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‐km2 study area. Denitrification was estimated by adjusting its value so the predicted and measured concentrations of NO3 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.