Groundwater nitrate in Nebraska: key factors and timescales of movement

September 6, 2024

Nebraska map of registered irrigation wells
Well density in Nebraska. 2023. UNL CSD based on NDNR data.

Factors influencing groundwater nitrate concentrations

Recent nitrate-N levels in groundwater are the result of several factors. Key factors include land use, the amount and timing of precipitation and irrigation water applied, subsurface conditions (including soils and geology), distance from ground surface to the water table (vadose zone thickness), and depth of the well. These factors vary significantly across Nebraska. Variability increases the complexity and challenge of monitoring and managing nitrate in aquifers and connected surface water bodies.

Land use

Land use for crop production (Ag Fields) depends on soil, typical annual rainfall and/or availability of irrigation water, and other factors such as topography. Application of nitrogen fertilizers is a major source of nitrate-N in soils, vadose zone, and groundwater in Nebraska.

Well density

Well density depends on productivity of the underlying geology (aquifer), suitability of land for row crop production, and other factors including groundwater regulations. Water application combined with unpredictable precipitation events can cause high infiltration rates that carry nitrate downward through the root zone and into the vadose zone and groundwater.

Depth to groundwater

Depth to groundwater depends on topography, where groundwater is usually closer to the surface in valleys than in surrounding uplands. Annual rainfall, irrigation use, soils, geology, and irrigation canal operations can also affect depth to groundwater. In general, areas with shallow groundwater and high permeability soils overlying high conductivity geological materials (e.g., sand/gravel) show the most rapid response in groundwater nitrate concentrations to increases or decreases in inputs of nitrogen. In upland areas, or areas where depth to groundwater is greater, the impact of nitrogen on groundwater nitrate concentrations may not be as immediate, but substantial nitrate may be temporarily stored in the vadose zone as it travels downward toward the groundwater.

Key factors and timescales of movement

The timescales for nitrate movement through the subsurface is variable at local and regional levels across Nebraska. McMahon et al. (2006) provide descriptions of fast and slow path scenarios and related timescales for water and chemicals to reach groundwater. The examples in the figure below give an approximate sense of timescales through the vadose zone. Once nitrate reaches the groundwater, it may be present for years, decades, or more, before exiting the aquifer through pumping or discharge into surface water.

 

Map of Nebraska with groundwater depths

Nebraska generalized depth of groundwater. Nebraska Department of Environment and Energy. 2023.

 

Diagram of nitrate movement

Factors and timescales of nitrate movement from McMahon et al. 2006.