What is Groundwater?

Groundwater is water that is stored under the earth’s surface within soil and rock layers. When rain falls, only a small portion of the water flows across the ground as runoff and enters streams or rivers. Much of this water remains trapped in the soil and percolates (seeps) deep into the ground and becomes groundwater. Once surface water becomes groundwater it can remain underground anywhere between a few days to thousands of years. In many cases, groundwater is a direct part of the surface water system and transitions between flowing above ground and underground regularly.
Groundwater is connected to surface water through complex interactions. The study of these interactions is a part of hydrogeology.

What is hydrogeology?

Hydrogeology is the study of groundwater, the rock layers that carry groundwater and the interaction of groundwater with surface water. The interaction of groundwater and surface water is also a part of hydrology. Hydrology is the study of surface water and the hydrologic cycle.
Groundwater is a vital part of the hydrologic cycle (see Figure 1).When water infiltrates into the ground it flows through soil and rock layers. Groundwater can be stored in layers called aquifers. The water flowing underground can return to the surface in springs, wetlands, streams, lakes, and oceans.

Figure 1: The Hydrologic Cycle: The hydrologic cycle, or the water cycle, includes hydrology above ground, and hydrogeology below ground [1].