Filling a Pool in Los Angeles: Your Tap Water’s Starting Numbers
Filling a pool in Los Angeles means starting from Los Angeles Department of Water and Power (LADWP)’s water — which is hard at roughly 179 mg/L total hardness (about 10.5 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
At a typical 84 F pool temperature the published Los Angeles Department of Water and Power (LADWP) numbers index at +0.32 — inside the balanced band (how this is calculated). That puts Los Angeles 11th of the 15 cities here that publish enough to compute an index, counting from the most scale-forming down.
Since utilities publish TOTAL hardness and the index wants the CALCIUM fraction, +0.32 is a ceiling and the true figure sits at or below it — which for a near-balanced reading means Los Angeles water can test slightly aggressive once it is in the pool. Worth knowing before you assume balanced water needs nothing.
The top-off arithmetic
Evaporation removes pure water and leaves the minerals behind, so make-up water re-adds Los Angeles hardness every time. Replacing one full pool volume through top-off adds roughly another 179 ppm; starting from a fresh fill at about 179 mg/L, that reaches the 400 ppm ceiling after about 1.2 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 14th 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
Los Angeles Department of Water and Power (LADWP) reports hardness spanning 78 to 280 mg/L — a 3.6x 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 Los Angeles water is like this
There is no single "LA hardness." LADWP publishes results by treatment plant, and the plants differ by more than 2x – this is the single most important fact for a pool owner in Los Angeles. 2025 total hardness as CaCO3: LA Aqueduct Filtration Plant 103 mg/L (range 96-117); Northern Combined Wells (San Fernando Basin) 115 mg/L (78-151); Southern Combined Wells 116 mg/L (83-159); MWD Weymouth 234 mg/L (189-280); MWD Diemer 236 mg/L (191-280); MWD Jensen 140 mg/L (137-142). The mechanism is watershed geology.
LAA water is Sierra Nevada snowmelt running over granitic rock that gives up very little calcium – calcium 27 mg/L, magnesium 9 mg/L. Weymouth and Diemer are the Colorado-River-heavy plants, and Colorado River water has crossed hundreds of miles of sedimentary and evaporite terrain – calcium 56 mg/L, magnesium 22 mg/L, roughly double. Jensen runs State Water Project water (Sacramento-San Joaquin Delta), which sits in between at calcium 32 mg/L. THE CRITICAL ASYMMETRY: alkalinity does NOT vary the way hardness does.
Total alkalinity is 105 (LAA), 98 (Northern Wells), 103 (Southern Wells), 110 (Weymouth), 108 (Diemer), 98 (Jensen) mg/L – essentially flat near 100 across the entire city. So moving across Los Angeles changes your calcium hardness dramatically while your carbonate alkalinity barely moves. A Westside pool and an Eagle Rock pool filled on the same day start with similar alkalinity but calcium hardness differing by more than 130 mg/L, which is the difference between needing to ADD calcium and needing to manage scale.
LADWP uses CHLORAMINES as its distribution-system disinfectant and says so directly: "LADWP has enhanced your drinking water through the use of chloramines." The report warns that "If you maintain a fishpond, tank, or aquarium, adequate treatment must be provided to remove chloramines, as they can be harmful to fish." That matters to pool owners in two ways: chloramine does not dissipate by standing or sunlight the way free chlorine does, and it arrives as combined chlorine, so a fresh fill can register combined chlorine on a test kit before any bather load exists.
Ammonia plus chloramines measured as N run 0.4 to 0.5 mg/L at the LAA plant and the well fields. The LAA plant uses ultraviolet treatment for Cryptosporidium and Giardia control. NO lime softening is used, so hardness passes through untouched.
Forward-looking: the Los Angeles Groundwater Replenishment Program (LAGWR) broke ground December 6, 2024 and will supply up to 45 million gallons daily of purified water; it is described as operational in time for the LA28 Olympics, which will raise the local share above today’s 1 percent.
Zonal variability is the dominant effect and LADWP maps it explicitly by community. WESTERN LOS ANGELES (Brentwood, Bel Air Estates, Pacific Palisades, Palisades Highlands, Castellamare, Venice, Mar Vista, Palms, Westwood, Century City, Cheviot Hills, Rancho-adjacent Sawtelle, West Los Angeles, Westchester, Playa del Rey, Beverly Glen, Culver City) is served by the Los Angeles Aqueduct ONLY – the softest water in the system, about 103 mg/L. EASTERN LOS ANGELES (Eagle Rock, Highland Park, Glassell Park, Mt.
