What is a water table?

A period of drought can cause the water table to drop significantly.

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 where the soil is completely saturated. It forms the upper boundary of groundwater deposits and can go up and down based on a number of factors. People often access it with wells, as there is more than 20 times more fresh water underground than on the Earth’s surface.

shape and location

Runoff from lakes, rivers and streams contributes to the water table.

People often think of the water table as a flat line that divides the two underground zones, but this is not the case. It usually floats in the landscape, getting closer to the surface in some places and deeper in others. The shape can also be determined by the surrounding rock or human activity. For example, a large piece of impermeable rock can deflect it and make it higher or lower, or humans can trigger a collapse of rock and soil by accessing a usable underground water store, called an aquifer, changing its shape. .


Wells must be deep enough to reach the water table.

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

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gaining access

It is often necessary to dig pumping wells to reach the water table and draw water to the surface. The location of a pit is important as it must be situated in a location where the table is close to the surface and there are underground deposits. In some cases, a sudden change in geography can cause the top of the ground to coincide with the waterline, which forms a natural spring.

Factors that affect water levels

The water table level can fluctuate considerably depending on environmental conditions such as seasonal drought and tidal changes, as well as human use. A period of drought, for example, can cause a significant drop. In some places, seasonal fluctuations are common enough to be predicted with any degree of accuracy. The water level near the oceans sometimes changes daily with the tides, getting highest during high tide and lowest when low tide.

Humans sometimes change water levels intentionally, usually for industrial purposes. For example, if there is an ore deposit below the water table, a mining company may install wells or pumps to draw water to get to the ore. After the project is complete, water can often flow back into the area, raising the level again.


The main threats to the water table come from pollution and overuse. Although it takes a long time for pollutants to filter out, it is very difficult to remove them once they are there. Common pollutants include runoff from large-scale factory and agricultural projects, leaking sewer pipes, and leaching from landfills. Another common problem is overuse of water, such as when a population suddenly increases, requiring more water than before. This type of depletion is especially common in areas where water is used for industrial production.

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