The invention relates to a process for decreasing the content of nitrate in ground water, artificial ground water, that is infiltrated surface water, or sarface water flowing through an aquifer or through a basin or a filter containing naturally occurring filter material. Water containing denitrification organisms and or substrate therefore is introduced intermitterially through a number of injection wells or injection pipes arranged around the or more extraction wells or extraction pipes for purified water. During each introduction of water containing denitrification organisms and/or substrate therfore said water is fed to only some of said injection welle or pipes and simultaneously water is drawn from nearby, adjacent or intervening injection wells or pipes or the purpose of creating a denitrification zone at a distance suficiently far from the extraction well or extraction pipe.