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What Is a Water Table?

A dry spell can cause the water table to drop significantly.
Runoff from lakes, rivers, and streams contribute to the water table.
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  • Originally Written By: Mary McMahon
  • Revised By: Meshell Powell
  • Edited By: O. Wallace
  • Last Modified Date: 11 August 2014
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The Earth's crust can be divided into two main zones: the unsaturated zone, which contains some water but has room for more, and the phreatic zone, in which all rocks and soil are completely surrounded and filled with water. The water table is the point between the two zones at which the ground becomes completely saturated. It forms the upper limit of deposits of groundwater, and can go up and down based on a number of factors. People often access it with wells, since there is over 20 times more freshwater underground than on the Earth's surface.

Shape and Location

People often think of the water table as a flat line that divides the two underground zones, but this isn’t the case. It generally fluctuates within the landscape, going closer to the surface in some places and getting deeper in others. The shape can also be determined by the surrounding rock or by human activity. For example, a large chunk of impermeable rock could divert it and make it higher or lower, or humans could trigger a collapse of rock and soil when accessing a usable groundwater deposit, called an aquifer, changing its shape.

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Development

A number of factors contribute to the formation of the water table. Every time it rains, for example, water trickles down through layers of soil, raising its level. Runoff from lakes, rivers, and streams also contributes, as does melted snow. The rock around the water table has to be porous so that it can be saturated with water. Impermeable rock, like granite or basalt, cannot collect water, although aquifers are often surrounded by deposits of impermeable rock which keep the water trapped inside. If an aquifer is completely surrounded by a shell of impermeable rock, it can become pressurized, in which case it will shoot to the Earth's surface if tapped with a well.

Gaining Access

It's usually necessary to dig pumping wells to reach the water table and pull water to the surface. The location of a well is important, since it must be situated in a location where the table is close to the surface and underground deposits are present. In some cases, a sharp change in geography can make the top of the soil coincide with the water line, which makes a natural spring.

Factors Affecting Water Levels

The level of a water table can fluctuate considerably, depending upon environmental conditions like seasonal dryness and tidal changes, as well as human use. A dry spell, for example, can cause it to drop significantly. In some places, seasonal fluctuations are common enough to be predicted with some degree of accuracy. The water level near oceans sometimes changes daily along with the tides, getting higher during high tide and lower as the tide goes out.

Humans sometimes change water levels intentionally, usually for industrial purposes. For instance, if there is a deposit of ore below the water table, a mining company might install wells or pumps to remove the water to get to the ore. After the project is complete, the water is usually allowed to flow back into the area, raising the level again.

Threats

The main threats to the water table come from pollution and overuse. Though it takes a long time for pollutants to filter down, it's very difficult to remove them once they're there. Common pollutants include runoff from manufacturing plants and large-scale agricultural projects, leaking sewage pipes, and leaching from landfills. Another common problem is excessive use of water, such as when a population increases suddenly, demanding more water than was used previously. This type of depletion is especially common in areas where water is used for industrial production.

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Russ36
Post 20

Back in 1993 I dug a hole down to the water table three feet to the water then another foot. I installed a 4 inch vertical pipe, took a plastic bottle and attached a 1/2 inch PVC pipe on the bottle as a float. I marked where the water table was and over 20 years later, the water table about the same. When it rains, the water table is up but over time it goes back down. I live about one mile from the ocean and not am concerned at this time about the sea rising.

anon241169
Post 5

The water table is so important to us.

Amphibious54
Post 3

@ Mentirosa- Just to put in perspective how little of the earths water is fresh water I would like to break down the ratios of salt water, frozen water, ground water, and surface water on the planet.

*Salt water- 97.5 percent of all water on the planet is salt water.

*Frozen Water- Another 1.85 percent of all water on the planet is frozen in glaciers, icebergs, and snow.

*Groundwater- Groundwater only accounts for 0.64 percent of all water on the planet. this includes all springs, aquifers, and everything below the water table that is not saline.

*Surface Water- 0.01% of all water is surface water. This includes water in lakes, rivers, and the atmosphere.

This illustrates your point about how important the water table is. This information came from notes that I took in my college geology class.@ Mentirosa- Just to put in perspective how little of the earth’s water is fresh water, I would like to break down the ratios of salt water, frozen water, ground water, and surface water within the earth’s hydrologic cycle.

*Salt water- 97.5 percent of all water on the planet is salt water.

*Frozen Water- Another 1.85 percent of all water on the planet is frozen in glaciers, icebergs, and snow.

*Groundwater- Groundwater only accounts for 0.64 percent of all water on the planet. This includes all springs, aquifers, and everything below the water table that is not saline.

*Surface Water- 0.01% of all water is surface water. This includes water in lakes, rivers, and the atmosphere.

This illustrates your point about how important the water table is. This information came from notes that I took in my college geology class.

mentirosa
Post 1

The water table is so important particularly on the farms. In the mid western United States, where there are many farms, the farmers have to drill for water to supply their household and farm needs.

The farms are large and scattered, so the possibility of having water supplied to the farm by a water department would be very expensive. Every farm digs and has to find their own water and water supply.

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