Nitrate-Contaminated Drinking Water Followback Study

Background: Nitrate is one of the most common contaminants found in Wisconsin’s groundwater. Groundwater naturally contains traces of nitrate that are produced by decaying vegetation and transported through soil by rainwater and snow melt. Under current landuse conditions, however, the major contributors of nitrate to aquifers are related to human activities. Seepage from septic tanks, discharges from municipal sewage facilities, waste from farm animals, and agricultural and lawncare fertilizers contribute millions of pounds of nitrate to Wisconsin’s groundwater each year.

The federal drinking water standard for nitrate-nitrogen (nitrate-N) which applies only to public water supplies was set at 10 mg/L in 1991. Private water supplies fall under a health advisory issued by the Wisconsin Department of Health and Family Services. This advisory recommends that water that exceeds the federal standard not be fed to infants that are less than six months of age. The advisory was expanded in 1993 to include pregnant women. A recent water supply survey conducted by the Wisconsin Division of Health and the National Center for Disease Control and Prevention found that nearly seven percent of Wisconsin’s 750,000 private wells had nitrate levels that exceeded the health advisory level. The problem varies regionally with the highest nitrate levels being found in the southern and central regions of the state where agriculture is most intense.