Utilizing natural nitrogen reduction in national regulation
Geological Survey of Denmark and Greenland (GEUS)
Since the mid 1980ties a suite of different nitrogen measures have been imposed in different action plans in Denmark. These measures have been successful in reducing the N surplus by 30-50 % and reversing an upward trend in nitrate groundwater concentrations in large parts of the country as well as nearly halving the nitrogen loads to estuaries. However, shallow oxic groundwater aquifers, covering approximately 45 % of the area in Denmark, still experience nitrate concentrations above the groundwater and drinking water standards of 50 mg l-1. Similarly, assessments of the ecological status of marine waterbodies indicates that further reductions in nitrogen loadings are required in some waterbodies to obtain a good ecological status according to the European Water Framework Directive.Past and current regulations have primarily relied on a general approach, applying same restrictions for all areas independent on drainage schemes, hydrogeochemical conditions in the subsurface and retention in surface waters. Although significant reductions have been achieved this way, general measures are not cost-effective, as nitrogen retention (primarily as denitrification) varies significantly depending on the physical and biogeochemical conditions. If areas with high and low retention can be identified, regulation can be targeted allowing less strict regulation in some areas and focus stronger regulation and mitigation measures in areas, where nitrate leaching is high and nitrogen retention is low. This was recognized by the Danish Commission on Nature and Agriculture, who recommended new regulation principles based on a spatially differentiated and targeted nitrogen regulation.As a first step in exploring how a differentiated approach can be integrated in national regulation, a national nitrogen model has been developed for Denmark. The model is constructed by linking existing models describing nitrate leaching from the root zone, groundwater transport and reduction as well as surface water retention models. The models are coupled at sub-catchment scale dividing the country into topographic catchments with a mean size of 15 km2, which constitutes the computational units in the national model. Model development, calibration and validation have been performed on measurements of nitrogen transport from 340 streams gauging stations covering approximately half of the total area in Denmark. The national nitrogen model was initially designed to compute the nitrogen loads to coastal areas and compute national maps displaying the estimated nitrogen retention in groundwater, surface water and the total retention from the field to the sea. Further developments are focusing on extending the model to describe also nitrogen loads to the 402 groundwater bodies (consisting of 2771 groundwater aquifers) delineated in Denmark. Currently, new regulation principles utilizing new and more spatially targeted nitrogen measures are implemented in Denmark. The national nitrogen model and the associated retention maps have been important building blocks in the dimensioning of new mitigation measures required to meet the environmental goals, while at the same time optimizing agricultural production.