How Wells Are Designed
The well's diameter must be large enough to take the pump equipment necessary to move the water to the surface. Most home wells use 4-inch diameter pumps. The well casing lining the drilled hole must extend down far enough to reduce the risk of any surface or near-surface contamination. The well may need a screen to allow for the efficient flow of water from the aquifer into the well. In some cases, it may be necessary to place additional casing to seal off parts of the drilled hole where, for example, the water may have high iron content or some other unwanted chemical attribute.
Not only do the above factors dictate some of well design considerations, there also may be decisions about whether to use steel or plastic casing, about whether to use steel, stainless steel or plastic screens, and whether the screens should be wire wrap, slotted or louver. In many states, there are regulatory requirements and construction codes that the well contractor has to follow, particularly concerning depth of casing and grout material used to seal the annular space between the hole and the casing. Critical decisions related to well depth, diameter and positioning of screens (if needed) are usually made on a site-by-site basis depending on the driller's experience in the local area, the drilling equipment selected for constructing the well, and on the specific information on water strikes and rock conditions found during the drilling process.
A well screen is an engineered device that may be used in wells to help maximize inflow from the aquifer and allow for long-term satisfactory operation of the well. Well screens typically are installed in wells where the aquifer is comprised of loose or unstable material. The screen prevents rock fragments from entering the well, helps support the wall of the well and allows water to enter slowly. Turbulent flow can more easily transport unwanted rock particles, and agitated water may release minerals and clog up the well. A commonly used screen type for water wells uses a continuous slot construction, made by wrapping and welding a continuous length of wire or plastic around vertical rods. Well screens also are made by precision machine slotting (vertical or horizontal slots) or by making louver openings. Screens are made in many different slot or opening sizes and usually are installed by fixing the screen to the end of the casing, which is then lowered down the well to the selected water-producing zone(s) of the aquifer.
For high-yield wells, a "gravel pack" sometimes is used to fill up the space between the well screen and the drilled hole. Placing the gravel in the well next to the screen can be a tricky business, but the highly permeable gravel can really help make the well efficient. Selecting the size of gravel to be used is yet another important well design decision.
For wells where freezing temperatures occur, there has to be a way of diverting water from the well to the home below ground. To achieve this, most contractors will use a special adaptor that connects the vertical pipe coming from the well inside the casing to a horizontal pipe that takes water from the well to the home. This connection usually is made about 6 feet below ground surface by means of a pitless adaptor. This device not only connects the well pipe to the house pipe, but also helps support the weight of the pump and pipes. The design of the adaptor fitting with an O-ring seal also allows the pump to be removed from the well for servicing or replacement.
All wells should have a secure cap to prevent insects or debris from entering the well. However, the well cap should not have a perfect airtight seal. There must be an air vent so that when the water level drops because of pumping, the space created by the falling water level can be replaced by air. If a well is constructed in an area prone to flooding, the casing should be extended up to above the likely flood level.
For more (non-commercial) consumer information about wells and ground water, please visit the following website www.privatewell.com.
The next article in the Trust's How To Talk Ground Water series will discuss the pumping equipment used in wells and the differences among monitoring, observation, test and production wells.