Estimation of Nitrate Contamination of an Agro‐Ecosystem Outwash Aquifer Using a Nitrogen Mass‐Balance Budget

A mass‐balance budget of N cycling was developed for an intensive agricultural area in west‐central Minnesota to better understand NO − 3 contamination of ground water in the Otter Tail outwash aquifer. Fertilizer, biological fixation, atmospheric deposition, and animal feed were the N sources, and crop harvests, animal product exports, volatilization from fertilizer and manure, and denitrification were the N sinks in the model. Excess N, calculated as the difference between the sources and sinks, was assumed to leach to ground water as NO − 3 . The budget was developed using ground water data collected throughout the 212‐km 2 study area. Denitrification was estimated by adjusting its value so the predicted and measured concentrations of NO 3 in ground water agreed. Although biological fixation was the largest single N source, most was removed when crops were harvested, indicating that inorganic fertilizer was the primary source of N reaching the water table. It was estimated that denitrification removed almost half of the excess NO − 3 that leached below the root zone. Even after accounting for denitrification losses, however, it was concluded that the ground water system was receiving approximately three times as much N as would be expected under background conditions.