David R. Sharpe

On the kriging of water table elevations using collateral information from a digital elevation model

In unconfined aquifers flowing under topographic gradients, the water table is a subdued replica of the ground surface above. This principle is the basis for using detailed collateral or secondary information from digital elevation models to supplement sparse observations from water wells in the mapping of phreatic surfaces. Data from DEM-derived secondary variables are incorporated into the estimation of water table elevations using the geostatistical method known as kriging with an external drift (KED). Two different KED models are proposed based on the choice of secondary variable. In the first, water table elevation is expressed as the sum of a deterministic trend given by topographic elevation and a residual random component representing depth to water table. In the second, depth to water table is expressed as a linear function of a deterministic trend, given by the TOPMODEL topographic index, and a residual random error. The relationship between water table depth and topographic index is derived from simplified groundwater dynamics and forms the basis of TOPMODEL-type rainfall-runoff models. The two KED models are applied to the mapping of water table elevations in the Oak Ridges Moraine (ORM), an unconfined aquifer near Toronto, Canada. Results show that KED with topographic elevation as external drift is the more robust of the two models. Despite its strong theoretical basis, the second model yields kriged water table elevations that are not always physically plausible. In part, this is because field observations of water table depth do not verify the predicated relationship with topographic index in large parts of the study area. However, this relationship may be valid in other cases and at other spatial scales. In such cases, the second model would provide a very powerful approach for mapping water table elevations and for calibrating distributed parameters of the TOPMODEL equations on water well observations.

Erosion of bedrock by subglacial meltwater, Cantley, Quebec

Several erosional forms on bedrock at Cantley, Quebec, differ from well-known glacial abrasion forms. The forms consist of obstacle marks, hollows, depressions, and channels, which are defined by sharp rims, smooth inner surfaces, divergent flow features, and remnant ridges. These forms are found on lee, lateral, and overhung rock surfaces. This assemblage of features is best explained by differential erosion produced by separation eddies along lines of reattachment. Rapid, sediment-laden, turbulent, subglacial melt-water flows likely produced the forms by corrasion and cavitation erosion. Sculpted fluvial forms in terrain subject to flooding in Australia are identical to some of the Cantley forms which confirms their formation by water erosion. Although glacial abrasion may not be eliminated as an explanation for sculpted forms, it is not necessary. Ice-abrasion forms, such as striations, and such plucked forms as gouges and crescentic fractures are also present at the Cantley site. Pitted forms, polishing, and carbonate precipitate are also found. The occurrence of abrasion, pitting, polishing, and carbonate precipitate with meltwater forms suggests that the meltwater flows were subglacial. Decoupling of abrading ice from its bed temporarily suspended glacial abrasion, whereas reattachment of ice to the bed may have led to the rounding of sharp edges and the production of striations superposed on the glacifluvial forms. The association of forms produced both by glacifluvial erosion and ice abrasion suggests that the glacier was alternately lifted from, and reattached to, the bed during periodic subglacial floods. These floods may have affected the dynamics of the ice sheet, and depositional sequences related to catastrophic meltwater outbursts probably were laid down in adjacent basins.

Evidence for rapid sedimentation in a tunnel channel, Oak Ridges Moraine, southern Ontario, Canada

Abstract In south-central Ontario, a Late Wisconsinan regional unconformity consisting of tunnel channels and drumlinized till crops out north of Lake Ontario. The tunnel channels are locally infilled and the unconformity buried by sediment of the Oak Ridges Moraine. Based on seismic reflection profiling and drillcore, the tunnel channels are known to continue beneath the moraine. Detailed sedimentological analysis of ∼300 m of continuous core from two drillholes located ∼7 km apart in subparallel tunnel channels identified three facies. The gravel facies is 17 m thick and occurs in only one of the cores. It directly overlies the unconformity and may include a number of upward-fining units. Seismic reflection data suggests the gravel forms stacked gravel mesoforms or quasi-horizontal gravel layers (bed load sheets deposited from fluidal flows). Along the deep channel axis, the graded massive sand facies up to 37 m thick is the most common facies and consists of silty, medium sand with minor coarse sand. Strata are reverse-graded, normal-graded, or massive with isolated silt intraclasts and evidence locally for dewatering. This facies is interpreted to have been deposited from hyperconcentrated dispersions downflow of a hydraulic jump (or major channel confluence). The third facies consists of medium-scale and small-scale cross-stratified sand. Medium-scale cross-stratification occurs predominantly in the lower 37 m of the core and is interbedded with the graded massive sand facies; small-scale cross-stratified sand is progressively more common upward in core. Medium- and small-scale cross-stratified sands was deposited, respectively, by subaqueous dunes and ripples formed in dilute fluid flows. The complete succession is interpreted to have been deposited very rapidly within a subglacial tunnel channel before discharge ceased along the channel. Deposition followed closely, and in part coincided with rapid expansion of the channel by erosion in a hydraulic jump or at a major channel confluence along the grounding line of a subglacial lake. Scour into unconsolidated sediment contributed to the sediment flux and quickly overloaded the flow with suspended sediment, which in turn resulted in extremely high rates of sedimentation immediately downflow. Such depositional conditions support the notion that tunnel channels in the study area formed and/or served as conduits for subglacial jokulhlaup discharges, and that the Laurentide Ice Sheet most probably did not generally drain by steady state processes, but instead by short-lived catastrophic events.

