1/ 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. 2/ 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.
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