Prairie Groundwater Quantity

Groundwater can be collected in dugouts and wells, and pumped to the surface for human use. The capacity of groundwater aquifers* to supply water depend upon how much water is within the aquifer, the rate of movement of groundwater discharge* within and from the aquifer, and the ability of the aquifer to be recharged* or continually supplied with a reliable water source in the future. As surface water percolates downward it replenishes the aquifers lying below it. Eventually the water will return to the surface via natural springs or by seeping into streams, lakes and marshes.

Groundwater availability will be determined by the area of the aquifer and the ability of the rock formation to be saturated with water. Some Prairie aquifers are small so may only be able to supply water locally. Many of these small aquifers are shallow aquifers. The amount of water in shallow aquifers may also be influenced by water supplied from rainfall and surface waterbody sources. Indeed, the dry climate and frequent droughts in the Prairies may sometimes limit the amount of groundwater available for human use. In contrast, larger permeable bedrock aquifers exist that contain large amounts of reliable water amounts. These large permeable bedrock formations have contained water for millions of years but they typically contain very saline or salty water that is not suitable for normal drinking water supplies.

Underground water moves from higher to lower elevation or from areas of higher to lower pressure. The rate of movement is determined by the differences in elevation and/or pressure. Unlike other regions, Prairie groundwater does not, however, recharge beneath the mountains and flow through the subsurface beneath the plains and all the way to the sea. There is no general west-to-east flow such as is the case for most surface streams. The direction of flow will depend upon the local topography and geology and can vary within aquifers and from one aquifer to another.

The rate of flow of water varies considerably when groundwater moves through different materials. In some, the water may move several metres in a day. In others, it may move only a few centimetres over centuries. Water movement within and from an aquifer may be limited if thick aquitards* lie below or above the aquifer. These aquitards have low permeability so water cannot move readily as the rock or soil layers are not very porous. Many Prairie aquifers have limited water recharge ability because impermeable aquitards are common, especially for deep bedrock aquifers. Some aquitards consist of thick layers of dense clay and clay-rich glacial till that is common throughout the Prairies. These types of aquifers are considered deeply confined aquifers as the upper level of groundwater is trapped between impermeable aquitard layers and is located deep in the ground and far from the water table.

All of these aspects of Prairie groundwater limit the amounts of water available for human uses. Groundwater pumped from a Prairie well generally has entered the aquifer as recharge within a few hundred meters to at most tens of kilometres distance from the well. Problems of water supply may also occur when multiple users draw water from the same aquifer. Heavy use in one area may lower the overall water level. If the level of an aquifer is reduced, then the level of any stream or lake that it discharges into may also be reduced. Wells drawing from that source will also fail if the water level falls below the depth of their intake points.

The amount of sustainable yield from a groundwater well is influenced by the type of aquifer because of differences in their recharge rates. Shallow aquifers (< 30 m deep) may receive relatively strong recharge (up to tens of mm of water on an areal basis, and the effect of pumping from such aquifers rarely extends more than a few km form the pumping well. Deeper aquifers receive very little recharge and the effects of pumping can slowly accumulate until regionally significant effects become apparent after perhaps decades of pumping. Buried channel aquifers are common and may be highly permeable, allowing large short-term well yields. However, this type of aquifers groundwater may have very large level drawdown* due to pumping, and effects can extend quickly along the buried channel over distances that can exceed tens of kilometres. Sustainable yields from such aquifers are therefore limited, particularly if they are also confined by thick layers of clay and clay-rich glacial till.

* Glossary:

Aquifer: a geologic formation that is permeable enough to yield useful amounts of water to water supply wells.

Aquitard: a geologic formation that has low permeability and does not yield useful amounts of water to supply wells. Most aquitards in the prairies have very low permeability and greatly restrict the recharge of water to underlying aquifers.

Base flow: the flow in a stream that is due to groundwater discharge. Such groundwater discharge is usually continuous year-round, but for small streams transpiration by vegetation along the stream can use up all the base flow in the summer time.

Discharge: volume of groundwater that moves or flows through an aquifer

Drawdown: the decline of the water level in a well due to pumping from the well itself or other wells.

Recharge: process where water from the surface moves down and enters a groundwater aquifer. For an aquifer undisturbed by pumping, the recharge is balanced by discharge of water from the aquifer. Pumping usually induces additional recharge and may reduce the amount of discharge from the aquifer.