WELL SCREENS

This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2000, Number 3]

WELL SCREENS - WHAT DO THEY DO?

A well screen is an engineered device used in many water wells to help maintain a good water supply from the aquifer and allow for long-term satisfactory operation of the well.

Water wells are constructed in either unconsolidated materials such as sand and gravel or comparatively hard and (usually) stable bedrock. Well screens are installed in wells where the zone containing the water for the well (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. Silt or sand particles may cause excessive abrasion and early failure of the well pump and/ or plugging of equipment in the home. In addition, sand may accumulate in a well and block off water-bearing zones.

In all wells, it is preferable to have water enter slowly. Turbulent flow can more easily transport particles and agitated water may release minerals and clog up the well. Selection of the right screen and the right length of screen can help improve a well’s efficiency and productive life. A commonly used and effective 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. Wrapped screens have an elongated triangle shape when viewed in cross-section with the “point” of the triangle oriented inwards so that the outside slot width is small compared to the inside spacing. This design allows particles that do penetrate the screen to continue through without getting stuck.

Screens are also made by precision machine slotting (vertical or horizontal slots) or by making louver openings. Louver type well screens (shutter screens) are typically used only for high yield wells. Screens are made in many different slot or opening sizes and are usually 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.

Many residential wells in bedrock do not need a well screen. Well screens are not a “one size fits all” item, and if one is needed, local well contractors know how to select a screen and choose the appropriate screen length. For most domestic wells a screen length of 4 to 5 feet is usually adequate. State or local well construction codes (in addition to cost) may influence screen selection.

By all means “screen” your wellhead with a flower display or a “wishing well” but don’t pile soil around the well casing and never use pesticides, manure or fertilizers around a water well.

[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2000, Number 3]

Well Components

The main components of a domestic well are shown in Figure 10 and are described below. They include the well casing, well screen, a pitless adaptor (for cold climates), an annular seal, and a well cap. The primary purpose of these components is to allow the well to function properly and to prevent contaminants from entering the well. Not all components are found in all wells, and the materials and design of the components can vary depending on the well type, geologic conditions, local regulations, and local practices.

• Casing – a pipe used to keep the ground open and prevent sediment and shallow groundwater from entering the well. The pipe is usually made of steel or plastic. A drive shoe (a hardened section of pipe with a beveled edge) is attached to the base of the casing to act as a cutting edge and protect the casing while it is driven into the ground. In modern dug wells, the casing is often made from several concrete rings joined together, while older dug wells may be lined with rocks or bricks instead of casing.

Figure 10 – The main components of a domestic well and water system. Two possible types of pumps are shown here: a submersible pump inside the well for deep wells and a jet pump in the home for shallow wells. Only one of these pumps is needed to operate the water system (modified from USEPA, 2019).

• Screen – a section of pipe at the bottom of the casing with openings that allow water to enter the well, while keeping aquifer material (sediment or broken rock) from entering the well. Common screen types include continuous slot (also called wire wrap screens) and slotted or perforated pipe with various opening designs. A screen is not usually used in domestic drilled bedrock wells unless the bedrock is unstable. Dug wells do not have screens because the water enters through the walls of the well (if the casing or lining is not sealed), and the bottom of the well, which is often filled with a layer of clean gravel. This

