Nitrate in California well and tap water

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What nitrate is and where it comes from

Nitrate is a form of nitrogen that dissolves easily in water and moves down through soil into groundwater. According to the State Water Board's nitrate and nitrite page, nitrate in groundwater is generally associated with septic systems, confined animal feeding operations such as dairies, and fertilizer use. It is also present in treated wastewater, so it can show up in surface water and in groundwater recharge projects that use recycled water.

Because those sources are tied to land use, nitrate is mostly a groundwater issue. A utility that relies on imported surface water usually reports very little; a utility or a private well drawing from groundwater under farmland or older septic areas can report much more. That difference is why a ZIP-level check, not a statewide average, is the right starting point.

California's nitrate limits

California sets its maximum contaminant levels (MCLs) for nitrate in Title 22 of the California Code of Regulations, section 64431. Per the State Water Board:

Watch the units. California's earlier nitrate standard was written as nitrate (NO3), and some older reports and lab sheets still use it. 10 mg/L as N is the same amount of nitrate as roughly 45 mg/L as NO3, so a well result of "30 mg/L nitrate as NO3" is under the limit, while "30 mg/L nitrate as N" is three times over it. Always check which form the lab or utility reported.

Unlike many contaminants, nitrate has no gap between the legal limit and California's health goal. The Office of Environmental Health Hazard Assessment set its Public Health Goals (PHGs) for nitrate and nitrite at the same values as the MCLs, because both are based on the same infant health effect (OEHHA nitrate and nitrite).

Why nitrate matters most for infants

The State Water Board explains that nitrite can interfere with the ability of red blood cells to carry oxygen, a condition called methemoglobinemia, often called "blue baby syndrome." Infants are most at risk because their stomachs convert nitrate to nitrite more readily. Utility reports carry the state's required health language: nitrate above 10 mg/L is a health risk for infants under six months, symptoms include shortness of breath and blueness of the skin, and high levels may also affect pregnant women and people with certain enzyme deficiencies.

The practical rule is simple: do not use water above the nitrate MCL to mix infant formula, and do not try to fix it by boiling. Boiling does not remove nitrate; it concentrates it as water evaporates.

Where nitrate shows up in California

Nitrate ranks high among the regulated contaminants detected above their MCLs in California. The State Water Board's historical review of 2002 to 2005 monitoring found nitrate (then measured as NO3) above its MCL at least once in 731 drinking water sources, with the most in Los Angeles (123 sources), San Bernardino (82), Riverside (67), Kern (41) and Monterey (30) counties. The board's GAMA program keeps a newer groundwater information sheet on nitrate with statewide maps, and the board ran pilot nitrate projects in the Tulare Lake Basin and Salinas Valley.

Public systems handle high-nitrate wells by treating them, blending them with cleaner water, or taking them out of service. The 2025 reports of three large Southern California utilities show how much results vary:

• Riverside Public Utilities, which ran entirely on groundwater in 2025, reported nitrate averaging 5.4 mg/L as N (range 4.7 to 6.5) and says it blends 45 wells to meet standards (RPU water quality).
• LADWP reported nitrate below 0.4 mg/L at its aqueduct filtration plant and up to 1.4 mg/L in its combined well supplies (LADWP 2025 report).
• City of San Diego, mostly on imported surface water, reported nitrate as not detected to 0.4 mg/L at its plants (San Diego 2025 report).

All three met the MCL. A result like Riverside's is legal, but it is the kind of number that federal Consumer Confidence Report rules flag with extra educational language once nitrate passes half the MCL, and it is worth knowing about if you have an infant at home.

Private wells: you are the water system

Private domestic wells are not regulated public water systems under the California Health and Safety Code, so nobody tests them for you; the state says the MCLs can be used as guidance. If your home is on a well, especially near farmland, dairies or septic systems:

1. Test at least for nitrate and bacteria through a lab accredited by the state's Environmental Laboratory Accreditation Program (ELAP).
2. Retest after changes: heavy rain, flooding, a new septic system nearby, or a change in taste or color.
3. Check for free programs. The Central Valley Water Board's Nitrate Control Program has sampled over 9,000 wells since 2021 and provided free replacement water to more than 1,800 households with nitrate-impacted wells, and it expanded to more basins in 2025 (State Water Board announcement). Partners such as the Valley Water Collaborative offer free well tests and deliver water if nitrate exceeds 10 mg/L. The state lists other well-assistance providers by county on its domestic wells programs page.
4. Read the GAMA guidance on testing domestic wells.

How to read a nitrate result

• Confirm the unit: mg/L as N (limit 10) or mg/L as NO3 (equivalent limit about 45).
• Look at the range, not only the average. Nitrate in groundwater can change with season, rainfall and pumping. Riverside notes that demand, flow and maintenance affect its nitrate levels during the year.
• Check nitrite too, which has a much lower limit (1 mg/L as N).
• Compare with the PHG. For nitrate it equals the MCL, so a result under 10 mg/L as N meets both.

Treatment that actually reduces nitrate

Nitrate is a small, dissolved ion, so ordinary carbon filters and pitcher filters sold for taste do not reliably remove it, and a water softener is not a nitrate treatment. Options that work, when certified for nitrate:

• Reverse osmosis (RO) under-sink systems certified under NSF/ANSI 58 for nitrate/nitrite reduction, for drinking and cooking water.
• Anion exchange systems designed and certified for nitrate.
• Distillation units certified for nitrate.

California's Residential Water Treatment Devices program requires devices sold with health claims to be independently certified, and says nitrate devices are generally acceptable for water up to 24 mg/L as N (108 mg/L as NO3), need at least 40 psi of water pressure, and should be checked with periodic lab tests. Every registered device has a Performance Data Sheet listing exactly what it is certified to reduce. You can search certified products in the NSF listings.

If nitrate is present, also check your report for arsenic, chromium 6 and PFAS; a certified RO system can address several at one tap, while a softener for hard water addresses none of them.

Related guides

• Hard water in California
• PFAS in California drinking water
• Chromium 6 in California water
• Arsenic in California drinking water
• Chlorine taste and smell in tap water
• How to check tap water by ZIP
• City water guides
• Regional water guides

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