Mixing-induced groundwater denitrification beneath a manured field in southern Alberta, Canada

Contamination of shallow groundwater by NO3- from manure may occur under fields where manure is spread as fertilizer and for disposal. Attenuation of NO3- in groundwater occurs through denitrification under certain conditions, or NO3--contaminated younger groundwater may mix with older groundwater, lowering the NO3- concentration. In this study, δ15N and δ18O values of NO3-, and δ18O and δ2H values in groundwater under a manured field were evaluated to determine if groundwater NO3- concentrations were influenced through mixing of shallower, manure-impacted groundwater with older groundwater, or if denitrification was reducing NO3- concentrations. The younger groundwater showed clear evidence of manure impact with elevated Cl− (∼85 mg L−1) and NO3- concentrations (∼50 mg NO3–N L−1), and δ15N and δ18O values of NO3- consistent with a manure source. Vertical hydraulic gradients and δ18O and δ2H values in groundwater suggest older, more reduced groundwater is upwelling locally and mixing with the shallow groundwater. Decreasing NO3:Cl ratios, decreasing dissolved O2 concentrations, and increasing δ15N and δ18O values of NO3- suggest that denitrification occurs locally in the aquifer. The extent of denitrification is proportional to the fraction of deeper groundwater in the aquifer. Denitrification apparently does not proceed in the younger, manure-impacted groundwater in the absence of mixing.