San Diego water by treatment plant
San Diego treats most of its water at three plants, and each mainly serves one section of the city. All three averaged about 240 ppm in 2025, which reads as very hard on the USGS scale.
Which one serves my home?
The city says each plant mainly serves one section of the city. Its 2025 report says the areas served by each plant overlap and shows them on a service-area map.
Alvarado plant: central San Diego
The Alvarado plant serves customers in the central section of the city. Source: City of San Diego water and wastewater facilities page.
- Average 237 ppm (13.8 grains per gallon), range 203-270 ppm for total hardness, Alvarado plant, calendar year 2025. Source: Annual Drinking Water Quality Report (2025 data).
The published figure here is 237 mg/L, which reads as very hard on the USGS scale.
Miramar plant: northern San Diego
The Miramar plant serves about 500,000 customers in the northern section of the city. Source: City of San Diego water and wastewater facilities page.
- Average 242 ppm (14.1 grains per gallon), range 200-285 ppm for total hardness, Miramar plant, calendar year 2025. Source: Annual Drinking Water Quality Report (2025 data).
The published figure here is 242 mg/L, which reads as very hard on the USGS scale.
Otay plant: southern San Diego
The Otay plant serves about 100,000 customers in the southern section of the city. Source: City of San Diego water and wastewater facilities page.
- Average 243 ppm (14.2 grains per gallon), range 201-271 ppm for total hardness, Otay plant, calendar year 2025. Source: Annual Drinking Water Quality Report (2025 data).
The published figure here is 243 mg/L, which reads as very hard on the USGS scale.
Purchased treated water
A small part of the supply is treated water bought from the Metropolitan Water District and the San Diego County Water Authority. The report does not say which neighborhoods get it. Source: City of San Diego 2025 Annual Drinking Water Quality Report.
- Average 145 ppm (8.5 grains per gallon), range 54.4-232 ppm for total hardness, purchased treated water, calendar year 2025. Source: Annual Drinking Water Quality Report (2025 data).
The published figure here is 145 mg/L, which reads as hard on the USGS scale.
Across the 4 San Diego areas above, the figures run 145 to 243 mg/L, which spans USGS classes from hard to very hard. Each is a plant, well or area figure, not a reading at one address. (USGS hardness classes)
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Sources last checked October 27, 2026. San Diego Water Systems is not a water utility.