Canadian Journal of Earth Sciences Geophysical, geological and hydrogeological characterization of a tributary buried bedrock valley in southern Ontario Journal: Canadian Journal of Earth Sciences Manuscript ID cjes-2016-0120.R2 Manuscript Type: Article Date Submitted by the Author: 05-Dec-2016 Complete List of Authors: Steelman, Colby; University of Guelph, School of Engineering Arnaud, Emmanuelle;Draft University of Guelph, Pehme, Peeter; University of Guelph Parker, Beth; University of Guelph, School of Engineering buried bedrock valley, electrical resistivity tomography, karst, water Keyword: supply, Silurian dolostone https://mc06.manuscriptcentral.com/cjes-pubs Page 1 of 59 Canadian Journal of Earth Sciences TITLE : Geophysical, geological and hydrogeological characterization of a tributary buried bedrock valley in southern Ontario AUTHORS : Colby M. Steelman 1,2 , Emmanuelle Arnaud 1,3 , Peeter Pehme 1,2 and Beth L. Parker 1,2 AFFILIATIONS : [1] G360 Centre for Applied Groundwater Research, University of Guelph, Guelph ON Canada [2] School of Engineering, University of Guelph, Guelph ON, Canada [3] School of Environmental Sciences, University of Guelph, Guelph ON, Canada *Correspondence to: C. M. Steelman ([email protected]) and B. L. Parker ([email protected]) Draft 1 https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 2 of 59 1 ABSTRACT 2 Buried bedrock valleys infilled with Quaternary-aged sediment have the potential to become productive 3 aquifers due to prevalent sand and gravel deposits often associated with these topographic lows. In 4 areas where groundwater is drawn from the underlying bedrock aquifer, buried bedrock channels may 5 significantly affect the spatial distribution of recharge and localized contaminant pathways. Therefore, 6 understanding the form, distribution and the nature of Quaternary infill sediments within these buried 7 bedrock river valleys, and their relationship to hydraulically-transmissive bedrock features is an important 8 aspect of groundwater resource management. Here, we evaluate the effectiveness of electrical resistivity 9 and seismic refraction collected over a partially urbanized 150 ha area with variable vegetation, roads, 10 and structures, to map the spatial distribution of sediments and delineation of a channel segment 11 associated with a regional bedrock valley. Electrical resistivity and seismic refraction was performed 12 along 13 (covering ~11.6 km) and 7 transects (coverDrafting ~0.9 km), respectively, to map and characterize 13 the bedrock surface morphology beneath a variable thickness of unconsolidated deposits. Three 14 continuously cored holes and downhole geophysical logs, supplemented with four nearby water well 15 records captured the in-channel as well as adjacent Quaternary stratigraphy (~15-40 m). Cores recorded 16 multiple glacial till deposits and ice marginal processes associated with ice advances and retreats. 17 Hydraulic transmissivity of the bedrock around the valley feature was evaluated using a FLUTe™ hydraulic 18 transmissivity profiling technique. This study demonstrates the potential of combining several surface 19 geophysical methods with sedimentological analysis of continuous cores and hydraulic data for 20 characterizing tributary bedrock channel morphology and Quaternary infill at a scale relevant to localized 21 studies of municipal production well recharge zones and contaminant transport and fate. 22 23 KEY WORDS : buried bedrock valley; electrical resistivity tomography; karst; water supply; Silurian dolostone 2 https://mc06.manuscriptcentral.com/cjes-pubs Page 3 of 59 Canadian Journal of Earth Sciences 24 1.0 INTRODUCTION 25 Buried bedrock valleys (BBVs) represent an important component of the hydrogeologic conceptual 26 model of southern Ontario and other previously glaciated terrains such as the Canadian Prairies, northern 27 United States, and Europe (e.g., Kehew and Boettger 1986; Sandersen and Jørgensen 2003; Russell et al. 28 2004; Cummings et al. 2012; Sharpe et al. 2013). In southern Ontario, these complex and variably 29 distributed features comprise an extensive network of bedrock valleys that formed through, or were 30 enhanced by, intense glacial and glaciofluvial erosion during the Pleistocene (Karrow 1979; Eyles et al. 31 1997; Kor and Cowell 1998; Gao 2011), and were subsequently infilled with Quaternary-age glacial and 32 interglacial sediment (Karrow 1968; Barnett 1992). Although province-wide mapping (Gao et al. 2006) 33 and regional mapping of some specific areas (e.g., Marich et al. 2011; Burt 2011; Bajc et al. 2012) have 34 been carried out, the location and geometry ofDraft these BBVs and their tributaries is not well constrained in 35 many parts of Ontario. 