Hard water in Alum Rock, CA
Reported by San Jose Water · October 2025
- CA typical
- 121 mg/L
- EPA health violations since 2016
- 1
- Softener for 3 people
- 24k grain
- Data
- Utility report
The water in Alum Rock carries roughly 178 mg/L of dissolved calcium and magnesium, or 10.4 grains per gallon. That is hard water. The number comes from San Jose Water, dated October 2025.
At 178 mg/L, Alum Rock is 57 mg/L above the typical California figure of 121. Of the 10 nearest cities we track, 2 have softer water and 4 have harder. The range nearby runs from 49 mg/L in Milpitas to 298 in Sunnyvale.
What 178 mg/L means at home
Expect white crust on faucets and showerheads, plus spots on dishes that come out of the dishwasher.
Kettles collect scale quickly. Water heaters build up sediment at the bottom of the tank, which makes them work harder over time.
Soap and detergent need a little more to lather, and soap scum shows up in the tub.
Hardness is not a health problem, and EPA sets no limit for it. It is a matter of scale and soap.
What to do about it
At 178 mg/L most of the trouble is scale, and only a few things deal with it.
Ion-exchange water softener
The standard whole-house fix. It swaps calcium and magnesium for sodium or potassium and needs salt plus a drain. Size it with the calculator below.
Salt-free conditioner
No salt and no drain line. It changes how minerals form scale instead of removing them, so water still tests hard.
Reverse osmosis or carbon filter
Under-sink reverse osmosis removes minerals from drinking water only. Plain carbon filters improve taste but do not soften.
3 people × 75 gallons a day × 10.4 gpg = 2,342 grains a day. A 24,000-grain unit regenerates about every 10 days.
EPA record for Alum Rock's water system
1 health-based violation since 2016
Serves about 1,039,920 people from surface water.
- Health-based
- 1
- Monitoring or reporting
- 1
- Open now
- 0
TapWaterData ties Alum Rock to this system. EPA's service-area records do not name Alum Rock, so check the utility printed on your water bill.
| Contaminant or rule | Violations | Period | Status |
|---|---|---|---|
| Surface Water Treatment Rulefiltration and disinfection | 1 | 2023 | Resolved |
Health-based means a contaminant above its legal limit or a missed treatment step. Monitoring and reporting violations are late or missing tests and notices. Source: EPA ECHO, 2026Q3 export, violations with periods starting 2016 or later. Full EPA record.
Questions
Is Alum Rock water hard?
Yes. It measures about 178 mg/L (10.4 grains per gallon), which the USGS classes as hard.
What is the water hardness in Alum Rock in grains per gallon?
About 10.4 gpg. One grain per gallon equals 17.1 mg/L, so 178 mg/L divided by 17.1 gives 10.4.
What size water softener do I need in Alum Rock?
For 3 people using 75 gallons each per day, 10.4 gpg works out to about 2,342 grains a day. A 24,000-grain softener covers that with a regeneration about every 10 days.
Has the Alum Rock water system had EPA violations?
EPA records show 1 health-based violation for San Jose Water since 2016, involving Surface Water Treatment Rule. There were also 1 monitoring or reporting violation, which are about late or missing tests and notices.
Nearby cities
- East Foothills178 mg/LHard
- San Jose185 mg/LVery hard
- Milpitas49 mg/LSoft
- Santa Clara248 mg/LVery hard
- Campbell178 mg/LHard
- Fremont65 mg/LModerately hard
- Sunnyvale298 mg/LVery hard
- Los Gatos178 mg/LHard
- Cupertino186 mg/LVery hard
- Saratoga178 mg/LHard
Hardness data: TapWaterData US Water Hardness (utility report, October 7, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.