The role of buried bedrock valleys on the development of karstic aquifers in flat-lying carbonate...

The role of buried bedrock valleys on the development of karstic

aquifers in flat-lying carbonate bedrock: insights from Guelph,

Ontario, Canada

Jason Cole & Mario Coniglio & Simon Gautrey

Abstract A spatial relationship between high capacitymunicipal production wells (>5,000m3/day), completed ina deep bedrock aquifer, and a buried bedrock valley wasrecognized in the city of Guelph, southwestern Ontario,Canada. Most production wells are completed in a discretezone, ∼60m below ground surface, within flat-lyingdolostones of the Silurian Amabel Formation. Thickoverburden and limited subsurface data make character-ization of the karstic aquifer difficult. This study integrateshydrogeologic data with models of karst formation,deriving a conceptual model of porosity development asit relates to valley incision. Bedrock valley incision likelyoccurred prior to the early Wisconsinan age (>60–75ka).Incision created steep hydraulic gradients within the flat-lying bedrock, and provided the driving force required tointegrate regional groundwater flow into karst conduitsthat drained at the base of the valley. Dissolution inproduction zone dolostones was favoured over dissolutionin shallower bedrock due to abundant bedding planepartings and fossiliferous facies with high intercrystallineporosity. Burial of the valley during subsequent iceadvances reduced the valley’s hydraulic influence andthe efficacy of the flow system to cause dissolution. The

high capacity municipal wells near the buried bedrockvalley tap into the now dormant karst aquifer system.

Keywords Karst . Carbonate rocks . Conceptualmodels . Water supply . Canada

Introduction

To properly understand a karst aquifer system, reconstruc-tion of the geologic and hydrologic history surroundingthe formation of the system is paramount, particularlywhere thick overburden drift deposits limit direct obser-vation of the bedrock. Without an understanding of thehistorical mechanisms of karst formation, a conceptualmodel of the present-day permeability distributions,recharge areas, and a mechanism to explain how water isstored, transmitted and discharged cannot be consideredcomplete. This type of information is necessary forgroundwater resource investigations, well-head-protectionstudies, and numerical models of flow and transport.

Paleozoic carbonates in southwestern Ontario are knownto form significant groundwater aquifers, particularly in theMiddle Silurian dolostones (Turner 1978a, b, c, d). MiddleSilurian dolostones exhibit secondary porosity developmentalong the Niagara Escarpment, an ∼800 km long scarplocated ∼30 km to the east of the study area, as well as alongother, less prominent bedrock outcrops, as jointed bedrockpavement and well-interconnected, vuggy and cavernouspores. The dissolution kinetics associated with aggressivegroundwater reacting with dolomite can result in theformation of long, interconnected karstic porosity (White1988) that has the capacity to form productive karsticaquifers (i.e. aquifer systems that can supply enough waterfor a municipality) such as the Winnipeg Aquifer, Manitoba,Canada (e.g., Render 1970; Ford 1983) and the Siluriandolostone aquifers in Door County, Wisconsin, USA(e.g., Muldoon et al. 2001). The location of horizontalhigh permeability features has been linked with strati-graphic horizons of contrasting lithologies and beddingplane fractures (Muldoon et al. 2001; Brunton et al. 2007).Few studies, however, have investigated the distributionof permeability within a particular zone of bedding planefractures in areas where bedrock and much of the geologic

Received: 17 October 2007 /Accepted: 23 January 2009Published online: 14 February 2009

© Springer-Verlag 2009

J. Cole ())AECOM,300 Town Centre Blvd,Suite 300, Markham,Ontario L3R 5Z6, Canadae-mail: [email protected].: +1-905-4778400

M. ConiglioDepartment of Earth and Environmental Sciences,University of Waterloo,Waterloo, Ontario N2L 3G1, Canada

S. GautreyAMEC,160 Traders Blvd.East, Suite 110, Mississauga,Ontario L4Z 3K7, Canada

Hydrogeology Journal (2009) 17: 1411–1425 DOI 10.1007/s10040-009-0441-3

evidence of the karst system development is generallyconcealed by extensive Quaternary sediments.

The present study focuses on characterizing the originand development of karstic porosity and permeabilitywithin a nearly flat-lying Silurian dolomite aquifer insouthern Ontario, near the City of Guelph (Fig. 1). TheCity of Guelph obtains potable groundwater from munic-ipal production wells completed within Silurian dolostoneaquifers of the Guelph and Amabel formations. Large,hydraulically interconnected cavernous pores, hereininterpreted as karst, are found at an average depth of∼60 m below ground surface (bgs) and have beenidentified using downhole geophysical logging, videologs, and hydraulic testing. The local area has beenextensively glaciated, and thick Quaternary depositsconceal much of the bedrock, generally limiting observa-tions of the karst aquifer to borehole information andinformation from distal locations. The fundamental prob-lem concerning the origin of karst aquifers in the Guelpharea is how deep circulation of aggressive water occurredin essentially flat-lying dolostone bedrock.

A study of bedrock outcrops and borehole informationby Kunert et al. (1998) of karst formation near theRockwood buried valley, located approximately 10 kmnorth of the study area, revealed three vertical tiers ofvuggy and cavernous porosity within the Amabel Forma-tion, adjacent to the Rockwood buried valley, above thebaselevel of the valley. Bedrock outcrops in this areaallowed correlation between cavernous features observedin boreholes to caves and solution-enlarged joints exposedon the surface. Kunert et al. (1998) suggested that karstdevelopment in the Amabel Formation resulted from

changing paleowater tables associated with the progressiveincision of the Rockwood buried valley. Porosity wasestimated to have developed pre-Wisconsin to EarlyWisconsin (>60 to 75 ka), based on a correlation of anundated till at the base of the Rockwood buried valley withthe Canning Till south of the study area (Greenhouse andKarrow 1994).

