Permeable layers of rock underground (i.e., they have openings that can contain liquids or gases) filled with water are called aquifers. Some examples of water-bearing rock are sandstone, sand, and gravel.
Aquifers fill with water that drains into them from the surface—from rivers, lakes, snow-melt, or rain. Sometimes this flow is reversed, with aquifers feeding natural springs. Many aquifers are tapped for agricultural and industrial uses.
- Unconfined aquifer: lies underneath permeable rock and draws surface water, but remains only partially filled because water also drains out of the aquifer.
- Confined aquifer: situated in between two layers of denser rock, and fills as water permeates cracks in the upper layer of rock from the surface, an underground lake or river, or an unconfined aquifer.
Water carries minerals and trace elements in specific quantities depending on the subsoil and rocks it encounters. It can therefore have a low, medium or high mineral content.