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

Groundwater samples from boreholes and springs in the unconfined Chalk aquifer of Cambridgeshire were analysed for N₂O 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 N₂O, with concentrations ranging from 13 to 320 times the saturation concentration with respect to air-equilibrated water.A very good positive correlation between N₂O and NO₃ concentrations was obtained (r²=0.80), but no relationship was established between N₂O and NO₂ or NH₄ concentrations. Concentrations of N₂O and NO₃ increased continuously in the direction of groundwater flow, with molar net gain ratios of NO₃ to N₂O varying between 204 and 410. These ratios are within the range reported in previous studies of nitrification. Corresponding dissolved O₂ levels in groundwater samples were moderately undersaturated, further indicating that the main source of N₂O in Chalk groundwater in Cambridgeshire is probably nitrification. No consumption of N₂O seems to take place within the unconfined aquifer with degassing to the atmosphere apparently being the sole mechanism for N₂O removal from groundwater.An estimated N₂O flux of around 0.05 kg N₂O 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 N₂O 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 N₂O budget.