Back to National Report

Nitrate in Drinking Water, 2003-2017: Maryland

Between 2003 and 2017, tests detected elevated levels of nitrate (3 milligrams per liter, or mg/L) in the tap water supplies of 143 towns and cities in Maryland, serving over 2.1 million people.

Contamination is getting worse in 77 of those community water systems1 – 54 percent, serving approximately 1.9 million people. Larger towns and cities tend to depend on surface water for drinking, whereas smaller, often rural, communities depend on groundwater. A large portion of Maryland’s population relies on surface water for drinking water. During the period studied, nearly 1.8 million residents of nine larger towns and cities that rely on surface water faced increased contamination. Sixty-eight mostly small, rural communities, serving approximately 89,000 people, that depend on groundwater also saw nitrate contamination increase.

Maryland Communities With Increases in Nitrate Contamination, 2003 to 2017
Explore the Map

Source: EWG, from Maryland Department of the Environment data.

Average nitrate contamination across these communities jumped by 26 percent between 2003 and 2017. In 2003, average contamination was 3.65 mg/L. By 2009, average contamination had increased to 4.56 mg/L and continued increasing to 4.60 mg/L in 2017.

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

Source: EWG, from Maryland Department of the Environment 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 studied, drinking water was getting worse in 47 Maryland communities, serving approximately 140,000 people, that already had tested at or above 5 mg/L at least once.

Drinking water for approximately 15,000 of those people, in 14 communities, had already tested at or above the legal limit at least once and had increasing nitrate levels during the period studied. Of the 24 communities where nitrate exceeded the Environmental Protection Agency’s legal limit at least once, four systems, serving approximately 400 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 Maryland page of EWG’s Tap Water Database.

Who Is Affected?

Contamination was more likely to get worse in smaller communities between 2003 and 2017. Seventy-nine percent of systems where contamination increased 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) 50 65%
Small (501-3,300) 11 14%
Medium (3,301-10,000) 9 12%
Large (10,001-100,000) 6 8%
Very large (>100,000) 1 1%

Source: EWG, from Maryland Department of the Environment data.

Maryland was one of three states where nitrate contamination was more likely to get worse in urban than in rural communities. In Maryland, only 34 percent of communities with growing nitrate levels were rural, whereas 66 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.

Share this page

Share Share

Share Share on Social Media

Search by Zip Code

Please type below a 5-digit Zip Code, then click on "SEARCH"