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.
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.
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.
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.