An aquifer consists of layers or units of sands, gravels and rocks that contain sufficient saturated and permeable geologic material to yield a useable, sustainable amount of potable groundwater. The supply of underground water originates from surface water infiltrating into the ground and accumulating in the geological deposits.
Over the course of the earth's history, geologic deposits have formed and been buried by subsequent deposits. The creation of deposits capable of holding groundwater has been the result of complex and dynamic interactions of natural earth processes, including weathering and erosion, streamflow, glaciation and deglaciation, and sea level fluctuations.
Confined and unconfined aquifers
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.
An unconfined aquifer is an aquifer that receives water directly from the surface. There are no overlying layers of low permeability which can isolate the groundwater system.
Main Types of Aquifers
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.
Transgressive depositional environments are periods when the shoreline of the sea retreats from the landmass. This is referred to as a regressive sea environment, and although the same mechanism of sediment deposition occurs, the vertical sequence is reversed with clay particles now being deposited.
During certain periods of the earth's history, such episodes of sea transgression and sea regression have repeatedly occurred. This has resulted in a geologic sequence consisting of alternating zones of predominantly silty sands which are now common sandstone formations representing potential aquifers, and predominantly sandy clays which are now common shale formations. These shales are generally unproductive as a groundwater supply.
Bedrock carbonate aquifers
Carbonate bedrock sediments are 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.
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 or 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. These usually extend 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 is 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 drop in water velocity 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.
Pleistocene glaciers have affected much of the world over the past three million years, particularly in the Northern Hemisphere. The last continental glacial advances commenced approximately 80,000 years ago and began retreating about 10,000 to 8,000 years ago. The ice sheets accumulated in the high plateau areas of Baffin Island and Labrador in Canada 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 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 kilometres.
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 retreats of the ice front occurred on multiple occasions within each of the major glacial events. This has resulted 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 area covered.
Preglacial buried valleys
Another type of sand and gravel deposit, which may represent a potential source of groundwater, is water bearing sand and gravel which infills 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.