Nitrates are one of the most common drinking water contaminants in the United States, and they come from agriculture. Fertilizer applied to fields, manure from livestock operations, and failing septic systems all contribute nitrogen that soil bacteria convert to nitrates—and those nitrates move with water, draining into rivers, reservoirs, and the groundwater that feeds public water systems.
For the full picture on what's in tap water and how the regulatory framework addresses it, see the complete tap water guide for businesses. This piece covers nitrates specifically: where they come from, what the EPA's 10 mg/L limit actually means, which markets are most affected, and what point-of-use purification removes that standard carbon filters do not.
Where Nitrates Come From
Nitrogen is the primary ingredient in agricultural fertilizer—and when it's applied in quantities that exceed what crops can absorb, the excess moves through soil as nitrate. Unlike many contaminants, nitrate doesn't bind tightly to soil particles. It dissolves readily in water and follows it—into groundwater, into rivers, into the reservoirs that feed municipal water intakes.
Three sources drive most nitrate contamination in U.S. drinking water.
Agricultural fertilizer. Row crop agriculture—corn, soybeans, wheat—depends on nitrogen at rates that routinely exceed plant uptake, particularly during wet spring conditions when fields drain fast. The Midwest corn belt (Iowa, Illinois, Indiana, Wisconsin, Ohio) carries the largest nitrate load of any agricultural region in the country.
Concentrated animal feeding operations. Large livestock facilities produce manure at volumes that surrounding land cannot absorb. Lagoon leaks, overapplication of liquid manure, and feedlot runoff all contribute nitrate to local groundwater. In high-density livestock counties in Iowa and North Carolina, private well nitrate contamination consistently exceeds the federal limit.
Septic systems. Septic systems process household waste but don't remove nitrogen. Effluent percolates into soil, and in areas with aging or high-density septic infrastructure, the cumulative nitrogen load in groundwater can be meaningful.
What the EPA Limit Actually Means
The EPA's maximum contaminant level for nitrates is 10 milligrams per liter. This limit was set specifically to prevent methemoglobinemia—a condition in which nitrates interfere with blood oxygen transport, most dangerous for infants under six months. In severe cases it causes bluish skin tone, which is why it's sometimes called blue baby syndrome.
That limit was not set by evaluating what long-term adult exposure at lower concentrations does. Research published in peer-reviewed literature has found associations between nitrate exposure below the MCL and increased colorectal cancer risk in adults—suggesting the regulatory ceiling may not reflect the full range of chronic health concerns.
What this means practically: water testing at 9 mg/L is legally compliant. The EPA didn't design that limit with long-term employee exposure in mind.
Which Markets Are Most Affected
Nitrate contamination concentrates heavily in agricultural regions. Wisconsin has over 42,000 private wells exceeding the federal limit—a figure from state groundwater monitoring data that reflects the scale of agricultural runoff there. Iowa operates public water systems that routinely require treatment to stay within the MCL during high-runoff spring months. In some years, systems near the Iowa River and Des Moines River have issued advisories when source water nitrate spikes.
Nitrate contamination is also seasonal. Spring planting and snowmelt create higher-contamination windows that don't always align with annual utility testing cycles. A business in an agricultural market drawing water that tests at 7 mg/L annually may see higher concentrations in spring—still potentially within compliance, but with less margin and a higher actual exposure window for employees.
What Reverse Osmosis Removes That Carbon Filtration Does Not
This is the part that matters most for businesses evaluating purification options: carbon block filters don't remove nitrates. Carbon filtration removes chlorine, chloramines, volatile organic compounds, and sediment—it does not address dissolved ionic compounds. A carbon-only system, regardless of how many stages it includes, doesn't change the nitrate level at the dispenser.
Reverse osmosis removes nitrates effectively. The RO membrane blocks dissolved ionic compounds at the molecular level—including nitrates, which carry a negative ionic charge. EPA documentation on nitrate treatment puts RO removal rates at 85 to 95 percent. A bottleless water purification system with an RO stage removes nitrates along with PFAS, lead, arsenic, and other dissolved contaminants in the same process.
Ion exchange—specifically anion exchange resin—is the other effective method. Some systems combine RO and ion exchange for markets with elevated nitrate levels. The key question when evaluating any purification system for a nitrate-affected water supply is whether the technology actually addresses dissolved ions, not just chlorine and particulates.
What to Check for Your Building's Water
Your utility's annual Consumer Confidence Report includes nitrate monitoring results. For businesses in agricultural markets, the annual figure may understate seasonal highs—nitrate concentrations peak in spring during fertilizer runoff season. The EPA's drinking water dashboard shows utility-level nitrate data going back several years, which gives a more complete picture than any single annual reading.
For the complete walkthrough on reading your utility's water quality report, see our Consumer Confidence Report guide. A certified lab can also test your tap water directly for nitrate for approximately $30 to $50.
Frequently Asked Questions
What are nitrates and why are they in tap water?
Nitrates are nitrogen-based compounds that form when bacteria break down nitrogen from fertilizer, organic matter, and animal waste. They're highly soluble and move easily into groundwater and surface water. Unlike many contaminants, they don't bind to soil—they follow water wherever it flows. Agricultural fertilizer runoff is the primary source in U.S. drinking water, with additional contributions from septic systems and livestock operations.
What is the EPA limit for nitrates and why was it set there?
The EPA's maximum contaminant level for nitrates is 10 mg/L. That limit was set specifically to prevent methemoglobinemia—a condition in which nitrates interfere with blood oxygen transport—in infants under six months. It was not derived from evaluating long-term adult exposure effects. More recent research has found associations between nitrate exposure below the MCL and increased colorectal cancer risk in adults.
Does a carbon filter remove nitrates?
No. Carbon filters remove chlorine, chloramines, and organic compounds—they don't remove dissolved ionic compounds like nitrates. An RO membrane is required to address nitrate contamination. When evaluating a purification system for a nitrate-affected water supply, confirm it includes an RO stage, not just carbon filtration.
Which regions have the most nitrate risk?
Businesses in agricultural regions—particularly the Midwest corn belt (Iowa, Illinois, Indiana, Wisconsin) and parts of the Great Plains—draw from water supplies with higher nitrate concentrations than national averages. Within those regions, spring and early summer are higher-risk periods when fertilizer runoff increases nitrate load in rivers and aquifers feeding municipal systems.
How do I find out if my building's water has elevated nitrates?
Start with your utility's annual Consumer Confidence Report, which includes nitrate monitoring results. The EPA's drinking water dashboard shows historical monitoring data by utility. A certified lab can also test your tap directly for approximately $30 to $50.