Within the present study area, a buried bedrock valleysystem, which is interpreted to be a continuation of theRockwood buried valley to the south, has been identifiedfrom bedrock topography maps, borehole records andshallow geophysical studies in the region (Karrow et al.1979; Miller et al. 1979; Greenhouse and Karrow 1994;Gartner Lee 2003). To avoid confusion, the buried valleypresent in the Guelph area will be referred to as theGuelph buried valley (Fig. 1). Southern Ontario wasextensively glaciated during the Pleistocene, having beenexposed to at least three glacial ice advances, with themost recent being the Wisconsin (115–10 ka), and thisbedrock valley system is interpreted to have formed fromerosional downcutting by glacial meltwater prior to andfollowing the last glacial ice retreat in southern Ontario(Karrow et al. 1979; Greenhouse and Karrow 1994).

Gautrey (2004) first recognized the spatial relationshipbetween the location of high-capacity municipal produc-tion wells and buried bedrock valleys within the City ofGuelph. Karst formation here occurs mainly in theAmabel Formation, as it does in Rockwood; however,zones of cavernous porosity are found below the baselevelof the Guelph buried valley. This difference is importantbecause is suggests that karst formation in the Guelph areamay require a different explanation than the one proposed

Fig. 1 a Location of the Guelph study area within southwestern Ontario. Silurian aged bedrock units are shown in detail. b Detailed mapof the City of Guelph, showing the location of municipal production wells and research boreholes (modified from Gautrey 2004). Bedrocktopography contours are shown to highlight the continuous buried bedrock valley system that is present throughout the study area (modifiedfrom Karrow et al. 1979 and Miller et al. 1979)

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by Kunert et al. (1998) for karst associated with theRockwood buried valley.

The focus of the present study is to examine moreclosely the relationship between the bedrock valleys andthe Silurian aquifers in the Guelph area, and to proposea qualitative, conceptual physical model that includesthe current understanding of the geologic and hydro-logic history of the area and how it relates to bedrockvalley incision and karst formation. The process-basedmodel draws upon classical and modern models forkarst development (e.g. Ford and Ewers 1978; Ford etal. 1988; Palmer 1991, 2000; Worthington 2001; White2003, 2007), and data compiled from numerous unpub-lished studies conducted on the water supply system forthe City of Guelph (JHL 1995, 1998a, b, c; Gartner Lee2003; Golder 2005). This study draws upon this pool ofdata, as well as observations from core samples toprovide a clearer framework for porosity distribution inthe area.

Methods

Most of the data for this study was collected as“secondary” data from consultant reports submitted tothe City of Guelph over the past two decades. A series ofreports by consultant Jagger Hims Limited (JHL 1995,1998a, b, c) contain geophysical and borehole logs for 23production wells and provide information on the lithologiccharacteristics of the stratigraphic units. An example ofthese logs is shown in Fig. 3. In borehole logs, Siluriandolostones are often incorrectly referred to as limestonesin the JHL logs. Downhole logs include video, naturalgamma, spontaneous potential, resistivity, and caliper;however, these tools were available for only a smallnumber of wells. A study of the Arkell spring grounds byGartner Lee Limited (Gartner Lee 2003) provided a modelfor regional groundwater flow that was useful forinterpretation of regional flow patterns. A series of reportscompleted by Lotowater Geoscience (Lotowater Ltd.Reports; 003–030, dated April 24, 2001; 033–033, datedJune 28, 2001; 033–010, dated May 12, 1998) providedinformation on groundwater flow rates from specificintervals of many of the municipal wells, using spinningflowmeters. The results of these reports were summarizedby S. Gautrey and provided to the authors of this study (S.Gautry, AMEC, unpublished data, 2004).

Two complete bedrock cores were made available inorder to confirm the results of the secondary datacollection, specifically the properties of the productionzone. Golder Associates (Golder 2005) provided a corethat was logged in detail as part of this study and a secondcore was provided by researchers at the University ofWaterloo, drilled as part of a separate study. This lattercore was logged in detail by L.-K. Dekeyser (L.-KDekeyser, University of Waterloo, unpublished data,2005) of the University of Waterloo and results weremade available to the authors of this study. The locationsof these two boreholes are shown in Fig. 1 as “Golder”

and “UW Boreholes”. The core logs were combined withthe information retrieved from the various consultantreports to create a detailed data set showing the elevationof each bedrock unit, the elevation of major solutioncavities or fractures, downhole flow profile results, and theinterpreted waterbearing interval (Table 1).

Development of karst aquifers: background

A clear understanding of the physical mechanisms andprocesses that formed the aquifer system in Guelph isgreatly hindered by the lack of outcrop and sparse boreholedata. However, established conceptual models of karstdevelopment provide a framework, which allows one tocompare the present day karst aquifer system to other, morethoroughly characterized karst aquifer systems.

The “karstification” of an aquifer is dependent upon theoriginal discontinuities, fractures, joints, bedding planes,intercrystalline porosity (matrix porosity), and the hydrau-lic flow field responsible for adding, transferring, storing,and discharging water from the developing karst system.For a karst aquifer system to form, especially one capableof supporting a groundwater extraction at a municipalscale, the system must: (1) have an area of recharge; (2) beable to transmit water; and (3) be able to continuouslydischarge water (Springfield and LeGrand 1966; LeGrand1983; Brahana et al. 1988; Ford et al. 1988). Fullydeveloped karst conduit aquifers originate within fracturedcarbonate rocks along bedding planes, joints, or faults,and only as they mature do they develop into conduitaquifers (Ford et al. 1988; White 2003).

The pattern of karst conduit systems is highlydependent upon the geological and hydrological condi-tions present during the initial phase of karst development(Dreybrodt et al. 2005). According to Palmer (2000), theconduit network pattern is influenced by two sets ofconditions: the type of porosity (intergranular, beddingplane, fracture) and the type of recharge (diffuse, via karstdepressions, hypogenic). The amount of recharge avail-able to a developing conduit will determine the true formof the cave in terms of length and width (Palmer 1991).Bakalowicz (2005) and White (2003) also point out theimportance of a hydraulic gradient, i.e. the difference inhead between the recharge area and the spring, and therelationship between the direction of the hydraulicgradient and the drainage planes. This provides the drivingforce for movement of aggressive water though a karsticaquifer on a regional scale.

