Effects of Glacial Sediment Type and Land Use on Nitrate Patterns in Groundwater

Abstract

Growing population centers such as those in southern Ontario rely on fractured bedrock aquifers for drinking water. A threat to these aquifers is posed by surficial nonpoint‐source pollution infiltrating with rainwater and moving through the overlying Quaternary glacial deposits. Investigation of local unconsolidated sediments, and the factors affecting contaminant transport through these, is needed to assess risks to the quality of underlying groundwater resources. In this study, sites with a variety of agricultural land management practices and glacial geologic settings were investigated by employing high‐resolution data collection methods. Geologic data from continuous sediment cores were combined with depth‐discrete hydrogeologic and geochemical parameter measurements using high‐resolution multilevel monitoring wells. Within a relatively small geographic area with three distinct glacially derived sediment types, the three sites exhibited greatly disparate vulnerability to nitrate contamination. The geologic setting, including surface topography and architecture and heterogeneity of sediment types at depth, influenced groundwater flow paths and water geochemistry, and subsequently, nitrate distribution. Although management practices influence the quantity of pollutants leaching to groundwater resources, the physical and chemical properties of the subsurface related to the geologic setting ultimately determine the persistence or attenuation of nitrate, and therefore become important to characterize when evaluating best nutrient and waste management practices.

Subsurface heterogeneity in the geological and hydraulic properties of the hummocky Paris Moraine, Guelph, Ontario

“…It is interesting to note that despite some variability in terms of silt content or degree of consolidation, the diamict units have relatively consistent hydraulic properties. In contrast, based on hydraulic data from site TGI-1A, monitoring well ports in the lowermost diamict are characterized by much slower pumping compared with coarser diamict units above 15 m (Best et al 2015). This is likely due to the loss of structure and consolidation; demonstrating how the use of disturbed sediment samples imposes important limitations on estimating K sat .…”

“…2), Best et al (2015) demonstrated how subsurface variability can affect groundwater recharge and the distribution of nitrate at depth (Fig. 12).…”

“…In contrast, the coarse-grained sediments of core ARS-1A and the laterally discontinuous basal diamict in the outwash plain allowed faster infiltration of surface water and contaminants at depth (Fig. 12b) as evidenced by modern groundwater in the top of bedrock and high concentrations of both nitrate and Escherichia coli (Best et al 2015). Lastly, a two-layered groundwater flow system exists at site VAN-1A, defined by a middle unit of mud (ϳ22 m depth) separating two aquifer units (sand and gravel) and modern groundwater at depth (Fig.…”

“…Lastly, a two-layered groundwater flow system exists at site VAN-1A, defined by a middle unit of mud (ϳ22 m depth) separating two aquifer units (sand and gravel) and modern groundwater at depth (Fig. 12c); this suggests that lateral changes in sediment types typical of the Paris Moraine allow the water to bypass this mud aquitard (Best et al 2015). Considering the variable succession of sediments encountered in the moraine, these three scenarios (Fig.…”

“…At the local scale, a detailed hydrogeological characterization of heterogeneity is warranted to define its impact on the groundwater flow regime or contaminant pathways, considering the wide range of hydraulic conductivities (8.48 × 10 −4 to 1.25 × 10 −7 m/s; Fig. 11) and to identify the complex geometry and connectivity of coarsergrained units in this type of ice-marginal setting (Boulton 1972;Paul 1983;Golder Associates Ltd. 2006;Blackport Hydrogeology Inc. et al 2009;Evans 2009;Best et al 2015). At the large scale, the hydraulic conductivity of the moraine sediments is relatively consistent (K Ga = 6.14 × 10 −7 m/s), even in places where the subsurface is lithologically variable (e.g., site .…”