A regional assessment of occurrences, sources and fate of nitrate in groundwater of Alberta

Nitrate contamination in groundwater is a widespread problem in Europe, Northern America, Asia, and elsewhere due to adverse health effects if the water is used for drinking purposes. The objectives of this study were to a) assess the occurrence of nitrate in groundwater throughout the province of Alberta (Canada), b) to use isotope techniques to assess the predominant sources of nitrate in groundwater, and c) to use a combination of chemical and multi-isotopic techniques to assess the fate of groundwater nitrate. The study utilized >60,000 NO₃-N concentration data points from domestic water wells supplemented by 986 chemical and isotopic data points collected from groundwater monitoring water wells in Alberta.

In Alberta, nitrate has been detected in 34% (22,943 out of 66,421 samples) of the groundwater samples collected from domestic wells and NO₃-N concentrations range from <0.005 to 421 mg NO₃-N/L. Nitrate was detected in 18% (180 out of 986 samples) of the groundwater samples collected from monitoring wells with nitrate concentrations ranging from <0.004 to 300 mg NO₃-N/L. Nitrate levels in 3.4% (2279 of 66,421 samples) of groundwater samples from domestic wells and <1% (8 out of 986 samples) of groundwater samples from monitoring wells were above the maximum allowable concentration (MAC) in drinking water of 10 mg/L NO₃-N. To determine the sources of nitrate in groundwater, we used a multi-isotope approach on a subset of groundwater samples (n < 70) from monitoring wells. Three different potential sources of nitrate in groundwater were identified: 1) nitrate from microbial nitrification of soil organic matter and/or synthetic fertilizers represented by samples with δ¹⁵N <+6‰ and δ¹⁸O <+5‰; 2) manure-derived nitrate characterized by δ¹⁸O values <+5‰ and elevated δ¹⁵N values of >+10‰ combined with elevated nitrate concentrations; and 3) in a few cases geogenic nitrate in weathered glacial tills where clay-derived ammonium was oxidized to nitrate resulting in the highest nitrate concentrations and unique isotopic compositions of nitrate with δ¹⁵N_NO3values near 26 ‰ and δ¹⁸O_NO3 values of +5 ‰. We also found that the occurrence and concentration of groundwater nitrate was critically dependent on the redox conditions in the aquifers. Geochemical analyses revealed that 66% of all groundwater samples were moderately to highly reducing suggesting that denitrification is a widespread NO₃ removal process in groundwater in Alberta. Compiling geochemical and multi-isotopic data enabled us to assess the occurrence and the origins of nitrate in groundwater in Alberta and identify denitrification as an important natural attenuation process that has the potential to remove nitrate from the investigated aquifers.