Education

Groundwater

Groundwater lies almost everywhere below the earth's surface. In Canada, an average 25% of the water supply comes from groundwater. In addition, groundwater is one of our most important sources of water for irrigation.

Understanding The Basics of Water Wells

A well is an access, a port to the water present in the subsurface. It is in most cases either cased to be in contact with the saturated soil or drilled to intercept fractures where water flows.

Water is present in the subsurface in the void between soil particles (pores) or in rock fractures or interstices that result from the formation or history of the rocks (e.g. earthquakes) or due to dissolution (for specific rock formations). When all the pores or fractures are filled with water (or saturated), the combination of the soil or rock formation and the water is called an aquifer.

When pumping a well, the water in the well is lowered. The drop in elevation in the well is called a drawdown. Due to the drawdown, a difference between the water level at a distance from the well which is higher (or presents a higher level of energy) and the water level in the well (lower and at a lower level of energy) is created. This translates in a flux of water from the highest to the lowest level of energy that results in a flow of water towards the well.

The flow of water towards a well can be compared to the flux of energy from a building to the outside. The drawing illustrates a building where a temperature is maintained at a higher level than the outside temperature. This creates a flow of energy from the building to the outside, through the walls. To maintain the temperature in the building, a source of energy is required, and it is the input of energy that creates the temperature rise in the building that triggers the flux of calories from the building to the outside.

In the case of a water well, the energy is provided to extract water from the well. This energy input indirectly creates a drawdown (or drop of water level) in the well. The resulting difference between the water level in the well and in the aquifer is similar to the difference of temperature between the inside and outside of the building. A flux of water will be created between the aquifer and the well. By keeping pumping, the drawdown will be maintained, thus keeping the movement of water towards the well.

The volume of water extracted from a well comes from a volume of aquifer in the subsurface. The area where infiltration due to precipitation or recharge associated with movement of groundwater contributes to this volume of aquifer is called the capture zone. From the well outward, the time taken by the groundwater within the capture zone to reach the well can be tracked in time and is referred to as travel time.