Sequence stratigraphy of a glaciated basin fill, with a focus on esker sedimentation

A large integrated data set of cores, outcrop data, and seismic transects from the mud-buried Vars-Winchester esker in the Champlain Sea basin, Canada, was studied to gain insight into how muddy glaciated basins fill with sediment, and how esker sedimentary systems contribute to this process. Three stratigraphic units—a till sheet over carbonate bedrock, the Vars-Winchester esker , and overlying Champlain Sea mud—are identified in the data set. The till is massive, mud rich, carbonate rich, and drumlinized. The esker is also carbonate rich, and rests erosively on till or bedrock. It consists of two elements, a narrow gravelly central ridge and a broad sandy carapace. Three units comprise the overlying mud package: gray carbonate-rich rhythmites, massive bioturbated mud, and carbonate-poor, red-and-gray rhythmites. A sequence stratigraphic model is proposed to explain these observations. Emphasis is placed on gradual ice-front translation superimposed by rapid meltwater events. The esker is interpreted to have been derived from the underlying till by water that flowed through a subglacial conduit (R-channel), within which the narrow gravelly central ridge was deposited. Most mud and finer sand bypassed the conduit and was deposited proglacially on the floor of the Champlain Sea, first as sandy outwash and, farther basinward, as muddy carbonate-rich rhythmites. Gradual ice-front retreat superposed distal facies over proximal facies, generating the upward-fining succession that starts with the esker gravel and ends with muddy rhythmites. Most esker sediment appears to have been deposited during rapid, jokulhlaup-like floods that punctuated gradual retreat. Discharges are estimated to have been high, possibly on the order of several hundred to, perhaps more commonly, several thousand cubic meters per second. The chaotic and random-looking appearance of the resultant sedimentological signatures in the esker sensu stricto is sharply contrasted with the regularity of the muddy rhythmites. If the rhythmites are indeed correlative to the esker, which seems reasonable given their geochemistry and the fact that their volume scales to the volume of mud in the till, the flood events that deposited the esker must have been seasonally mediated, and the basin water must have attenuated the flood signal, resulting in a rhythmic “on-off” signature in more distal portions of the system. The regularity of the rhythmites does not betray the chaotic nature of the esker sensu stricto, and vice versa. Studying either one in isolation would lead to a very different “end-member” impression of how eskers form and how esker sedimentary systems operate during the infilling of glaciated basins.

A 3-dimensional geological model of the Oak Ridges Moraine area, Ontario, Canada

Abstract Please click here to download the map associated with this article. The Oak Ridges Moraine area, southern Ontario, includes most of the Greater Toronto Area, which is the most populated region of Canada. The ∼ 11,000 km2 region is bounded to the south by Lake Ontario and to the north where Paleozoic bedrock abuts Precambrian Canadian Shield. The area extends 160 km eastward from the Niagara Escarpment, a prominent 100 m high regional bedrock scarp. The surficial sediment is up to 200 m thick, and reveals exposures of the oldest Quaternary sediment in southern Canada. Population growth has caused land use conflicts and increased pressure on groundwater resources. Construction of a regional 3-D geological model of the glacial stratigraphy was needed to support a better understanding of aquifer distribution, scale, and resource potential and protection. Mapping of the regional glacial geomorphology and sediment succession identified a number of distinct landforms: tunnel channels, drumlins, eskers, moraines, and till and lacustrine plains. Using sequence stratigraphic concepts, strata have been grouped into four principal units that unconformably overlie Paleozoic bedrock: Lower sediment, Newmarket Till, Oak Ridges Moraine, and Halton Till. These four Quaternary units plus bedrock have been mapped in the subsurface as a succession of interpolated surfaces using an innovative stratigraphic database-GIS approach. The model-building process involved stratigraphically coding high-quality data, then integrating an extensive and diverse array of subsurface geological and archival datasets using an expert system (geological rules). Stratigraphic data subsets were then extracted and merged with DEM-controlled surface mapping and interpolated in a GIS.

