Review and assessment of nitrate reduction in groundwater in the Baltic Sea Basin

Study region: Six countries within the Baltic Sea Basin: Denmark, Sweden, Finland, Lithuania, Poland and Germany Study focus: During transport from the field to the sea nitrogen undergoes natural reduction, but with large spatial variations. Mapping this variation would allow more optimal regulation strategies, by imposing most restrictions in areas with low natural reduction. In the present study, a map for spatially variable nitrate reduction in groundwater is developed covering six countries in the Baltic Sea Basin. In contrary to previous studies within the Baltic Sea Basin, the present study provides an independent estimate of the nitrogen reduction in groundwater, based on review of national data and studies. Depending on availability, different approaches were used for the countries ranging from national modelling to expert judgement. Through a joint workshop the assessments were, to the extent possible, harmonised. New hydrological insights for the region: 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 optimised, if variation in the natural reduction of nitrate is considered