What is an Aquifer?
Aquifers are natural formations that store groundwater, but these vital structures are threatened
The word “aquifer” means “water bearer.” In the context of hydrogeology, it refers to a natural underground formation of permeable rock or sediment that stores and conducts the groundwater we access through wells and springs. A healthy aquifer not only supplies water, but also helps purify groundwater.
Aquifers can be either confined or unconfined. Confined aquifers are bounded by impermeable layers, so the groundwater will rise up under pressure when tapped, whereas unconfined, or water-table, aquifers allow groundwater to find its own level, that is, the water table.
Aquifers can also be classified by how they formed:
- Alluvium (river deposited sand, gravel, and silt)
- Sedimentary bedrock (consolidated sediments)
- Glacial sediments (glacier-deposited unconsolidated material)
- Igneous/metamorphic bedrock
They can further by classified as composed of either consolidated rock or unconsolidated sediment. Consolidated rock includes sandstone, shale, granite, and basalt, while unconsolidated sediment refers to granular material like sand, gravel, clay, or silt. In more detail, according to the United States Geological Survey, there are six major classifications of aquifer materials:
- Sand and Gravel Aquifers: There are four types of unconsolidated sand and gravel aquifers, basin-fill or valley-fill aquifers, blanket sand and gravel aquifers, glacial-deposit aquifers, and stream-valley aquifers. All four types generally have highly conductive intergranular porosity, and all are unconfined. High hydraulic conductivity makes them highly productive but more susceptible to contamination.
- Sandstone Aquifers: In sandstone aquifers, the hydraulic conductivity is low or moderate because of compaction and cementation that reduce the pore space. Joints, fractures, and bedding planes are left to store and move most of the groundwater, but such aquifers can still surrender great yields because of their expansive size.
- Sandstone and Carbonate Aquifers: Although not as common, some aquifers have carbonate sedimentary rock deposits that originate in marine environments that are present in roughly equal proportions with water-yielding sandstone. The carbonate rocks yield significantly more water than the sandstone, although it tends to be slightly acidic.
- Carbonate-Rock Aquifers: Most carbonate-rock aquifers are of limestone, but they may be of dolomite and marble. Yields vary widely among carbonate-rock aquifers, from negligible to some of the highest.
- Igneous and Metamorphic-Rock Aquifers: In igneous and metamorphic-rock aquifers, porosity is insignificant because the rocks are not porous. Relatively few unconnected, microscopic spaces between mineral crystals are all that store groundwater, but they can still be valuable aquifers because they can be quite large and are the only reliable source of water in many areas.
- Other Types of Rock Aquifers: Other types of rock aquifers include areas of varying size that are called minor aquifers because of their low-permeability deposits that yield little groundwater.
Aquifers in Crisis
Groundwater is Earth’s most extracted natural resource, with approximately 982 km³ of groundwater used annually. About 70% is used for agricultural irrigation, and it accounts for 50% of drinking water.
Human activities are depleting aquifers far more quickly than natural systems can recharge them through percolation of precipitation through the ground.
Overdrafts from aquifers not only contribute to water scarcity, but also dramatically diminish river flows. To make matters worse, once an aquifer is depleted, the ground above tends to sink, or subside, with serious consequences for aquifer capacity and infrastructure. It can also create fissures that damage surface and subsurface structures and serve as pathways for contaminant infiltration. In coastal areas, a depleted aquifer draws in seawater, rendering it unusable for most purposes.
We can lighten the burden on aquifers by reducing our water use. Increasing water efficiency is important, in particular through the maintenance of pipe networks to prevent lost, or non-revenue, water, and through water reuse. Managed aquifer recharge (MAR) can help the natural recharge process. And aquifers that fall victim to saline intrusion can provide water highly suitable for desalination.