Read this first. Utility figures are from the reports linked below, read October 22, 2026. Water changes by season, source and neighborhood. Your tub maker's instructions come first: a dosing chart written for one tub can be wrong for another, and some makers tell you not to add chemicals at all. This page is not health advice.
What comes out of the tap
| Seattle, Cedar supply (most of the city) | Seattle, Tolt supply (north Seattle) | Portland, Bull Run (main supply) | Portland, groundwater (backup) | |
|---|---|---|---|---|
| Total hardness | 1.73 gpg at the plant, 1.55 gpg in distribution | 1.53 gpg in distribution | 7 mg/L; typically 7 to 11 ppm, about 0.5 gpg | 68 mg/L average; "about 80 ppm, or about 5 grains per gallon" |
| Calcium hardness | 24.3 to 26.4 mg/L as CaCO3 | 24.0 to 26.2 mg/L as CaCO3 | See total hardness | See total hardness |
| Total alkalinity | 26.3 to 29.9 mg/L | 20.1 to 26.5 mg/L | 29 mg/L (target 28 to 30 ppm at entry) | 91 mg/L |
| pH | 7.89 to 8.20 | 8.20 to 8.52 | 8.8 (typically 8.0 to 9.0) | 8.2 |
| Disinfectant | Free chlorine 1.08 to 1.61 mg/L | Free chlorine 0.63 to 1.58 mg/L | Chlorine plus ammonia (chloramine); chlorine target 2.2 to 2.5 ppm at entry | Chloramine |
| Sample or report date | May 19 and 20, 2026 | May 19 and 20, 2026 | Test results table updated July 28, 2026; April 2026 data | Same |
Seattle's figures are SPU's quarterly analysis at the treatment plant outlet and one distribution site per supply. SPU says more than 80 percent of Seattle residences get Cedar water and lists Broadview, Bitter Lake, Crown Hill, Northgate, Haller Lake, Pinehurst and parts of Maple Leaf, Wedgewood, Greenwood, View Ridge and Roosevelt as regularly receiving Tolt water. Portland uses groundwater from the Columbia South Shore Well Field to supplement Bull Run in summer and fall and to replace it during turbidity events; from December 19 to 26, 2025, groundwater was the sole source. One grain per gallon equals 17.1 ppm.
What the tub makers ask for
| Maker | Home sanitizing approach | Published targets | Hard or soft water notes |
|---|---|---|---|
| Renu Therapy | Ozone plus filtration; "no chemicals are needed for home use"; optional chlorine-free, bromine-free kit | pH 7.2 to 7.8; alkalinity 80 to 120 ppm (with the optional kit) | "Calcium Water Hardness does not apply to our tanks" |
| BlueCube | 20-micron filtration, automatic ozone, monthly enzyme treatment; residential units ship chemical-free | For commercial chlorine: 3 to 10 ppm free chlorine at pH 7.0 to 7.8, per BlueCube citing CDC | Never add casual chlorine, pool shock, bromine tablets, fragrances, essential oils or vinegar to the tub; inline carbon filter on the fill hose in hard-water areas |
| Plunge | Residential non-chlorine and commercial chlorine protocols | All-In Commercial Max: pH target 7.35 (7.10 to 7.80); free chlorine 1 to 3 ppm target, 5 ppm max; adjust alkalinity first | Well water: reduce calcium before filling, high calcium can damage the pump; commercial Max protocol adds calcium below 250 ppm hardness |
What this means for your tub
1. Scale is not the issue, so skip the softener
Hardness of 0.4 to 1.7 grains per gallon is very soft. The hard-water warnings you read online, like Plunge's note on well water calcium damaging the pump or BlueCube's carbon pre-filter for staining, are written for water many times harder than Seattle or Portland city water. If you are on a private well in rural Washington or Oregon, test it; well water can be very different.
2. Expect low alkalinity and high pH on your first test
Fresh city water reads about 20 to 30 mg/L alkalinity, roughly a quarter to a third of the 80 to 120 ppm range Renu and Plunge publish, and a pH that is often above 7.8. Low alkalinity means pH moves easily. If your maker's program calls for balancing, do it in the maker's order. Plunge says alkalinity first, then pH, waiting between additions. If your maker says no chemicals for home use, as Renu and BlueCube do, the test numbers are not a reason to start dosing on your own.
3. The chlorine in tap water is not a sanitizer program
Seattle's water leaves the plant with about 1.6 mg/L of free chlorine and Portland's with a chloramine residual. That keeps pipes safe, not a tub that people sit in for weeks. Home tubs rely on filtration, ozone, a lid, a rinse before you get in, and fresh water on the maker's schedule. Gyms, spas and clinics are a different case: health departments want a measurable sanitizer residual, and CDC's hot tub guidance is at least 3 ppm free chlorine and a pH of 7.0 to 7.8.
4. Watch for Portland's groundwater weeks
When Portland blends or switches to groundwater, hardness and alkalinity rise (68 mg/L hardness and 91 mg/L alkalinity on the bureau's table). A tub filled during a groundwater period will test differently from one filled with pure Bull Run water. Portland offers source-change notifications on its test results page.
