| Relatively high manure-derived nitrate concentrations (∼32 mg NO3-N/L) are found below an irrigated and manured farm field located in the Battersea drainage basin in Southern Alberta. The field overlies a shallow unconsolidated sandy aquifer adjacent to an agricultural drainage canal. The purpose of this study was to quantify the attenuation of nitrate concentrations along the horizontal subsurface flowpath within the aquifer. Geochemical and isotope analyses of groundwater samples collected along the flowpath reveal that manure-impacted young groundwater is mixing with a localized older, low redox groundwater upwelling from the bottom clay layer of the aquifer. Decreasing NO3−/Cl− ratios are accompanied by a decrease in dissolved oxygen concentrations and by an increase of δ15N-NO3 and δ18 O-NO3 values in the residual nitrate. These trends are particularly evident where mixing with low redox groundwater occurs. Approximately 34% of the manure-derived nitrate is attenuated in this mixing zone by denitrification thereby improving the groundwater and subsequently surface water quality with respect to nitrate. However, denitrification appears to result from decreased redox conditions associated with upwelling of deeper water and, due to more oxidizing conditions, would not likely occur in the absence of upwelling. |
