Groundwater begins with rainfall and snow melt that seeps or infiltrates into the ground. The amount of seepage varies with the type of land surface – water seeps more readily through porous material such as sand or gravel and less readily through material such as clay. The water that does not seep into the ground either runs off into lakes and streams or evaporates into the atmosphere.
The water that seeps into the ground descends by force of gravity until it reaches a depth where it fills all the openings in the soil or rock. This is called the saturated zone and typically includes water-filled crevices in the upper layer of bedrock. The top of the saturated zone is called the water table. The water table rises and falls with the seasons and the seasonal amount of rain and snow.
Another zone is found between the water table and the surface of the land. This is the unsaturated zone, where the openings in the soil are only partially filled with water. Plant roots can capture the water passing through this zone on its way to the water table.
An aquifer is a water-bearing soil or rock formation that is capable of yielding usable amounts of water. When groundwater becomes trapped under impermeable soil or rock it is called a confined or artesian aquifer. A well piercing a confined aquifer is known as an artesian well. Aquifers that are not confined under pressure are called unconfined or water table aquifers, and the water level in an unconfined well is the same at the water table outside the well.
Water seeping into an aquifer is known as recharge. Recharging occurs intermittently both during and immediately after rainfall or snow melt. The areas where permeable soil or rock allows the water to seep into the ground are called recharge areas. Groundwater enters the ground in recharge areas and leaves the ground at discharge points, usually as seepage into wetlands, lakes, and streams. Streams that receive groundwater discharges are known as gaining streams. The level of water in the stream (or wetland or lake) is the water table level for the adjacent aquifer.
Groundwater moves very slowly from recharge areas to discharge points. The rate of flow may take years, decades, or even centuries to move long distances through some (less permeable) aquifers. However, it may take only a few days or weeks for groundwater to move a short distance through loose soil.
Pumping water from a well lowers the water table near that well. This is known as the cone of depression. The groundwater is diverted toward the well as it flows into the depression cone. The cone of depression formed by a pumping well may extend to a nearby stream or lake. When the level of the water table is lower than that of the stream or lake, the stream or lake loses water to the adjacent groundwater aquifer. This is known as induced recharge. Streams and wetlands have been known to completely dry up as a result of the induced recharge from pumping wells.