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Understanding Groundwater

In this Section:

What Is Groundwater?

In the simplest sense, groundwater is water located below the surface of the ground in the pore spaces, fractures and voids of sediments and/or rock formations. Water infiltrates into the ground and seeps downward and can return to the ground surface through springs and streams. This is part of the continuous circulation of water between ocean, atmosphere and land, termed the hydrologic cycle. For a more detailed overview of the hydrologic cycle the reader is referred to an excellent site prepared by Environment Canada.

What Are Aquifers?

An aquifer consists of layers or units of geologic materials that contain sufficient saturated and permeable material to yield a useable, sustainable amount of groundwater. There are two basic types of aquifers, unconfined and confined.

Unconfined aquifer

In an unconfined aquifer water flows directly between the ground surface and the saturated zone of an aquifer. An unconfined aquifer does not have an overlying confining layer and water infiltrating from the ground surface, seeps downward until it reaches the saturated groundwater zone or water table. The water table is in direct contact with the atmosphere through the pore spaces of the overlying soil materials and groundwater moves in direct response to gravity.

What is the water table?

The water table, or the phreatic surface, is the depth below ground at which the soil pore spaces or fractures and voids become completely saturated. The depth of the water table may range from immediately beneath the ground surface to tens of metres and is a result of several factors, including surface soil textural characteristics, topography and recharge rates due to precipitation or runoff.

The depth of the water table may vary over time due to natural changes in recharge and discharge rates, but may also change as a result of developmental pressures. There may also be significant seasonal fluctuations in the water table, mainly due to changes in precipitation.

Confined aquifer

A confined aquifer is an aquifer which occurs immediately beneath an impermeable layer of geologic material. This overlying unit may be either an aquiclude, which is a layer or unit of geologic material of such low permeability that it is virtually impermeable to groundwater flow, or an aquitard, which is a layer or unit with low permeability which still allows for limited transmission of groundwater.

The water in a confined aquifer can be under pressure. The level (or elevation) below ground to which groundwater will rise in a confined aquifer is called the potentiometric surface. If the water level rises above the top of the aquifer, the aquifer is described as an artesian aquifer and if the water level rises above the ground surface the aquifer is a flowing artesian aquifer.

How Are Aquifers Created?

The creation of aquifers, or aquifer systems, is the result of complex and dynamic interactions of natural earth processes, including weathering and erosion, streamflow, glaciation and deglaciation, and sea level fluctuations. The erosion of rocks exposed at or near the surface of the earth results in their breakdown into progressively finer and finer particles, and the redistribution and deposition of these particles by running water, glacial ice and wind produces material which eventually comprises aquifers and aquifer systems.

Aquifers Types

The following sections provide a brief summary of these complex earth processes and how they may result in the creation of aquifers and aquifer systems. The main aquifer types include:

Bedrock sandstone aquifers

In the geologic past, ancient seas covered most of the central part of North America. Periods when the sea encroached upon the shoreline of the landmass are referred to as transgressive sea periods. As the sea transgressed over the land, sand carried to the sea by rivers and streams, was continually deposited as beach deposits along the margin of the advancing shoreline. As opposed to transgressive depositional environments, periods when the shoreline of the sea retreats from the landmass are referred to as regressive sea environments, and although the same mechanism of sediment deposition occurs, the vertical sequence is reversed with clay particles now being deposited.

During certain periods of earth history, such episodes of sea transgression and sea regression have repeatedly occurred, resulting in a geologic sequence consisting of alternating zones of predominantly silty sands, now commonly sandstone formations representing potential aquifers, and predominantly sandy clays, now commonly shale formations, which are generally unproductive of groundwater.

Bedrock carbonate aquifers

Carbonate bedrock sediments are also an important source of groundwater in some areas of Canada. Shells and secretions from marine organisms were deposited in the shallow warm waters of ancient seas to form great thicknesses of organic calcium carbonate coral reefs. These organic remains, usually mixed with variable amounts sand and silt, form what are commonly referred to as limestone aquifers.

Alluvial aquifers

As rocks are degraded by erosion, streams and rivers transport the eroded rock particles overland as either bedload, suspended load or in solution. Bedload consists of the more coarse-textured particles moving along the channel bottom. Suspended load is the more fine textured sediments (usually clay to coarse sand) carried along in suspension and dissolved material is carried in solution. The bedload and suspended sediments form alluvial aquifers, with the groundwater development potential dependent largely on the thickness and coarseness of the deposit. Typical alluvial landforms comprising aquifers are floodplains, terraces, alluvial fans, and deltas.

Floodplain deposits, often referred to as river alluvium, form in river valleys when the river jumps its banks (or floods), with subsequent deposition of the suspended sediments. Over time as the river meanders back and forth across the bottom of the valley, these deposits coalesce into a plain, usually extending across most, if not the entire width of the valley. Generally, due to the well sorted nature of the sediments, and the high recharge rates provide by the river, floodplain deposits often represent good aquifers.

