Nitrates in Well Water: Agricultural Sources and High-Risk Regions
Health disclaimer: This page provides general information about nitrate sources and contamination risk in private wells. It is not medical advice. If you have concerns about nitrate exposure, especially for infants or pregnant individuals, contact a healthcare provider.
Infants under 12 months old should NOT drink water or formula made with water that contains nitrates above 10 mg/L. Nitrate can cause methemoglobinemia (blue baby syndrome), a condition that prevents blood from carrying oxygen normally. Symptoms include bluish skin, difficulty breathing, and unusual fatigue. This is a medical emergency, call 911.
Nitrate contamination in US groundwater is largely an agriculture story. Nitrogen fertilizers applied to fields for decades have moved through soil into aquifers across the Corn Belt and beyond. This isn’t a new problem. The USGS has been tracking it since the 1990s. But it’s a persistent one, and it gets worse in wet years when rainfall accelerates the movement of nitrogen through the soil.
If you’re on a private well in an agricultural region, you’re in the highest-risk category. You bear the risk that municipal customers don’t, because nobody monitors your well but you.
How Fertilizers Become Groundwater Contaminants
The chemistry is straightforward. Nitrogen fertilizers applied to fields, whether synthetic or organic, release nitrogen compounds into the soil. Soil bacteria convert those compounds into nitrate. Unlike many other contaminants that bind to soil particles, nitrate is soluble and mobile. Rain and irrigation water carry it down through the soil profile.
Eventually it reaches the saturated zone, the groundwater table. From there it moves laterally with the groundwater flow and can travel significant distances. A well upgradient from a field gets less of that nitrogen load. A well that sits where water drains from surrounding farmland accumulates it.
The process isn’t instant. It can take years or even decades for nitrate applied to a field surface to reach a deep well. That means contamination you detect today may reflect fertilizer applications from years ago. And it also means reducing fertilizer use now doesn’t clean up groundwater quickly. It’s a slow-moving problem in both directions.
Tile drainage systems, common in Midwest row-crop agriculture, accelerate the process. These buried perforated pipes collect water that would otherwise pool in fields and route it to drainage ditches or streams. They’re effective for farming. They also create a fast path for nitrate to exit fields and enter surface water and shallow groundwater before the soil can buffer it.
The High-Risk Regions
The USGS National Water-Quality Assessment Program (NAWQA) has mapped groundwater quality in domestic wells across the country for decades. The data consistently shows the same pattern: the highest nitrate concentrations cluster in the agricultural Midwest.
Iowa ranks near the top of every assessment. Intensive corn and soybean production, heavy fertilizer application, and a shallow water table combine to create widespread nitrate problems. The Iowa Department of Agriculture and Land Stewardship tracks nitrate trends in rivers and wells because it’s a documented public health issue in the state, not a hypothetical one.
Illinois, Indiana, Nebraska, and Kansas show similar patterns. These states share intensive row-crop agriculture, nitrogen-dependent crop systems, and, in many areas, geology that allows relatively rapid movement of water from the surface into shallow aquifers.
California’s Central Valley is a different geography but the same basic problem. Decades of intensive vegetable, fruit, and nut farming, with heavy irrigation and fertilizer use, have elevated nitrate levels in groundwater across much of the valley. The State Water Resources Control Board has documented communities in the Central Valley where nitrate in drinking water is a serious and ongoing issue.
The USGS found in one analysis that nitrate levels in domestic wells in agricultural areas are roughly 10 times higher than in wells outside agricultural land. That gap reflects the direct relationship between land use and groundwater quality.
Livestock Operations as a Second Source
Fertilizers get most of the attention, but confined livestock operations contribute separately and sometimes more acutely.
Concentrated animal feeding operations (CAFOs) produce large volumes of manure that’s stored in lagoons or spread on land. When those lagoons fail or overflow, or when manure is applied to land at rates the soil can’t absorb, nitrogen loads reach groundwater fast and at high concentrations.
Hog, poultry, and cattle operations all contribute. Hog operations tend to produce the highest nitrogen concentrations in waste. States with large hog industries, North Carolina, Iowa, and Missouri among them, have documented nitrate problems in areas near large operations.
Manure applied to crop fields as fertilizer is, on its own, a well-established agricultural practice with real benefits. The problem is over-application or poor timing. Applying manure in fall before a wet spring, for example, leaves nitrogen sitting in soil through the winter when crops aren’t taking it up. Rain then moves it into groundwater.
Unlike synthetic fertilizer contamination, which tends to be diffuse and spread across large areas, CAFO-related contamination can create hotspots. A well near a lagoon that breached, or downhill from fields receiving heavy manure applications, can see very high nitrate levels that don’t reflect the broader regional average.
Well Construction Matters as Much as Location
Two neighboring wells in the same county can show very different nitrate levels depending on how they were built and how deep they go.
Shallow wells, generally those less than 100 feet deep, tap younger groundwater that reflects recent land use. Water that’s been in the ground for five to ten years carries more of the nitrate from recent agricultural applications. Deeper wells tap older water that predates more intensive farming, though even some deep wells show elevated nitrate in heavily farmed regions.
Well casing depth and integrity matter. A well casing that extends well below the surface with intact grouting around it prevents surface water and near-surface groundwater from entering directly. A well drilled decades ago with minimal casing or grouting that has since degraded can allow near-surface water, and the nitrate it carries, to short-circuit into the well.
Well location on the property is a factor too. A well at the low point of a field or near a drainage tile outlet is in a worse position than one on higher ground uphill from the agricultural activity.
None of this means you can assess your risk by looking at your well or your neighbors’ test results. Geology varies, well construction varies, and nitrate concentrations can differ dramatically across a short distance.
What to Do If You’re in a High-Risk Area
The answer starts with a test, not an assumption.
A certified lab test for nitrates costs $20 to $50 at most state-certified labs. Your state’s department of environmental quality or public health department may offer free testing, especially in agricultural regions where nitrate is a documented problem. That’s worth checking before you pay out of pocket.
If you have an infant under 12 months old in your home, or a pregnancy, and you haven’t tested recently, use bottled water now for any infant feeding. Don’t wait for the test results. The consequences of high nitrate exposure for an infant are serious enough that a week of bottled water is the right precaution while you wait.
If you test above 10 mg/L, you need treatment at your drinking water tap. Reverse osmosis (NSF/ANSI 58 certified) is the most practical option for most households. It reduces nitrates at the point of use, typically the kitchen sink. See How to Remove Nitrates from Water for a breakdown of your options.
Retest your well annually. Nitrate levels in agricultural areas can shift with seasonal rainfall, land use changes nearby, and the cumulative buildup over time. A single clean test from two years ago doesn’t tell you what’s in your water today.
For guidance on what to test and how to find a certified lab, see Well Water Testing Guide and Best Mail-In Water Tests.
Sources:
- USGS National Water-Quality Assessment Program: https://www.usgs.gov/mission-areas/water-resources/science/national-water-quality-assessment-nawqa
- EPA National Primary Drinking Water Regulations, Nitrates: https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- PMC, Blue Baby Syndrome and Nitrate-Contaminated Well Water: https://pmc.ncbi.nlm.nih.gov/articles/PMC1638204/
Health disclaimer: WaterAnswer.com provides general information only. If you have questions about nitrate exposure or infant health, contact a healthcare provider. For emergencies, call 911.