Los Angeles County Nitrate in Drinking Water: What 2025 Data Shows by Water District

Los Angeles County draws its tap water from more than 90 public water systems that blend local groundwater with imported supplies from Metropolitan Water District. Nitrate is monitored in every public system under California and federal law, and the San Gabriel Valley groundwater basin carries a documented history of elevated nitrate from agricultural and industrial activity. The federal Maximum Contaminant Level for nitrate is 10 milligrams per liter as nitrogen, set by the EPA to protect against acute health risks in infants under six months. California sets the same standard expressed as 45 milligrams per liter as nitrate. Most large LA County utilities serving blended surface and groundwater report levels well below this limit. San Gabriel Valley communities drawing more heavily on local groundwater have historically seen concentrations closer to the threshold. This article compiles what recent Consumer Confidence Reports and state monitoring data show by district and explains what homeowners should do if nitrate is a concern.

Get a free water test in Los Angeles if you want to know your actual nitrate level before making any treatment decision.

Where does LA County drinking water come from?

Los Angeles County is served by more than 90 separate public water systems, ranging from the Los Angeles Department of Water and Power, which serves about four million people, to small systems serving fewer than a few hundred connections in unincorporated areas.

Most large utilities in the county blend two types of source water. Imported surface water arrives through Metropolitan Water District of Southern California from the Colorado River Aqueduct and the State Water Project, which draws from the Sacramento-San Joaquin Delta. Local groundwater is pumped from several distinct basins, the largest of which are the Main San Gabriel Basin, the Central Basin, and the West Coast Basin.

The source blend matters for nitrate because MWD imported surface water typically carries lower nitrate concentrations than groundwater from basins with a history of agricultural or industrial land use. Utilities that rely more heavily on local groundwater, particularly in the San Gabriel Valley, tend to see higher average nitrate in their finished water.

Source data: California State Water Resources Control Board Division of Drinking Water utility list, searchable at drinc.ca.gov.

How this nitrate data was compiled

This article draws from three public sources: Consumer Confidence Reports filed by individual water utilities and searchable through the California Drinking Water Information Clearinghouse at drinc.ca.gov; the SWRCB's Groundwater Ambient Monitoring and Assessment (GAMA) program database at waterboards.ca.gov/gama; and the EPA's National Primary Drinking Water Regulations for nitrate at epa.gov.

CCR data represents what was distributed to customers at the tap after treatment. GAMA data represents ambient groundwater monitoring and may reflect source conditions before treatment. Both are relevant for understanding where nitrate risk exists in the county.

Nitrate levels in groundwater change over time as land use evolves and well blending ratios shift. Verify current data for your specific utility directly through drinc.ca.gov before making any health or treatment decisions.

Nitrate levels by water district

The table below summarizes nitrate data for major LA County service areas. Levels are expressed as mg/L as N (the federal standard). For utilities where a specific recent CCR figure is not available in this review, the table directs readers to the utility's own report.

The San Gabriel Basin entries reflect monitoring data from the SWRCB GAMA program, which documents ambient groundwater quality and has recorded nitrate concentrations in basin wells ranging from near zero to levels approaching the MCL, depending on the specific well location and depth. Retail utilities drawing from the basin blend well water with MWD imports and treat before distribution, so finished tap water concentrations are typically lower than raw groundwater readings.

What do these nitrate levels mean for homeowners?

For most LA County households on large utilities that blend MWD imported water, nitrate is unlikely to be a primary water quality concern. MWD surface water is treated and monitored to federal standards before delivery to member agencies, and the blending dilutes any localized groundwater contributions.

The picture is different for households in the San Gabriel Valley served by utilities that draw a larger share from local basin wells. The SWRCB GAMA program has documented areas of the Main San Gabriel Basin where ambient groundwater nitrate is elevated due to legacy agricultural fertilizer use and industrial contamination, some of which dates to the mid-20th century when the basin had heavy manufacturing activity.

Health risk from nitrate is most acute for infants under six months. Elevated nitrate in drinking water can cause methemoglobinemia, sometimes called blue baby syndrome, by reducing the blood's ability to carry oxygen. The EPA set the 10 mg/L MCL specifically to address this risk, as noted in the EPA's primary drinking water regulations. Adults and older children face lower acute risk at concentrations near the MCL, though some research examines long-term exposure effects at lower levels. Consult your physician with specific health questions.

Which water treatment options address nitrates?

Not all home water treatment technologies remove nitrate. Standard pitcher filters, refrigerator filters, and most countertop carbon filters do not address nitrate. A water softener removes calcium and magnesium for hardness but does not remove nitrate.

Two treatment approaches are certified for nitrate reduction:

Reverse osmosis. Point-of-use RO systems certified to NSF/ANSI Standard 58 remove 85 to 95 percent of nitrate from water at the treated tap. This is the most common home treatment approach for nitrate because it also addresses a broad range of other contaminants including PFAS, arsenic, and lead. An under-sink RO system handles drinking and cooking water. Whole house RO is available but typically not necessary solely for nitrate at levels near the MCL.

Anion exchange. Dedicated nitrate-selective ion exchange systems use a specialized resin to remove nitrate from water. These systems can be used at point of use or as a dedicated whole house treatment. They require periodic regeneration and disposal of concentrated brine, which should be done in compliance with local wastewater regulations.

Before purchasing any treatment system, test your tap water to confirm current nitrate levels and identify any co-occurring contaminants that should influence system selection.

When to get a free water test

A free water test is the starting point for any treatment decision. Consumer Confidence Reports give you a utility-wide average, but your individual tap result can vary based on your location in the distribution system, the age of your plumbing, and seasonal shifts in the source blend your utility uses.

A water test is particularly worthwhile if you live in the San Gabriel Valley or another part of LA County where local groundwater contributions to your utility's supply are higher, if you have an infant under six months in your household, if your CCR shows nitrate above 5 mg/L, or if you use a private well rather than a public water system.

Request a free water test from Purest Water Solutions for an initial assessment at your Los Angeles County tap. Our team serves communities throughout LA County and can review the monitoring data for your specific service area.