Denitrification in carbonate aquifers: a literature review

Denitrification is the most important process for nitrate removal in groundwater. Although carbonate aquifers are important for drinking water supply, the denitrification process in these systems is less understood than for unconsolidated sedimentary aquifers. We reviewed studies that (1) provide evidence for denitrification in carbonate aquifers, (2) discuss the specific location where it occurs, and (3) quantify denitrification rates. Literature sources were identified by a systematic search of scientific databases and follow-up reference tracking. The key finding is that denitrification in carbonate aquifers is possible, supported by microbiologic and/or isotopic evidence for several locations. Pore size and connectivity are major limiting factors for denitrification within the matrix. Denitrification in fissures/fractures would be limited by electron donor availability and unfavourable redox conditions. However, it has also been hypothesised that denitrification could occur in micro-anaerobic environments and biofilms. Denitrification rates for carbonate aquifers varied by several orders of magnitude (0.01–36 792 mg N/L/y). High concentrations and very reactive organic carbon from waste-water contamination and high groundwater temperatures are characteristic for some of the studies. Thus, rates may not be transferable to cooler oligotrophic conditions. Future research is needed to fill identified knowledge gaps.