Agricultural Practices Can Affect Levels Of Nitrate In Groundwater

U.S. farmers embraced nitrogen-based fertilizer at a dramatic pace during the 1960s and '70s. Since then, its use has played a key role in boosting agricultural productivity. But as a consequence, nitrogen's more soluble form, nitrate, has become a common drinking water contaminant in Wisconsin and around the country.

In a Jan. 20, 2016, talk at the Wednesday Nite @ The Lab series on the University of Wisconsin-Madison campus, UW-Extension groundwater education specialist Kevin Masarik laid out the agricultural context in which nitrate is used and explained how it often can end up contaminating some of Wisconsin's drinking water. The talk was recorded for Wisconsin Public Television's University Place.

Nitrogen is an essential nutrient for most plants, and therefore nitrate will continue to play an integral role in agriculture, Masarik said. But Wisconsin's varied bedrock geology and the specific choices farmers make about applying nitrate play a big role in determining how much of it actually ends up in the water supply, he explained. Nitrate contamination can present a serious public health threat, especially in the form of methemoglobinemia, colloquially called blue baby syndrome.

In his talk, Masarik outlined what Wisconsinites can do to protect their drinking water, including testing private wells and looking up information on the Well Water Quality Viewer, developed by the Center for Watershed Science and Education (a partnership between UW-Extension and UW-Stevens Point's College of Natural Resources).

Masarik also discussed some of the solutions he and other Wisconsin water quality experts have proposed. If farmers introduced different kinds of crop rotations, grew cover crops and implemented other land-use practices — and if those practices become economically viable — he said Wisconsin could vastly improve its nitrate picture.

Masarik also shared his advice for private well owners in a recent online Q&A.

Key facts

• As an element, nitrogen is not created or destroyed, but is present in many chemical forms that natural and artificial processes can convert from one form to another in a complex cycle — including organic and manmade sources. As a negative ion, nitrate is more likely to move with water than positive ions, which tend to adhere to soil molecules.
   
• In agriculture, the more nitrogen fertilizer the better — but only up to a point. Studies show that farmers must strike a tricky balance between fertilizer costs and crop yields to earn the highest profit. Studies have shown that rotating different types of crops, rather than constantly focusing on one, can reduce nitrate leaching into groundwater.
   
• Groundwater is highly localized. Although Wisconsin can be divided up into about 20 regional watersheds, variations in groundwater are still pronounced at a much smaller geographic scale.
   
• About 75 percent of Wisconsin residents rely on groundwater as a primary drinking water source.
   
• Groundwater's susceptibility to nitrate contamination varies throughout the state, depending on local geology and what kinds of crops farmers are planting. Wet years (high-precipitation), especially those following dry years, offer prime opportunities for nitrate to leach into groundwater.
   
• High levels of nitrate in drinking water can suggest that other contaminants are also moving in a water source.
   
• According to studies by state regulators, roughly 9 percent of groundwater wells in Wisconsin exceed the acceptable level of nitrate in drinking water. This number gets as high as 21 percent in agriculture-heavy areas. Other studies show that extreme highs in nitrate contamination have decreased over time in Wisconsin, but median levels of concentration haven't gone down much.
   
• Women, children and older people face the greatest health risks from nitrate exposure. When consumed via drinking water, nitrate can cause blue baby syndrome, which has been linked to central nervous system damage and non-Hodgkin's lymphoma later in life.
   
• A lone septic system has a relatively low nitrate footprint, but developments with a high concentration of septic systems present greater risk of groundwater contamination. Overall, septic systems don't do a good job of getting rid of nitrate.

Key statements

• On the definition of groundwater: "It's not an underground lake, it's not an underground river. It's just the water that occupies the empty spaces in the geologic material underneath our feet."
   
• On interpreting nitrate levels: "Anything above 1 [milligram per liter] is giving us some indication that the groundwater in that area is being impacted to some degree by the local land use."
   
• On health concerns related to nitrate: "When we talk about the current nitrate standard of 10 milligrams per liter, our concerns really are infants and pregnant women because of a condition called methemoglobinemia."
   
• On the balancing act that nitrogen demands: "The thing that makes it difficult is it's a critical resource for agriculture. We wouldn't have the same agricultural productivity on the land surface if it wasn't for nitrogen and its role in increasing production. If we don't replace the nitrogen in agricultural soils, we are in a sense mining that nutrient and other nutrients from the soil."
   
• On current trends in the use of nitrogen fertilizer: "We have reached a point where we're not seeing the dramatic increase in nitrogen use like we once did."
   
• On nitrate leaching and farming practices: "I think there's this assumption that people have when they find nitrate in their water, that it's because (of) something that a farmer's doing that's illegal or they're not following best management practices. What we find (in our research) is that even under optimal management practices, we are gonna expect some leaching lost below these systems ... Nitrate leaching to some extent is inevitable."
   
• On practices that offer a way forward: "When we talk about the landscape, it doesn't necessarily have to be a completely natural system that's good for groundwater. Just integrating more diverse crops, more perennial crops, lower-nutrient-intensive crops, is one way (to reduce nitrate contamination)."