Ground water as it moves through soil and rock is actually being naturally filtered. Some substances like sand can help filter pollutants and particles easily. Clays in soil and rock also act as a way to "capture" and exchange some elements and compounds when they are dissolved in water. This helps to eliminate other pollutants that filtration cannot handle.

 Water is stored in aquifers, which have similar characteristics of reservoir rocks that store soil and gas. Well-sorted sediments with high porosity (A) and highly fractured (C) rocks are excellent candidates for aquifers. If poorly sorted rocks are partially cemented (B), they are not good reservoir rocks. The fluid must have room to move around. An aquifer is not an underground river, it just retains the water until it is pumped out through a well or naturally emerges as a spring.