Worthington (2001) suggested that depth of conduitlength is proportional to flow path length and strataldip. In steeply dipping beds, flow is concentrated alonga single bedding plane, which promotes deep flowbelow the water table (Ford and Ewers 1978). At lowerbedrock dip angles, deep flow can only be achieved byfollowing joints or faults to depth, then concentratingflow along a single bedding plane (Worthington 2001).White (2003) suggests that nearly flat-lying bedrocktends to develop two-dimensional flow paths that

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concentrate along bedding plane fractures, forminghighly integrated drainage patterns. Lithologic andstratigraphic controls on karstic aquifer formationinclude: the thickness of the bedrock succession, depthto confining layers, lithologic character of the carbonaterock, and structural influences. Variations in carbonatelithology tend to focus groundwater flow and conduitdevelopment into rock units that have the most highlydeveloped secondary porosity (White 2003).

Geologic and hydrogeologic setting

The City of Guelph is located in southwestern Ontario,between Toronto and Lake Huron, and is underlain bymiddle Silurian aged dolostones, which form an importantregional aquifer system along the eastern edge of theMichigan Basin (Fig. 1). The regional bedrock dip istowards the southwest at approximately 0.2 to 0.3%(Karrow 1968). Stratigraphic correlations can be made

Table 1 Complied City of Guelph municipal supply well data References for each of the data sources are listed below

Well Elevation(masl)

Depth(m)

Downhole flow profile resultsa (masl) Major solution cavities or fracturesb (masl)

Sacco 339.15 95.7 NA 322.55; 315.55–295.15Calico 324.14 64 303.74–299.74 306.14–304.24; 305.84; 303.44; 302.84Paisley Rd 322.18 80.2 NA 304.48–290.48; 284.38–241.98Queensdale 325.93 74.3 269.53 282.83–270.33; 279.83; 277.03; 274.03;

255.83–251.63Smallfield 343.98 102.1 NA 333.28–300.38; 268.08; 267.48; 249.98Silvercreek Parkway 318.31 67.1 NA 272.01–251.21Hauser test well 324.2 96.1 NA 309.6–305.6Park well No.1 328.74 56.3 282.44–272.44 302.14–301.84; 290.24–286.54; 285.64–283.54;

282.34–280.44Park well No. 2 328.82 57.9 294.32; 286.22–270.92 312.02–311.42; 300.12–298.62; 282.82–281.52Emma St 328.71 47.2 307.41–299.21; 284.51–281.51 312.81–309.81; 308.31–306.41; 304.61–301.91;

300.01Clythe Creek 326.96 64.0 304.46; 298.36; 282.16–278.16 312.86–312.26; 302.46–302.16; 282.66–281.76;

279.26–274.76; 272.56–271.96; 270.2–269.0Eastview Rd 343.44 82 NA 298.64–295.04Water St 315.02 59.4 NA 298.82–297.62; 277.82–274.52; 272.32–272.02Edinburgh Rd 315.6 69.5 NA 293.7; 268.7–266.2; 255.2–254.7; 264.1–258.3Dean Ave 323.33 57.2 NA 290.43–290.13; 276.43–268.73; 272.13–271.53University of Guelph 329.5 64.3 274.5–265.20 301.8; 286.2; 281.3; 278.6; 271.6–265.8Downey Rd 317.35 73.8 NA 300.05; 295.75; 261.65–260.65; 254.25–252.15Membro 316.37 73.8 NA 273.67Arkell PW6/63 330.27 44.2 298.57; 295.87; 292.47–290.67 314.77–313.8; 302.6–302.0; 298.9–298.6;

296.2–294.3; 292.8–292.2Arkell PW7/63 330.21 43.8 292.21–286.41 303.71–301.4; 296.1–295.8; 293.9–293.6;;

291.2–289.1Arkell PW8/63 330.21 42.1 295.21–294.21; 292.21–288.11 311.31–309.2; 303.7–303.1; 300.3–299.7;

296.2–291.5; 290.6–289.2Burke PW2/66 335.61 78.9 308.81 308.51; 296.9; 295.4; 295.1; 272.5; 256.0Scout Camp PW1/87 316.47 45.7 303.97–301.27; 284.17–282.07 289.97–288.5; 287.8–286.3; 285.1–282.5;

280.6–279.3; 273.8–271.5Carter PW1/89 324.15 20.1 313.15 316.85–316.0; 314.4–314.2; 313.0–312.4;

304.7–304.1Carter PW2/62 323.97 20.7 312.97 317.57–317.3; 316.4–315.8; 314.2–313.7;

313.2–312.7Univerisity of Waterloof 341.76 105.45 NA 331.23–327.4; 325.4–322.8; 317.8; 313.8–307.5;

301.2–297.1; 296.4; 252.3; 250.9; 249.2Goldersg 320.89 79.4 NA 317.69–316.70; 314.09–312.89; 294.76–293.02;

278.09; 261.69–258.19; 255.78–246.22

Notes:aInterval selected from downhole flow profile testing conducted by Lotowater Geoscience Ltd. where greater than 50% of total flow occurred (S.Gautrey, AMEC, unpublished data, 2004)bResults obtained and interpreted from downhole geophysical surveys, including video logging, from a series of reports by Jagger Hims Ltd.(JHL 1995, 1998a, b, c)cWaterbearing interval selected as either the interval from the flow profile results where >50% of flow is from or as the primary interval of majorsolution cavities or fractures or as the waterbearing interval from the original well drillers logsdProduction zone interval selected as the interval recognized from downhole video logging as having karstic porosity or by core logging resultsfrom intervals with high intercrystaline porosity or by primary water bearing interval within the middle Amabel Fm.eApproximate distance between the interpreted center of the Guelph Buried Valley and the location of each municipal supply wellfCore was logged by L.-K Dekeyser (L.-K. Dekeyser, University of Waterloo, unpublished data, 2005) and the results were interpreted by theauthors of this studygCore was provided by Golder Associates (Golder 2005) and was logged in detail by the authors of this studyNA no information available; NP unit or interval not present according to the author’s interpretation of data provided

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between bedrock exposures along the Eramosa River, nearRockwood, as described by Kunert et al. (1998), butgenerally, subsurface correlations are derived from coredata.

These massive, fossiliferous dolostones represent anextensive, shallow-water carbonate platform that formedon a Precambrian basement high, known as the AlgonquinArch, between the subsiding Michigan and Appalachianbasins in the middle Silurian (Fig. 2; Brett et al. 1990;

Johnson et al. 1992). Sedimentary facies are varied andcomplex, and reflect a diversity of paleoenvironmentsranging from high-energy reefal settings to quiet water,back-reef lagoons.

