HR: 0800h
AN: H41D-0328    [Abstracts]
TI: Evaluation of Stable Isotope Analysis as a Tool to Determine Nitrate Sources in Irish Groundwaters
AU: * Minet, E
EM: eminet@tcd.ie
AF: Centre for the Environment, Geology Department Trinity College Dublin, Dublin, 2 Ireland
AU: Coxon, C
EM: cecoxon@tcd.ie
AF: Centre for the Environment, Geology Department Trinity College Dublin, Dublin, 2 Ireland
AU: Kalin, R
EM: r.kalin@qub.ac.uk
AF: Questor Centre, Queen's University of Belfast, Belfast, BT9 5AG United Kingdom
AB: Natural abundance of stable isotope ratios of nitrogen (\delta$^{15}$N) and oxygen (\delta$^{18}$O) in NO$_{3}$ were measured in a nitrate-vulnerable aquifer in eastern Ireland underlying an intensive agricultural area. The aim was to determine whether the dual stable isotope approach (\delta$^{15}$N and \delta$^{18}$O) could provide a more power tool than \delta$^{15}$N measurements alone for determining the source of groundwater nitrate. Forty-five private wells and boreholes representing a range of potential N sources (diffuse sources such as synthetic fertiliser spreading and/or farmyard effluents and/or septic tank effluents) were sampled on seven occasions between 2002 and 2004. Additionally, nitrate leachate samples were collected from the unsaturated zone under small-scale experiments simulating different agricultural practices. Nitrate was extracted from each water sample using an anion exchange resin technique (Silva et al., 2000), and \delta$^{15}$N and \delta$^{18}$O values were both measured by Continuous-Flow Isotopic Ratio Mass Spectrometry. The initial hypothesis was that, in a shallow gravel aquifer where nitrate leaching is important and fast, groundwater nitrates are likely to display the isotopic characteristics of their source (low \delta$^{15}$N and high \delta$^{18}$O for diffuse sources, compared to high \delta$^{15}$N and low \delta$^{18}$O for point sources). The first set of results indicates that the \delta$^{15}$N values only follow this trend, although the \delta$^{18}$O values suggest the occurrence of denitrification. The levelling of \delta$^{18}$O values may in fact be caused by the Mineralisation-Immobilisation Turnover processes (MIT) that have previously been found to occur in the unsaturated zone, incorporating new oxygen atoms during the remineralisation of the nitrate (Mengis et al., 2001). References Mengis, M.; Walther, U.; Bernasconi, S.M.; Wehrli, B. (2001) Limitations of using \delta$^{18}$O for the source identification of nitrate in agricultural soils. Environmental Science & Technology, 35, 1840-1844. Silva, S.R.; Kendall, C.; Wilkison, D.H.; Ziegler, A.C.; Chang, C.C.Y.; Avanzino, R.J. (2000) A new method for collection of nitrate from fresh water and the analysis of nitrogen and oxygen isotope ratios. Journal of Hydrology, 228, 22-36.
DE: 1806 Chemistry of fresh water
DE: 1831 Groundwater quality
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting