I had a family friend from Indiana ask me the other day “What percentage of nitrates in the water come from farmers versus homeowners?”. I wrote this up as an answer to him. If you have more up-to-date information, please follow up with me! I’d love to learn more.

That’s a great question but it’s hard to answer that directly. People are trying to trace nitrogen back to their source (using nitrogen and oxygen isotopes) but the chemical transformations of nitrogen in the environment makes it really hard to accurately trace or “fingerprint” the source of nitrogen in the environment. Furthermore, while we have decent estimates of how much nitrogen comes from agricultural sources, we don’t have as good of a sense of how much nitrogen is applied to lawns by homeowners, or to golf courses and other manicured green spaces, because domestic sources of fertilizer are not tracked as closely.

But here’s what I can tell you given the information that is available:

At the state or US-scale, lawn fertilizer isn’t as big of a concern compared to agricultural fertilizer because the mass of nitrogen applied to agricultural land is much higher than the mass being applied to lawns. I did a rough calculation comparing nitrogen fertilizer use on turf grass versus cropland across the U.S., including lawns, golf courses, and athletic fields. The area of cropland (163 million hectares) far exceeds that of turf grass (16 million hectares). If we assume the all turf grass area is fertilized at the recommended rate of for basic lawn maintenance—in reality, about 52-71% of homeowners actually fertilize their lawns, but those who do probably over-fertilize because of carelessness and unclear instructions—turf grass fertilizer would still only account for around 25% of the total inorganic nitrogen fertilizer used in the U.S. (“inorganic” excludes nitrogen from livestock manure, which is another issue altogether). So, farmers rightfully get a lot of the attention when it comes to nutrient management because agricultural land use is a much larger source of nitrogen to the environment than turf areas.

However, lawn fertilizer can still be problematic at the localized scale. While there aren’t many studies on the specific impacts of urban fertilizer on water quality, nitrate concentrations in shallow groundwater are usually 2-3 times higher in row crop agricultural areas than in residential properties or golf courses. A study from the 2000 of groundwater wells across the US shows that median groundwater concentrations in agricultural areas was 3 mg-N/L while urban areas had median concentrations closer to 1 mg-N/L. Similarly, a study of river concentration in the Baltimore area showed that river nitrogen concentrations in agricultural regions had the highest concentrations (4-5 mg-N/L), while residential areas were lower yet still elevated (1.5-3 mg-N/L) compared to the forested areas (<0.5 mg-N/L). There are also other sources of nutrients in urban watersheds besides fertilizer that are remain underexplored. For instance, there was a study of the impact of a cemetery on water quality (nitrogen from bodies), which found that the groundwater concentration under the cemetery was about 6 mg-N/L, compared to 0.5 mg-N/L in nearby residential areas. For context, the World Health Organization (WHO) has set 10 mg-N/L as the “maximum allowable limit” because above these concentrations it can cause blue baby syndrome. Typically, these high concentrations are exclusively found in groundwater in agricultural areas. But chronic exposure to drinking water with nitrogen concentrations even as low as 1 mg-N/L can still increase the risk of some cancers. So, while urban sources of nitrogen to water are less of a concern, in localized urban regions, they can definitely pose a local risk to water quality.

A couple of things to keep in mind when thinking about the health risks of nitrogen exposure in drinking water: public groundwater wells typically draw from “confined aquifers.” These aquifers are deeper and protected with an impermeable clay layers (well, technically permeable, but water takes hundreds to thousands of years to pass through them), which makes it less likely for human-source pollutants from the last 100 years to have contaminated groundwater drinking water sources. And thus, this water source is generally of higher quality than what you’d find in shallow groundwater or unconfined aquifers, which can have pollutants leach directly into them because they don’t have that protective clay layer. Of course, there are exceptions, especially when wells are drilled through those protective layers. Polluted groundwater migrate through newly made routes if the wells are improperly sealed. However, both public groundwater and surface water go through water treatment, and some treatment plants even have a reverse osmosis step which can remove most nitrogen. That being said, if I had a private well, especially in agricultural areas like much of Indiana, I’d definitely check the nitrate concentrations, just to be safe.