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California water profile

Nitrate in California community water systems

California's nitrate story is overwhelmingly the Central Valley, where decades of intensive irrigated agriculture have made nitrate the most common pollutant in the aquifer. Higher public-system results also appear in parts of the arid south. See where the pattern concentrates and what each reporting system measured in the three years from August 9, 2023 to August 9, 2026.

Browse all 2,405 California systems →

Search by the utility or system name shown on a water bill. Example: East Valley Water District. Private wells are not in this inventory.

85%
of the state’s 2,816 active community water systems have a current result here
2,405
systems with a current nitrate result
42,591,451
people served by those reporting systems
14.1%
of reporting systems at or above 5 mg/L
How to read these numbers Each system is placed into a band by its most recent reported result within a rolling three-year window. The bands describe how strongly human activity, read from nitrate, has marked a water source; they are not a safety verdict. Figures describe system reporting, not nitrate measured at an individual tap.

How California's systems fall across the five bands

Most systems report below 3 mg/L

Natural background
0.00–0.99 mg/L as N
1,239 systems · 17,059,122 served
Human impact likely
1.00–2.99 mg/L as N
538 systems · 14,084,740 served
Human impact clear
3.00–4.99 mg/L as N
288 systems · 4,821,498 served
Health-study range
5.00–9.99 mg/L as N
297 systems · 5,601,467 served
Above the federal limit
10.00 mg/L and above
43 systems · 1,024,624 served

Each system appears in one band only, placed by its most recent reported result in the current three-year window. Counts sit beside people served so a small number of larger systems cannot hide behind a row of tiny ones. Of the 340 systems reporting 5 mg/L or more, 89 stayed at or above 5 across the whole three-year range; the other 251 reached it only on a high sample and otherwise ran lower.

Most recent reading Jun 2026; the typical system was last tested around Feb 2026. 2,249 systems sampled their own water; 156 systems carry a supplier’s result.

The named systems

Every reading here belongs to a utility with a name

Highest recent readings
SystemMost recent resultBandBased onPeople served
Psmcsd - Springfield Ws56.20 mg/LMay 2026Above the federal limit12 samples200
River Rd Ws #2538.00 mg/LFeb 2026Above the federal limit10 samples65
Konocti Shores36.00 mg/LMay 2026a high point — runs lowerAbove the federal limit40 samples277
Lake Morena Views Mwc34.00 mg/LMay 2026a high point — runs lowerAbove the federal limit107 samples360
Ray Water Company27.00 mg/LMay 2026Above the federal limit32 samples40
Son Shine Properties25.00 mg/LApr 2026a high point — runs lowerAbove the federal limit12 samples438
Bonfire Village Hotel24.00 mg/LApr 2026a high point — runs lowerAbove the federal limit14 samples44
Woodland Park Mutual Water Co22.00 mg/LMay 2026a high point — runs lowerAbove the federal limit46 samples538
Largest systems
SystemMost recent resultBandBased onPeople served
Los Angeles-City, Dept. Of Water & Power1.29 mg/LApr 2026Human impact likely1740 samples7,751,132
East Bay Mud0.20 mg/LMay 2026Natural background471 samples1,442,800
San Diego, City Of0.20 mg/LMay 2026Natural background143 samples1,385,379
San Jose Water0.20 mg/LMar 2026Natural background6 samples1,039,920
Sfpuc City Distribution Division11.40 mg/LJul 2025a high point — runs lowerAbove the federal limit18 samples848,019

Each result shows its date and what it rests on, so a lone sample is never taken for a settled measurement. A single high sample is not a complete account of what a household drinks.

Where higher nitrate concentrates

Higher results gather in the Central Valley and the arid south

Human impact is clear on drinking water served in 46 of California’s 58 counties. Each is shaded by how many of its systems reported 3.00 mg/L as N or above. Shading counts systems, not severity — a county with many small utilities can outrank one holding a single larger system that reads higher. The table beside the map splits the two bands so that difference stays visible.

CA counties shaded by number of community water systems reporting nitrate at or above 3.00 mg/L as NCounties are shaded by how many community water systems reported nitrate at or above 3.00 mg/L as N in the three years from August 9, 2023 to August 9, 2026. The same counts appear in the table below this map.San Benito County — 1 system at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveFresno County — 5 systems at 3.00–4.99 mg/L as N; 15 systems at 5.00 mg/L as N or aboveEl Dorado County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveMonterey County — 20 systems at 3.00–4.99 mg/L as N; 25 systems at 5.00 mg/L as N or aboveSan Bernardino County — 22 systems at 3.00–4.99 mg/L as N; 23 systems at 5.00 mg/L as N or aboveTehama County — 2 systems at 3.00–4.99 mg/L as N; 10 systems at 5.00 mg/L as N or aboveTuolumne County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSanta Barbara County — 6 systems at 3.00–4.99 mg/L as N; 10 systems at 5.00 mg/L as N or aboveStanislaus County — 8 systems at 3.00–4.99 mg/L as N; 24 systems at 5.00 mg/L as N or aboveColusa County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSiskiyou County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveDel Norte County — 3 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveInyo County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSan Joaquin County — 13 systems at 3.00–4.99 mg/L as N; 23 systems at 5.00 mg/L as N or aboveCalaveras County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveButte County — 2 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveMadera County — 11 systems at 3.00–4.99 mg/L as N; 5 systems at 5.00 mg/L as N or aboveVentura County — 8 systems at 3.00–4.99 mg/L as N; 5 systems at 5.00 mg/L as N or aboveOrange County — 4 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveKern County — 27 systems at 3.00–4.99 mg/L as N; 31 systems at 5.00 mg/L as N or aboveContra Costa County — 0 systems at 3.00–4.99 mg/L as N; 4 systems at 5.00 mg/L as N or aboveMendocino County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSonoma County — 8 systems at 3.00–4.99 mg/L as N; 6 systems at 5.00 mg/L as N or aboveYuba County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveMerced County — 4 systems at 3.00–4.99 mg/L as N; 9 systems at 5.00 mg/L as N or aboveTulare County — 11 systems at 3.00–4.99 mg/L as N; 19 systems at 5.00 mg/L as N or aboveSacramento County — 7 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSolano County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSan Francisco County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveMariposa County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSutter County — 1 system at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveRiverside County — 14 systems at 3.00–4.99 mg/L as N; 18 systems at 5.00 mg/L as N or aboveSanta Clara County — 13 systems at 3.00–4.99 mg/L as N; 19 systems at 5.00 mg/L as N or abovePlacer County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveSan Luis Obispo County — 13 systems at 3.00–4.99 mg/L as N; 10 systems at 5.00 mg/L as N or aboveSan Mateo County — 2 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveSan Diego County — 8 systems at 3.00–4.99 mg/L as N; 8 systems at 5.00 mg/L as N or aboveNapa County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveAlameda County — 4 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveYolo County — 4 systems at 3.00–4.99 mg/L as N; 4 systems at 5.00 mg/L as N or aboveLake County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveLos Angeles County — 24 systems at 3.00–4.99 mg/L as N; 32 systems at 5.00 mg/L as N or aboveGlenn County — 6 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveSanta Cruz County — 3 systems at 3.00–4.99 mg/L as N; 4 systems at 5.00 mg/L as N or aboveHumboldt County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveModoc County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or above
none1–23–56–1112–2526+Number of drinking water systems reporting ≥3.00 mg/L NO₃–N per county as of August 9, 2026

Los Angeles leads this layer with 32 reporting systems, followed by Kern, Monterey, Stanislaus. Higher results gather in the Central Valley and the arid south. Coverage note: the map places 94% of California’s reporting systems. All 140 of the rest buy their water and carry their supplier’s result, which was measured at the supplier’s source — shading their own county would put the reading in the wrong place. They are counted in the figures above.

Counties with the most systems at or above 3 mg/L as N
County3–4.99≥5Total ≥3
Kern273158
Los Angeles243256
Monterey202545
San Bernardino222345
San Joaquin132336

Counts use each system's most recent reported result in the current three-year window. A system appears in only one of the two bands. Coverage note: the map places 94% of California’s reporting systems. All 140 of the rest buy their water and carry their supplier’s result, which was measured at the supplier’s source — shading their own county would put the reading in the wrong place. They are counted in the figures above.

