Factors affecting denitrification during infiltration of river water into a sand and gravel aquifer in Saxony, Germany

River infiltration into a sand and gravel aquifer was investigated to assess the importance of denitrification in maintaining low-NO−3 groundwater supplies. Samples from the River Elbe and groundwater sampling points along a section of the aquifer were analysed for dissolved organic carbon, major ions and the 15N/14N isotopic ratio of dissolved NO−3. Input of NO−3 to the aquifer is influenced by seasonal, temperature-dependent denitrification in the river bed sediments. Along an upper flowpath in the aquifer from the River Elbe to a sampling point at a distance of 55 m, the median NO−3 concentration decreased by 4.8 mg litre−1 and the δ15N composition increased by +9.0‰, consistent with denitrification. Similar isotopic enrichment was demonstrated in a laboratory column experiment with a reduction in NO−3 of 10.5 mg litre−1 for an increase in δ15N of +9.8‰, yielding an isotopic enrichment factor of −14.6‰. A mass balance for denitrification shows that oxidizable organic carbon required for denitrification is derived from both the infiltrating river water and solid organic matter fixed in the river bed sediments and aquifer material.