Author: Mario Rebolledo
Text published in the book: Cenotes, «Imprints of Water and Light in the Jungle», Cancun, Mexico 2008
Introduction
The Yucatan Peninsula does not have important surface rivers, with the exception of the Hondo River, near Chetumal. However, the region has extensive aquifers which constitute the most important underground river system in the world. The great ecological diversity which still characterizes the area is sustained by this hidden reserve of water. But it would seem the fact that this water flows beneath the surface prevents us appreciating its vulnerability. We lose sight of the connections that link the cenotes, lagoons and waterholes which appear on the surface. These windows onto the liquid soul of the Peninsula must be cared for; it is vital to avoid contaminating and over-exploiting them.
In fact the wonderful cenotes of the Yucatan Peninsula have recently become a must for visitors and locals, unable to pass up the opportunity for a relaxing swim or some cave-diving. On the other hand, the characteristics of the coastal aquifers of the Peninsula mean that practically anyone can dig a shallow well in the backyard of their house and obtain drinking water. This has created the false impression that “there’s more than enough water in the region”. However, it is worth posing a few questions on this point. How much water is there in the subsoil? Is the subterranean water drinkable? How does this system of living water beneath our feet function? This essay seeks to provide the reader with some answers to these questions.
Origin of the Yucatan Peninsula
To explain the presence and characteristics of the aquifer in the Yucatan Peninsula we must look briefly at its origin. A study of the geological history of the Peninsula reveals that we are dealing with one of the youngest parts of Mexico.
Approximately 120 million years ago, the Peninsula was under the ocean. At this time, what is now the Atlantic Ocean was beginning to form. Later (around 70 million years ago) the area was a shallow sea barely 300 ft deep, with climatic conditions very similar to the present Caribbean. (The Pacific Ocean today has an average depth of 12,000 ft).
Gradually a reef system developed in this warm tropical sea, as well as a sea bed rich in limestone. However, the story is not without interruptions; in fact, study of the sequence of rocks in the Peninsula indicates that there were several short periods when the sea level fell enough to leave the region above the surface of the sea. These periods during which the Peninsula emerged (in fact it was still an island, rather than a peninsula) allowed the formation of a type of rock known as “evaporites”, because many of their properties derive from the evaporation of seawater. Gypsum and halite, or rock salt, are examples of this type of rock.
During the last two million years of the Quaternary period there occurred the so-called “ice ages” – when the planet’s climate was much colder than today–. During these periods a large quantity of water was trapped in ice, causing a drop in sea levels. At the end of the glacial era, the sea level rose again. During the Quaternary, the planet underwent 16 glaciations, averaging 100,000 years in length. The last of these, called the Wisconsin, began 75,000 years ago and ended 10,000 years ago. In some of these glacial periods, the sea-level dropped 360ft, although during the Wisconsin period, the sea level is believed to have fallen an average of 120 ft (Nova Scotia Museum of Natural History, 1996).
Karst topography
During the periods when the limestone platform which we now know as the Yucatan Peninsula emerged from the sea, a system of caves began to develop, evolving over hundreds of thousands of years. These caverns are highly permeable, permitting liquids to pass through. The fact that the rock is easily water-soluble allowed the caves to grow ever larger, until eventually many of them became so big that the roofs collapsed, leaving the caves exposed to the light of day. These structures are known locally as “cenotes” –from the Maya word dzono’ot.
The phenomenon described here is well known to geologists: these landscapes formed by the dissolution of carbonate rock are called “karst topography”.
The dissolution of limestone, which is made of calcium carbonate, is also a well-known phenomenon. It is the result of a series of chemical reactions that mildly acidify the water which filters through fissures and slowly dissolves the carbonate rock.
Water falling as rain reacts with the carbon dioxide found both in the atmosphere and in the soil, around the roots of plants, producing the following reaction:
H20 + CO2 —> H2C03 + CaCO3 —> Ca++ + 2(HCO3)–
In other words, one molecule of water and one of carbon dioxide produce a molecule of carbonic acid. This in turn reacts with the calcium carbonate, dissolving it. The result is a free calcium ion and two molecules of acid dissolved in water. In a dry cave the free calcium will be precipitated again as calcium carbonate, producing stalactites and stalagmites.
