An understanding of the fate of nitrate in groundwater is vital for managing risks associated with nitrate pollution, and to safeguard groundwater supplies and groundwater-dependent surface waters. One of the main measures to control nitrate is the designation of nitrate vulnerable zones (NVZs). Recent review of the designation process in England has highlighted that in some locations measured groundwater nitrate concentrations were not as high as expected. A possible reason for this is that the nitrate is being transformed through denitrification. This review forms the first stage in the development of a tiered approach to assessing denitrification potential across groundwater in England (and Wales) using three main lines of evidence: groundwater quality indicators, geological mapping of confined aquifers and superficial deposits which can influence redox status and a mass balance approach
Denitrification can take place either heterotrophically, where microorganisms use a sequence of ions as electron acceptors to enable the consumption of organic carbon, generating intermediates or by-products that can be used as process indicators, or autotrophically where pyrite is used as an electron donor and is oxidised to sulphate. Other nitrogen cycle processes which can produce some similar intermediates can include dissimilatory nitrate reduction directly to ammonia (DRNA), assimilatory nitrate reduction to organic nitrogen, anaerobic ammonium oxidation (anammox) and nitrification.
Demonstrating and quantifying denitrification is difficult, requiring specialist measurements and with different problems in environment and scale. Stable isotopic methods have been the most widely applied. Only a mass balance approach appears to be feasible in groundwater without new specialist measurement