The study area is covered by Late Wisconsin glacialsediments, which range in thickness between 0 and 50 m.These sediments are thought to form a regional aquicludewhere they overlie the Silurian bedrock, except in placeswhere glacial outwash directly mantles the bedrock.

Guelph Fm.b (masl) Eramosa Mb.b (masl) Amabel Fm.b (masl) Clinton Gp.b (masl) Waterbearing intervalc (masl)

333.1 NP 295.6 251.2 315–295307.6 NP 279.1 NA 306–299308.0 NP 280.2 NA 284–277317.7 NP 282.4 NA 282–269

338.7 NP 292.2 246.48 333–300NA NA NA NA 272–251312.6 NP 280.2 228.6 310–305319.2 NA 305.0 NA 282–272

319.3 NA 305.0 NA 286–270322.6 NA 305.0 NA 285–281

323.4 314.8 299.0 263.0 282–278

325.9 312.3 299.9 261.5 299–295309.8 306.2 293.4 NA 278–272308.3 298.5 285.5 NA 269–254311.4 296.7 288.9 NA 276–268302.7 294.4 274.0 NA 275–265311.9 296.1 282.3 247.3 262–252311.2 297.3 280.1 243.2 273.0NP 319.9 304.7 286.1 299–290

NP 307.6 301.2 286.4 292–286

NP 318.6 314.0 288.1 295–288

315.8 306.6 292.9 256.7 308.0NP 316.5 306.5 270.8 304–282

321.5 NA NA NA 316–313

321.3 NA NA NA 317–312

332.9 NP 290.2 244.7 331–296

311.3 293.8 275.5 NA 261–256

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Silurian aquifer of southwestern OntarioIn terms of water supply, the Guelph and Amabelformations have been grouped together as the Guelph-Amabel Aquifer (Turner 1978a, b, c, d). This regionallyextensive aquifer extends from the Niagara Peninsula,where it is referred to as the Guelph-Lockport Aquifer, tothe Bruce Peninsula, which separates Lake Huron fromGeorgian Bay (Fig. 1). Shales from the Clinton Groupform a regional lower aquitard below the Guelph-AmabelAquifer. In the present study, when it is useful todifferentiate the Amabel and Guelph aquifers, they arereferred to as the Amabel Aquifer and the Guelph Aquifer.

The recharge for the bedrock aquifers is thought to bediffusive through the overburden sediments that overlie thebedrock (JHL 1995, 1998a, b, c; Gartner Lee 2003). Ground-water presently discharges from the regional bedrock aquifersinto the valleys of the Speed River and Eramosa River, andfrom pumping from wells completed in the aquifers (Turner1978a; JHL 1995, 1998a, b, c; Gartner Lee 2003).

Lithofacies and porosity development in SilurianbedrockStratigraphic nomenclature and bedrock geology for thestudy area are summarized in Fig. 2. Although thedolostone studied in core exhibited a wide variety oftextural types, three distinct lithostratigraphic units can beidentified. In ascending order there are the AmabelFormation, the Eramosa Member of the Guelph Forma-tion, and the undifferentiated Guelph Formation. Table 2summarizes the geological and textural descriptions for

each of these units as well as the overburden sediments,and provides a brief explanation of porosity developmentand hydraulic properties for each unit.

The Amabel Formation outcrops east of the study areaalong the Niagara Escarpment (Fig. 1). The AmabelFormation is the primary water-bearing unit in the studyarea. A discrete hydrostratigraphical zone is recognizedover the full extent of the study area within the middlestrata of the Amabel Formation, which has been referredto informally as the “production zone” (Gartner Lee 2003;Golder 2005). Video logs reveal large solution cavernsand an abundance of bedding plane fractures, many ofwhich show a significant degree of solution enhancement(Fig. 3a). Most of the groundwater produced from theAmabel Aquifer comes from this production zone, whichranges in depth from 28 to 72 m below ground surface(average depth of ∼60 m) and is typically 8–30 m belowthe top of the Amabel Formation. Pumping tests by localconsultants show almost instantaneous positive hydraulicresponses (drawdown) to pumping between groups ofwells completed within the production zone at distancesgreater than 1 km, implying significant lateral intercon-nectivity (JHL 1995; Gartner Lee 2003; Golder 2005).Core samples from the production zone evaluated as partof this study and by Golder (2005) and Dekeyser (L.-K.Dekeyser, University of Waterloo, unpublished data,2005), indicate that these strata have a moderate to highfossil content indicative of reefal sedimentary facies, andimportantly have high intercrystalline and vuggy porosity(up to 20% based upon visual estimates), and highbedding plane fracture porosity (Fig. 4b). Large cavernous

Well Production zone intervald (masl) Distance from buried valleye (m) Capacityb (m3/day)

Sacco NA 5,500 1,037Calico NA 6,600 2,160Paisley Rd NA 3,400 1,555Queensdale 269 2,500 3,024Smallfield 271–268 5,200 1,296Silvercreek Parkway NA 2,400 <2,000Hauser test well NA 5,700 1,175Park well No.1 290–272 2,400 3,888Park well No. 2 286–270 2,400 34,56Emma St 285–281 2,900 1,296Clythe Creek 282–270 <100 3,888Eastview Rd NA 1,200 1,037Water St 278–265 1,000 3,923Edinburgh Rd 269–254 900 1,961Dean Ave 276–268 700 2,290University of Guelph 275–265 <100 2,618Downey Rd 262–252 700 5,236Membro NA 1,600 7,076Arkell PW6/63 298–290 200 6,540Arkell PW7/63 292–286 <100 7,862Arkell PW8/63 295–288 300 8,813Burke PW2/66 279–274 200 8,813Scout Camp PW1/87 284–271 600 5,789Carter PW1/89 NA <100 7,862Carter PW2/62 NA <100 >5,000Univerisity of Waterloof 256–248 4,800 NAGoldersg 262–246 400 NA

Table 1 (continued)

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karst has been identified in the production zone by caliperlogs and downhole video (Fig. 4c). The remaining portionsof the Amabel Formation show very limited potential forgroundwater production (Fig. 4a), and do not show thesetypes of secondary porosity features in abundance.

