An estimated 9 to 10 percent of Wisconsin wells have tested over safe limits for nitrate.
Studies have estimated that 90% of nitrate in groundwater comes from spreading of synthetic fertilizers and dairy manure on farm fields, with most of the remainder from septic systems.
Nitrate behaves differently. Relatively little lingers near roots where it can be absorbed. Water washes it down into shallow groundwater that is the source of drinking water for one-quarter of Wisconsin residents.
Legumes and alfalfa, take up nitrogen before it can reach groundwater, but are not as profitable as corn. When prices rise for corn, which requires heavy applications of nitrogen-based fertilizer, farmers quickly convert acreage and boost spreading. Wisconsin farmers applied over 200 million pounds of nitrogen in excess of UW-Extension recommendations.
Not only does the fertilizer create toxic drinking water, but the phosphorus in fertilizer and manure contributes to abnormal algae growth in lakes and streams when it runs off the land with rain and snowmelt.
Drinking water contaminated with more than 10 milligrams per liter of nitrate poses acute risks to infants and women who are pregnant, a possible risk to fetuses in early stages of pregnancy, and a longer-term risk of serious disease in adults.