What is groundwater, and what is an aquifer?
Groundwater is water held beneath the surface in the spaces within rock and sediment; an aquifer is a body of rock permeable enough to yield usable quantities of it.
Where it sits. Rainfall that neither runs off nor evaporates infiltrates downward. Above the water table, pores contain both air and water — the unsaturated zone. Below it, they are fully saturated. The water table is not level; it broadly follows the topography and rises and falls with recharge.
The two properties that matter, and they are different:
Porosity — the proportion of the rock that is void space, determining how much water it can hold.
Permeability — how well those voids are connected, determining how readily water moves through.
Clay has high porosity and very low permeability — it holds a great deal of water and releases almost none, which is why it forms barriers rather than aquifers. This distinction is the key to understanding groundwater.
The types of aquifer:
Unconfined, open to the surface above, recharged directly by infiltration, and vulnerable to surface contamination.
Confined, sandwiched between impermeable layers. Water within can be under pressure, so a well drilled into it may rise without pumping — an artesian well.
An aquitard or aquiclude is a layer restricting flow.
A spring occurs where the water table meets the surface.
Why it matters enormously: groundwater supplies a large share of the world's drinking water and irrigation, sustains river flow between rainfall events — baseflow, which is why rivers keep running in dry weather — and maintains wetland ecosystems.
Over-abstraction is the central problem. Where extraction exceeds recharge, water tables fall, wells deepen, springs and wetlands dry, and rivers lose their baseflow. Land subsidence can follow as sediments compact, and it is largely irreversible — the storage capacity is permanently lost.
Coastal aquifers face saline intrusion as seawater moves inland to replace abstracted fresh water.
Fossil water in some aquifers accumulated over thousands of years and is effectively non-renewable at any useful timescale.
Contamination is slow to appear and extremely slow to clear, because groundwater moves slowly.