Nitrous oxide production and consumption in British limestone aquifers

Nitrous oxide and other nitrogen species were measured in groundwater from the most important limestone aquifers in the UK. Nitrous oxide levels were generally very high, with concentrations exceeding the concentration of air-equilibrated water by up to 320 times. The correlations between N₂O, NO₃ and dissolved oxygen, as well as the spatial distribution of these chemical parameters, were used to identify nitrification as the main N₂O production mechanism in the investigated aquifers. Most of the N₂O in groundwater appears to be produced via nitrification in the unsaturated zone, which is probably strongly supplemented by nitrogenous compounds from anthropogenic land applications. Nitrous oxide production in the saturated zone is less substantial and can also be denitrification mediated; denitrification under very reducing aquifer conditions can result in nitrous oxide consumption. The observed high N₂O concentrations in groundwater, which are most likely caused by agricultural aquifer pollution, can potentially lead to large N₂O emissions from aquifers to the atmosphere, which will contribute to the climatic forcing induced by higher atmospheric N₂O concentrations.