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Nitrate in Drinking Water, 2007-2017: Oklahoma

Between 2007 and 2017, tests detected elevated levels of nitrate (3 milligrams per liter, or mg/L) in the tap water supplies of 219 towns and cities in Oklahoma, serving approximately 438,000 people. For the other nine states in this analysis, EWG had data between 2003 and 2017, but we only received data for the years between 2007 and 2017 from our Oklahoma public records request.

Contamination is getting worse in 110 of those community water systems1 – 50 percent, serving approximately 235,000 people. Larger towns and cities tend to depend on surface water for drinking, whereas smaller, often rural communities depend on groundwater. Oklahoma relies on groundwater more than many states do. Close to 52,000 residents of 11 towns and cities that rely on surface water faced increased contamination. But 99 communities, serving approximately 183,000 people, that depend on groundwater saw nitrate contamination increase.

Oklahoma Communities With Increases in Nitrate Contamination, 2007 to 2017
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Source: EWG, from Oklahoma Department of Environmental Quality data.

Average nitrate contamination across these communities jumped by 44 percent between 2007 and 2017. In 2007, average contamination was 3.71 mg/L, and climbed to 5.60 in 2017.

Average Nitrate Levels in Oklahoma Communities Where Contamination Rose, 2007 to 2017

Source: EWG, from Oklahoma Department of Environmental Quality data.

Health Hazards of Nitrate

Nitrate is a primary chemical component of fertilizer and manure that can run off farm fields and seep into drinking water supplies. Under the federal Safe Drinking Water Act, the legal limit for nitrate, measured as nitrogen, in drinking water is 10 mg/L. This limit was set in 1962 to guard against so-called blue baby syndrome, a potentially fatal condition that starves infants of oxygen if they ingest too much nitrate.

But more recent studies show strong evidence of an increased risk of colorectal cancer, thyroid disease and neural tube birth defects at levels at 5 mg/L or even lower. During the period analyzed, drinking water got worse in 71 Oklahoma communities, serving approximately 162,000 people, that already had tested at or above 5 mg/L at least once.

Drinking water for approximately 36,835 of those people in 28 communities had already tested at or above the legal limit at least once and had increasing nitrate levels during the period analyzed. Of the 69 communities where nitrate exceeded the Environmental Protection Agency’s legal limit at least once, 24 systems, serving approximately 27,000 people, tested at or above the legal limit in 2017.

People who want to know the level of nitrate in their community’s water system can visit the Oklahoma page of EWG’s Tap Water Database.

Who Is Affected?

Contamination was more likely to get worse in smaller communities between 2007 and 2017. Ninety-one percent of systems with worsening nitrate contamination served 3,300 people or fewer. For our analysis, systems were put into the EPA-designated size categories based on how many water customers they serve.

Number and Percent of Systems With Increasing Nitrate Levels, by System Size, 2007 to 2017

System size System count Percent of systems
Very small (<501) 39 35%
Small (501-3,300) 61 55%
Medium (3,301-10,000) 4 4%
Large (10,001-100,000) 6 5%
Very large (>100,000) 0 0%

Source: EWG, from Oklahoma Department of Environmental Quality data.

Nitrate contamination was also more likely to get worse for people living in rural areas. In Oklahoma, 79 percent of communities with growing nitrate levels were rural, whereas 21 percent were urban, as defined by the U.S. Census Bureau.


1 Community water systems are public water supplies that serve residents in cities and towns year-round.

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