About Groundwater

In Ontario, 23 percent of the population relies on groundwater as a source for their drinking water. In some areas, groundwater is the only source of water.

Groundwater is an important source of drinking and is found in the cracks and spaces in soil, sand and rock. It moves through geological formations called aquifers. Eventually it reaches a spring, stream, lake or wetland, where it discharges to the surface, becoming surface water. This may take as little as a few days or as much as centuries. Groundwater is always on the move.

Conservation authorities monitor groundwater using wells within their long-standing partnership program with the Provincial Groundwater Monitoring Network. They rely on the Ontario Drinking Water Quality Standard for Nitrite + Nitrate and for measuring and reporting on chloride concentrations.

Two indicators are used to assess groundwater quality for the Conservation Authority watershed report cards:

Nitrite + Nitrate

Although nitrogen occurs naturally in rocks and groundwater, the concentration of nitrogen can be significantly increased by the overuse of fertilizer, spilled manure, and leaky septic systems. Conservation Authorities rely on the Provincial Groundwater Monitoring Network as one source for nitrite + nitrate data.


Chloride is a naturally occurring element that can be found in concentrations that exceed drinking water quality standards under natural circumstances, sometimes due to the type of rock the groundwater is coming from. It can be difficult to determine whether high chloride levels are due to natural or human causes.

Threats to Groundwater

Contamination of groundwater sources is a growing concern. Cracks in the exposed bedrock can allow contaminants from industrial, agricultural and residential activities to easily enter the groundwater below. This can be impossible to clean up.

Specifically, groundwater sources can be threatened by activities such as leaky fuel or chemical storage containers, road salt spreading, spreading of sewage treatment sludge, spilled fertilizers or chemicals such as dry cleaning solvents, accidental spills, septic systems, underground sewers, landfills, manure, or even from overuse. Uncapped wells can also provide a direct pathway to groundwater sources.

The sustainability of groundwater can also be threatened. Underground aquifers are recharged mainly by rainfall and snow. As long as the water contained in these aquifers is not extracted faster than it is replenished, groundwater is a renewable resource.

Aquifers and Recharge Areas Connect Surface and Groundwater Sources

An aquifer is an area of soil or rock under the ground that has many cracks and spaces and has the ability to store water. Water that seeps into an aquifer is called recharge. Much of the natural recharge of an aquifer comes from rain and melting snow.

The land area where the land or snow seeps down into an aquifer is called a recharge area. Recharge areas often have loose soil such as sand or gravel which allows water to seep slowly into the ground. Areas with shallow fractured bedrock are also often recharge areas.

A recharge area is considered significant when it helps maintain the water level in an aquifer that supplies a community with drinking water. Under the Clean Water Act 2006, it may also be considered significant if it plays a necessary role in recharging cold water streams that support specific fish.