Due to changes in the stream gradient, the river may resume erosion along the course of the valley, resulting in down-cutting into and through the existing floodplain deposits. With the deepening of the valley, the former floodplain surface(s) are left behind as terraces along the valley walls; these terraces may be either paired, or existing along both sides of the valley, or unpaired, existing along only one side of the valley. Numerous terrace levels are often encountered along the course of a valley. If saturated, terrace deposits may represent a good aquifer, however, they are often impacted by spring drainage into the adjacent valley.

Alluvial fans are formed at the base of mountains when high velocity streams and rivers carrying large sediment loads encounter relatively flat-lying valleys adjacent to the mountain front. As a result of the sharp water velocity drop when the swift moving mountain stream enters the flanking valley, the sediment load is abruptly dropped and large aprons of sediment are developed at the base of the mountain. As multiple fans develop and grow they can coalesce into an almost continuous wedge of sediment lying along the mountain front. The porosity and permeability (hydraulic conductivity) characteristics of alluvial fan deposits are lower as compared to river alluvium, however, due to their often great thicknesses they can also often represent a good source of groundwater.

Deltaic landforms develop when rivers or streams enter bodies of standing water, either freshwater or saltwater, or when high velocity, high sediment loaded streams enter a slower moving stream. These deposits usually have good porosity due to the generally well sorted and well rounded nature of the deposits however, the hydraulic conductivity may vary greatly depending on the average grain size distribution.

Glacial aquifers

Pleistocene glaciers have affected much of the world over the past 3 million years, particularly in the Northern Hemisphere. The last continental glacial advances commenced approximately 80,000 years ago and began retreating about 10000 to 8000 years ago. In Canada the ice sheets accumulated in the high plateau areas of Baffin Island and Labrador and radiated out from these areas.

Rock debris carried within the ice is eventually transported to the terminal (end), or to the lateral (side) locations of the glacier and deposited as till. Till is defined as unsorted and unstratified glacial drift comprised of a heterogeneous mixture of clay, silt, sand and gravel, containing cobbles and boulders of variable size.

The rock debris melting out of the ice is known as ablation till and forms a landform referred to as a moraine. Small lenses of sand and gravel occur when the ablation till is reworked by running water. These sand and gravel deposits generally occur as small, pockets or lenses, randomly distributed through the till and are sources of groundwater for many individual wells.

On a larger scale, the sediment carried as bedload or in suspension in large meltwater or spillway channels issuing from the melting ice front may be deposited within the meltwater channel as glaciofluvial sands and gravels or as stratified (layered) outwash sand and gravel deposits.

Outwash accumulates in front of the moraine in standing water and depending on the residence time of the ice-moraine contact, may develop into an outwash plain extending for several kilometers. Outwash deposits are extremely variable in terms of thickness, textural variability, lateral extent and continuity. As multiple glaciations occurred, these outwash deposits were buried under later tills and are now encountered as intertill sands and gravels occurring between till units.

Although yields from glacial outwash deposits will vary greatly depending on the thickness and areal extent of the sediments, wells completed in intertill sands and gravels will generally be higher than yields from outwash pockets and lenses scattered throughout individual till units.

With the advance and retreat of continental ice sheets on several occasions in North America, this process has resulted in a complex succession of interlayered till and outwash sequences. This distribution of till and outwash is further complicated by the fact that short term pulses (advances) and retreat of the ice front occurred on multiple occasions within each of the major glacial events thus resulting in similar inter-layered till and outwash deposits on a smaller scale within the major units.

Although not deposited directly from the ice, or from flowing meltwater, wind blown deposits known as loess are also related to glacial activity and in some areas may represent a potential source of groundwater supply. Loess consists of surficial deposits of unconsolidated and non-stratified silt, formed by high velocity winds coming off the ice front blowing across the outwash plains lying at the front of the ice sheet. These windblown silt deposits may achieve substantial thickness and extent.

Preglacial buried valleys

Another type of sand and gravel deposit which may represent a potential source of groundwater are water bearing sands and gravels which infill preglacial drainage channels. These preglacial valleys, commonly referred to as buried valleys, are ancient drainage channels which were carved into the preglacial bedrock landscape and were subsequently buried by later glacial deposits.

Although they may be regional in extent, the distribution and hydraulic continuity of water-bearing sediments within these ancient drainage channels often vary greatly over short distances, resulting in extremely variable groundwater development potential.

Igneous and metamorphic aquifers

The Canadian Shield is a vast area comprised of complexly folded to flat-lying metamorphic and igneous rocks of Precambrian age, mantled by a thin layer of heterogeneous glacial drift. These solid rocks do not have the necessary hydraulic characteristics of water storage and transmission to provide adequate groundwater supplies. Any groundwater obtained from these rocks comes from fractures and voids.