Brief description

Nitrogen loads in surface water often do not reflect the actual input situation. This retention of nitrogen can be explained by chemical transformations in the soil and groundwater (e.g. denitrification) and hydrological factors (e.g. transition time, mixing) in soil and groundwater and depends strongly on the geological and chemical patterns within the catchment areas (e.g. reactive substances, conductivities).

Major goals are (1) the investigation of temporal and spatial characteristics of nitrogen transport and turnover on the path soil-unsaturated zone-groundwater until the exfiltration to the surficial waters in two typical catchment areas in the Northern German pleistocene lowland, (2) the parametrisation and calibration of a suitable modeling approach based in the experimental results on different spatial scales (lysimeter, field, catchment) and (3) the investigation of the relation between catchment characteristics like drainage system, landuse and fertilizer application, geochemical properties etc. and Nitrogen-stress in the river system, Identification of sensitive factors. Study areas are the Schaugraben catchment in the North of Saxony Anhalt and the Buschgraben catchment in Brandenburg.

Field studies include a survey of geological and pedological characteristics and landuse in the study catchments and a monitoring of discharge and nutrient loads. Tracer experiments have been conducted in soils and in surface near groundwater, using bromide and nitrate as conservative and reactive tracer, in order to assess transport and decay of nitrate in the vadose zone and in groundwater. In order to facilitate modelling studies on the relation between nitrogen transport and catchment characteristics a modelling approach has been developed, that allows simulation of the complete nitrogen transport path from the soil input until the exfiltration into the surface water system. This approach is based on the one-way coupling of a soil model (mRISK-N) with a groundwater flow model (MODFLOW) and a multi-species reactive transport model (RT3D). Groundwater nitrogen turnover is represented as full reactive transport including oxidation of organic matter and pyrite oxidation by several electron acceptors as the main reactive pathways, in order to link nitrogen turnover directly to the availability of the substances involved in the chemical reactions. Model applications focus on the interactions of catchment characteristics and distribution of nitrate in the catchment.

The modelling approach developed here is a tool for the assessment of transport-turnover interaction and may help to improve experimental studies and measurement strategies and to provide useful information for managing purposes.