The typical source of well water is groundwater, which is water stored beneath the Earth’s surface. A well is a bore that is dug, driven, or drilled into the ground to intercept this water. This underground water is a constantly replenished resource, originating from precipitation that has seeped into the soil. Well water, unlike municipal water, is drawn directly from these subsurface reservoirs.
The Journey from Surface to Subsurface
The process begins as rain and melted snow fall onto the land and move downward through the soil and rock layers. This downward movement is called infiltration, governed by gravity and the porosity of the material. Water first passes through the zone of aeration, the layer closest to the surface where spaces between soil particles and rock fractures are filled with both air and water.
As the water continues to descend, it reaches a point where all the pore spaces are completely saturated with water. This lower region is known as the zone of saturation, and the water within it is defined as groundwater. The boundary that separates the unsaturated zone of aeration from the saturated zone of groundwater is called the water table.
The depth of the water table is not fixed; it fluctuates based on local weather conditions, rising during periods of heavy rainfall and falling during droughts. This movement of water through the subsurface is slow, with flow rates often measured in feet per year. The rate of this percolation, or slow movement through the ground, depends heavily on the type of sediment or rock it passes through.
The Groundwater Reservoir: Aquifers
The primary source for well water is a geological formation called an aquifer, which is a layer of permeable rock, sand, or gravel that stores and transmits groundwater. For a layer to be a productive aquifer, it must exhibit high porosity, which is the measure of the empty space within the material that can hold water. It also requires high permeability, which is the measure of how easily water can flow through that material.
Aquifers are categorized into two main types: unconfined and confined. An unconfined aquifer is the most common source for residential wells and has the water table as its upper boundary. Because it is close to the surface and is not capped by an impermeable layer, it is directly recharged by water infiltrating from above.
A confined aquifer exists between two layers of impermeable material, such as dense clay or solid rock, known as aquitards or confining layers. The water in a confined aquifer is under pressure because of the weight of the overlying layers and the hydrostatic pressure from its recharge area, which may be miles away. When a well penetrates this type of aquifer, the pressure can cause the water level in the well to rise above the top of the aquifer.
The water table’s position directly dictates the depth a shallow well must reach to access a reliable water supply. Because unconfined aquifers are closer to the surface, they are more susceptible to changes in precipitation and are typically the first to experience water level drops during a dry season. Confined aquifers, due to their depth and protective layers, are more stable, though they take much longer to recharge once water is withdrawn.
Accessing the Water: Well Types
The three basic types of wells—dug, driven, and drilled—are distinguished by their construction method and the depth they reach into the aquifer, intercepting the saturated zone at different levels. The physical structure of a well determines which part of the groundwater reservoir is accessed.
Dug wells are the oldest type, typically shallow, usually extending only 10 to 30 feet below the surface. These wells draw water directly from the unconfined aquifer, tapping into the water just below the water table. Because of their shallow depth, they are the most vulnerable to surface contamination and seasonal fluctuations, sometimes running dry during droughts.
Driven wells are constructed by driving a small-diameter pipe into soft ground, like sand or gravel. They are slightly deeper than dug wells, often reaching depths of 30 to 50 feet, and access the shallower regions of the unconfined aquifer. Like dug wells, their proximity to the surface makes them susceptible to contamination from surface activities.
Drilled wells are the most common type of modern well and are created using powerful rotary or percussion drilling equipment. These wells can be hundreds of feet deep, sometimes exceeding 1,000 feet, allowing them to penetrate deep into rock formations to access confined aquifers. The depth and continuous casing provide protection from surface pollutants and offer a more consistent water source.