Washington, Cypress Park, Atwater Village, Echo Park, Lincoln Heights, Boyle Heights, El Sereno, Montecito Heights, Monterey Hills) and the HARBOR COMMUNITIES (San Pedro, Wilmington, Harbor City, Harbor Gateway, East San Pedro/Terminal Island) are served by MWD State Water Project AND Colorado River Aqueduct water through Weymouth/Diemer/Jensen – the hardest, about 234-236 mg/L.
SAN FERNANDO VALLEY (Van Nuys, Sherman Oaks, Studio City, Encino, Tarzana, Woodland Hills, Northridge, Chatsworth, Granada Hills, Porter Ranch, Reseda, Canoga Park, West Hills, Winnetka, Sun Valley, Sunland, Tujunga, Pacoima, Sylmar, Arleta, Mission Hills, North Hills, North Hollywood, Toluca Lake, Valley Village, Panorama City, Lake View Terrace, Olive View, Warner Center, Hollywood Hills) draws LAA plus local groundwater plus MWD Jensen.
CENTRAL LOS ANGELES (Hollywood, Los Feliz, Silverlake, Koreatown, Hancock Park, Mid City, Westlake, Crenshaw, Baldwin Hills, Hyde Park, Park La Brea, Chinatown, Little Tokyo, Watts, Griffith Park, Country Club Park, Mt. Olympus, Rancho Park, West Hollywood) draws LAA plus local groundwater. LADWP footnotes that in Culver City, West Hollywood, Harbor Gateway and the L.A. City Strip "sources of drinking water may fluctuate …
depending on operational needs and source water availability." Year-to-year variation is also real: the LAA share depends on Sierra snowpack, so a dry year pushes the city toward the harder MWD supply.
Background sources: ladwp.com, water.ca.gov.
Before you drain or refill in Los Angeles
THE KEY FACT, VERIFIED AND FREQUENTLY MISREPORTED: Los Angeles CAN prohibit filling residential pools, but that prohibition is in PHASE V of the Emergency Water Conservation Plan Ordinance – and the city is currently in PHASE 2. LADWP’s own ordinance page states plainly: "City of Los Angeles is Currently in Phase 2 of Water Conservation." Reading the actual ordinance text (Ordinance 184250, effective May 3, 2016, LAMC Sec. 121.08), the pool provisions escalate as follows.
PHASE II (current): no pool provisions at all – filling and refilling a residential pool is unrestricted. PHASE III: "Recommend use of pool covers to decrease water loss from evaporation" (a recommendation, not a mandate).
PHASE IV: "Mandate use of pool covers on all residential swimming pools when not in use" – this is the phase where a pool cover becomes legally required – plus "No filling of decorative fountains, ponds, lakes, or similar structures used for aesthetic purposes, with potable water" (note: fountains and ponds, still NOT pools). PHASE V: "No filling of residential swimming pools and spas with potable water." PHASE VI: the Board may add further prohibitions by newspaper publication.
CAUTION: several widely-cited secondary summaries place the residential pool-filling ban in Phase III. That is wrong against the ordinance text published by LADWP itself – it is Phase V, three phases above the current one. CURRENT PHASE 2 RULES that still affect a pool project: landscape watering is limited to Mondays, Wednesdays and Fridays for odd-numbered addresses and Sundays, Tuesdays and Thursdays for even-numbered addresses; ALL outdoor watering is prohibited from 9:00 a.m.
to 4:00 p.m.; no runoff onto streets, driveways and gutters; all leaks must be repaired in a timely manner; no washing vehicles without a self-closing shutoff nozzle. ENFORCEMENT: first offense is a written warning, then for meters under two inches $100 / $200 / $300 for the second, third and fourth violations, and flow restriction on the fifth; for meters two inches and larger $200 / $400 / $600 then flow restriction.
VARIANCES: LADWP accepts a temporary variance or hardship application where a specific requirement would cause undue hardship. DRAINING – UNVERIFIED. LA Sanitation’s "Los Angeles Swimming Pool Discharges" guidance PDF and the City of Los Angeles Stormwater Program’s pools/spas/fountains pages both returned HTTP 404 on 2026-08-14, and the lastormwater.org site appears to be defunct or compromised.
Search engines still index the old content, but I could not open a live official page, so LA’s pool DRAINING requirements are reported as UNREACHABLE, not as nonexistent. A pool owner should call LA Sanitation’s Industrial Waste Management Division before draining. 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 179 here.
- Filling a pool in Charlotte — The other end of the scale: Charlotte gets about 11.5 pool volumes of top-off against 1.2 pool volumes here.
Source: LADWP 2026 Drinking Water Quality Report (Calendar Year 2025 monitoring results, Tables II-III) (2026 (CY2025 data) 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.