A three-dimensional hydrostratigraphic model of the Waterloo Moraine area, southern Ontario, Canada

Aquifers of the Waterloo Moraine play a key role as the main source of drinking water for the Region of Waterloo. For the effective management of this water source, a sound understanding of the aquifers contained within and below the Moraine is essential. Critical knowledge required for this understanding includes the definition of the sediment facies distribution, architectural elements and geological origin of the Quaternary-aged deposits. A basin analysis approach has been applied to geologic data collection and interpretation to unravel the paleogeographic history of the study area and to provide a predictive framework for understanding its geological variability. Coarse (sand and gravel) sediment within the Waterloo Moraine was deposited during a series of high-energy meltwater discharge events from several sediment input corridors (eskers), into a deep, large, ice-supported glacial lake. This depositional setting led to a complex three-dimensional architecture comprising sand-gravel and mud units that are increasingly interbedded away from the multi-directional influx sources around the perimeter of the Moraine. A recently completed digital, three-dimensional geologic model of the area provides details of the various geological units that help refine the understanding of the hydrostratigraphy. This information has improved the understanding of groundwater flow (including interaction between surface and groundwaters) and has provided valuable information critical for source water protection. Information on the distribution, thickness, geometry and properties of these units has resulted in a better understanding of the potential linkages between near-surface recharge areas and deep aquifers across the region. This geological information is important in developing predictive models, for example, determining the location of high transmissivity zones within the moraine. Derivative products such as aquifer vulnerability and recharge maps may help inform policy makers in developing land use and nutrient management plans in the vicinity of well fields and sensitive lands.

The Waterloo Moraine: Water, science and policy

Growth is widely seen as the means to create employment and achieve economic prosperity. Growth also consumes natural resources. As a consequence, growth can create the potential for societal conflict where resources are limited. For example, urban development or aggregate extraction can conflict with ecosystem preservation and recharge zone protection. In the Canadian context, Waterloo Region (the Region) is a classical case study for this potential conflict. Economically, the Region is changing rapidly. While much of the Region’s traditional manufacturing industry has been devastated by globalization, a new high-tech industry has emerged, resulting in considerable urban growth. The provincial government has supported this trend by designating Waterloo Region as one of a number of growth centres in the province, and as a result, the population is expected to increase by 50% over the next 30 years. Growth increases the pressure on the community’s natural resources. In addition to land, the most critical natural resource for the Region is water. Although the Region is situated between two of North America’s Great Lakes, access to potable water from these lakes is complicated and expensive. Fortunately, the Region already has an excellent source of water beneath the urban landscape – the groundwater of the Waterloo Moraine. In addition to providing the Region’s drinking water, the Moraine also assures the ecological health of streams and wetlands, and it supports a healthy agricultural sector in the rural areas. The Moraine water resource is adequate for present use; however, it is also limited. Growth not only increases the demand on water but can potentially diminish the resource itself. Sprawling subdivision developments over the aquifer recharge areas can affect the quantity of water available, urban contaminants such as road salt can impact the groundwater quality and aggregate pits can weaken the protection of the aquifers from contaminants. Consequently, conflict can potentially arise when the growing demand tests the limits of the resource. Water managers at the Region of Waterloo have so far been successful in striking a balance between growth, water use and the protection of the water source, using a multi-faceted approach including demand management, delineating groundwater sensitivity zones and constraining the urbanized area by means of a “countryside line”. The sensitivity zone concept is science-based, while the countryside line has a political origin. The latter concept is being challenged by development interests, which seek to expand residential subdivisions across the countryside line into rural areas that also contain the main recharge areas of the Waterloo Moraine. Allowing urban sprawl into the main Moraine recharge areas has the associated risk of upsetting the recharge-withdrawal balance. The considerable effort to preserve and protect the Region’s water source has produced a large amount of knowledge over the past 40 years, involving different branches of groundwater science. The idea for this Special Issue arose when researchers at the Geological Survey of Canada and the University of Waterloo decided to compile an authoritative source for this body of knowledge, to be readily available to others confronted with similar land-water conflicts. The result is a set of 11 research papers by authors from government, universities and private consultants, all with different backgrounds and expertise, but all with a passion for groundwater. These papers cover both science and societal aspects of groundwater, they are all interrelated, but each stands on its own merit. Frind and Middleton (2014, this issue) examine the Region’s overall strategy for managing the complex Moraine groundwater resource and providing a reliable water source for the growing community. This successful strategy integrates the principles of sustainable water governance with a science-based understanding of the complex Moraine system. Bajc et al. (2014, this issue) lay the foundation for the science of the Waterloo Moraine by unravelling its depositional history during the Quaternary age. Understanding this history and applying a full range of exploratory tools including