An aquifer is a geological formation, part of a formation, or group of formations that stores and transmits groundwater in sufficient quantities to supply wells or springs. It consists of saturated rock or sediment through which water moves along interconnected pores, fractures, or dissolution channels. An aquifer is therefore both a storage medium and a pathway for underground flow, rather than necessarily an underground lake or river. Its usefulness depends on how readily water can enter, move through, and be extracted from the material. (usgs.gov)
Geological materials and pore structure
Aquifers occur in unconsolidated deposits, including sand and gravel, and in consolidated rocks such as sandstone, limestone, and fractured crystalline rock. Their properties reflect the geology of the material and its history of deposition, cementation, fracturing, and dissolution. Loose sediment commonly stores water between grains; otherwise dense rock may transmit water principally through fractures. (usgs.gov)
Porosity is the proportion of a material’s volume occupied by voids. Permeability describes its ability to transmit fluid through connected openings. These properties are related but not interchangeable: a material can contain substantial pore space while transmitting water poorly. Clay, for example, may store considerable water yet release it slowly because its pores are small and flow is restricted. (pubs.usgs.gov)
In karst aquifers, dissolution of soluble rock produces enlarged fractures and conduits. These can support rapid groundwater movement and long-distance contaminant transport. Extensive caves are not essential: England’s Chalk aquifer contains networks of smaller dissolutional openings that influence flow despite relatively limited cave development. (bgs.ac.uk)
Confined, unconfined, and perched aquifers
An unconfined aquifer has a water table as its upper boundary. At that surface, water pressure equals atmospheric pressure; above it lies an unsaturated zone containing both air and water. The water table rises and falls as recharge, discharge, and pumping change. (usgs.gov)
A confined aquifer lies beneath a poorly permeable confining layer. Its water is under pressure, so water entering a well can rise above the top of the aquifer. The elevation to which it rises represents the aquifer’s potentiometric surface. An artesian well flows without pumping only when this surface stands above the well outlet; confinement alone does not guarantee flow at ground level. (usgs.gov)
A low-permeability layer that restricts groundwater movement is called an aquitard. Such layers need not be completely impermeable and can allow leakage between aquifers. Perched groundwater occurs above the main groundwater body, commonly over a localized low-permeability bed, with an unsaturated interval separating it from the regional water table. (pubs.usgs.gov)
Groundwater flow and storage
Groundwater moves from higher to lower hydraulic head, a measure combining elevation and pressure. Flow therefore does not simply follow the downward slope of the land surface. Under appropriate conditions, it is described by Darcy’s law:
[ Q=-KA\frac{dh}{dl}, ]
where (Q) is volumetric discharge, (K) is hydraulic conductivity, (A) is the cross-sectional area, and (dh/dl) is the hydraulic-head gradient. The minus sign indicates movement toward decreasing head. (pubs.usgs.gov)
Hydraulic conductivity depends on both the porous medium and fluid properties, including viscosity. Transmissivity describes the capacity of the aquifer’s saturated thickness to transmit water; for a uniform layer, it equals hydraulic conductivity multiplied by saturated thickness. Storage properties describe how much water is released when head declines. Unconfined aquifers release water mainly through drainage of pores, whereas confined aquifers initially release it through compression of the aquifer framework and expansion of water. (pubs.usgs.gov)
Recharge and discharge
Aquifers participate in the water cycle through groundwater recharge and discharge. Natural recharge includes precipitation that infiltrates through the unsaturated zone, and water lost from streams, lakes, or other surface-water bodies. Discharge occurs through springs, seepage into streams, evaporation and plant use where groundwater is shallow, and withdrawal from wells. (pubs.usgs.gov)
An aquifer and a river may exchange water in either direction, depending on their relative hydraulic heads. Groundwater discharge can sustain streamflow between rainfall events, while a losing stream can recharge an aquifer. Pumping may intercept groundwater that otherwise would reach a stream or induce additional stream water to enter the aquifer. Connected wetlands can also be affected by changes in groundwater levels. (usgs.gov)
Water quality and depletion
Aquifer water is not necessarily fresh or uncontaminated. Its composition reflects recharge-water quality, minerals in the rock and sediment, geochemical conditions, and residence time. Agriculture can introduce dissolved constituents from fertilizers and pesticides, while natural water–rock reactions also influence contaminant concentrations. Rapid karst pathways can increase vulnerability by transporting pollutants through an aquifer quickly. (water.usgs.gov)
Groundwater depletion occurs when withdrawals reduce stored groundwater over time. Pumping is supplied by some combination of removal from storage, increased recharge, and reduced natural discharge—not simply by the aquifer’s pre-existing recharge rate. Consequences can include declining well yields, greater pumping costs, reduced surface-water flows, and land subsidence. Compaction of fine-grained layers can permanently reduce storage capacity. (pubs.usgs.gov)
In coastal aquifers, reduced freshwater heads can cause saltwater intrusion, including inland migration of saline water or upward movement toward pumping wells. The freshwater–saltwater boundary is generally a mixing zone rather than a sharp barrier. (usgs.gov)
Investigation and managed recharge
Aquifer investigations combine geological records, water-level observations, laboratory measurements, and hydraulic testing to characterize subsurface conditions. Core samples provide information about pore structure and permeability, while groundwater records document changes over time. (bgs.ac.uk)
Managed aquifer recharge deliberately adds water through infiltration or injection for underground storage. Its effects depend on aquifer structure and water chemistry: introduced water can alter geochemical conditions and mobilize naturally occurring constituents, including arsenic. Evaluation therefore includes both changes in groundwater quantity and changes in quality. (pubs.usgs.gov)