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.

Supplementary information: The online version contains supplementary material available at 10.1007/s10533-025-01209-8.

Keywords: Agriculture; Fertilizers; Groundwater quality; Manure; Nitrate.

Fig. 1

Location map of Southern Alberta showing a the location of the different groundwater sample types, b the manure production index (AAFRD 2005) and location of confined feeding operations (CFOs, shown as light grey dots) (ABMI 2019), c bedrock geology modified from Prior et al. (2013), and d thickness of surficial sediments modified from Atkinson et al. (2020). In three panels, the location of irrigation districts is shown as outlined-black polygons

Fig. 2

Spatial distribution of median NO₃⁻–N concentrations in groundwater samples per township: a wells completed in surficial sediments, b wells in bedrock formations, and c NO₃⁻–N concentrations in spring samples in Southern Alberta. The Representativity Index (RI) of the median NO₃⁻–N concentration per township is depicted as follows: hashed squares indicate low RI, bold-outlined squares indicate high RI, and squares with no additional symbology indicate intermediate RI. Histograms of NO₃⁻–N concentrations for individual groundwater samples analyzed in this study are shown to the right of the maps

Fig. 3

Depth (m) vs. Ca + Mg to Na mass ratio in groundwater samples associated with surficial sediments (blue circles) and bedrock formations (red circles). The size range represents the NO₃⁻–N concentrations of the samples

Fig. 4

Distribution of NO₃⁻–N concentrations at different depth intervals for groundwater samples obtained from wells completed in surficial sediments (a) and bedrock formations (b) in Southern Alberta. The blue values indicate the percentage of samples with NO₃⁻–N concentrations above detection limits. The red dashed line represents the maximum acceptable concentration of 10 mg/L NO₃⁻–N for drinking water in Canada (Health Canada 2013). Boxes represent 25th, 50th, and 75th percentile values, whiskers extend to the smallest and largest values within Q1/Q3 − / + 1.5 × IQR, and closed circles represent outliers beyond that range; the star symbol indicates the mean NO₃⁻–N concentrations

Fig. 5

δ¹⁸O vs. δ¹⁵N values of nitrate in groundwater from monitoring wells completed in bedrock formations (circles) and surficial sediments (triangles) from Southern Alberta. Numbers from 1 to 7 indicate the individual GOWN wells and repeated numbers represent replicate samples from the same well. Boxes indicate ranges of stable isotope ratios of nitrate sources (Kendall et al. ; Proemse et al. ; Kruk et al. 2020). The solid and dashed blue arrows represent denitrification lines of 1:1 and 2:1 ratio, respectively. The legend shows the hydrochemical facies associated with each groundwater sample

Fig. 6

Theoretical evolution of δ¹⁵N–NO₃⁻ values and NO₃⁻–N concentrations. Blue arrows represent theoretical evolution during denitrification for a source A (SA) with a NO₃⁻–N concentration of 300 mg/L and a δ¹⁵N–NO₃⁻ of 18‰ with N isotope enrichment factors of ε =  − 4‰ (dashed blue line), ε =  − 9‰ (dotted blue line), and ε =  − 14‰ (solid blue line). Numbers from 1 to 7 indicate the individual GOWN wells and repeated numbers represent replicate samples