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14.4.4. Karst modeling. Groundwater

Groundwater

The groundwater are water from precipitation of melting, streamsrivers or the sea, that infiltrate the soil through the cracks and pores permeable rocks and stored in the subsoil forming aquifers.

In order to extract the water, we must make a well until we reach the water in the aquifer. The water table is the upper level of the water table.

Some of the factors on which the infiltration of water depends are:

  • The weather. The greater the rainfall, the more water can infiltrate.
  • The slope. If the terrain is very sloping, the water will circulate on its surface with little time to infiltrate.
  • The vegetation. The vegetation retains the water making its movement difficult, which facilitates its infiltration.
  • The permeability. Permeable materials allow the passage of water.

The volume of groundwater is much greater than that of rivers and lakes (about 20 times more), and although it is less than that of glaciers, it is an important water reserve. In many places, it is the only source of drinking water available.

Some of the main risks of groundwater are:

  • The pollution. The contamination of groundwater is very dangerous because its self-purification is much more difficult than in surface waters. Contaminants can come from agricultural, livestock, mining, industrial, landfill, etc.
  • The overexploitation of aquifers. If more water is withdrawn than is recharged in the aquifer, it is said that it is being overexploited and may become depleted. If this drop in the water table occurs in a coastal area, the aquifer may suffer saline intrusion and the fresh water may become saline. The salinization of the aquifer occurs because the marine water occupies the void left by the exploited fresh water.

Karst modeling

The waters that infiltrate the soil can also form karst patterns, characteristic of limestone and gypsum areas.

The limestone rock is formed by calcium carbonate (CaCO3), it is resistant and insoluble in water. But the water that infiltrates is loaded with CO2 (from the respiration of living beings) and is transformed into carbonic acid. Carbonic acid reacts with insoluble calcium carbonate, transforming it into soluble calcium acid carbonate (or calcium bicarbonate). In this way, the limestone dissolves, generating a karst landscape with erosive forms.

The water will be loaded with calcium bicarbonate which, when it reaches a medium poor in CO2, will precipitate the calcium carbonate leaving sedimentary forms.

The karstificación produces both forms exokársticas  as endokársticas:

Exokarst forms

The water dissolves the limestone rocks that are on the surface leaving characteristic structures:

Lapiaces

By Ymaup (Own work) [GFDL or CC BY-SA 3.0], via Wikimedia Commons

  • Dolinas. They are circular depressions generated by subsidence of the roof of internal cavities produced by dissolution.

Dolina en Pozondón (Teruel)

Dolina del Hoyón in Pozondón (Teruel)

  • Grapes. Originated by the union of several sinkholes.
  • Poljés: depressions in the ground of irregular shape and of great extension.
  • Calcareous tuff: the water comes out of the karst with a high carbonate content, and as the CO2 concentration is lower, it precipitates on mossesrootsstems, etc., giving rise to a rock called tuff, which reflects the shape of the plant .

Endokarst forms

The water takes advantage of the cracks that may be in the ground to infiltrate and widen them by dissolution. Thus it leaves a set of vertical cavities (chasms), horizontal (galleries) or large (rooms).

  • Sinks. Places located at the bottom of sinkholes or poljes through which surface waters enter the karst complex .
  • Siphons. Cavities are filled with underground water, which when emptying gives rise to complex caves.
  • Caverns or vadose caves. Cavities located between the water table and the surface through which the water circulates dissolving and precipitating in calcium carbonate.
  • Stalactites. Vertical structures with an elongated shape that hang from the ceiling of the vadose caverns. They are formed by the precipitation of carbonate crystals that leaves the water drop by drop, so their growth is very slow.
  • Stalagmites. When the drop that forms the stalactite falls to the ground, it again leaves calcium carbonate crystals, so another vertical structure grows from the ground, the stalagmite.
  • Columns. When a stalactite joins a stalagmite.

We have some examples of karst modeling in the Crystal Grottoes of Molinos (Teruel) , the As Güisas cave in Villanúa (Huesca) or the Nerja cave (Málaga) .

Cueva de Nerja (Málaga)

Nerja Cave (Malaga)

Answer in your notebook

14.9.- Explain how a sinkhole is formed.

Answer in your notebook

14.10.- Gypsum is a soluble rock. Will it erode further, by dissolution, in an arid climate or in a humid climate?