HR: 09:45h
AN: H51E-07    [Abstracts]
TI: Isotopic Tracing of Sources of Riverine Nitrate: The Example of the Oldman River Basin, Alberta, Canada
AU: * Mayer, B
EM: bmayer@ucalgary.ca
AF: University of Calgary, Department of Geology and Geophysics, 2500 University Drive NW, Calgary, AB T2N 1N4 Canada
AU: Rock, L
AF: University of Calgary, Department of Geology and Geophysics, 2500 University Drive NW, Calgary, AB T2N 1N4 Canada
AU: Ellert, B
AF: Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, AB T2N 1N4 Canada
AU: McCallum, J
AF: University of Calgary, Department of Geology and Geophysics, 2500 University Drive NW, Calgary, AB T2N 1N4 Canada
AU: Ryan, C
AF: University of Calgary, Department of Geology and Geophysics, 2500 University Drive NW, Calgary, AB T2N 1N4 Canada
AB: The isotopic composition of dissolved nitrate in aquatic ecosystems can be a valuable tracer for its sources and its biogeochemical history. Nitrate derived from atmospheric deposition, synthetic fertilizers, soil nitrification, and from sewage or manure can often be distinguished if both nitrogen and oxygen isotope ratios are determined. Additionally, denitrification result in characteristic patterns of progressively increasing nitrogen and oxygen isotope ratios as nitrate concentrations decrease. The objective of this study was to determine the sources of riverine nitrate and the processes it may have undergone in the Oldman River Basin in southern Alberta, Canada, using a combination of hydrological, chemical, and isotopic techniques. In the mainly forested upstream portion of the Oldman River Basin, riverine nitrate was characterized by low concentrations (0.2 mg/L) and low nitrogen isotope ratios (+3 per mil). In the agriculturally used lower part of the Oldman River Basin, concentrations and nitrogen isotope ratios of riverine nitrate increased significantly, particularly in the winter months. Sampling of tributaries revealed that canals and drains in the irrigation districts were often characterized by nitrate concentrations exceeding 35 mg/L and nitrogen isotope ratios near +15 per mil. Many of the fields in the irrigation districts in the lower part of the Oldman River Basin are regularly manured at rates exceeding 15 Mg/ha/year. Topsoils had total N concentrations of 4 mg/g and N isotope ratios approaching +15 per mil as a consequence of repeated manure applications. Concentrations of dissolved nitrate in an aquifer underneath an agricultural field approached 50 mg/L and nitrogen isotope ratios ranged between +11 and +14 per mil, whereas oxygen isotope ratios were negative. This suggests that manure applied to agricultural fields in the lower part of the Oldman River basin is the dominant source of groundwater nitrate in the underlying aquifers. This nitrate infiltrates into the irrigation and drainage canals particularly in the non-irrigation season, when water levels are low. Agricultural return-flow provides a feasible explanation for elevated concentrations and nitrogen isotope ratios of dissolved nitrate in the drainage canals, which constitute tributaries to the Oldman River. The increased nitrate load in the tributaries is ultimately responsible for increased concentrations and elevated nitrogen isotope ratios of riverine nitrate in the Oldman River during the winter months.
DE: 1831 Groundwater quality
DE: 1860 Runoff and streamflow
DE: 1871 Surface water quality
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting