Drilled wells (Figure 4 and Figure 5) are the most common type of domestic well in developed counties. They are usually installed with a truck-mounted drill rig (Figure 6) that uses either a rotary or percussion drill bit to create a relatively small-diameter borehole (< 200 mm diameter). A rotary drill uses a rotating drill stem and drill bit (or a combination of rotation and percussion action) to create a borehole. There are several types of rotary drilling methods, including air-rotary, mud-rotary, and downhole hammer. Rotary drill rigs are powerful enough to drill deep holes (100 m depth) through bedrock in one day, depending on drilling conditions. Percussion drilling, also known as cable tool drilling, is one of the oldest known drilling methods and was developed in China over 4,000 years ago. Cable tool rigs make a borehole by using a cable to repeatedly raise and drop a heavy drill stem and bit into the ground. It is much slower than rotary drilling but is still widely used in some areas because the equipment is simple to operate and less expensive than rotary drilling equipment. This shows how a drilled well is installed and discusses the main components of a well and water system.
Domestic drilled wells are typically constructed by drilling a borehole with a diameter of 200 mm or less and lining the borehole with a 150 mm diameter casing. A casing diameter of 150 mm is large enough to allow a commonly used 100 mm diameter submersible pump to be installed through the casing. Depending on the drilling method, the casing is either installed as drilling proceeds or after drilling is completed. The open space between the casing and the geologic formation (i.e., well annulus) is usually sealed with bentonite or cement grout (or a mixture of the two) to prevent surface contaminants from entering the well and aquifer. A drilled well in unconsolidated sediments is cased for its entire depth and has a well screen or slotted casing throughout its length where groundwater is intended to enter the well.
A domestic well drilled in bedrock will normally only have casing in the top section of the well to keep unconsolidated sediments from collapsing and to provide a space for the grout or bentonite seal. Typically, the casing extends from the ground surface to the bedrock, although it may be extended deeper into the bedrock to provide additional protection from shallow groundwater contamination. If the bedrock is stable, the bedrock section of the well will stay open without casing and is often left as an open hole. If the bedrock has sections with loose, broken rock that may collapse into the well, these sections will be cased and screened.
Drilled wells have several advantages over other well types. Because of their greater depths, they are less vulnerable to shallow groundwater contaminants originating from human activities, and to declining groundwater levels during droughts. They can be installed in both unconsolidated aquifers and bedrock aquifers, and they can be used in settings with deep water tables or deep aquifers that are not accessible by shallow wells.
One of the main disadvantages of drilled wells is that they may access deeper, older groundwater that has had more time in the subsurface to dissolve naturally occurring materials that may produce higher concentrations of dissolved constituents, such as arsenic and fluoride.