Background and Objective

In Europe, about 20 million tons of nitrogen (N) were applied as mineral and organic fertilizers to agricultural soils. A substantial part of the N input is discharged into surface and ground water providing a significant hazard for water resources.

This nitrogen load can be reduced by denitrification in soils and groundwater. Denitrification is therefore of central importance for the protection of groundwater and of surface waters. Similarly, denitrification is important for the control of emissions of the greenhouse gas N₂O from aquatic systems ("indirect N₂O emissions").

The aim of the project is to validate existing models for denitrification in soils and groundwater (DENUZ and WEKU, respectively (Kunkel Wendland 1997 and 2006)) by measuring dissolved N₂ from  denitrification in groundwater (N₂/Ar-method). Moreover, the potential lowering of indirect N₂O emissions by denitrification in groundwater will be derived from the determined removal of N loads.

Target Group

Science, Federal Ministry of Food and Agriculture, professionals in the field of water supply, agriculture and environmental protection.

Approach

During the project 612 groundwater monitoring wells distributed all over Lower-Saxony, Germany were sampled. Major groundwater parameters are analyzed and quantified. Denitrification elapsed during groundwater residence time is quantified using the N₂ / Ar method, which is based on the measurement of gaseous reaction products of denitrification in groundwater samples. At selected groundwater monitoring wells, groundwater age is determined (Helium-Tritium method). From these data, denitrification rates in groundwater as well as nitrate input are derived.

The collected data will be processed in a database and loaded into ArcGIS. The modeled nitrate leaching (DENUZ) and the modeled nitrate reduction on the groundwater flow path (WEKU) are then compared with the project data. Based on this, model adjustments are finally performed.

Our Research Questions

Concerning the comparison of the N₂ / Ar-method with DENUZ:

• Are modeled nitrate inputs comparable with the results of the N₂ / Ar method?
• What is the influence of the setting of the groundwater measuring wells on comparability of measured and modeled values​?
• Are there regional differences in the goodness of fit between modeled nitrate emissions and the N₂/Ar-method?
• How does the heterogeneity of soil properties and/or of land use in the vicinity of a measuring well influence the comparability of model and measurement?

Concerning the comparison between the N2/Ar-method and WEKU:

• How is the agreement of the modeled and the measured nitrate reduction in groundwater?
• What is the influence of aquifer types on the comparability between model and measurement?