Nitrogen Sources and Pathways for Surface Water Contamination.

Nitrogen in surface waters not only creates aquatic life toxicity concerns for Minnesota’s waters, but nitrogen transport from the Midwest into the Mississippi River system is a major contributor of hypoxia (oxygen deficiency) in the Gulf of Mexico. High inputs of nutrients from the Atchafalaya and Mississippi Rivers to the northern Gulf of Mexico contribute to nutrient over-enrichment and the creation of a seasonal zone of hypoxic waters. The extent of this zone is seasonally variable, and sometimes covers 20,000 square kilometers. The federal-state interagency Mississippi River/Gulf of Mexico Watershed Nutrient Task Force in 2001 established a goal of reducing the hypoxic zone to 5,000 square kilometers. In 2008, Minnesota agreed to follow the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force recommendations by developing a state-specific strategy. Guidance for state strategy development recommends that states conduct assessment work prior to establishing quantitative targets and identifying the needed management practices/strategies. The guidance suggests that each state: characterize watersheds, identify sources, prioritize geographic areas, document current loads, and estimate historical trends. This project has the following goals: 1) Assess soil nitrogen budgets (N additions to soil and losses from soils) for combinations of soils, climates and land uses representative of the most common Minnesota conditions, 2) Assess nitrogen contributions to Minnesota rivers from each of: a) the primary land use sources (excluding point source municipal and industrial), and b) the primary hydrologic pathways, 3) Determine the watersheds which contribute the most nitrogen to the Mississippi River, and combination of land uses and hydrologic factors having the greatest influences on the elevated nitrogen. The results of this effort will be useful as Minnesota works to establish state-specific goals and strategies to address its contribution to Gulf of Mexico hypoxia. The results will also be useful as Minnesota develops strategies to address nitrate concentration impairments in surface waters throughout the state and nitrate drinking water standard exceedances in well water.