Groundwater nitrate in three distinct hydrogeologic and land-use settings in southwestern Ontario, Canada

Groundwater nitrate is a concern in the Great Lakes Basin (GLB) of North America. Concentrations vary based on hydrogeologic setting and surface contamination pressures. The first objective of this study was to characterize groundwater nitrate trends in a spatial and temporal context across three unique hydrogeologic and land-use settings in the GLB, at research sites in Norfolk, Guelph, and Acton, in southwestern Ontario, Canada. The second objective was to compare groundwater nitrate trends at the sites to determine the potential influence of hydrogeologic setting on seasonal nitrate variability. Groundwater nitrate-N concentrations and additional parameters were measured at 25 monitoring wells at the three sites, once every 2 months from June 2014 to July 2016 (13 sampling events). The dominant land-use pressure at all three sites is agricultural; however, the type of agriculture and the amount of natural and urban land vary. The Guelph site had the highest nitrate concentrations, while the Acton and Norfolk mean concentrations were similar. Seasonally, nitrate was found to be highest during the spring and early summer periods at all three sites. Several monitoring locations had cyclical patterns in nitrate concentrations, while others remained consistent except for isolated peak events. Comparison of multiple years reveals the expression of a relationship between climatic conditions during winter months and nitrate concentrations, indicating the importance of antecedent conditions to groundwater nitrate concentrations. The combination of climatic conditions, land-use pressure, and hydrogeologic setting plays an important role in the characteristics of groundwater nitrate time-series trends.