The Eramosa Member is generally less susceptible tochemical dissolution than other bedrock units in the studyarea possibly due to its finely crystalline, bituminouscomposition (Fig. 4d). Where present, the EramosaMember is a distinctive marker bed between the Guelphand Amabel formations and can be clearly recognized byits characteristic high natural gamma count in downholegeophysical logs (Fig. 3a). In many cases, the EramosaMember shows significant bedding plane fracture porosity,but these fractures show limited carbonate dissolution.This unit is generally considered to act as a regional semi-confining unit between the Guelph Aquifer and theAmabel Aquifer (JHL 1995, 1998a, b, c; Gartner Lee2003; Golder 2005).

Observations from this study and from Dekeyser (L.-K.Dekeyser, University of Waterloo, unpublished data, 2005)indicate that in the northwestern portion of the study area,Eramosa-type facies are absent. The discontinuous nature

of the Eramosa Member in the study area could reflectlateral sedimentary facies changes (Liberty and Bolton1971) or non-deposition or pre-Wisconsin erosion (Bruntonet al. 2007; Bancroft et al. 2008).

The Guelph Formation (Fig. 4e) forms the secondaryaquifer unit within the study area. Groundwater produc-tion from the Guelph Aquifer comes from the upperweathered zone (JHL 1998b) and from zones of poorlyconnected vugs and fractures (JHL 1998c). Groundwateryields from wells completed in the upper weathered zone,which may be a buried epikarst pavement, can exceed5,000 m3/day and are often hydraulically connected tosurficial aquifers (Fig. 3b), whereas production rates fromwells completed in zones of vugs and fractures rarelyexceed 2,000 m3/day and show little lateral hydraulicconnection (JHL 1995, 1998b, c). Large, sub-verticalfractures have been observed in the Guelph Formationfrom downhole video logs (Fig. 4f), which confirms theexistence of vertical flow conduits in the study area.

Buried bedrock valleysA deeply incised bedrock valley system (Guelph buriedvalley) has been clearly identified from bedrock topogra-phy maps (Karrow et al. 1979; Miller et al. 1979) withinthe study area (Fig. 1) and is believed to have been formedby the paleo-Speed and paleo-Eramosa rivers (Karrow etal. 1979). This valley system ranges in depth along itscourse, but typically is between 10 and 30 m deep relativeto the elevation of the top of the adjacent bedrock. Thetrend of the Guelph buried bedrock valley, as estimatedfrom bedrock topography maps, is approximately south-west, which corresponds to other valley trends insouthwestern Ontario measured by Scheidegger (1980)and to the regional neotectonic stress field (Scheidegger1980; Eyles and Scheidegger 1995; Eyles et al. 1997).

Kunert et al. (1998) measured joint orientations nearRockwood (Fig. 5) and found that the trends in theAmabel Formation (northeast) and the Eramosa Member(northeast and north–northeast) corresponded to the trendof the Rockwood buried valley and to the regional stressfield. Joints were found to be open at the surface andshowed signs of solution enhancement, especially in theAmabel Formation. Extrapolating the structural conditionsof the bedrock from the Rockwood area where outcropsare abundant allows one to make reasonable inferencesabout the Guelph area, where outcrops are few. Thecorrespondence in the Rockwood area between themeasured joints in the Silurian bedrock to the regionaltrend of the buried bedrock valleys and the correlation tothe regional neotectonic stress field together support theconclusion that similar vertical to sub-vertical joints arepresent in the Guelph area and are likely continuousthrough all bedrock units.

No evidence is available to provide chronologicinformation about the formation of the Guelph buriedvalley, but the interpretation that the Guelph buried valleyis an extension of the Rockwood buried valley to thesouth suggests that they may be of the same age. As

Fig. 2 Middle Silurian stratigraphy for southern Ontario and thestudy area. Although the study area lies between the Michigan andAppalachian basins, the stratigraphic nomenclature of the Michiganbasin is generally used for the Guelph area. The Wiarton-ColpoyBay and Lions Head Members of the Amabel Formation were notdifferentiated as part of this study. Diagram is modified fromJohnson et al. (1992)

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Tab

le2

Sum

maryof

thebedrockgeolog

y,basichy

draulic

characteristics,

andsecond

arypo

rosity

developm

entfrom

theAmabel

Formation,

theEramosaMem

berof

theGuelph

Formationandtheun

differentiatedGuelphFormation,

from

with

intheGuelphstud

yarea

Geologicalun

itGeologicdescriptiona

,bPorosity

a,b

Thickness

cHyd

raulic

cond

uctiv

ityK

(m/s)d

Produ

ctioncapacity

(m3/day)c

Overburden

Glacial

outwash

Glaciofl

uvialsand

andgravel.Fou

ndin

topo

graphiclowland

sandfloo

dplain

ofSpeed

andEramosarivers

30%

0–60

m1×10

−5–1

×10

−4Upto

5,00

0

Glacial

till

Stony,sand

y,silty

till(W

isconsinan

age).May

beun

derlainby

olderclaysor

silts

15–45%

1×10

−6Aqu

itard

GuelphFm.

Lith

olog

yLight

brow

nto

mottledgrey

andblue-grey,

fine

tomedium

crystalline

doloston

e.Dom

inant

litho

facies

includ

ebioh

ermal

fram

estones

andinter-reefal

bioclastic

grainstonesand

packston

es

Matrix<1%

0–30

m<1×10

−71,00

0–7,80

0average=

2600

Secon

dary

porosity

Develop

edas

poorly

conn

ectedvu

gsand

fractures.The

upperweathered

zone

(epikarst?)

contribu

tessign

ificantly

tothebu

lkpo

rosity

and

prod

uctio

ncapacity

1–20

%1×10

−5

EramosaMb.

Lith

olog

yBrownto

black,

very

fine

tomedium

crystalline,

bituminou

s,no

n-fossiliferous

tomod

erately

fossiliferous,laminated

doloston

e.Chertno

dules

andsilicified

fossils

occasion

ally

foun

din

lower

beds.Minor

shaleoccurs

asthin,flaggy

beds

interbeddedwith

thedo

loston

es

Matrix<1%

0–20

m<1×10

−8Aqu

itard

Secondary

porosity

Dom

inated

byintercrystallin

eandthin

fracture

pores

1–5%

5.0×10

−8

Amabel

Fm.

