Groundwater is the water present beneath the Earth’s surface, filling the small spaces within rock and soil layers. This resource represents nearly all of the planet’s readily available fresh water, making it a globally important supply for agriculture and public use. The journey of water from a raindrop to a well is a natural process of replenishment and storage. This process involves the slow movement of water through different underground zones before it can be intercepted for human use.
The Journey from Surface to Subsurface
The movement of water toward the storage area begins with precipitation, such as rain or snowmelt, that does not run off the surface. This initial sinking of water into the soil surface is called infiltration. The rate of infiltration is influenced by the soil type; sandy or gravelly soils have a higher capacity for absorption than dense clay soils. If the ground is already saturated, the infiltration rate slows down considerably, leading to more surface runoff.
Once water has entered the soil, its downward journey continues through a process known as percolation. Percolation describes the slow, vertical movement of water through the deeper layers of soil and rock due to the force of gravity. This movement occurs through the unsaturated zone, which is the region above the main water supply where the pores in the soil and rock contain both air and water.
The depth of the unsaturated zone can vary significantly, from a few feet to hundreds of feet below the surface. As the water percolates downward, it is filtered by the geological material it passes through. This slow, steady movement replenishes the deeper reserves, ensuring the continuity of the underground supply.
Defining the Groundwater Storage System
The downward-moving water eventually reaches the saturated zone, where all the open spaces, or pores, in the rock and soil are completely filled with water. This fully saturated area is the geological storage system from which wells draw their supply. The upper surface of this saturated zone is called the water table, and its level rises and falls seasonally depending on the amount of precipitation and the rate of extraction.
The geological layer that can store and transmit water in usable quantities is known as an aquifer. Aquifers are typically composed of materials like sand, gravel, sandstone, or highly fractured rock, which possess specific properties that make them effective storage units. The capacity of an aquifer to hold water is defined by its porosity, which is the total volume of pore space within the material.
The ease with which water moves through the aquifer material is called permeability, which depends on how well the internal pores are connected. A good aquifer must have high porosity to store water and high permeability to allow water to flow toward a well. Unconfined aquifers have the water table as their upper boundary, while confined aquifers are trapped between layers of low-permeability rock, creating a pressurized system.
How a Well Intercepts the Water Supply
A well is an engineered vertical shaft, or borehole, drilled from the surface deep enough to penetrate the water table and extend into the saturated aquifer. The well must be constructed with a casing, which is a pipe inserted into the borehole. This casing prevents the surrounding geological material from collapsing inward and seals off the well from surface contamination.
At the bottom of the casing, adjacent to the water-bearing layer, a well screen is installed. The screen is a slotted or meshed pipe that allows water from the aquifer to flow freely into the well while filtering out sand and sediment. This component maintains the well’s efficiency and prevents damage to the pumping equipment.
When a pump is activated, it draws water up from the wellbore, lowering the water level inside and creating a pressure difference between the well and the surrounding aquifer. This pressure gradient causes the groundwater to flow from the aquifer toward the well, which creates a temporary, cone-shaped depression in the water table centered on the well. The size and shape of this “cone of depression” depend on the rate of pumping and the permeability of the aquifer material.