Leaching of nitrate (NO<sub>3</sub><sup>−</sup>) from animal waste or fertilizers at agricultural operations can result in NO<sub>3</sub><sup>−</sup> contamination of groundwater, lakes, and streams. Understanding the sources and fate of nitrate in groundwater systems in glacial sediments, which underlie many agricultural operations, is critical for managing impacts of human food production on the environment. Elevated NO<sub>3</sub><sup>−</sup> concentrations in groundwater can be naturally attenuated through mixing or denitrification. Here we use snapshots of the stable isotope values of NO<sub>3</sub><sup>−</sup> to quantify denitrification in groundwater at two confined feeding operations overlying glacial sediments in Alberta, Canada. Uncertainty in <i>δ</i><sup>15</sup>N<sub>NO<sub>3</sub></sub> and <i>δ</i><sup>18</sup>O<sub>NO<sub>3</sub></sub> values of the NO<sub>3</sub><sup>−</sup> source and denitrification enrichment factors are accounted for using a Monte Carlo approach. When denitrification could be quantified, we reconstructed the initial NO<sub>3</sub>-N concentration and NO<sub>3</sub>-N / Cl<sup>−</sup> ratio at the point of entry to the groundwater system. The addition of NO<sub>3</sub><sup>−</sup> to the local groundwater system from temporary manure piles and pens equalled or exceeded NO<sub>3</sub><sup>−</sup> additions due to leaching from earthen manure storages at these sites. Nitrate attenuation at both sites is attributed to a spatially variable combination of mixing and denitrification, but is dominated by denitrification. On-site denitrification reduced agriculturally derived NO<sub>3</sub><sup>−</sup> concentrations by at least half and, in some wells, completely. These results indicate that infiltration to groundwater systems in glacial sediments where NO<sub>3</sub><sup>−</sup> can be naturally attenuated is likely preferable to off farm export via runoff or drainage networks. The application of isotopes of nitrate to constrain a mixing model based on concentrations of Cl<sup>−</sup> and NO<sub>3</sub><sup>−</sup>, which can be routinely monitored in groundwater, provides a relatively simple method to assess the sources and fate of agriculturally derived NO<sub>3</sub><sup>−</sup> in these settings.