California Cities With Riskiest Drinking Water

By Sam Stebbins  •  May 4, 2026
a body of water with ducks swimming in it
Photo by Linda Pomerantz Zhang on Unsplash

Water – whether too much of it or too little – is a perennial problem in California. According to the U.S. Drought Monitor, over 90% of the state was in a severe drought or worse for most of 2021 and 2022. Additionally, moderate or worse drought conditions impacted over one-third of California’s land area for much of 2025. That same year, December brought record-breaking rainfall to parts of Los Angeles, prompting flooding, multi-county emergency declarations, and mass evacuations. 

While threats posed by natural disasters such as floods and drought are often well publicized, water in California also presents a lesser-known, invisible threat – nitrate contamination. 

Nitrate is an inorganic pollutant contained in fertilizer, and in many parts of the country, agricultural runoff can contaminate ground and surface water. A recent report from the the Environmental Working Group found that more than 22.7 million people in California, by far the most of any state, have likely been exposed to tap water containing at least 3 milligrams of nitrate contaminants per liter – a level of concentration that can increase the risk of certain types of cancer, including thyroid and colorectal. According to EWG, as many as 10.7 cancer cases for every 100,000 people in the state could be attributed to such exposure. 

Notably, the Environmental Protection Agency only limits nitrate concentrations in public water supplies to 10 milligrams or less per liter. Even though these federal standards are potentially inadequate in protecting public health, there are many parts of California where tested tap water still contains at least 10 milligrams of nitrate per liter. At these levels, nitrate contamination has been linked to dangerously low blood-oxygen levels in infants. 

Using data from EWG’s Tap Water Database, Climate Crisis 247 identified the places in California with the most dangerous drinking water. We reviewed test results in each of the 40 cities and towns in California that have had at least one water test that revealed concentrations of at least 10 milligrams of nitrate contaminants per liter. Places are ranked on the share of all reported test results that revealed nitrate concentrations exceeding this threshold. Supplemental data on water tests yielding concentrations of at least 5 milligrams of nitrate contaminants per liter are also from the EWG. Supplemental data on agriculture as a share of local GDP is from the Bureau of Economic Analysis. 

10. City of Parlier; Fresno County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 13.0% (3 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 43.5% (10 total)
  • Population served: 15,250 people in and around Parlier area, California
  • Annual agricultural economic output in Fresno County: $5.8 billion (9.1% of local GDP)

9. Casitas Municipal Water District; Ventura County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 13.3% (25 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 19.7% (37 total)
  • Population served: 10,464 people in and around Oak View area, California
  • Annual agricultural economic output in Ventura County: $2.1 billion (3.0% of local GDP)

8. Bakman Water Company; Fresno County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 13.8% (12 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 62.1% (54 total)
  • Population served: 16,756 people in Fresno County, California
  • Annual agricultural economic output in Fresno County: $5.8 billion (9.1% of local GDP)

7. City of Ripon; San Joaquin County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 18.5% (10 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 72.2% (39 total)
  • Population served: 15,847 people in and around Ripon area, California
  • Annual agricultural economic output in San Joaquin County: $2.1 billion (4.5% of local GDP)
aerial photo of city
Photo by Josh Miller on Unsplash

6. California State Polytechnical University; Los Angeles County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 20.0% (159 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 51.9% (412 total)
  • Population served: 30,746 people in and around California State Polytechnic University area, California
  • Annual agricultural economic output in Los Angeles County: $347.9 million (0.03% of local GDP)

5. City of Corona; Riverside County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 20.7% (576 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 59.2% (1648 total)
  • Population served: 172,836 people in and around Corona area, California
  • Annual agricultural economic output in Riverside County: $1.2 billion (0.9% of local GDP)

4. Valley Water Company; Los Angeles County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 32.9% (56 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 82.9% (141 total)
  • Population served: 10,070 people in and around La Canada Flintridge area, California
  • Annual agricultural economic output in Los Angeles County: $347.9 million (0.03% of local GDP)

3. Morro Bay Public Works Department; San Luis Obispo County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 33.6% (197 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 46.6% (273 total)
  • Population served: 10,234 people in and around Morro Bay area, California
  • Annual agricultural economic output in San Luis Obispo County: $719.7 million (2.9% of local GDP)

2. San Francisco City Water System; San Francisco County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 37.8% (14 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 78.4% (29 total)
  • Population served: 884,363 people in and around San Francisco area, California
  • Annual agricultural economic output in San Francisco County: $13.7 million (0.01% of local GDP)

1. La Verne Water Department; Los Angeles County, California

  • Tests with at least 10mg of nitrate contaminants per liter: 84.6% (225 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 99.6% (265 total)
  • Population served: 33,200 people in and around La Verne area, California
  • Annual agricultural economic output in Los Angeles County: $347.9 million (0.03% of local GDP)


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