The hydrologic system (sometimes called the hydrologic cycle or water cycle), consists of a series of transfers of water involving the atmosphere, soils, plants, rocks, rivers, lakes, oceans and glacial ice. Water may occur as a liquid, solid (ice) or a gas (water vapor). Water drops are made up of thousands of water molecules, two atoms of hydrogen and one of oxygen (H2O). An individual water molecule may move rapidly through the hydrologic system in a few days or may be in storage (for example as ice or groundwater) for hundreds of years.
Groundwater can occur in pore spaces in some sedimentary rocks (and in sediments such as sands and gravels) and in cracks and fissures in any type of solid rock. Aquifer is the correct technical word to describe water saturated rock formations. Some groundwater is tens of thousands of years old, but most groundwater used for drinking supplies in the US has only been underground for a few months or years. Some groundwater may be recharged from leakage through rivers and lakes, but rainfall and snowmelt soaking into the ground is the principal source of recharge. However, not all rainfall recharges groundwater. Water from light rain is likely to evaporate from soils or used by plants before it has a chance to soak deep underground.