Chapter 15 Groundwater
Except in areas where groundwater comes naturally to the surface at a spring (a place where the water table intersects the ground surface), we have to construct wells in order to extract it. If the water table is relatively close to the surface, a well can be dug by hand or with an excavator, but in most cases we need to use a drill to go down deep enough. There are many types of drills that can be used; an example is shown in Figure 14.3.1. A well has to be drilled at least as deep as the water table, but in fact must go much deeper; first, because the water table may change from season to season and from year to year, and second, because when water is being pumped, the water level will drop, at least temporarily.
Where a well is drilled in unconsolidated sediments or relatively weak rock, it has to be lined with casing (steel pipe in most cases) in order to ensure that it doesn’t cave in. A specially designed well screen is installed at the bottom of the casing. The size of the holes in the screen is carefully chosen to make sure that it allows the water to move into the well freely, but prevents aquifer particles from entering the well. A submersible pump is typically used to lift water from within the well up to the surface where it is needed. The well shown in Figure 14.3.1 has casing that is about 40 centimetres in diameter, which might be typical for a municipal water supply well, or a very large well for irrigation. Most domestic wells have 15 centimetre casing.
Pumping water from the well removes water from inside the well at first. That lowers the water level inside the well. This means that water will flow from the surrounding aquifer toward the pumping well where the water level is now lower.
That is how a well gets water from the ground. The water table, or potentiometric surface, will slope in toward the well where the water is being withdrawn. That indicates the energy gradient that is allowing water to flow toward the well. This creates a shape known as a cone of depression surrounding the well, as illustrated in Figure 14.3.2. If pumping from a well continues for hours to days, the cone of depression may result in a loss of water in nearby wells. As shown in Figure 14.3.3, pumping of well C has contributed to well B going dry because the water level is now below the bottom of well B. If pumping continues in well C, it too may go dry.
Exercise 14.2 Cone of depression
The two diagrams here show the same well before (left) and after (right) long-term pumping. A cone of depression has developed. This provides the energy gradient for water to flow toward the well so that it can be pumped out.
How will this likely affect the rate of flow into the well?
See Appendix 3 for Exercise 14.2 answers.
Like other provinces in Canada and states in the US, British Columbia has a network of observation wells administered by the Ministry of the Environment. These are wells that are installed to measure water levels; they are