Groundwater flows below the surface of the earth, between cracks in rocks and soil particles. Of all the fresh water in the world, two-thirds is underground, making groundwater one of the most valuable resources.

Groundwater: Saturated and Unsaturated ZonesGroundwater is water that has percolated through the soil until it reaches an aquifer at which point it flows with gravity downhill and discharges to a surface water source. An aquifer is comprised of sand and gravel or porous rock, for example limestone or sandstone, which rests above an impervious surface such as bedrock or clay that the water cannot infiltrate.

With one in four people in Canada relying on groundwater for their water supply, it is an important source of water. One concern with using groundwater is that it may be used faster than it can recharge. Due to the nature of groundwater, it is difficult to accurately assess how much is available, and how fast it can be recharged. Some areas that rely heavily on groundwater for their water supply have seen the amount of water drop as a result of over withdrawal. This can pose a serious problem for areas that do not have other reliable sources of water supply.

The problem of over withdrawal can also affect the quantity of surface water, as groundwater eventually discharges to the surface contributing to the water flow in streams, rivers, marshes, and lakes. In some areas, groundwater is the only source of water that keeps many streams flowing during dry weather periods of the year.

Click here to learn more about Groundwater in Quebec and Groundwater in Ontario

Adapted from Pollution Probe's Drinking Water Primer