Prairie Groundwater Quality

Groundwater quality is affected by factors different from those which affect surface lakes and streams. The quality of groundwater can vary considerably among different Prairie aquifers* and within aquifers at different depths or locations. Some shallow aquifer wells contain high quality drinking water that is naturally hard with high amounts of Iron, Calcium and Magnesium but these compounds do not pose health risks. In contrast, many deep aquifers do not contain water that is suitable for drinking water. Deep aquifers often have low hardness or soft water that has poor water quality because of high salinity and high amounts of Sodium or Sulphate. High levels of arsenic, uranium and other harmful compounds can also be found to occur naturally in some deep aquifers.

The quality of groundwater may be compromised if poor quality surface water or contaminants enters or discharge* into groundwater aquifers. For example, pollutants may seep into groundwater from chemical spills, runoff from agricultural lands or other sources. The type and depth of groundwater aquifers greatly influences their sensitivity to surface water stressors and their ability to buffer effects. The rate of flow of groundwater will also influence how much and quickly groundwater quality can be compromised by surface contaminants. Shallow unconfined aquifers are more vulnerable than deeply confined bedrock aquifers that are located deep in the ground, especially if movement is limited by aquitards*. Indeed, shallow aquifers with high recharge* rates can be directly related to the exposure of contaminants on the surface of the land and effects can occur within a short period. In contrast, deep aquifers may not be contaminated for decades or longer. Because of the slow rate of flow, pollutants may take years to reach a monitoring well only a short distance away.

In addition, drilled water and other types of wells can act as a conduit to potentially introduce surface contaminants into an aquifer. Aquifer contamination may occur if water or other types of drilling wells (eg. oil/gas) have not been sealed or decommissioned properly, which allows the mixture of poor and good quality water within aquifers and between the borehole and casing pipe that may contain contaminants. This may be of concern, especially as about 40% of wells in Canada have been found to be contaminated by nitrates, bacteria and pesticides[1]. Numbers are comparable within the Prairies for nitrate and bacteria levels, and private wells have higher average contamination rates than municipal wells[2]. Many of these private or other drinking wells may not have any or have limited water treatment.

Moreover, because aquifers are isolated underground, spills or contaminants that have reached them are extremely difficult to clean up. These two factors greatly increase the seriousness of contamination and underline the importance of prevention in maintaining groundwater quality.

If protected from contamination, groundwater may be of high quality. The filtering action of the soil plus the long time it is underground leaves the water almost free from bacteria and sediment, although in the Prairies it often contains high concentrations of dissolved minerals. However, this seldom poses a health risk and many aquifers remain excellent sources of drinking water.

* 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.

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

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.

[1] François Côté, 6 February 2006. Freshwater Management in Canada IV. Groundwater. Library of the Parliament of Canada. Parliamentary Information and Research Services.