A preliminary assessment of nitrous oxide in Chalk groundwater in Cambridgeshire, U.K.

Abstract

Groundwater samples from boreholes and springs in the unconfined Chalk aquifer of Cambridgeshire were analysed for N 2O and other N species on a monthly basis between March 1995 and February 1996. Land use in the study area is devoted to intensive arable farming supported by the application of N-based fertilisers. All groundwater samples were strongly oversaturated with N 2O, with concentrations ranging from 13 to 320 times the saturation concentration with respect to air-equilibrated water. A very good positive correlation between N 2O and NO 3 concentrations was obtained ( r 2=0.80 ), but no relationship was established between N 2O and NO 2 or NH 4 concentrations. Concentrations of N 2O and NO 3 increased continuously in the direction of groundwater flow, with molar net gain ratios of NO 3 to N 2O varying between 204 and 410. These ratios are within the range reported in previous studies of nitrification. Corresponding dissolved O 2 levels in groundwater samples were moderately undersaturated, further indicating that the main source of N 2O in Chalk groundwater in Cambridgeshire is probably nitrification. No consumption of N 2O seems to take place within the unconfined aquifer with degassing to the atmosphere apparently being the sole mechanism for N 2O removal from groundwater. An estimated N 2O flux of around 0.05 kg N 2O ha −1 a −1 from the sampled groundwater discharge points to the atmosphere was calculated for the study area. This figure is likely to be much higher, since it does not account for diffuse N 2O emissions from groundwater seepage areas or any degassing from the unconfined aquifer through the unsaturated zone. Both these processes will contribute substantially to the total aerial flux, thus suggesting that groundwater may be a significant contributor to the global N 2O budget.