Nelson Issue Brief: Nitrate contamination in drinking water and groundwater

How Does the Residential Development of Farmland Affect Nitrate in Groundwater?

Contact the authors

Kenneth R. Bradbury, Director and State Geologist, Wisconsin Geological and Natural History Survey, UW-Madison Division of Extension, ken.bradbury@wgnhs.uwex.edu

Stephanie DeVries, Wisconsin Geological and Natural History Survey, UW-Madison Division of Extension, stephanie.devries@wgnhs.uwex.edu

What happens to groundwater when rural farmland is converted to residential homes with on-site septic systems? We use monitoring and flow modeling to understand the relationship between land use and nitrate contamination in groundwater.

Savannah Valley — Near Sun Prairie

We monitored groundwater quality for 10 years in Savannah Valley, a 78-acre unsewered subdivision in south-central Wisconsin. We used groundwater monitoring wells measuring nitrates in both a shallow gravel and a deep bedrock aquifer. Monitoring began in 2002 while the site was primarily used for corn, soybean and alfalfa production, and continued as it was converted to residential homes.

Prior to development, groundwater quality showed high variability in both space and time. Nitrate in the shallow wells exceeded 10 mg/l in some wells, and there was evidence of other effects from agricultural use and highway salting in many wells. Concentrations in deeper wells, although lower and less variable, also showed evidence of impacts from land use.

A group of researchers testing groundwater
Groundwater testing near Sun Prairie, Wisconsin. Photo by Ken Bradbury

Between 2002 and 2013, nitrate concentrations showed statistically significant decrease in 6 of 12 (50 percent) of the wells most frequently sampled. In 2002, seven wells exceeded the 10 mg/l (as N) nitrate standard; in 2013 only one well exceeded the standard. With decreased use of agricultural fertilizers and manure in farm fields as land use has changed, nitrate levels have substantially decreased in over half the wells sampled.

City of Waupaca

Nitrate concentrations in a municipal well field serving Waupaca, Wisconsin have demonstrated large spatial and temporal variability, and data between 2006 and 2018 show that maximum annual concentrations are increasing. In order to understand how nitrate reaches Waupaca’s municipal supply we need to identify land areas where water infiltrates and flows toward the well, areas known as the well’s “capture zone.”

We developed a model to simulate groundwater flow through a 26 square mile watershed contributing to the Waupaca wells in order to find the capture zone. We found that the capture zone providing water to Waupaca’s well within one year (the “short-term capture zone”) is covered by unsewered residential areas and cultivated cropland. Changes to this pattern of land uses may thus result in significant changes to well water quality. Other research has shown that targeted land management changes have proved effective in reducing groundwater nitrate concentrations.

Our research finds that long-term monitoring is necessary in order to draw conclusions about how nitrates in groundwater respond to land use changes and provides some guidance for land management approaches to reducing groundwater nitrate.

Learn more about nitrate contamination