An aquifer is a subsurface layer or layers of rock or other geological strata of sufficient porosity and permeability to allow either a significant flow of groundwater or the abstraction of significant quantities of groundwater.
The term also describes layers of rock, sand or gravel that can absorb water and allow it to flow.
An aquifer acts as a groundwater reservoir when the underlying rock is impermeable. This may be tapped by wells for domestic, agricultural or industrial use. A serious environmental problem arises when the aquifer is contaminated by the seepage of sewage or toxins from waste dumps. If the groundwater in coastal areas is over-used salt water can seep into the aquifer.