsbad Desalination Plant – is dramatically softer, averaging 145 mg/L and dipping as low as 54.4 mg/L, because desalinated seawater carries almost no calcium hardness after reverse osmosis.
It is under 10 percent of supply, so it barely moves the citywide average, but where it lands it softens the fill noticeably. The high pH is also notable and pool-relevant: all three City plants deliver water above pH 8, and the purchased stream averages 8.40 with an upper range of 9.00, because "the corrosivity of the water is controlled by adjusting the pH" for pipe protection.
High pH plus 237-243 mg/L calcium plus 110-121 mg/L alkalinity is a scale-forming combination straight out of the hose – a San Diego fill typically starts with a positive Langelier index before a single chemical is added.
San Diego uses an unusually advanced disinfection train, and it varies by plant. "At the Alvarado and Miramar Water Treatment Plants, ozone is used as the primary disinfectant"; the Otay Water Treatment Plant uses CHLORINE DIOXIDE as its primary disinfectant. Miramar also runs pre-ozone.
All three then apply CHLORAMINES as the secondary/distribution disinfectant: "Chloramines are created by adding chlorine and ammonia to the water." Coagulation uses ferric chloride and polymer; filtration is through anthracite coal and silica sand; sodium hydroxide and fluoride are added. Corrosion control is by pH adjustment, which is why the delivered pH sits at 8.07-8.40. NO lime softening is used – hardness passes straight through.
For pool owners the chloramine point is the operational one: the fill water arrives with combined chlorine rather than free chlorine, so a fresh fill can show combined chlorine on a test kit immediately. The desalination contribution (Carlsbad, within the purchased-treated stream) is the only part of the supply that meaningfully reduces mineral content, via reverse osmosis.
BOTH seasonal and zonal, and the utility warns about both. SEASONAL: "Throughout the year, the ratio of water from each source changes. The constituents that make up the City’s source water are influenced by the water source, climate, geology and the land activities that they flow through." That is why the ranges are wide – Miramar hardness swings 200-285 mg/L and Otay’s alkalinity swings 83.3-155 mg/L within a single year. ZONAL, WITH A TWIST: San Diego does not have clean plant boundaries.
The report states that "the City’s distribution system is interconnected, and the areas served by each treatment plant overlap. To gather information about the water quality at your residence, you may need to refer to multiple columns in the data tables." Broadly: Alvarado serves the central city (adjacent to Murray Reservoir), Miramar serves the northern city (Scripps Miramar Ranch, adjacent to Miramar Reservoir), Otay serves the southern city (adjacent to Lower Otay Reservoir).
The practical consequence is that the three City plants are close enough in hardness (237/242/243) that zone matters LESS in San Diego than it does in Los Angeles – the real variability is seasonal source-ratio drift plus the softer purchased-treated stream. A structural change is coming: Pure Water Phase 1 begins delivering purified water in early 2027, which will add a new, very low-mineral stream to the blend.
Background sources: sandiego.gov, water.ca.gov.
Before you drain or refill in San Diego
FILLING – PERMITTED BUT WITH A SPECIFIC PERMANENT RULE THAT NAMES POOLS. The City of San Diego has year-round PERMANENT mandatory water restrictions (not drought-stage-dependent), and one of the enumerated restrictions is: "Customers shall not overfill swimming pools and spas." That is an unusual, pool-specific, always-in-force provision – it does not ban filling, but overfilling is a citable violation.
Other permanent restrictions that touch a pool project: no potable water applied to outdoor landscapes causing runoff onto adjacent property, walkways, roadways or structures; leaks must be repaired within 72 hours of notification; no washing down driveways, sidewalks, parking areas or other paved areas without a power washer or a shutoff-nozzle hose, and only to alleviate immediate safety or sanitation hazards, with water collected and prevented from entering the storm sewer; no non-recirculating potable water in ornamental or cascading fountains; landscape watering only before 10 a.m.
or after 6 p.m.; no outdoor landscape watering during and within 48 hours after measurable rainfall. Water waste is reported via the Get It Done app or waterwaste@sandiego.gov; the Stormwater Enforcement Team responds if over-irrigation reaches the gutter, street or storm drain inlet.
Per the City’s Pools, Spas and Fountains storm water fact sheet: "It is illegal to discharge swimming pool, spa, fountain, reflective pool, and pond water; filter backwash; or their construction materials such as plaster, gunite, and cement into the Municipal Separate Storm Sewer System (MS4) (San Diego Municipal Code Sec. 43.0304 and Sec. 43.035(e)(3)).
Penalties associated with these violations can be up to $10,000 per day per incident." Discharge directly to the storm drain "is only permissible if the surface flow path is cleared of trash, debris and sediment; the water is dechlorinated; has a pH in the 7-8 range; is within ambient temperature; has no algae, algaecide or suspended solids; and is not saline. The path to the storm drain inlet must be swept, even if it is a few blocks away.
You must be able to prove this has been completed." SALTWATER POOLS SPECIFICALLY: "Discharge of saline swimming pool water to City storm drains, curb gutters, or any other part of the City’s storm drain system is prohibited unless the saline water is prepared as above and can be discharged through a pipe or concrete channel directly to a naturally saline water body (e.g., the Pacific Ocean)." So a salt-chlorine-generator pool effectively cannot be drained to the street in San Diego.
Permitted alternatives: gradual irrigation of lawn or landscape over several days provided the soil absorbs it without runoff or erosion (check for yard drains that would route it to the storm drain), or discharge to the sanitary sewer through an onsite sink or lateral cleanout – "Contact the Public Utilities Department at (858) 654-4100 for approval." Water quality must be monitored over 3 to 5 days to reach pH 7-8, and the water may not contain chlorine, saline, chemicals used to counteract chlorine, hydrogen-peroxide-based products, algae or algaecides, or debris and suspended solids.
BACKWASH: "Back washing or acid cleaning is not allowed to enter the storm drain system"; clean filters over lawn, planter box or vegetated hillside, and all filter backwash fluids must be at pH 7-8 before discharge into the sanitary sewer via a legal connection. No pool fill-water rebate or sewer credit was found. NO REBATE FOUND.
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 170 here.
Source: City of San Diego 2025 Annual Drinking Water Quality Report (CY2025 data) + 2025 Additional Characteristics table (2025 (published June 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.