Lith

olog

yLight

grey-tan,thicklybedd

edto

massive,medium

tocoarse

crystalline

doloston

e.Precursor

sediment

was

predom

inantly

well-sorted

bioclastic

grainstone,andlocally

reefal

sedimentsare

representedby

coralandstromatop

oroidfram

estones

Matrix<1%

∼50m

<1×10

−71,00

0–8,80

0average=

4,30

0

Secon

dary

porosity

Formsalon

gbedd

ingplanefractures,jointsand

joint-bedd

ingplaneintersectio

ns.Possible

dissolutionof

reefal

fram

ework.

Caverno

uspo

rosity

developedin

the“Produ

ctionZon

e”contribu

tessign

ificantly

tobu

lkpo

rosity

and

hydraulic

cond

uctiv

ity

1–20

%1×10

−5–1.7×10

−3

Notes:

aKun

ertet

al.(199

8)bGreenhouseandKarrow

(199

4)cJH

L(199

5),JH

L(199

8a,b,

c)dGolder(200

5)

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mentioned earlier, the Rockwood buried valley andporosity formation related to valley incision were estimat-ed to be pre- to early Wisconsin (>60–75 ka) (Kunert et al.1998). The sediments at the base of the Guelph buriedvalley were not assessed as part of this study, although itis probable that much of the Guelph buried valley is filledwith low permeability glacial sediments, similar to the lowpermeability tills at the base of the Rockwood buriedvalley

Results

Relationship between buried bedrock valleysand municipal production wellsGautrey’s (2004) observation of the spatial relationshipbetween high capacity municipal groundwater productionwells and buried bedrock valleys in the Guelph arearequires further consideration. The distance from eachmunicipal production well to the edge of the nearestburied bedrock valley was estimated and plotted againstthe rated production capacity for each well (Fig. 6). Thisplot shows not only that the number of production wellsincreases towards the buried valleys, but so does the ratedgroundwater capacity of the wells. The authors suggestthat the bedrock valley and karst conduit formation in theregion may be related as cause and effect, respectively.The area of hydraulic influence of the valleys as it relatesto significant karst conduit formation is approximately4 km based upon the distance from the farthest municipalwell that derives its water from the production zone andthe Guelph buried valley. Of course there is bias in using

data only from municipal wells, as heterogeneities in karstaquifers are not represented. For example, a boreholedrilled directly beside the Guelph buried valley may notintercept a flow conduit, and therefore would not be amunicipal well, whereas a borehole drilled many kilo-metres away from the buried valley may intercept a well-interconnected conduit and produce sufficient water toconstitute a municipal well. The spatial relationshipexpressed in Fig. 6 suggests there is a higher probabilityof encountering a major karst conduit near the buriedbedrock valley system rather than at a distance from theburied valley.

Current understanding of the stratigraphyand hydrostratigraphyCore and geophysical logs from the Guelph area (JHL1995, 1998a, b, c; Golder 2005; L.-K. Dekeyser, Univer-sity of Waterloo, unpublished data, 2005) provide keyinformation on the stratigraphy and hydrostratigraphy ofthe study area (Fig. 7). Waterbearing intervals wereselected from data in Table 1, where either downholeflow profile results showed the discrete interval where themajority of groundwater flow was derived or where theinterval where most of the major solution cavities/fractures were recognized from downhole video logs.The production zone interval was identified where thewaterbearing interval corresponded to the interval ofmajor solution cavities or fractures within the AmabelFormation. The non-waterbearing production zone stratawere recognized from intervals that were not deemed to bewaterbearing, but where facies typical of the production

Fig. 3 Core logs from a the University of Guelph (“University”) Municipal Well and from b Calico Municipal Well. The results ofdownhole video, natural gamma, and interval flow rates studies are shown for both wells. The Eramosa Member of the Guelph Formation ismost easily identified by an increase in the natural gamma. Downhole video logs show frequent fractures or solution cavities in each of therock units, but the results from downhole flow profiles show that in a flow is derived from a specific interval between 55.0 and 64.3 mbgs,and b flow is derived from a specific interval between 20.4 and 24.4 mbgs. Figure is modified from JHL (1995) and JHL (1998c)

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zone were identified from downhole video logs and/orcore logs.

The position of primary karst development in theAmabel Formation (production zone) does not seem tomimic the probable paleowater table elevations within thestudy area and is present deep below the baselevel of theGuelph buried valley within the Amabel Formation. Incontrast, karst development in the northwest portion of thestudy area (located ∼5 km west of the closest buriedbedrock valley) formed within the Guelph Formation, andalthough bedrock lithologies similar to those of theproduction zone are present, karst development is limited.This suggests that the hydraulic influence of the bedrockvalley was not strong enough to induce deep groundwaterflow at a distance of >5 km from the valley.

Discussion

Lithologic control on karst development in flat-lyingcarbonate bedrockDownhole video logs, core samples, and well hydrauliccharacteristics show that karst formation varies signifi-cantly among the Amabel Formation, the EramosaMember of the Guelph Formation, and the overlying,undifferentiated Guelph Formation. The physical andchemical conditions of their burial diagenesis and morerecent exposure to meteoric waters were likely similar,and therefore original textural differences, possibly alongbreaks of changes in the stratigraphy, among these unitslikely were the dominant controls on secondary porositydevelopment. Within the production zone in the middle

Fig. 4 Downhole camera and core photographs. The core diameter for a, b, d, e is 7.62 cm (3”), and the borehole diameter is 50.8 cm(20”) and 30.5 cm (12”) for c and f respectively. a Light grey-tan, thickly bedded to massive, fine-to-medium crystalline dolostone of theundifferentiated Amabel Formation from above the production zone strata. b Highly porous strata of the production zone. Core sampleshows significant intercrystalline porosity estimated to be approximately 20% and solution enhanced bedding plane porosity consistent withkarst development. c Significant karst cavern from the production zone. Based upon drilling records and caliper logs, the void isapproximately 2.5 m in length and at least 0.7 m in diameter. d Brown to black, bituminous dolostone of the Eramosa Member. Beddingplane partings are observed in the core samples. e Light brown to mottled grey and blue-grey, fine to medium crystalline dolostone of theGuelph Formation. Small vugs are present in core samples. f Sub-vertical fracture from the Guelph Formation. Sub-vertical fractures appearelliptical in downhole video. Downhole video photographs were provided by Gautrey (2004). Core photographs were taken by the authorsof this study from the Golder (2005) core

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Amabel Formation, the medium crystalline texture of thedolomite and the abundant broken fossils increasedsurface area roughness, and therefore enhanced thebedrock solubility (Kunert et al. 1998). By contrast, thefiner crystalline texture (originally muddy), bituminous,and siliceous lithology of the Eramosa Member appears to

have limited the degree of dissolution that could occur inthis unit. Little to no macroscopic carbonate dissolutionwas observed in core samples or downhole video logs.

