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Nitrate in Drinking Water, 2003-2017: Iowa

Between 2003 and 2017, tests detected elevated levels of nitrate (3 milligrams per liter, or mg/L) in the tap water supplies of 236 towns and cities in Iowa, serving 1.3 million people.

Contamination is getting worse in 126 of those community water systems1 – 53 percent, serving approximately 551,000 people. Larger towns and cities tend to depend on surface water for drinking, whereas smaller, often rural communities depend on groundwater. Iowa, however, depends on groundwater more than most states. Just over 155,000 residents of nine mostly larger towns and cities that rely on surface water faced increased contamination. But 117 mostly small, rural communities, serving approximately 396,000 people, that depend on groundwater also saw nitrate contamination increase.

Iowa Communities With Increases in Nitrate Contamination, 2003 to 2017
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Source: EWG, from Iowa Department of Natural Resources data.

Average nitrate contamination across these communities jumped by 29 percent between 2003 and 2017. In 2003, average contamination was 3.66 mg/L. By 2009, average contamination had increased to 3.90 mg/L and continued climbing to 4.71 mg/L in 2017.

Average Nitrate Levels in Iowa Communities Where Contamination Rose, 2003 to 2017

Source: EWG, from Iowa Department of Natural Resources 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 was getting worse in 84 Iowa communities, serving approximately 521,000 people, that had already tested at or above 5 mg/L at least once.

Drinking water for approximately 58,000 of those people in 17 communities had already tested at or above the Environmental Protection Agency’s legal limit at least once and had increasing nitrate levels during the period analyzed. Of the 41 communities where nitrate exceeded the federal legal limit at least once, one system, serving 160 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 Iowa page of EWG’s Tap Water Database.

Who Is Affected?

Contamination was more likely to get worse in smaller communities between 2003 and 2017. Eighty-two 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, 2003 to 2017

System size System count Percent of systems
Very small (<501) 51 40%
Small (501-3,300) 53 42%
Medium (3,301-10,000) 11 9%
Large (10,001-100,000) 10 8%
Very large (>100,000) 1 1%

Source: EWG, from Iowa Department of Natural Resources data

Nitrate contamination was also more likely to get worse for people living in rural areas. In Iowa, 73 percent of communities with growing nitrate levels were rural, whereas 27 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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