Aquifer
An aquifer is a body of permeable rock or sediment that holds and transmits enough groundwater to supply wells and springs.
Definition
An aquifer is an underground layer of sand, gravel, or fractured rock with enough porosity and permeability to hold and transmit useful amounts of groundwater, unlike clay or shale, which store water but transmit it so poorly that they act as confining barriers called aquitards. Unconfined aquifers are open to the surface with the water table as their top, while confined aquifers sit sandwiched between confining layers under pressure, sometimes producing artesian wells that flow without a pump. Performance is described by hydraulic conductivity, transmissivity, and storativity, which for unconfined aquifers equals specific yield, often $0.1$ to $0.3$, and is far smaller for confined aquifers since water there is released only by compression of the rock matrix and expansion of the water itself, parameters typically obtained from pumping tests analyzed with the Theis solution.
Example
A layer of sand beneath a town can serve as an aquifer, with rain soaking down into it and wells pumping the water back out, and North Carolina's Coastal Plain has a stack of sandy aquifers separated by clay, including the productive Castle Hayne limestone aquifer, while Piedmont wells tap fractures in granite and gneiss that yield far less water. The Floridan aquifer, a karst limestone system beneath Florida and southern Georgia, discharges through first-magnitude springs exceeding $2.8\ \text{m}^3/\text{s}$ each, and over-pumping of coastal aquifers near Wilmington draws seawater inland, monitored with sentinel wells.
Key Insight
An aquifer is not a cave full of water but solid material riddled with tiny connected spaces that the water fills, and confined aquifers are especially easy to deplete and slow to recover, since their water can be thousands of years old and once pressure drops, compaction of the surrounding clay layers can permanently reduce how much water the aquifer can ever hold again, as seen in the subsidence of California's Central Valley.