Anker Lajer Højberg1, Anne Lausten Hansen1, Przemysław Wachniew2, Anna J. Żurek2, Seija Virtanen3, Jurga Arustiene4, Johan Strömqvist5, Katri Rankinen6 and Jens Christian Refsgaard1
1Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark
2AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland
3Drainage Foundation sr, Simonkatu 12 B 25, 00100 Helsinki, Finland
4Lithuanian Geological Survey, S. Konarskio str. 35, LT-03123 Vilnius, Lithuania
5Swedish Meteorological and Hydrological Institute, SMHI, 60176 Norrköping, Sweden
6Finnish Environment Institute, P.O.Box 140, 00251 Helsinki, Finland
Riverine load of nitrogen to the Baltic Sea has been reduced in recent years, but further amendments are required to meet the goal of the EU Water Framework Directive. The largest contributor from anthropogenic activities is agriculture and reduction in the load from farming praxis is inevitable. Regulation of nitrogen has typically relied on uniform regulation, i.e. imposing the same restrictions on farming all over. During transport from the field to the sea nitrogen undergoes natural reduction, but with large spatial variations, due to variation in the hydrogeological and hydro-geochemical conditions. Mapping this variation would allow more optimal regulation strategies, where most restrictions are imposed in areas with low natural reduction. Most assessments on nitrogen transport and reduction take a catchment scale approach in which the total removal of nitrogen for the catchment is estimated, not differentiating between surface water and groundwater. Discriminating between the two domains is nevertheless important in order to identify the correct type of mitigation measure to implement. In the project BONUS Soils2Sea (www.soils2sea.eu) a map for nitrate reduction in groundwater was developed by groundwater experts from five countries in the Baltic Sea Basin based on a review of national data and previous studies. The review revealed large variations in the hydro-geochemical conditions important for transport and degradation of nitrogen in groundwater. This includes the hydrogeology, the reducing conditions of the subsurface, and the fraction of water transported by drainage systems bypassing the reducing subsurface environments. Significant variations in groundwater reduction between the countries and within most of the countries were thus found, indicating that strategies for nitrogen regulation and mitigation measures may be optimized, if variation in the natural reduction of nitrate is considered.