Isotopic Assessment of Sources of Surface Water Nitrate within the Oldman River Basin, Southern Alberta, Canada

Concentrations and isotopic compositions of NO3- from the Oldman River (OMR) and some of its tributaries (Alberta, Canada) have been determined on a monthly basis since December 2000 to assess temporal and spatial variations of riverine NO3- sources within the OMR basin. For the OMR sites, NO3--N concentrations reached up to 0.34 mg L-1, δ15N-NO3- values varied between -0.3 and +13.8 per mille , and δ18O-NO3- values ranged from -10.0 to +5.7 per mille . For the tributary sites, NO3--N concentrations were as high as 8.81 mg L-1, δ15N-NO3- values varied between -2.5 and +23.4 per mille , and δ18O-NO3- values ranged from -15.2 to +3.4 per mille . Tributaries in the western, relatively pristine forested part of the watershed add predominantlyNO3- to the OMR with δ15N-NO3- values near +2 per mille indicative of soil nitrification. In contrast, tributaries in the eastern agriculturally-urban-industrially-used part of the basin contribute NO3- with δ15N-NO3- values of about +16 per mille indicative of manure and/or sewage derived NO3-. This difference in δ15N-NO3- values of tributaries was found to be independent of the season, but rather indicates a spatial change in the NO3- source, which correlates with land use changes within the OMR basin. As a consequence of tributary influx, δ15N-NO3- values in the Oldman River increased from <+3 per mille to >+6 per mille in the downstream direction (W to E), although [NO3--N] increased only moderately (generally <0.5 mg L-1). This study demonstrates the usefulness of δ15N-NO3- and δ18O-NO3- values in identifying the addition of anthropogenic NO3- to riverine systems