U.S. Cities With the Most contaminated Drinking Water

By Sam Stebbins  •  April 30, 2026
person holding clear glass cup with half-filled water
Photo by Manki Kim on Unsplash

Responsibility for the regulation of clean drinking water in the United States falls to the Environmental Protection Agency – and according to the EPA, the U.S. has some of the most reliably safe drinking water supplies in the world. But a recent report published by the Environmental Working Group shows that water purity varies widely across the country, and in multiple American cities, water from the tap may contain hidden health hazards. 

Nitrate, an inorganic contaminant contained in fertilizer, is one of the most harmful pollutants threatening public water supplies. In many parts of the country, agricultural runoff can flow into ground and surface water, contaminating these sources with nitrate. At high enough levels, nitrate contaminants can cause dangerously low blood-oxygen levels in infants, and increase the risk of certain types of cancer, including thyroid and colorectal.

To mitigate these dangers, EPA standards limit nitrate concentrations in public water sources to less than  10 milligrams per liter. Notably, however, multiple studies have shown that even at concentrations as low as 2 milligrams per liter, nitrate can pose a grave threat to public health. While these studies suggest that the decades-old federal guidelines may be insufficient, there are also multiple cities across the country where residents are regularly exposed to nitrate concentrations that exceed even the EPA’s own guidelines. 

Using data from EWG’s Tap Water Database, Climate Crisis 247 identified the cities with the most dangerous drinking water. We reviewed test results in the more than 50 U.S. cities and towns that have had at least one water test that revealed concentrations of at least 10 milligrams of nitrate contaminants per liter. Cities 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 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, California
  • Annual agricultural economic output in San Joaquin County: $2.1 billion (4.5% of local GDP)

9. Atascosa Rural Water Supply Corporation; Bexar County, Texas

  • Tests with at least 10mg of nitrate contaminants per liter: 18.8% (3 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 56.3% (9 total)
  • Population served: 11,007 people in and around Atascosa, Texas
  • Annual agricultural economic output in Bexar County: $22.2 million (0.01% of local GDP)
green palm trees near city buildings during daytime
Photo by Venti Views on Unsplash

8. 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)

7. 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, California
  • Annual agricultural economic output in Riverside County: $1.2 billion (0.9% of local GDP)

6. Saint Peter; Nicollet County, Minnesota

  • Tests with at least 10mg of nitrate contaminants per liter: 22.2% (8 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 33.3% (12 total)
  • Population served: 11,784 people in and around St. Peter, Minnesota
  • Annual agricultural economic output in Nicollet County: $124.6 million (6.3% of local GDP)

5. Espanola Water System; Rio Arriba County, New Mexico

  • Tests with at least 10mg of nitrate contaminants per liter: 24.0% (6 total)
  • Tests with at least 5mg of nitrate contaminants per liter: 64.0% (16 total)
  • Population served: 12,012 people in and around Espanola, New Mexico
  • Annual agricultural economic output in Rio Arriba County: $31.1 million (2.3% 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, 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, 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, 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, California
  • Annual agricultural economic output in Los Angeles County: $347.9 million (0.03% of local GDP)


Similar Posts