Karst aquifers and cave systems

Danilo Antón

Limestones (carbonated rocks) are abundant throughout the world. Some are found on the seabed and near the coast, at different depths (organic ocean muds, corals, tidal plains and calcareous beaches). Others are found in lake, palm or even alluvial environments. In some cases their origin may be igneous (carbonatites) or they may have been subject to metamorphic transformations (marble).

Some carbonated aquifers contain materials of high primary porosity, such as coral formations, lumachelles, calcarenites and other poorly consolidated detritic limestones. Most of the calcareous aquifers, on the other hand, base their potential on secondary porosity, developed through chemical dissolution processes along fractures, as in compact limestones and dolomites.

Hydrogeologically, carbonate rocks are usually very dynamic. Over time, diagenetic processes tend to reduce their primary porosity. In the rock mass, gradual solutions and recrystallizations of carbonated minerals are produced. 

At the level of fractures, the circulation of water tends to dissolve the minerals in the walls, “eroding them” and forming underground courses, the size of which increases gradually. 

Since these processes can occur simultaneously, some carbonated aquifers have a relatively high primary porosity, not yet completely affected by diagenetic processes, and a secondary porosity developing, in fractures.

These rocks can contain considerable volumes of water in their intergranular spaces and fracture networks. The action of water increases the size of the fracture and, therefore, tends to facilitate its own circulation. These mechanisms are known as karst processes, so aquifers contained in such formations are often referred to as karst aquifers. When wells or springs are connected to the main karst courses, these aquifers can be extremely productive and very convenient as water sources

for use intended for the consumption of large cities and irrigation.

However, it is worth pointing out a series of limitations for the use of this resource. In first place, since carbonated aquifers tend to be discontinuous, not all drilling turns out to be productive. In these cases, wells can dry quickly if they are not in contact with fracture systems.

Second, while immediate yields can be impressive, they are sometimes unable to sustain the extraction of large volumes of water. In many cases their reservoirs contain less water, sometimes much less, than other types of formations that have lower yields.

Finally, another element to consider is the rapid flow of groundwater through open fractures. This rapid flow does not allow the degradation of contaminants that can reach the surface and be consumed without time to take corrective action.

Even in spite of these problems, karst aquifers are still the best and most reliable sources of water for urban consumption.

Carbonated formations are widespread throughout the world. They are especially abundant in almost all of Europe, from the Iberian Peninsula to the Alpine region. Karst systems are common along the Adriatic coast, in the Jura and in many other areas of the Mediterranean perimeter (eg the Maghreb, in the Aegean Sea basin).

In Latin America, highly productive carbonated aquifers are found mostly in the Caribbean and the Gulf of Mexico. There are karst systems in Barbados, in Cuba, in Jamaica, in Puerto Rico, on several islands of the Bahamas archipelago, in the vicinity of the Yucatan and Florida peninsulas, in various areas of the Mexican interior (Nuevo León, Tamaulipas, Coahuila, Guerrero, Morelos and in the state of Mexico itself) and in the coastal areas of northern South America.

Bridgetown (Barbados), Havana (Cuba), Montego Bay (Jamaica), Merida (Mexico) and Miami (USA) rely exclusively on groundwater obtained from carbonated aquifers. Other cities that depend heavily on this type of aquifer are: Nassau (The Bahamas), where desalinated water is also used, Kingston (Jamaica) and several of the largest cities in Puerto Rico, such as San José, Ponce and Arecibo .

3.22 Potential of karst aquifers. 

The carbonate formations of Latin America are heterogeneous due to their composition and genesis, they have varied porosity, degree of fracture and consolidation.

Equally diverse are their hydrogeological properties. Some of them are very compact, with low porosity and poor yields. 

On the contrary, there are numerous carbonated formations of high porosity, intensely fractured, that can supply large volumes of water and have excellent potential for the supply of high consumption areas.

In the molasse basins (mountain slopes conglomerates) of the Sierra Madre del Sur, in Mexico (for example, in the Huacapa River basin, near Chilpancingo), on the low slopes of the highlands of Jamaica towards the northern part of the island , in the south of Puerto Rico and along the coast of Venezuela, are highly porous carbonated aquifers. Typical karst aquifers, with fracture flow, occur in numerous places on the continent: in southern Havana (Cuba), in Montego Bay (Jamaica), in the Yucatan Peninsula, in the Torreón-Gómez Palacio area, Mexico and in many other areas.

These aquifers are very vulnerable to pollution due to the rapid circulation of water inside the system that does not allow adequate infiltration and purification of recharge water, especially when they are located in the vicinity of cities. In these cases, underground reservoirs can be easily reached by industrial and omestic waste.

In agriculture areas there are also risks due to the use of agrochemicals.

These problems are present in all karst regions of the continent.

Thus, the urban aquifers of Kingston, in Jamaica, and Mérida, in Mexico, are contaminated by industrial and domestic waste. It is believed that the intense agricultural activity that takes place in the recharge area of ​​southern Havana can damage the quality of the karst aquifer, which is the main source of water for that city and neighboring areas.

Carbonated aquifers are very sensitive to anthropogenic interference, so they require careful handling for continuous use.

Partially reproduced from the book "The aquifers of Latin America", Danilo Antón, Piriguazú Ediciones.