  video

  shows an example of what a well screen looks like and how it is installed in a water well.
• Pitless adaptor – an adaptor commonly used in cold regions to provide a frost-free, water line connection while allowing convenient access to the well. It is located below the frost line and allows the water pipe inside the well to pass through the casing and connect to the household water system. Pitless adaptors are designed to provide a watertight seal through the casing to prevent shallow, potentially contaminated groundwater, from entering the well. Historically, well pits were used instead of pitless adaptors to prevent water line connections from freezing. Well pits were constructed over the top of the well, with a typical depth of 2 to 3 m, and may have housed the pump and pressure tank. Well pits are no longer permitted in many jurisdictions because they are vulnerable to flooding which can allow surface water and contaminants to enter the well. However, they may still be in use in older wells.
• Annular seal – a seal placed in the annular (ring-shaped when viewed from above) space between the casing and the sides of the borehole to prevent contaminants, surface water, and shallow groundwater from entering the well. The seal is made with grout, which is a low permeability material (such as bentonite, cement, or a bentonite-cement mixture). It is usually installed along the entire length of the casing by pumping a grout slurry through a small diameter pipe (tremie pipe) from the bottom of the casing up to ground surface. Not all jurisdictions require annular seals, and some require only a portion of the casing to be sealed, rather than its entire length. In those cases, the drill cuttings are relied on to seal the annular space.
• Well cap – a cap on the top of the well casing that provides access for well maintenance and prevents debris, animals, and insects from entering the well. The cap usually includes a screened vent to allow the passive removal of naturally occurring gases and equalize the pressure inside the well with the outdoor atmospheric pressure. This prevents a vacuum from developing inside the well during pumping.

Water System Components

The main components of a domestic water supply system are shown in Figure 10 and are described below. They include a pump, a pressure tank, and if needed, a water treatment system. An example of a domestic water system is shown in the photograph in Figure 11. In this example, the water system uses a jet pump to draw groundwater from a dug well with a shallow water table and an ultraviolet light for treating microbial contaminants.

Figure 11 – An example of a domestic water system used to provide water from a dug well (photograph by John Drage).

• Pump – a pump draws groundwater from the well and delivers it to the house via a water supply line that runs from the well to the house. The two most popular types of pumps for domestic wells are submersible pumps and jet pumps. In deep drilled wells, the most common type of pump is a submersible pump which is placed inside the well and connected to a power source in the house via a power cable. In shallow wells, the most common type of pump is a jet pump which is located above ground, usually inside the house in colder climates. Jet pumps use suction to draw water from the well and, therefore, can only draw water from depths of less than about eight meters. However, deep well jet pumps are available that use two water lines to draw water from greater depths (up to about 30 m depth). Regardless of the pump type, a check valve (foot valve) is usually placed in the water supply line in the well to prevent the water from running back into the well when the pump shuts off. Note that submersible and jet pumps can both be used in deep and shallow wells, although as discussed above, jet pumps are limited to shallow water levels. The choice of pump type will depend on the site conditions and the well owner’s preferences (e.g., cost, installation and maintenance requirements, noise tolerance for pumps located inside the house).
• Pressure tank – a tank used to store water and provide water pressure to the household. The tank provides water storage that allows some water to be used without the need to turn on the pump. This keeps the pump from running every time water is used and extends the life of the pump. The larger the tank, the larger the volume of water that can be drawn without turning the pump on. Most modern pressure tanks contain a bladder which is filled with air to help regulate the water pressure. For low-yield wells that cannot meet a household’s peak water demands from well yield and well storage, the water system may include an additional storage tank that can provide more water than the pressure tank.
• Water treatment system – a device used to improve water quality. Treatment units can be “point-of-entry”, which are placed immediately after the pressure tank and provide treated water to the entire house. Alternatively, they can be “point-of-use” devices, which are placed at the tap where the treated water is needed. Point-of-use treatment units are normally used to provide potable water for drinking and cooking and are commonly installed at the kitchen tap. This approach provides a cost-effective way to provide potable water and avoids treating water that is to be used for non-potable needs, such as washing and toilet flushing. There is a wide range of possible contaminants that may need to be treated in domestic well water, including contaminants that can cause aesthetic problems (e.g., hardness, iron) and contaminants that cause health problems (e.g., microbial contaminants, arsenic, fluoride, lead, uranium). The most common types of treatment systems include water softeners to treat hardness (and low levels of iron and manganese), adsorptive media and reverse osmosis units to remove trace constituents (e.g., arsenic, fluoride, lead, uranium), and filtration combined with ultraviolet lights to treat microbial contaminants (e.g., bacteria, protozoa, viruses).