What Is the Connection Between the Water Table and Groundwater?

Groundwater and Water Tables

• Rain or snow water percolates into the ground and migrates downward under the influence of gravity. It moves through the upper unsaturated soil zone until it reaches the water table and the saturated soil zone. The water table surface is the boundary between unsaturated and saturated soil. In the unsaturated soil zone, water only partially fills openings between soil or rock particles, and the rest of those spaces is filled with air. In the saturated soil zone, the openings are completely filled with groundwater.

Water Table in Unconfined Aquifers

• Groundwater is stored in geologic layers called aquifers with openings that allow groundwater to flow through them. Unconfined aquifers, sometimes called water table aquifers, receive water replenishment from precipitation or surface water above them. They are directly affected by atmospheric pressure, and may lose water by evaporation and plant use. Water levels in water table aquifer wells will be initially the same as that of the groundwater surrounding them, but may go down in the immediate vicinity due to use. Alternatively, aquifers that are confined beneath less permeable layers of rock are less affected by atmospheric pressure and receive the most replenishment from higher land some distance away.

Depth of the Water Table

• Climate and human use can affect the depth to the water table. The water table surface may be deep in arid areas or closer to the soil surface in areas with higher rainfall. Perched water -- found in areas where the groundwater is held closer to the surface by a layer of impermeable rock -- also measures as a high water table, but may well yield comparatively little water. Water levels in confined aquifers, some of which travels under pressure through fractured rock, may vary drastically between nearby points as the cracks travel up and down. The water table may fall or rise in drought or high rainfall and flooding conditions, during periods of low or high barometric pressures, and when large volumes of groundwater are pumped or used for crop irrigation using water from sources other than the local aquifer.

Replenishment of Water Table Aquifers

• Natural replenishment occurs when it rains or snows. Some of the water evaporates, some falls into lakes, streams or oceans, and some percolates into the ground to become groundwater. Water in lakes, streams and oceans also percolates into the ground. Artificial recharge is used to help replenish some aquifers, especially in areas with large populations and low rainfall. Unlined ponds can be used to capture rainwater or hold recycled water and allow it to percolate into the ground.

Movement of Groundwater in Water Table Aquifers

• Groundwater in water table aquifers is not static but flows through the ground toward lakes, streams and oceans. Many streams and lakes receive their water from water table aquifers. Wetlands that contain surface water are located where the water table is at or above the ground surface. Springs commonly flow from water table aquifers where the water table surface meets ground surface. The downward slope of the water table surface points in the direction that groundwater flows.

Access to the Water Table for Human Use

• Less than 2 percent of the Earth's 330 million cubic miles of water is groundwater, and less than 1 percent is fresh water. Humans using more groundwater than is replenished could eventually use all of the available groundwater. When too much groundwater is used, the water table falls, causing higher costs to install wells. In some areas where water table aquifers are overused, local laws limit the amount of groundwater used to the amount that is annually replenished.