HR: 0800h
AN: H41D-0326    [Abstracts]
TI: A Dual Isotope Study of Nitrates in Aquifer and Surface Waters: Initial Results at the Watershed Scale
AU: * Savard, M M
EM: msavard@nrcan.gc.ca
AF: Geological Survey of Canada, 880 chemin Sainte-Foy, suite 840, Qu‚bec, QC G1S 2L2 Canada
AU: Simpson, S
EM: shawna.simpson@NRCan.gc.ca
AF: Geological Survey of Canada, 601 Booth Street, Room 117, Ottawa, ON K1A 0E8 Canada
AU: Smirnoff, A
EM: asmirnof@nrcan.gc.ca
AF: Geological Survey of Canada, 880 chemin Sainte-Foy, suite 840, Qu‚bec, QC G1S 2L2 Canada
AU: Somers, G
EM: ghsomers@gov.pe.ca
AF: Environment and Energy of Prince-Edward Island, 11 Kent St., C.P. 2000, Charlottetown, PEI C1A 7N8 Canada
AU: van Bochove, E
EM: vanbochovee@agr.gc.ca
AF: Agriculture and Agri-Food Canada, 2560 Bld Hochelaga, Sainte-Foy, QC G1V 2J3 Canada
AB: This dual isotope-nitrate study is aimed at contributing to the quantification of the annual N budget of the Wilmot River watershed on Prince-Edward Island (PEI, Canada). Aquifers constitute the only source of freshwater on PEI. In many areas, nitrate concentrations in groundwater (GW) have been increasing over time. It is assumed that mineral fertilization for potato cropping constitutes a major source of nitrates. However, a better understanding of the transfer dynamics of nitrates from soils to GW is required to reduce their detrimental effects. Here we report on N concentrations ([N-NO3-]), nitrate isotope analyses (N and O), and water isotope ratios (H and O) obtained after one year of seasonal sampling of surface water and groundwater (GW). The analyses of the nitrate isotopes were performed on silver nitrates using EA-CF- and Pyrolysis/EA-IRMS systems for N and O, respectively. Water isotopes were analysed with an equilibration system in continuous flow. Our 2003 summer and fall results indicate that 23% of the samples have N-NO3 concentrations above the threshold established for human health (10 mg/L), whereas 10% have concentrations within natural ranges (<1 mg/L). Combined nitrate and water isotope results suggest that during summer and fall most nitrates in the Wilmot River are derived from GW, and that about 75% of the GW samples contain nitrates from chemical fertilizers while the remaining 25% of nitrates are from natural soils, manures or septic wastes. Moreover, the N isotopes-nitrate concentration trend for GW departs significantly from the curve expected to result from microbial denitrification and corresponds better to natural attenuation due to dilution of nitrate-rich waters with water devoid of nitrates. The oxygen isotopes in GW nitrates indicate that summer rain and evaporated vadose water likely represent the O sources involved in soil nitrification. Future work will involve characterization of potential nitrate sources and of N-species from the vadose zone, characterization of vertical distribution of the isotopic tracers following sampling of discrete intervals along wells, as well as 2D and 3D modelling.
DE: 1803 Anthropogenic effects
DE: 1806 Chemistry of fresh water
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting