Nitrate in California well water
In California, 16% of 57,577 tested private well samples exceeded the EPA limit for nitrate (10 mg/L).
Nitrate reaches wells from fertilizer, septic systems and manure. Above the limit it can cause methemoglobinemia (blue baby syndrome) in infants under six months, and long-term exposure is associated with some cancers. Shallow and dug wells near farmland or dense septic systems are most at risk.
Check your nitrate number
Type the value from your lab report. It is compared to the EPA limit, the same way the full interpretation does it.
Test your California well for nitrate
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
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Systems that remove nitrate
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Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.
For nitrate
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For nitrate
Aquasana SmartFlow reverse osmosis
under-sink or countertop, about $225
Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.
Rather have it installed and sized for your well? Book a free in-home water test with a licensed local company.
Where to get a well tested for nitrate in California: certified labs, state programs and prices.
By county, from individual samples
Public monitoring samples against the EPA limit (10 mg/L); counties with at least 30 samples.
| County | Above EPA limit | Samples | Detected | Median | Highest |
|---|---|---|---|---|---|
| Fresno | 2,352 | 91% | 6.6 | 2,030 mg/L | |
| Tulare | 676 | 95% | 6 | 384 mg/L | |
| Santa Clara | 129 | 90% | 5.04 | 42 mg/L | |
| Stanislaus | 997 | 84% | 5 | 193 mg/L | |
| Merced | 1,533 | 92% | 3.61 | 1,200 mg/L | |
| Contra Costa | 200 | 97% | 3.8 | 55 mg/L | |
| Alameda | 7,632 | 95% | 3.8 | 155 mg/L | |
| Madera | 343 | 89% | 3.61 | 81 mg/L | |
| San Joaquin | 776 | 88% | 3.08 | 330 mg/L | |
| Monterey | 239 | 84% | 2.02 | 465 mg/L | |
| San Luis Obispo | 1,102 | 89% | 2.02 | 325 mg/L | |
| Kings | 268 | 72% | 0.567 | 487 mg/L | |
| Glenn | 430 | 95% | 3.18 | 130 mg/L | |
| Yolo | 180 | 98% | 2.54 | 82.5 mg/L | |
| Butte | 302 | 96% | 3.23 | 120 mg/L |
- Jurupa CCD, Riverside County82% of 200
- Firebaugh CCD, Fresno County71% of 404
- Livingston-Delhi CCD, Merced County63% of 229
- Guadalupe CCD, Santa Barbara County60% of 342
- Riverside CCD, Riverside County59% of 44
- Turlock CCD, Stanislaus County58% of 74
- Tulare CCD, Tulare County57% of 89
- Parlier-Del Rey CCD, Fresno County56% of 48
- Manteca CCD, San Joaquin County54% of 71
- Dinuba CCD, Tulare County53% of 40
- Salida CCD, Stanislaus County51% of 53
- Fresno CCD, Fresno County49% of 410
- South Santa Clara Valley CCD, Santa Clara County48% of 31
- Sanger CCD, Fresno County48% of 88
- Patterson CCD, Stanislaus County48% of 130
- Porterville CCD, Tulare County45% of 40
- Escalon CCD, San Joaquin County45% of 40
- Selma CCD, Fresno County45% of 92
- Hughson CCD, Stanislaus County44% of 32
- Mendota CCD, Fresno County42% of 419
Sources
- Individual samples from the USGS Water Quality Portal and state monitoring, summarized per geography. nitrate counts and shares are listed on each county and town page with their sources.
Analysis: Nitrate in private wells, decade by decade, from the same public records.