Why laterally interconnected karst formation favouredthe Amabel Formation over the Guelph Formationremains unclear, as both units have very similar lithologiesand textures. Detailed logging of core from the study areashows that the Guelph Formation and the shallower partsof the Amabel Formation have less intercrystalline andvuggy porosity than the production zone strata in themiddle Amabel Formation. This may make these units lesssusceptible to dissolution and therefore less likely todevelop the continuous horizontal karst features like thosethat characterize the production zone. In the Guelph area,phreatic water travelled deep into the Amabel Formationrather than taking the shorter, more direct path within theGuelph Formation, presumably because the AmabelFormation initially contained a more highly developedsecondary porosity network.

Structural control on karst development in flat-lyingcarbonate bedrockDue to a low dip angle of the bedrock in the study area,flow along bedding plane partings to depth is not possible.As Worthington (2001) suggested, deep flow in flat-lyingbedrock can only be achieved by following joint or faultsto depth, and then concentrating flow along bedding planefractures or lithologies that favour dissolution.

The measurement of northeast-trending vertical to sub-vertical joint sets measured in the Eramosa Member andthe underlying Amabel Formation near Rockwood (Fig. 5;Kunert et al. 1998), and the observation of sub-verticaljoints that show signs of solution enhancement within theGuelph Formation in the present study (Fig. 4f), impliesthat joints crosscut all three bedrock units. Stress releasefracturing resulting from pressure release during valleyincision may have increased the number of open jointsnear the bedrock valleys (e.g., Goss and Engelder 1991;Sasowsky and White 1994). Throughgoing joints withinthe study area allowed for diffusive recharge water toreach depths averaging 60 metres below ground surface(mbgs), under-saturated with respect to dolomite, wheredissolution of the Amabel Formation within the produc-tion zone strata encouraged the formation of long,interconnected karst conduits. Acidification of infiltratinggroundwater from oxidation of pyrite found in theEramosa Member may have further enhanced the aggres-siveness of the groundwater (Ford and Williams 1989).

For cave development, the depth of a flow path belowthe water table has been postulated to be proportional tothe flow path length and the stratal dip angle (Worthington2001), using the following formula,

D ¼ 0:18 Lqð Þ0:79

where, D is the mean depth of flow in metres below thewater table, θ is the dimensionless dip (equal to the sine of

Fig. 5 Measured joint orientations in the Amabel Formation andthe Eramosa Member of the Guelph Formation, near RockwoodOntario (modified from Kunert et al. 1998)

Fig. 6 A plot showing the relationship between the approximateddistance between the municipal supply wells and the Guelph buriedvalley, and the rated production capacity of the wells. Productioncapacity of the wells and the number of wells increase towards the buriedvalleys. Most of the wells near the buried valley obtain the majority oftheir flow from the production zone interval within the AmabelFormation, whereas wells at a distance from the valley obtain themajority of their flow from the upper weathered zone within the GuelphFormation

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the dip in degrees), and L is the flow path length in metres.Shales of the Clinton Group underlie the AmabelFormation and act as a regional aquitard, constrainingmaximum flow depth to ∼65 m below the current landsurface. This limits the utility of Worthington’s (2001)relationship between flow depth, stratal dip and flowlength, as it does not include a variable for stratigraphiclimits on karst development. However, assuming therelationship derived by Worthington applies to beddingplane karstic features present in the Guelph area and flowis parallel to the structural dip of the bedrock, the flowpath length from the input area to the discharge area mustbe >2 km.

Hydraulic control on karst development in flat-lyingcarbonate bedrockThe development of steep vertical gradients under non-pumping conditions is usually created by a combination oftopographic change in elevation between recharge anddischarge areas, and the presence of one or more lowpermeability layers to restrict groundwater flow betweenthe recharge and discharge area. Progressive incision ofthe Guelph buried valley prior to infilling with glacialsediments provided the topographic relief necessary tocreate large hydraulic gradients in flat-lying carbonatebedrock near the City of Guelph. Bedrock topographymaps indicate that prior to filling of the bedrock valleywith Quaternary sediments, elevation differences rangingfrom 10 to 30 m existed between the base of the valleyand the top of the valley walls (Fig. 1). This regionaltopographic low would have drained large areas of land

surrounding the exposed valley, directing flow to thedischarge area at the base of the valley and providing thedriving force for rapid movement of aggressive water toreach the fossiliferous dolomites of the Amabel Formationat ∼60 mbgs. The finely crystalline Eramosa Member thatunderlies parts of the study area is a semi-confining layerthat would have restricted the vertical flow of water andallowed steep gradients to develop between the AmabelAquifer and the Guelph Aquifer, which would havefurther increased the hydraulic influence of the bedrockvalley. Diffusive infiltration though exposed bedrock orglacial deposits with low carbonate content contributednearly uniform amounts of flow to throughgoing verticalfractures. This type of infiltration would cause a branch-work type conduit network to form along bedding planefractures and sub-vertical joints, and likely develop a karstaquifer system with significant lateral continuity asobserved in the Guelph area.

Conceptual model of karst developmentA conceptual model for karst formation in the Guelph areaas it relates to incision of the Guelph bedrock valley isshown in Fig. 8. Timing of the various stages ofdissolution is poorly constrained, but karst formationwould have occurred prior to infilling of the bedrockvalleys with low permeability tills in pre- to lateWisconsinan time (>60–75 ka). Plugging of the dischargeareas with relatively low permeability sediments wouldhave reduced the efficacy of the groundwater flow systemto develop karst by blocking the discharge areas anddecreasing hydraulic gradients.

