Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

Leaching of nitrate (NO3-) from animal waste or fertilisers at agricultural operations can result in NO3- 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 NO3- concentrations in groundwater can be naturally attenuated through mixing or denitrification. Here we use isotopic enrichment of the stable isotope values of NO3- to quantify the amount of denitrification in groundwater at two confined feeding operations overlying glacial sediments in Alberta, Canada. Uncertainty in delta(NNO3)-N-15 and delta(ONO3)-O-18 values of the NO3- source and denitrification enrichment factors are accounted for using a Monte Carlo approach. When denitrification could be quantified, we used these values to constrain a mixing model based on NO3- and Cl concentrations. Using this novel approach we were able to reconstruct the initial NO3-N concentration and NO3-N/Cl ratio at the point of entry to the groundwater system. Manure filtrate had total nitrogen (TN) of up to 1820 mg L-1, which was predominantly organic N and NH3. Groundwater had up to 85 mg L-1 TN, which was predominantly NO3-. The addition of NO3- to the local groundwater system from temporary manure piles and pens equalled or exceeded NO3- additions from earthen manure storages at these sites. On-farm management of manure waste should therefore increasingly focus on limiting manure piles in direct contact with the soil and encourage storage in lined lagoons. Nitrate attenuation at both sites is attributed to a spatially variable combination of mixing and denitrification, but is dominated by denitrification. Where identified, denitrification reduced agriculturally derived NO3- concentrations by at least half and, in some wells, completely. Infiltration to groundwater systems in glacial sediments where NO3- can be naturally attenuated is likely preferable to off-farm export via runoff or drainage networks, especially if local groundwater is not used for potable water supply.