A groundwater well is a vertical pipeline drilled into the earth to access naturally stored water. We withdraw this water for various uses, such as household needs or irrigation. The well’s water supply is restored by the continuous, slow movement of water through the subsurface layers of the earth. This availability relies on a sustained natural cycle that brings water from the atmosphere back into the ground storage area.
The Source: The Hydrologic Cycle
All water found in a well begins as precipitation, such as rain, snow, or hail. This continuous global movement of water is known as the hydrologic cycle. The sun’s energy drives the cycle, causing surface water to evaporate, form clouds, and return to the earth as precipitation.
When precipitation reaches the land, some flows across the surface as runoff, but a significant amount soaks into the soil. This soaking process, called infiltration, is the first step in replenishing the underground water supply. Water that infiltrates the ground is pulled downward by gravity toward deeper geological storage zones.
Underground Storage: Aquifers and the Water Table
The water that infiltrates the ground is stored in geological formations known as aquifers. An aquifer is a permeable body of rock, sand, or gravel with interconnected pores that allow it to store and transmit groundwater. These formations act as the natural reservoir that the well taps for its supply.
Aquifers are classified based on their relationship with the surface and confining layers. An unconfined aquifer, often called a water table aquifer, has its upper boundary defined by the water table and communicates directly with the atmosphere. A confined aquifer is sandwiched between two layers of low-permeability material, such as clay or shale, which puts the water inside under pressure.
The water table is the upper surface of the zone of saturation, the region below ground where all pore spaces in the rock and soil are filled with water. A well must be drilled deep enough to intersect this zone to consistently draw water. The volume of water available is directly related to the size and characteristics of the aquifer it penetrates.
The Process of Recharge
The mechanism that restores the water level in the aquifer and the well is called groundwater recharge. Recharge is the process by which water moves downward from the surface through the unsaturated zone into the saturated zone. Water initially enters the soil through infiltration, followed by the slower, deep movement called percolation.
The speed and volume of recharge are dictated by the characteristics of the overlying soil and rock layers. Highly porous materials, like sand and gravel, have large empty spaces to hold water. Highly permeable materials allow water to move through them quickly. Where surface materials are less permeable, such as clay or dense rock, the rate of deep percolation and thus recharge is slower.
When a well is pumped, it temporarily lowers the water level nearby, creating a depressed shape in the water table known as the cone of depression. The slow, continuous movement of water from the surrounding aquifer works to fill this depression. The well replenishes as the aquifer’s natural flow dynamics push water back into the area drawn down by the pump.
Factors Influencing Well Recovery
The rate at which a well recovers after pumping is a measure of the aquifer’s ability to transmit water and its overall recharge dynamics. A primary factor is the balance between the pumping rate and the natural recharge rate of the aquifer. If water is withdrawn faster than the aquifer can replenish it, the water level will progressively drop, potentially causing the well to run dry.
Seasonal and climatic conditions influence the recharge rate. Prolonged drought reduces the precipitation available to infiltrate the soil, leading to a drop in the water table. Conversely, periods of heavy rainfall increase the recharge rate, causing the water level in the well to rise.
Land use and the presence of nearby wells also impact recovery. High-volume pumping, such as for agricultural irrigation or municipal water systems, draws water from the same underlying aquifer. This shared use competes for the resource and can lower the regional water table, slowing the recovery of all wells.
The geological makeup of the aquifer determines the upper limit of the recovery rate. Aquifers composed of highly permeable materials, like coarse sand, transmit water quickly and recover rapidly after pumping. Wells drilled into dense bedrock or clay, which have low permeability, will have a slow recovery rate because water moves through the formation at a slower pace.