A PRELIMINARY ASSESSMENT OF NITROUS-OXIDE IN CHALK GROUNDWATER IN CAMBRIDGESHIRE, UK

Groundwater samples from boreholes and springs in the unconfined Chalk aquifer of Cambridgeshire were analysed for N2O and other N species o n a monthly basis between March 1995 and February 1996. Land use in th e study area is devoted to intensive arable farming supported by the a pplication of N-based fertilisers. All groundwater samples were strong ly oversaturated with N2O, with concentrations ranging from 13 to 320 times the saturation concentration with respect to air-equilibrated wa ter. A very good positive correlation between N2O and NO3 concentratio ns was obtained (r(2) = 0.80), but no relationship was established bet ween N2O and NO2 or NH4 concentrations. Concentrations of N2O and NO3 increased continuously in the direction of groundwater flow, with mola r net gain ratios of NO3 to N2O varying between 204 and 410. These rat ios are within the range reported in previous studies of nitrification . Corresponding dissolved O-2 levels in groundwater samples were moder ately undersaturated, further indicating that the main source of N2O i n Chalk groundwater in Cambridgeshire is probably nitrification. No co nsumption of N2O seems to take place within the unconfined aquifer wit h 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 f rom groundwater seepage areas or any degassing from the unconfined aqu ifer through the unsaturated zone. Both these processes will contribut e substantially to the total aerial flux, thus suggesting that groundw ater may be a significant contributor to the global N2O budget. (C) 19 97 Elsevier Science Ltd. All rights reserved.