Porosity, permeability and fissures
Another key element in the formation of caves and cenotes is the presence of fissures and fractures in the rocks. Although acid plays an extremely important role in the process (which takes thousands and perhaps millions of years), it would require much more time to dissolve the rock without the help of these fissures and fractures. They create what is known in hydrogeology as “secondary porosity”, which facilitates the circulation of acidic water.
The fissures contribute to the dissolution of the rock, and gradually increase in size, until they are transformed into true underground rivers.
Given the importance of understanding the process by which cenotes are formed, we should clarify and distinguish the twin concepts of porosity and permeability. A pore is defined as “the space between two solid particles in a rock” (Bear, 1972); the total of these spaces in a given volume of rock is termed its “porosity”, and is generally expressed as a percentage of the total volume. The permeability of a rock, on the other hand, is a function of the size of the pores and the number of them which are interconnected, thus allowing circulation of fluids (ibid). To clarify the difference between the two, we can refer to a classic example: clays have a high degree of porosity, reaching up to 60% of the total volume, while in sands it does not exceed 50%. However, the reader will probably realize that clays, although extremely porous, are impermeable; that is, they do not permit liquids or gases to circulate, or only very slowly. Sands, meanwhile, have excellent permeability, and are in fact used in filters. If the porosity is not so dissimilar, what causes the difference? It is precisely the size of the pores, which in clays are so small that they do not allow fluids to circulate.
What we have described in the preceding paragraph is known as “primary” porosity and permeability. If we revise the definition of a pore, we can add that when a rock fractures, the resulting fissures become “secondary pores”. If several fractures interconnect they produce “secondary permeability”. Limestone, being rock formed from the precipitation of a chemical compound like calcium carbonate, has a very low primary porosity (less than 25%). However, through a series of physical processes it acquires a very high porosity and, in consequence, a secondary permeability which is the main focus of this section.
Like all materials, rocks expand when heated and contract when cool. This leads to what is called “thermal fatigue”, a phenomenon which produces fissures and cracks in materials. This is one of the causes of fissures, although they tend to be small (measuring from a few inches to a few yards). Another type of fracture is caused by relative shifting of the affected rocks. In geology these are known as faults, and are the result of more complex geological processes which space does not permit us to examine in detail.
Although there is no formal definition of a cenote, we can divide them into three main groups. Two of these are the semi-open, in which the roof has only partially collapsed, and the open, in which the roof has completely fallen in. It is interesting to note that all the open cenotes are almost perfect circles, for example the Sacred Cenote at Chichen Itza, although there have been no studies of why this should be. The third group of cenotes within this informal classification are the “dry” cenotes. The word dry is in inverted commas because this is a mistaken impression. In fact what happens is that the material resulting from the collapse of the roof, along with other sediments washed down by rainfall, accumulates in the interior of the cenote, raising the floor above the level of the water table. In the center of some cenotes along the Mayan Riviera it is common to see a small “island” surrounded by water; this is merely a consequence of the same phenomenon.
In the Yucatan Peninsula there are two regions with particularly interesting fractures and faults. One is in the northwest, around the famous “Chicxulub Crater”, which is 112 miles in diameter and over 18½ miles deep at its deepest point (Rebolledo-Vieyra, et al, 2000). Owing to the geological processes which operated around the edge of the crater during the 65 million years that have passed since the impact, an extensive system of faults appeared. This gave rise to one of the most remarkable hydrological phenomena in Mexico: the “Chicxulub ring of cenotes”.
If we were to take an aerial photograph of the region and join together the cenotes to the south of Merida, we would see that they form an almost perfect semi-circle, stretching from Celestun to Dzilam de Bravo. These cenotes are the result of water circulating through the fractures at the edge of the crater and dissolving the rock. The system thus formed is extremely important to the region’s hydrology, as studies indicate that water flowing from the south of the Peninsula is “trapped” in this ring, which behaves like an underground river and distributes the water throughout its length, finally discharging it into the ocean at Celestún and Dzilam de Bravo (Perry, et al., 2002).
