Dug wells are wells excavated by backhoe or shovel, and are generally only 10 to 30 feet deep. Because they are shallow, dug wells are the type of well most prone to contamination. Features such as watertight concrete casing, a concrete well cap, and a soil gradient which slopes away from the well, all help to prevent dug well contamination.
In the past, these wells were cased with brick, stones, or tile to ensure a long, sturdy life.
Driven wells run further down than dug wells, averaging 30 to 50 feet in depth. They are often found in areas of sandy or gravel-laden soil, where the ground water table is less than 20 feet from the surface of the ground, making access to groundwater easier.
Though driven wells are 20 to 40 feet deeper than dug wells, they are still considered shallow wells, and still pose a moderate risk of contamination. A filter is secured to one end of the pipe that is driven down into the soil, in order to filter sediment out of incoming water once the ground water table has been reached.
Drilled wells have the most depth of the three types of water wells, extending to depths of 100-400 feet, and into bedrock rather than loose soil. They are the most safe from environmental contamination. Drilled wells have to cross bedrock fractures which possess ground water in order to produce a water supply.
Casings for drilled wells are metal or plastic pipes, half a foot in diameter, extending at least 18 of the 100 to 400 feet down (a minimum of 5 feet of casing pipe has to extend into bedrock). Surface water is prevented from entering a drilled well by means of a cap.