Underground water, a vast subsurface resource, fills spaces within soil, sand, and rock formations. Unlike visible rivers and lakes, this water moves slowly through geological materials. It represents the largest reservoir of unfrozen fresh water on Earth, supporting ecosystems and supplying a significant portion of global drinking water. Understanding where and how this water is stored provides insights into its availability and the processes that sustain it.
The Main Reservoirs: Aquifers
The most significant locations for underground water storage are geological formations known as aquifers. An aquifer is an underground layer of water-bearing material, such as permeable rock, fractured rock, or unconsolidated materials like gravel, sand, or silt. These formations are saturated with water and can yield usable quantities to wells and springs. Aquifers are porous rock or sediment saturated with water, similar to a sponge holding liquid.
Aquifers are broadly categorized into two main types: unconfined and confined. An unconfined aquifer, often found closer to the surface, has its upper boundary defined by the water table and is open to the atmosphere through permeable material. Water in an unconfined aquifer is not under pressure, and its level can fluctuate with recharge and discharge. In contrast, a confined aquifer is situated between two layers of impermeable material, such as clay or non-porous rock, which restricts water flow. Water within a confined aquifer is under pressure due to this overlying confining layer.
Beyond Aquifers: Other Underground Storage
While aquifers are primary storage sites, water also exists in other underground locations. Soil moisture refers to water held within the unsaturated zone, the area above the water table where pore spaces contain both air and water. This water is crucial for plant growth and agricultural productivity. Fractured rock formations can also hold water within cracks and fissures, even if the rock matrix itself is not highly porous.
Water can also be found as very deep groundwater in less permeable bedrock, sometimes referred to as immobile water. Permafrost, permanently frozen ground, stores significant amounts of water as ice. When permafrost thaws, this stored water can become available.
The Journey Underground: How Water Reaches Storage
Water primarily reaches underground storage areas through a process linked to the broader water cycle. Precipitation, such as rain or snowmelt, is the main source. Once precipitation reaches the ground, a portion seeps into the subsurface, a process known as infiltration.
Following infiltration, water moves downward through soil and rock layers, a process called percolation. This movement continues until the water reaches a saturated zone or an impermeable layer, where it then accumulates or flows laterally. The rate at which water infiltrates and percolates depends on factors like soil type, vegetation cover, and precipitation intensity. This natural replenishment, known as groundwater recharge, is essential for sustaining underground water reserves.
Characteristics of Water-Bearing Formations
The ability of geological formations to store and transmit water depends on two main physical properties: porosity and permeability. Porosity quantifies the amount of empty space within a rock or soil material where water can reside. This space includes gaps between individual grains, as well as cracks or cavities within the rock. Sedimentary rocks, for example, typically exhibit porosities ranging from 10% to 30%.
Permeability, on the other hand, measures the ease with which water can flow through these interconnected spaces. A material can be highly porous but have low permeability if its pores are not well connected, trapping the water. Conversely, a material with fewer, but well-connected, spaces will have higher permeability, allowing water to move more freely. For a geological formation to be considered a good aquifer, it must possess both sufficient porosity to store water and adequate permeability to allow that water to be extracted.