A well is a hole in the earth from which fluid is withdrawn. A water well is an artificial excavation or structure put down by any method such as digging, boring or drilling for the purposes of withdrawing water.

Well water may be drawn via mechanical pump (such as an electric submersible pump) from a source below the surface of the earth, or drawn using containers, such as buckets, that are raised mechanically, or by hand. Wells can vary greatly in depth, water volume and water quality. Well water typically contains more minerals in solution than surface water and may require treatment to soften the water. Although water wells are the most common type, oil, gas, and mining wells also exist. A well is made by reaching the water table. Wells can be made in a variety of ways: digging, driving, boring, or drilling.

Wells draw water up from the ground, called ground water. Ground water is stored naturally below the earth’s surface. Most ground water originates as rain or snow that seeps into the ground and collects as underground aquifers.

An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Two broad classes of drilled-well types may be distinguished based on the type of aquifer which the well is completed in: Shallow or unconfined wells are completed in the uppermost saturated aquifer at that location (the upper unconfined aquifer); or deep or confined wells, which are sunk through an impermeable stratum down into an aquifer which is sandwiched between two impermeable strata (aquitards or aquicludes). The majority of confined aquifers are classified as artesian because the hydraulic head in a confined well is higher than the level of the top of the aquifer. If the hydraulic head in a confined well is higher than the land surface it is a “flowing” artesian well (named after Artois in France). There clearly are many cases that fall in between these two end-members; often unconfined wells may be very deep (what is often called a shallow well can be over 150 m deep) and many times wells are completed across all aquifers from their top to their bottom (especially agricultural or industrial wells), being open to both unconfined and confined aquifers.

Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. A formation of rock or soil is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces become fully saturated with water is called the water table. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology. Ground water does not stay in one place. Gravity causes ground water to flow downward and outward. Porosity-the size and number of void spaces in the formation-determines how much water can be stored in an aquifer. Permeability-the ability of water to move through void spaces-indicates how quickly the water will travel through the aquifer.

Unconsolidated aquifers usually transmit water more efficiently than bedrock aquifers. Ground water flows easily through the spaces between loose sand and gravel particles. Water wells drilled into sand and gravel aquifers are often very productive. Water flows out of pores and through fractures in consolidated bedrock aquifers. Productive water wells drilled into bedrock penetrate aquifers in fractured limestone or shale, or porous sandstone.

1. Dug Wells
2. Drilled Wells

Until recent centuries, all artificial wells were pumpless dug wells. Such primitive dug wells were excavations with diameters large enough to accommodate muscle-powered digging to below the water table. Relatively formal versions tended to be lined with laid stones or brick; extending this lining into a wall around the well presumably served to reduce both contamination and injuries by falling into the well. The iconic American farm well features a peaked roof above the wall, reducing airborne contamination, and a cranked windlass, mounted between the two roof-supporting members, for raising and lowering a bucket to obtain water. More modern dug wells may be hand-pumped, especially in undeveloped and third-world countries. Note that the term “shallow well” is not a synonym for dug well, and may actually be quite deep. Prepared by Richard Simpson CWD