A new concept for assessing groundwater nitrate reduction rates on a national scale

Assessing the agricultural impact on groundwater quality is important for evaluation and selection of mitigation measures for regulation of agricultural nitrogen management and fertilization. Many European countries with intensive farming, including Denmark, have successfully lowered the level of N pollution in the aquatic environment by introducing different mitigation measures aimed at agricultural regulation and management. In Denmark, successful one-size-fits-all national general nitrogen regulation has been implemented since the mid 1980’s resulting in a trend reversal of nitrate in oxic Danish groundwater. Since 2016 focus has been on developing cost-efficient geographically targeted mitigation measures as a supplement to the general regulation to further decrease the reactive nitrogen impact on streams and the marine environment.
Targeted mitigation measures require detailed hydrogeological and geochemical knowledge about the redox conditions and the denitrification rates in the subsurface. This knowledge is crucial for modelling the nitrate transport from agricultural fields to streams and coastal areas. A new Danish national map of the subsurface redox conditions has been developed using machine learning techniques. Determining the rate of denitrification in an aquifer is challenging as laboratory analyses often show high rates with very high variation likely not representing actual in-situ rates. Assessing the extent and the denitrification rates in the anoxic nitrate reducing zone will be presented, and a new national concept for determining denitrification rates will be presented. This national concept relies on assessment of nitrate trends in oxic and anoxic nitrate reducing groundwater using long-term groundwater monitoring data on the nitrate content and ages of the groundwater dated with CFC or the 3H/3He method.