Fig. 7 A generalized geologic cross-section A–A’ (Fig. 1b) between the northwest corner and the center of the study area. Core recordssuggest that the Eramosa Member is locally discontinuous in the northwest portion of the study area. The interpreted waterbearing intervaland the interpreted production zone strata are shown for each well. A generalized present-day water table is presented to show that thebedrock valley no longer has a hydraulic influence on the area

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The various stages of karst development in the Guelpharea can be summarized as follows:

1. Prior to pre- to late Wisconsinan time, glacial meltwa-ter began to carve the paleo- Speed and paleo-Eramosariver valleys. Groundwater at this time moved throughthe matrix and fracture porosity of the carbonatebedrock, because discrete flow conduits had not yetbeen established (Fig. 8a).

2. Progressive bedrock valley incision created steeperhydraulic gradients between the upland recharge areasand the lowland discharge areas at the base of the rivervalleys (Fig. 8b). The higher hydraulic gradientsprovided the driving force that focused more diffusiverecharge along sub-vertical joints that extended to theporous “production zone” in the Amabel Formation.

Karst conduits formed as dissolution occurred alongbedding plane partings and in the adjacent solublebedrock. Numerous smaller flow conduits coalescedinto fewer, larger flow conduits as the karst systemmatured.

3. As the baselevel of the bedrock valley continued tolower to present-day levels, hydraulic gradients steep-ened and the groundwater flux between the rechargeand discharge areas increased. Dissolution rates in bothvertical conduits and production zone strata increased,forming large cavernous karst, concentrated along longflow paths within the area of hydraulic influence of thebedrock valley (Fig. 8c).

4. During Late Wisconsin ice advances, the Speed andEramosa river valleys were filled with low permeabilityglacial sediments, effectively clogging the discharge

Fig. 8 Conceptual model of karst development as it relates to the historical geologic and hydraulic conditions of the Guelph area. Scalesare approximate, but are based upon observations from the study area. a Beginning stage of karst development. Groundwater flow isthrough matrix and fracture porosity towards the discharge area of the paleo-river valleys. b As valley incision progressed, steep hydraulicgradients were created between the recharge and discharge areas that directed regional groundwater flow towards the valleys. The hydraulicgradient established by the bedrock valley provided the driving force for water to travel along sub-vertical joints to the production zonestrata. Karst conduits began to form along bedding plane partings and dissolution of the soluble facies of the production zone strata asgroundwater flow was focused within discrete flow conduits. c Further incision of the bedrock valley increased the groundwater fluxbetween the recharge and discharge areas and formed a groundwater flow regime that was able to create a productive, interconnected, karstconduit aquifer system. d Prior to and during the most recent glacial episode, the bedrock valleys were filled with low permeability glacialsediments. This blocked the discharge area of the flow system and greatly reduced the rate of karst formation by flattening the regionalwater table

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area at the base of the bedrock valleys (Fig. 8d). Thisclosed the karst aquifer system by removing thehydraulic influence of the bedrock valley and createda flat-lying regional water table.

Conclusions

A compilation of data from numerous unpublished reportsand recent bedrock cores from the Guelph area providesnew insights into the relationship between karstic porosityin flat-lying Silurian carbonates, Quaternary history, andthe location of high capacity municipal production wellsnear buried bedrock valleys. Dissolution in “productionzone” dolostones in the middle of the Amabel Formation at∼60 mbgs, which is the primary water producing horizon, isfavoured over dissolution in shallower bedrock. Themedium to thickly bedded, massive, medium crystalline,reefal dolostones of the Amabel Formation were especiallysusceptible to dissolution along bedding plane partings anddissolution of fossiliferous facies. Dolostones of theGuelph Formation and Eramosa Member of the GuelphFormation were less susceptible to dissolution due to lessintercrystalline porosity, less bedding plane fracture poros-ity, and in the case of the Eramosa Member, perhaps also itsbituminous and locally siliceous composition.

Diffusive recharge through exposed bedrock followedthroughgoing joints from surface to the Amabel Formation,rapidly carrying water undersaturated with respect todolomite to depth. Incision of the Guelph bedrock valleycreated steep hydraulic gradients within flat-lying bedrock,by the difference in elevation between the upland rechargeareas and the lowland discharge areas, and increased paleo-groundwater gradients in the direction of the valley. Thisgradient created the driving force required to integrate flowinto karst conduits within a discrete zone (the productionzone) of laterally continuous karstic cavities.

This qualitative, conceptual physical model relatesobservable present-day porosity trends to the geologichistory and hydraulic conditions at the time of karstformation. This study provides an explanation regardingthe formation of highly productive, deep bedrock aquifersin flat-lying carbonate bedrock near buried bedrockvalleys. Furthermore it also provides insight for futuregroundwater exploration as well as land-use planning thatattempts to minimize the risk from surface contamination.

The above hypothesis to explain the relationship betweenburied bedrock valleys and high capacity municipal produc-tion from bedrock wells in the Guelph area clearly requiresfurther testing. Additional subsurface data are required tobetter understand the relationship between local and regionalporosity trends. In addition, the lithological controls onporosity distribution, including development and dissolutionof fractures and joints, especially within the production zonestrata, need to be addressed.

Acknowledgements This study is an outgrowth of Cole’s BSchonours thesis, carried out at the University of Waterloo, which was

partially funded by Coniglio’s NSERC Discovery Grant. Theauthors are indebted to D. Belanger and colleagues at the City ofGuelph Waterworks Division for providing many of the key reportsused in this project. We would also like to thank B. Parker forproviding access to research boreholes made possible by fundingfrom Guelph Tool Inc. and NSERC. S. DiBiase from GolderAssociated Ltd. provided access to cores drilled as part of a watersupply study for the City of Guelph. K. Dekeyser is thanked formaking available her detailed core logs of the University ofWaterloo research boreholes. The authors would like to thank S.Worthington and the other reviewers of this manuscript for theirinsightful ideas that contributed greatly to the report.

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