Find a specific California system

What the statewide pattern says

A Central Valley pattern

California's higher nitrate is concentrated in the San Joaquin Valley and the Tulare Lake Basin, where long-term nitrogen loading, shallow water tables, and decades-long travel times coincide — and where the burden falls hardest on small, rural communities.

The San Joaquin Valley and the Tulare Lake Basin to its south are the core. In sampled domestic wells there, on the order of 42 to 46 percent have exceeded the federal limit — far above the gentler Sacramento Valley to the north (Lockhart, Ransom and others, 2021). Nitrate decreases with groundwater age and well depth: shallow, young, oxic water runs high; older, deeper water runs lower.

The sources are distinctly Californian. Synthetic fertilizer supplies most of the cropland nitrogen and dairy manure much of the rest; the valley holds the great majority of the state's roughly 1.8 million dairy cows, which is why manure is a defining pathway here rather than the corn-and-tile story of the Midwest. Manure dominates near the dairy regions; synthetic fertilizer dominates in the citrus and orchard districts; septic matters on urban fringes (Nolan and Hitt, 2006).

The supported source reading is agricultural — fertilizer- and manure-heavy, varying by district — not a certified source at any individual system. Saturated-zone denitrification removes nitrate in localized anoxic zones, including under some dairies, but it is not reliable everywhere (Green and others, 2016).

California manages groundwater through the 2014 Sustainable Groundwater Management Act, but the lag is measured in decades — even strong changes at the surface take years to reach the wells (Dubrovsky and others, 2010). And the cost is uneven: small, rural, largely Latino communities in the valley are the most affected, many on domestic wells or small systems.

The general science — how nitrate moves, why some ground is vulnerable, and why the legacy lingers — is laid out with its sources on Where nitrate comes from.

Questions this page answers

What the California pattern does—and does not—say

Where does nitrate concentrate in California community water systems?

Los Angeles leads this layer with 32 reporting systems, followed by Kern, Monterey, Stanislaus. Higher results gather in the Central Valley and the arid south.

Why do deeper Central Valley wells tend to run lower?

Nitrate decreases with groundwater age and well depth: shallow, recently recharged, oxygen-rich water carries the highest nitrate, while older, deeper water has often been denitrified or predates heavy loading. Wells fed by river-water recharge also run lower through dilution.

Can this page say the source is agriculture?

At the zone level, yes — carefully: synthetic fertilizer and dairy manure dominate, in proportions that vary by district, with septic on urban fringes. The evidence does not certify a source at each individual system.

Does a high public-system result mean a nearby private well is high?

No. It is a local sentinel, not a well-specific result. It shows that nitrogen and a transport pathway have reached groundwater in the area; a certified-lab test is still the only way to know a specific private well.

Sources and provenance

What this page rests on

  1. NitrateSmart inventory: 2,405 California community water systems with a current nitrate result in the three years from August 9, 2023 to August 9, 2026. Figures count people served by reporting systems, not measured tap exposure.
  2. Nolan BT, Hitt KJ (2006). Vulnerability of Shallow Groundwater and Drinking-Water Wells to Nitrate in the United States. Environmental Science & Technology 40(24):7834–7840.
  3. Dubrovsky NM, and others (2010). Nutrients in the Nation's Streams and Groundwater, 1992–2004. USGS Circular 1350.
  4. Lockhart KM, Ransom KM, and others (2021). Effect of groundwater age and recharge source on nitrate concentrations in domestic wells in the San Joaquin Valley. Environmental Science & Technology 55(4):2265–2275.
  5. Ransom KM, and others (2017). A hybrid machine learning model to predict and visualize nitrate concentration throughout the Central Valley aquifer, California. Science of the Total Environment 601–602:1160–1172.
  6. Green CT, and others (2016). Regional oxygen reduction and denitrification rates in groundwater from multi-model residence time distributions, San Joaquin Valley. Journal of Hydrology 543:155–166.