Geophysical, geological, and hydrogeological characterization of a tributary buried bedrock valley in southern Ontario

Abstract

Buried bedrock valleys infilled with Quaternary-aged sediment have the potential to become productive aquifers owing to prevalent sand and gravel deposits often associated with these topographic lows. In areas where groundwater is drawn from the underlying bedrock aquifer, buried bedrock channels may significantly affect the spatial distribution of recharge and localized contaminant pathways. Therefore, understanding the form, distribution, and the nature of Quaternary infill sediments within these buried bedrock river valleys, and their relationship to hydraulically transmissive bedrock features is an important aspect of groundwater resource management. Here, we evaluate the effectiveness of electrical resistivity and seismic refraction collected over a partially urbanized 150 ha area with variable vegetation, roads, and structures, to map the spatial distribution of sediments and delineation of a channel segment associated with a regional bedrock valley. Electrical resistivity and seismic refraction was performed along 13 (covering ϳ11.6 km) and seven transects (covering ϳ0.9 km), respectively, to map and characterize the bedrock surface morphology beneath a variable thickness of unconsolidated deposits. Three continuously cored holes and downhole geophysical logs, supplemented with four nearby water well records captured the in-channel as well as adjacent Quaternary stratigraphy (ϳ15-40 m). Cores recorded multiple glacial till deposits and ice-marginal processes associated with ice advances and retreats. Hydraulic transmissivity of the bedrock around the valley feature was evaluated using a FLUTe hydraulic transmissivity profiling technique. This study demonstrates the potential of combining several surface geophysical methods with sedimentological analysis of continuous cores and hydraulic data for characterizing tributary bedrock channel morphology and Quaternary infill at a scale relevant to localized studies of municipal production well recharge zones and contaminant transport and fate.Résumé : Les vallées ensevelies taillées dans le roc remplies de sédiments d'âge quaternaire ont le potentiel de devenir des aquifères productifs en raison des importants dépôts de sable et de gravier souvent associés à ces dépressions topographiques. Dans les régions où l'eau souterraine est tirée de l'aquifère rocheux sous-jacent, les chenaux ensevelis taillés dans le roc pourraient avoir une importante incidence sur la répartition spatiale de la recharge et des voies de propagation des contaminants. La compréhension de la forme, de la répartition et de la nature des sédiments quaternaires qui remplissent ces vallées fluviales ensevelies et leur lien avec des caractéristiques du substrat rocheux transmissif est un aspect important de la gestion des ressources d'eau souterraine. Nous évaluons l'efficacité des méthodes de résistivité électrique et de sismique réfraction utilisées dans une zone partiellement urbanisée de 150 ha présentant une répartition variable de la végétation, des routes et des ouvrages, pour c...

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

“…9). The basal gravel in core GDC-1A likely formed by in situ weathering of the underlying bedrock or is colluvium deposited along the steep walls of the bedrock valley (Steelman et al 2017). Stratigraphically, this rubble may be preglacial, associated with pre-Catfish Creek ice or may be associated with the overlying diamict (Table 1).…”

Using a multiple variogram approach to improve the accuracy of subsurface geological models

“…1; black hatched lines represent the location of the cross sections in Fig. 4 (Karrow 1968(Karrow , 1987Gao et al 2006;Cole et al 2009) and observed in recently drilled boreholes within the study area (Steelman et al 2018). Relief on the bedrock topography surface is important in both local and regional groundwater applications, as it has an impact on the distribution of recharge to underlying bedrock aquifers.…”

“…5), which is also consistent with previous findings based on standard geological cross sections through the area (Blackport et al 2009;McGill 2012;Arnaud et al 2018), where subsurface heterogeneity, lateral discontinuity of geologic units, and (or) the relief on the underlying bedrock surface lead to rapid changes in unit thicknesses. For example, the basal diamict that underlies the outwash plain is not always preserved (McGill 2012), and the regional Catfish Creek Till thickens considerably from <5 m to over 25 m as it infills a topographic low in the underlying bedrock surface (Steelman et al 2018). …”

Quaternary geology of southern Ontario and applications to hydrogeology

“…The first paper on buried valleys in this issue, by Steelman et al (2018), provides a geophysical investigation of a gorge-style valley in the city of Guelph. An evaluation of the effectiveness of electrical resistivity and seismic refraction methods is supported with borehole and hydraulic information; this was used to map the spatial distribution of sediments and the delineation of a valley segment of a regional bedrock valley.…”

“…More recently, using the regional bedrock topographic model (Gao et al 2006), three classes of buried valleys have been identified by Russell et al (2007) on the basis of valley length, width, and position in the landscape, which from smallest to largest are (i) gorges, (ii) escarpment re-entrants, and (iii) escarpment troughs. An additional class of buried valleys is sediment-hosted tunnel valleys, most extensively developed in the Greater Toronto Area (e.g., Brennand et al 2006).The first paper on buried valleys in this issue, by Steelman et al (2018), provides a geophysical investigation of a gorge-style valley in the city of Guelph. An evaluation of the effectiveness of electrical resistivity and seismic refraction methods is supported with borehole and hydraulic information; this was used to map the spatial distribution of sediments and the delineation of a valley segment of a regional bedrock valley.…”