Distribution, sources, and fate of nitrate in groundwater in agricultural areas of Southern Alberta, Canada

Abstract

Nitrate pollution frequently impacts groundwater quality, particularly in agricultural regions across the world, but identifying the sources of nitrate (NO₃⁻) pollution remains challenging. The extensive use of nitrogen-containing fertilizers, surpassing crop requirements, and livestock management practices associated with the spreading of manure can lead to the accumulation and transport of NO₃⁻ into groundwater, potentially affecting drinking water sources. We investigated the occurrence and distribution of NO₃⁻ in groundwater in Southern Alberta, Canada, a region characterized by intensive crop cultivation and livestock industry. Over 3500 samples from a provincial-scale groundwater quality database, collated from multiple projects and sources, involving domestic wells, monitoring wells, and springs, coupled with newly obtained samples from monitoring wells provided comprehensive geochemical insights into groundwater quality. While stable isotope compositions of NO₃⁻ (δ¹⁵N and δ¹⁸O) were exclusively available for groundwater samples obtained from monitoring wells, the stable isotope data were instrumental in constraining NO₃⁻ sources and transformation processes within the aquifers of the study region. Among all samples, 49% (n = 1746) were associated with NO₃⁻ concentrations below the detection limits. Ten percent (n = 369) of all groundwater samples, including samples with concentrations below detection limits, exceed the Canadian drinking water maximum acceptable concentration of 10 mg/L for nitrate as nitrogen (NO₃⁻–N). Elevated NO₃⁻ concentrations (> 10 mg/L as NO₃⁻–N) in groundwater were mainly detected at shallow depths (< 30 m) predominantly in aquifers in surficial sediments and less frequently in bedrock aquifers. Statistical correlations between aqueous geochemical parameters showed positive associations between concentrations of NO₃⁻–N and both potassium (K⁺) and chloride (Cl⁻), indicating the influence of synthetic fertilizers on groundwater quality. In addition, isotope analyses of NO₃⁻ (δ¹⁵N and δ¹⁸O) revealed three NO₃⁻ sources in groundwater, including mineralization of soil organic nitrogen followed by nitrification in soils, nitrification of ammonium or urea-based synthetic fertilizers in soils, and manure. However, manure was identified as the dominant source of NO₃⁻ exceeding the maximum acceptable concentration in groundwater within agriculturally dominated areas. Additionally, this multifaceted approach helped identify denitrification in some groundwater samples, a process that plays a key role in reducing NO₃⁻ concentrations under favorable redox conditions in shallow aquifers. The methodological approach used in this study can be applied to other regions worldwide to identify NO₃⁻ sources and removal processes in contaminated aquifers, provided there are well networks in place to monitor groundwater quality and drinking water sources.