Assessment of the usefulness of the isotopic composition of surface water nitrate (delta-nitrogen-15 and delta-oxygen-18) for identifying its sources in a watershed with differing land use

The overall goal of this research project was to assess the usefulness of the nitrogen (N) and oxygen (O) isotopic composition of surface water nitrate for identifying its sources in a watershed with differing land use. The Oldman River Basin, located in southern Alberta, provided an ideal case study to reach this goal. This watershed has almost pristine headwaters in its western part, and increased anthropogenic activities in its eastern part. Data collected included isotopic compositions of H2O, and of carbon (C), sulphur (S), N, and O of solutes in conjunction with chemical and hydrometric data. It was found that groundwater influx dominates surface water flow within this basin. A N budget indicated that anthropogenic N inputs (primarily manure) dominate N inputs, especially within the eastern region. Based on delta 13C values of dissolved inorganic carbon, it was concluded that riverine DIC is primarily controlled by carbonate dissolution via carbonic acid, besides carbonate dissolution by sulfuric or humic acids, photosynthesis, and exchange with atmospheric CO2. Chemical and isotopic data of sulphate clearly indicated a spatial control on riverine sulphate, partly due to the land use in the eastern region. Sulphate within the latter region is primarily derived from the oxidation of reduced sulphur species. Based on the chemical and isotopic composition of nitrate, it was concluded that the isotopic composition of riverine nitrate was primarily consistent with reflecting the original source signal instead of nitrate transformation processes. A positive correlation was observed between the N isotopic composition of nitrate and percent manure of total N input to sub-basins. Furthermore, a shift from a soil to a manure dominated nitrate source with downstream distance was identified in the Oldman River. Hence, this study provides strong qualitative evidence that stable isotopes are extremely useful in tracing riverine nitrate sources in a watershed with differing land use between its upstream and downstream regions.