Types of wells
Almost half of the United States depends on ground water for its drinking water supply. If you have a well, it’s important to understand the different materials that comprise your well system.
The most common water supply for the home that is not served by a public system is a drilled well. They are constructed by either percussion or rotary-drilling machines that penetrate about 100-400 feet into the bedrock. Where you find bedrock at the surface, it is commonly called ledge. To serve as a water supply, a drilled well must intersect bedrock fractures containing ground water.
The upper part of a well is lined with casing to prevent well walls from collapsing and contaminants from entering the water supply. The casing is usually metal or plastic pipe, six inches in diameter that extends into the bedrock to prevent shallow ground water from entering the well. The casing must extend at least 18 feet into the ground, with at least five feet extending into the bedrock. The casing should also extend a foot or two above the ground’s surface. A sealant, such as cement grout or bentonite clay, should be poured along the outside of the casing to the top of the well. The well is capped to prevent surface water from entering the well.
Submersible pumps, located near the bottom of the well, are commonly used in drilled wells. Wells with a shallow water table may have jet pumps inside the home. Most modern drilled wells incorporate a pitiless adapter designed to provide a sanitary seal at the point where the discharge water line leaves the well to enter the home. The device attaches directly to the casing below the frost line and provides a watertight subsurface connection, protecting the well from contamination.
Dug wells are one of the oldest water supply technologies available. They are created by digging a hole in the ground with a shovel or backhoe. Dug wells have usually been excavated below the groundwater table until incoming water exceeded the digger’s bailing rate. The well was then lined (cased) with stones, brick, tile, or other material to keep it from collapsing. It was covered with a cap of wood, stone, or concrete. Since it is so difficult to dig beneath the ground water table, dug wells are not very deep. Typically, they are only 10 to 30 feet deep.
Dug wells are used extensively on many low-lying islands and are often used as a supplement to rainwater harvesting systems. However, because they are so shallow, dug wells have the highest risk of becoming contaminated. These wells also tend to go dry during a drought when the ground water table drops.
To minimize the likelihood of contamination, a dug well should be cased with a watertight material and a cement grout or bentonite clay sealant poured along the outside of the casing to the top of the well. It should be covered by a concrete curb and cap that stands about a foot above the ground. The land surface around the well should be mounded to allow surface water to run away from the well.
Driven wells are made by driving a tube into the earth to a water table above the bedrock. Also referred to as a sand point well, the driven well can only be constructed in areas with loose or sandy soil. Lengths of pipe with a well-point at the end are driven into the ground to reach the water, which flows into the pipe through the screened openings in the well-point. The driven well is typically 2 inches in diameter and up to 30 feet in depth. Driven wells are commonly used for irrigation. Similar to dug wells, driven wells are relatively shallow and have high risk of contamination.
To minimize this risk, the well cover should be a tight-fitting concrete curb and cap with no cracks and should sit about a foot above the ground. Slope the ground away from the well so that surface water runs away from the well.