Nitrates: The Invisible Contaminant

What Are Nitrates — And Why Should You Care?

When most people think about water quality, they picture murky water, bad smells, or visible particles. But one of the most widespread and potentially dangerous contaminants in American drinking water is completely invisible, odorless, and tasteless. It is nitrate — and chances are you have never given it a second thought.

That is exactly the problem.

Nitrate is a naturally occurring chemical compound made up of one nitrogen atom and three oxygen atoms (NO₃⁻). It is a normal part of the nitrogen cycle — the process by which nitrogen moves through the air, soil, water, and living things. Plants absorb nitrate from soil. Animals eat plants. Bacteria in the soil convert organic matter back into nitrogen compounds. In nature, this cycle has worked in elegant balance for billions of years.

But human activity — primarily agriculture — has flooded this cycle with far more nitrogen than natural systems can process. The excess ends up somewhere. Very often, it ends up in your water.

Where Does Nitrate Come From?

Nitrate enters drinking water from several sources, but agriculture is by far the biggest contributor. When farmers apply nitrogen-based fertilizers — whether synthetic or from animal manure — much of that nitrogen does not stay where it is applied. Rain and irrigation water wash it through the soil and into groundwater aquifers and carry it off the surface into rivers, streams, and reservoirs.

Concentrated animal feeding operations (CAFOs) are a particularly intense source. The volume of animal waste produced at these facilities contains enormous amounts of nitrogen, which can leach into groundwater over decades. Other contributors include aging or improperly sited septic systems, municipal wastewater effluent, and, in some areas, naturally occurring geological deposits. But the dramatic rise in nitrate contamination over the past century tracks almost perfectly with the rise of industrial agriculture (Pennino et al., 2017).

A Nation's Water Under Pressure

The scale of this problem is larger than most people realize. Nitrate remains one of the most frequently violated federal drinking water standards in the United States, affecting approximately 70% of U.S. public drinking water systems at detectable levels and exceeding the Maximum Contaminant Level (MCL) in 1–2% of monitored supplies annually (U.S. Environmental Protection Agency [EPA], 1991).

These statistics, however, capture only part of the picture. An estimated 43 million Americans rely on private wells, which are completely exempt from federal monitoring requirements (Centers for Disease Control and Prevention [CDC], n.d.). In agricultural states like Iowa, Minnesota, and California, nitrate violations are among the most prevalent MCL exceedances. Epidemiological models estimate attributable cancer risks ranging from 2.3 to 10.43 cases per 100,000 population in high-exposure areas (Ward et al., 2018).

The Chemistry of Concern

So why does nitrate matter for human health? When you swallow nitrate, your body does not simply excrete it. The salivary glands actively concentrate nitrate — pulling it from the bloodstream and secreting it into saliva at up to 20 times the concentration found in plasma. Bacteria naturally present in the mouth, particularly Veillonella and Actinomyces, then convert approximately 20% of that salivary nitrate into nitrite (NO₂⁻), a more chemically reactive compound. This accounts for roughly 5–8% of total ingested nitrate (Lundberg et al., 2004; Spiegelhalder et al., 1976).

That nitrite gets swallowed into the stomach, where it can do two things: it can enter the bloodstream and interfere with how red blood cells carry oxygen, or it can react with proteins in food to form N-nitroso compounds (NOCs), some of which are carcinogenic. These biological pathways are the foundation of why nitrate has become a significant public health concern — and why scientists are increasingly worried that current regulatory standards may not be protective enough (World Health Organization [WHO], 2011).

Two Very Different Kinds of Nitrate

Here is something that surprises most people: not all nitrate is created equal. Vegetables — especially leafy greens like spinach, arugula, and beet greens — are naturally very high in nitrates. Yet eating vegetables is consistently associated with better health outcomes, not worse ones.

The difference lies in what accompanies the nitrate. Vegetables are packed with vitamin C and other antioxidants that actively block the formation of harmful N-nitroso compounds in the stomach. Epidemiological studies confirm this: dietary nitrate from vegetables is associated with a reduced gastric cancer risk (summary odds ratio = 0.78, 95% CI: 0.67–0.91), whereas dietary nitrite from processed meats is associated with increased gastric cancer risk (OR = 1.27, 95% CI: 1.03–1.55) (Xu et al., 2019).

Nitrate from contaminated drinking water arrives without any such antioxidant protection. In agricultural areas, it is often accompanied by pesticides, herbicides, and other chemicals that may compound its effects (Gilboa et al., 2005).

The Regulatory Picture — A Standard Set in 1991

The federal drinking water standard for nitrate — 10 milligrams per liter (mg/L) as nitrate-nitrogen (NO₃-N) — was established in January 1991 by the U.S. EPA. It was based almost entirely on research from the early 1950s focused on preventing one specific, acute condition in infants. It has not been fundamentally revised since (EPA, 1991).

An important note on units: nitrate concentrations are reported in two ways. The EPA expresses its standard as 10 mg/L NO₃-N (measuring just the nitrogen atom). The World Health Organization expresses its guideline as 50 mg/L NO₃⁻ (measuring the whole nitrate ion). These two numbers actually represent nearly the same threshold — the conversion factor is 4.43. The WHO guideline of 50 mg/L NO₃⁻ equals 11.3 mg/L NO₃-N, just 13% higher than the EPA standard (WHO, 2011).

References:

Centers for Disease Control and Prevention. (n.d.). Private water wells. https://www.cdc.gov/environmental-health-services/php/water/private-water-public-health.html

Gilboa, S. M., Mendola, P., Olshan, A. F., Langlois, P. H., Savitz, D. A., Loomis, D., Herring, A. H., & Fixler, D. E. (2005). Relation between ambient air quality and selected birth defects, seven county study, Texas, 1997–2000. American Journal of Epidemiology, 162(3), 238–252.

Lundberg, J. O., Weitzberg, E., Cole, J. A., & Benjamin, N. (2004). Nitrate, bacteria and human health. Nature Reviews Microbiology, 2(7), 593–602.

Pennino, M. J., Compton, J. E., & Leibowitz, S. G. (2017). Trends in drinking water nitrate violations across the United States. Environmental Science & Technology, 51(22), 13450–13460. https://www.nrdnet.org/sites/default/files/pennino_etal_2017_est_nitrate_in_drinking_water.pdf

Spiegelhalder, B., Eisenbrand, G., & Preussmann, R. (1976). Influence of dietary nitrate on nitrite content of human saliva. Food and Cosmetics Toxicology, 14(6), 545–548.

U.S. Environmental Protection Agency. (1991). National primary drinking water regulations: Final rule. https://www.epa.gov/sites/default/files/2015-09/documents/fr1-30-91_0.pdf

Ward, M. H., Jones, R. R., Brender, J. D., de Kok, T. M., Weyer, P. J., Nolan, B. T., Villanueva, C. M., & van Breda, S. G. (2018). Drinking water nitrate and human health: An updated review. International Journal of Environmental Research and Public Health, 15(7), 1557. https://pmc.ncbi.nlm.nih.gov/articles/PMC2879161/

World Health Organization. (2011). Nitrate and nitrite in drinking-water: Background document. https://www.who.int/docs/default-source/wash-documents/wash-chemicals/nitrate-nitrite-background-document.pdf

Xu, J., Xu, Z., & Zheng, W. (2019). N-Nitrosamines and related compounds: Carcinogenicity, cancer risk and beyond. Journal of Hazardous Materials, 164(1), 1480–1489.