36 Source water protection has become an integral component of a community’s groundwater 37 management plan since the introduction of the Clean Water Act (2006), S.O. 2006, c.22. This has led to 38 significant investments in three-dimensional sediment and bedrock surface mapping across Ontario both 39 by the geological surveys (e.g., Sharpe and Russell, 2006; Bajc 2008; Davies et al. 2008; Bajc and Rainsford 40 2010; Gao et al. 2006; Bajc et al. 2012; Burt 2012; Bajc et al. 2014) and municipalities and conservation 41 authorities that were tasked with creating source water protection plans (e.g., Gartner Lee 2004; Golder 42 Associates 2006; AquaResource 2010). These activities have supported the conceptualization of aquifer 43 and aquitard units and the delineation of regional-scale groundwater flow systems within and around 44 BBVs. 45 In some cases, here in Ontario and elsewhere, thick successions of sand and gravel deposits have 46 infilled these valleys resulting in highly-productive aquifers (Kehew and Boettger 1986; Russell et al. 2004; 3 https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 4 of 59 47 Wiederhold 2009). Alternatively, buried valleys can be infilled or covered by low-permeable Quaternary- 48 age diamicton or a combination of sediment types with complex geometries and very different hydraulic 49 properties (Meyer and Eyles 2007; Jørgensen et al. 2003a; Cummings et al. 2012), further complicating 50 hydrogeologic conceptualizations (van der Kamp and Maathuis 2012, Cummings et al. 2012). Therefore, 51 these environments require more detailed characterization of both form and infill to ensure accurate 52 quantitative analysis of local groundwater flow system geometries. 53 The underlying sedimentary bedrock across southern Ontario contains ubiquitous interconnected 54 fractures with variable lengths and apertures (Johnson et al. 1992; Kennel 2008; Munn 2012). In addition 55 to these fractures, the Silurian dolostone aquifer – a prolific aquifer for private and municipal water 56 across southern Ontario – contains dissolution enhanced features (e.g., fractures or vugs) and conduits 57 that increase its groundwater resource potential,Draft but also its susceptibility to surface contamination (e.g., 58 Perrin et al. 2011). Many communities across southern Ontario rely almost exclusively on these fractured 59 and dissolution enhanced bedrock aquifers for their drinking water supply. The complex interdependent 60 relationship between Quaternary deposits and these fractured sedimentary rocks has become an 61 important component in the development of effective groundwater management strategies for these 62 communities (e.g., Gartner Lee 2004; AquaResource 2010). Given the potential association between 63 BBVs and increased recharge and transmissivity of bedrock aquifers (Cole et al. 2009), successful 64 groundwater management strategies will depend on sufficiently high-resolution (spatial and temporal) 65 quantitative information, enabling accurate and well-constrained geologic and hydrogeologic 66 conceptualizations of buried valley features (e.g., Shaver and Pusc 1992; Smerdon et al. 2005). 67 Geophysical methods have the capacity to unravel complex geologic relationships and can play a 68 critical role in advancing our understanding of shallow groundwater flow systems in these settings (van 69 Dam 2012). Studies utilizing multiple geophysical methods combined with geologic and hydrogeologic 4 https://mc06.manuscriptcentral.com/cjes-pubs Page 5 of 59 Canadian Journal of Earth Sciences 70 data are particularly effective for three-dimensional mapping of hydrostratigraphic units (e.g., Gabriel et 71 al. 2003; Jørgensen et al. 2003a and 2003b; Steuer et al. 2009; Stumpf and Ismail 2013; Cassidy et al. 72 2014; Sapia et al. 2014). In Ontario, shallow seismic reflection, gravity and electromagnetic surveys have 73 been used at mostly regional scales to delineate the nature of the infilled sediment and bedrock valley 74 geometry (e.g., Greenhouse and Monier-Williams 1985; Greenhouse and Karrow 1994; Zweirs et al. 75 2008; Burt 2011; Bajc et al. 2012; Pugin et al. 2013). Elsewhere, airborne transient electromagnetic 76 (TEM) methods are mostly used (e.g., Sandersen and Jørgensen 2003; Jørgensen et al. 2003a and 2003b; 77 Sørensen and Auken 2004; Sapia et al. 2015). 78 This paper focuses on a local-scale high-resolution electrical resistivity study conducted within the 79 City of Guelph, Canada, with a primary goal of demonstrating the capability and limitations of the 80 technique for mapping a buried bedrock valleyDraft at a relatively local scale. Surface electrical resistivity and 81 seismic refraction results are combined with sedimentological analysis of cores, downhole geophysical