Groundwater denitrification in Denmark—evolution, extent and reaction rates

Globally, nitrogen (N) pollution of the environment is a widespread consequence of intensive agriculture. Denmark has combated this challenge from mid-1980 by implementing effective national N regulation of agriculture. Further reduction is needed to meet legislative objectives, and more targeted farm-level regulation schemes are under development. Assessing the necessary farm-level reduction of agricultural N losses to the aquatic environment such as groundwater requires detailed knowledge of denitrification in the groundwater system. In this paper we demonstrate how determinations of the extent of the anoxic nitrate-reducing zone and associated denitrification rates can be done by combined use of national and local level multiscreen groundwater monitoring data representing different agro-hydrogeochemical conditions across Denmark. A new concept for evaluating groundwater denitrification both nationally and locally is presented. The new concept relies on the assessment of nitrate trends in oxic and anoxic nitrate-reducing groundwater using up to 35 years of groundwater monitoring data on nitrate content and groundwater age, dated with CFC or the ³H/³He method. The results from the anoxic nitrate-reducing zone indicate that the probability of denitrification increases with groundwater residence time. On average, the national level assessment shows that in the anoxic nitrate-reducing zone approximately 47% of the nitrate is being denitrified while in the local areas the assessments show variation between 14%-59%, and estimations of denitrification rates range from 0.008-0.032 µmol l⁻¹ day⁻¹. Results show that the variation in the thickness of the anoxic nitrate-reducing zone can be up to 16 m, indicating that the denitrification rates can vary from very fast (thin zone or sharp interface) to slow (thick zone) in a homogenous aquifer layer with the same magnitude of residence time. These results are important for groundwater modeling of nitrate transport from agricultural fields to streams and coastal areas through the groundwater system.