How Hard Is San Diego County's Water? Hardness by City and Water Utility

How hard is the water from each San Diego County utility?

The U.S. Geological Survey classes water above 180 mg/L as very hard, and most of the supplies below average above that line. Several utilities blend water from more than one treatment plant and publish a number for each rather than one system average; for those, we show each plant and you should test at your tap.

A few notes on reading the table. Hardness is reported as calcium carbonate, which is the standard way to express it. Where a utility buys water from several plants, like Carlsbad, Vallecitos, Ramona and Padre Dam, your tap gets some blend of those sources that the report does not pin down, which is why a tap test beats any average.

Desalinated water from the Carlsbad Desalination Plant is far softer than imported water, around 61 mg/L. It shows up in several North County utilities' supply, which is part of why their numbers vary more across the year.

How hard is hard? What do mg/L and grains per gallon mean?

The U.S. Geological Survey classes water by its hardness as calcium carbonate: 0 to 60 mg/L is soft, 61 to 120 is moderately hard, 121 to 180 is hard, and above 180 is very hard. Water treatment equipment is usually rated in grains per gallon; one grain per gallon is about 17.1 mg/L.

Hardness is not a health standard, and none of the reports in this guide treat it as one. Hardness is about calcium and magnesium, and its cost shows up as scale on fixtures, in water heaters and in appliances, and as soap that does not rinse off cleanly.

Why does San Diego County's water hardness change from year to year?

Because most of the county's water is imported, and the import is a changing blend. The Metropolitan Water District says Colorado River water has the highest salinity of its supplies, averaging about 625 mg/L of dissolved solids since 1976, while State Water Project water runs about 250 to 325 mg/L. More Colorado River water means harder water.

The San Diego County Water Authority puts it this way: Metropolitan's water comes from the Colorado River and the State Water Project, and the blend of the two sources fluctuates depending on hydrologic conditions and how Metropolitan operates its system. Local utilities then add their own reservoir water, desalinated water or groundwater.

You can see the effect in individual utilities. Helix Water District's supply was 26% Colorado River water in 2023 and 58% in 2024 (details in our El Cajon post). Poway's imported water averaged 117 mg/L in its 2017 report and 284 mg/L in others (year-by-year table in our Poway post). A number from three years ago may not describe this year's water at all.

Why do the hardness numbers you find online disagree?

Because many of them are not from a utility. Water treatment sellers often publish a single county average, or a high figure, without a report year or a source, and some pages have not been updated in years. Hardness moves enough between years and between treatment plants that an undated number can be off by several grains either way.

We have done this ourselves. Pages on our own site used a rounded countywide figure, and this guide replaces it with each utility's reported number. If a company quotes you a hardness number to sell you a system, ask which report it came from and which year.

The fix is simple: use the utility's own report, check the year, and if the decision matters, test the water at your tap. A test strip or a lab test tells you what is actually coming out of your faucet this month.

Does San Diego County tap water have chlorine or chloramine?

Mostly chloramine. Ten of the fourteen utilities in this guide name chloramine in their latest reports. Three report only a total chlorine residual without naming the form, and Poway named chloramine through CY2024 and changed its wording in CY2025. Chloramine is safe to drink at regulated levels but needs the right carbon to filter.

Our chloramine post covers which carbon handles it, and why it matters for aquariums and home dialysis.

What if you are on a private well?

Then none of these numbers apply to you. San Diego County's Department of Environmental Health and Quality says wells are commonly the only potable water supply in the rural parts of the county, and no utility report covers them. A private well needs its own lab test, and the state recommends testing every year.

Read our East County well water post for what to test for and how often.

What should you do with your hardness number?

Decide whether scale is costing you money. Water in the USGS hard and very hard ranges, above 120 mg/L or about 7 grains per gallon, builds scale. A softener removes hardness and is sized on people × 75 gallons × grains per gallon. A salt-free conditioner stops scale without removing minerals and is sized on bathrooms and peak flow.

Our salt-free vs softener comparison explains what each system does and does not do, and our cost post covers what each costs installed and to run. If you are in Poway or El Cajon, our Poway and El Cajon posts go deeper on those utilities.