Formerly it was believed that groundwater they came from the sea and had lost their salinity by seeping through the rocks. Today it is known that it is water from the rain.
Groundwater forms large deposits that in many places constitute the only source of drinking water available.
Sometimes, when they circulate underground, they form large systems of caves and galleries. In some places they return to the surface, sprouting from the earth in the form of fountains or springs. Others, you have to go to collect them at different depths digging wells.
The difference between the amount of precipitation and the amount of water carried by the rivers seeps under the ground and forms the aquifers. Filtration depends on the physical characteristics of the rocks. Porosity is not synonymous with permeability, as certain rocks such as clay, although they have a large porosity, are practically impermeable since they do not have conduits that communicate.
If the impermeable layer forms a depression, an underground lake may appear. On the other hand, if the impermeable layer is inclined, an underground river can be formed.
When a permeable layer is disposed between two impermeable layers, it forms what is called a captive or confined aquifer. Under these conditions the water is subject to considerable pressure. If, under any circumstances, a crack is created in the impermeable layer, then the water rises rapidly to the water table to balance pressure differences.
On the other hand, if the permeable layer finds no limit other than depth, then it is called a free aquifer.
Wells and springs
A spring is a natural flow of water that arises from the interior of the earth from a single point or through a small area. They can appear on dry land or go to water courses, lagoons or lakes. The springs can be permanent or intermittent, and have their origin in the rainwater that is filtered or have an igneous origin, giving rise to hot springs.
The water composition of the springs varies according to the nature of the soil or the bedrock. The flow of the springs depends on the season of the year and the volume of rainfall. The filtration springs are often dried in dry periods or with low rainfall; however, others have a copious and constant flow that provides an important local water supply.
Artesian wells, where water springs superficially like a spout, are the result of drilling a confined aquifer whose groundwater level is higher than ground level. When these sources are hot springs (hot water), they are called caldas or hot springs. The mineral salts that have dissolved the broths are recognized medicinal properties, which is why many spas have been built in these areas. This practice is ancient, and in Roman times public baths with mineral waters were much appreciated.
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