5. Change the water on your maker's schedule
Renu says every 4 to 6 months for the Cold Stoic. BlueCube says every 6 to 8 weeks for light use, down to 2 to 4 weeks for heavy use. Soft water does not change those intervals. Drain to the sanitary sewer, not the street; see our Seattle and Portland permit and drainage guide.
Quick checklist
- Find your supply: Cedar or Tolt in Seattle, Bull Run or groundwater in Portland, or your own utility's annual water quality report elsewhere.
- Read your tub maker's water care page before you buy any chemicals.
- Fill, run the filtration and ozone, and test once so you know your baseline.
- Balance only if your maker's program asks for it, alkalinity first.
- Keep the lid on, rinse before you plunge, and change the water on schedule.
Sources
Utility reports and maker pages read October 22, 2026.
- Seattle Public Utilities: water quality analyses (Cedar and Tolt service areas)
- Seattle Public Utilities quarterly surface water analysis, samples May 19 and 20, 2026 (PDF)
- Portland Water Bureau: 2026 drinking water quality report (hardness, pH, treatment)
- Portland Water Bureau: drinking water test results (Bull Run vs groundwater, updated July 28, 2026)
- Portland Water Bureau: supplemental water quality data, April 2026 (PDF)
- Renu Therapy Water Treatment Kit (pH and alkalinity targets, calcium note)
- Renu Therapy FAQs (ozone, no chemicals for home use, water change interval)
- BlueCube: how to keep cold plunge water clean
- Plunge Support: water maintenance (well water and calcium)
- Plunge Support: All-In Commercial Max water management (hardness, alkalinity, pH, chlorine tables)
- CDC: what you can do to stay healthy in hot tubs (chlorine, bromine and pH levels)
Related guides
- Renu Cold Stoic 3.0 vs BlueCube C1
- Cold plunges through a Pacific Northwest winter
- The best cold plunges of 2026, ranked by use case
Frequently asked questions
Is Seattle and Portland tap water good for a cold plunge?
For scale, yes. Seattle Public Utilities measured Cedar River water, which serves more than 80 percent of Seattle homes, at about 1.6 to 1.7 grains per gallon of hardness in May 2026, and Portland says its Bull Run water typically has 7 to 11 ppm of hardness, about half a grain per gallon. That is very soft, so mineral scale on the chiller and tub is unlikely from city water. The catch is that soft water also has low alkalinity and a high pH, so if your tub maker asks you to balance the water, expect to adjust it.
What pH and alkalinity should cold plunge water have?
Follow your tub maker. Renu Therapy lists a pH target of 7.2 to 7.8 and total alkalinity of 80 to 120 ppm for owners who use its optional water kit. Plunge lists a pH target of 7.35 with a 7.10 to 7.80 range and an 80 to 119 ppm alkalinity band for its commercial All-In model, and says to adjust alkalinity first and then pH. Seattle and Portland tap water arrives with a pH near 7.9 to 9.0 and alkalinity of about 20 to 30 mg/L, so it starts outside both ranges.
Do I need a water softener for a cold plunge in Seattle or Portland?
Not on city water. The water is already soft. Plunge warns that high calcium from well water can damage its pump over time and tells well owners to reduce calcium before filling, and BlueCube suggests an inline carbon filter on the fill hose in hard-water areas to prevent staining. Those tips are for hard water. Portland's groundwater, used at times in summer, fall and turbidity events, is harder at about 68 to 80 ppm, but that is still moderate.
Does chlorine in tap water sanitize my cold plunge?
No. Seattle adds free chlorine and Portland adds chlorine plus ammonia (chloramine) to keep drinking water safe in the pipes. The residual is low, about 0.6 to 1.6 mg/L free chlorine in Seattle's May 2026 samples, and it is not a sanitizer program for a tub people sit in. Renu and BlueCube both rely on filtration and ozone at home and say chemicals are not needed; BlueCube tells owners not to add casual chlorine, pool shock or bromine tablets. Commercial tubs are different: CDC recommends at least 3 ppm free chlorine and a pH of 7.0 to 7.8 for hot tubs.
Should I add calcium to cold plunge water if my water is soft?
Only if your tub maker says so. Renu says calcium hardness "does not apply to our tanks." Plunge's commercial All-In Max protocol adds a calcium booster when total hardness reads below 250 ppm. BlueCube's residential tubs run chemical-free. Using a dosing chart written for a different tub can do more harm than good.
How often should I change cold plunge water?
It depends on the tub. Renu tells Cold Stoic owners to change the water every 4 to 6 months. BlueCube says every 6 to 8 weeks for light use, 4 to 6 weeks for daily use or two users, and 2 to 4 weeks for heavy use. Keep the lid on and rinse before you get in to stretch either interval. Drain to the sanitary sewer, not a storm drain.
Keep reading
Browse our cold plunges collection, see all guides, or ask us a question about your project.