What is groundwater?
When rain soaks into the ground, it trickles down through soil and rock until it meets a layer it cannot get through, or can cross only very slowly.
Water then collects in the rock above that barrier, filling cracks and the tiny spaces between grains. A rock layer that holds water this way is known as an aquifer.
Rock Waterways
Chalk, limestone and sandstone are examples of rock that can store water. Granite, basalt and mudstone hold little water themselves, but they can act as a seal that keeps water from draining out of more porous rock nearby. Groundwater feeds springs and wells, and during dry spells it is often groundwater seeping into rivers, not runoff from the land, that keeps them flowing. Exposed rock in road cuts and stream banks shows where groundwater can travel. In bedrock, the gaps between layers are especially easy to spot and may stretch for miles; water percolates into these openings and fills the pores between individual rock particles.
In most deposits of sand and gravel, water fills the spaces between the grains and moves through them easily.
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