In the east of the Peninsula there is another system known as the “Holbox-Xel Há fracture zone” (Butterlin and Bonet, 1962), which runs from north to south practically through the entire state of Quintana Roo. Recent studies carried out by researchers from the National Water Center, in collaboration with their colleagues in the Autonomous University of Mexico, have found that, like the “Chicxulub ring of cenotes”, the Holbox-Xel Há fracture system “captures” water from the east of the Peninsula and distributes it along its length, feeding cenotes, lagoons, underground rivers and inlets throughout the state and discharging in the sea at Holbox, Sian Ka’an and other points.
In fact, we are still studying this system, because we need to know more precisely how it functions, how much water it contains and where exactly it goes. Although the origins of the fractures are completely different from those in the “Chicxulub ring of cenotes”, it is still interesting to note that the cenotes here are aligned with the fractures. In the Tulum area, it is clear that the Muyil, Unión and Chunkopó lagoons are all perfectly aligned, which suggests that they all owe their existence to one of the fractures of the Holbox-Xel Há system.
Another intriguing feature of the region is the way fresh water flows into the ocean in the form of “inlets”. These are of great ecological importance because of the diversity of species which congregate there. The discharges of underground rivers also form ojos de agua – small freshwater springs flowing directly up into the ocean. Such springs provide nutrients to the water and it is common to find concentrations of fish and other marine life around them.
Saline intrusion
A critical aspect of the hydrology of the Yucatan Peninsula is what is known as “saline intrusion”. It is vital to take this phenomenon into consideration if we wish to exploit a coastal aquifer appropriately (and this applies anywhere in the world). On every coastline, the pressure of the ocean tends to force seawater into the landmass in direct proportion to the porosity and permeability of the rocks in the region.
The magnitude of the intrusion will also depend on the characteristics of the aquifer in question. Seawater has a high content of salts and other suspended solids, which gives it greater density than fresh water. Thus, when two bodies of water with greatly differing densities meet, the fresh water of the aquifer tends to “separate” from the salt water, and literally float on top of the seawater, creating a mixing “interface” called a “halocline”.
In some cenotes and inlets such as Yal-Kú and Xel Há, in Quintana Roo, saline intrusion can be observed with nothing more than a mask; the salinity increases or decreases according to perspective, creating a visual phenomenon in which visibility is reduced and the water becomes cloudy and “oily” in appearance.
The equilibrium established between the two phases, saline and non-saline, is very fragile and easily upset. When planning the drilling of a well for fresh water, therefore, we need to know the time required for the well to recover its normal water level. If water is extracted at a rate faster than the well’s capacity to recover, the interface may lose equilibrium and contaminate the freshwater lens with saltwater.
It is therefore extremely important to take saline intrusion into consideration when drilling and using wells. The same phenomenon affects “injection wells”, which place “treated waste water” in the subsoil. This injection must be below the level of saline intrusion, otherwise the difference in density will cause the treated water to float on top of the saline level. As we have mentioned, this would bring about contamination of fresh water sources with waste water, a situation which is made worse when the water is not treated adequately.
How much water is a lot?
Finally, let us turn our attention briefly to the topic of management or exploitation of aquifers in the Yucatan Peninsula. Owing to some of the characteristics already mentioned (for example, the abundance of clear water cenotes combined with the lack of surface rivers), there has been a widespread but false belief that there is plenty of water in the Peninsula. It would seem that this mistaken impression is due in part to the fact that almost anyone can dig a small well in the patio of his house without ever finding the underground water contaminated.
Beyond the image presented to tourists, it is necessary to publicize the fragility of this system by means of an objective vision which creates a critical awareness in both visitors to the Peninsula and its residents.
Given the fragility of the system and its “high level of vulnerability” in technical hydrological terms, it is clearly necessary that both the authorities and the general public take measures regulating exploitation of the aquifers. Equally, technical regulations for the treatment and disposal of waste water must be applied, taking into account the characteristics we have mentioned.
In the Yucatan Peninsula, as throughout the world, our most valuable resource is the environment. Therefore, investing financial resources and effort in it is neither a waste of time nor a “politically correct” gesture. It is an investment with short- and long-term benefits, since both the tourist industry and the general population depend on an equilibrium in the environment.