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The Hydrogeology of the Yucatan Peninsula
Oscar A. Escolero Fuentes,
Universidad Nacional Autonoma de Mexico
The Yucatan Peninsula consists of an enormous limestone platform rising about 15
meters above sea level covering an area of 43.000 km2. The peninsula is bounded on the
south by the high territories of Chiapas and Guatemala; on the north, by the Caribbean
Sea and on the west by the Gulf of Mexico. The area near Sierrita de Ticul reaches an
elevation of about 150 meters above sea level. Annual precipitation on the peninsula
varies between 500 and 1500 mm, increasing from the north coastal zone inland (SARH,
1988). The rainy season extends from May to September (INEGI, 1992); with the
temperature remaining more or less constant throughout the year, varying from 23°C in
January to 28°C in May (Ward, et al., 1985).
The aquifer system of the Yucatan Peninsula consists of carbonate and evaporitic rocks
of marine origin dating back millions of years. The aquifer is found in the oldest of these
formations: crystalline limestone. These rocks can be observed on the Sierrita de Ticul.
Overlying these rocks one finds microcrystalline fossilíferous covered limestone covered
in some parts by crystalline limestone. In this area, calcarenites and coquinas1 are also
found. The most recent rocks and sediments consist of calcarenites, coquinas, sands and
caliche2 in a band parallel to the coast (SARH, 1988; Perry et al., 1989).
Hydrogeological characteristics
The aquifer of the Yucatan Peninsula consists of a mature karstic 3system. Groundwater
circulation occurs throughout the primary, secondary and tertiary (fractures and
dissolution conduits) porosity.
In the northeastern portion of the peninsula, surface water is very limited (Alcocer et al.,
2000). The aquifer consists of a thin fresh water lens that is only 60 meters thick in the
vicinity of Merida. Saltwater intrusion has been detected up to 110 km from the coast
(Marin and Perry, 1994; Steinich and Marin, 1996).
Due to the karstic nature of the terrain, precipitation infiltrates quickly reaching the
shallow water table, typically located less than 30 meters from the surface. The aquifer is
unconfined except for a band parallel to the coast, where it is confined by a layer of
caliche (Perry et al, 1989). The hydraulic gradient is very low, on the order of 7-10
mm/km. (Marin et al, 1987; Marin, 1990). Back and Hanshaw (1970) suggested that this
aquifer had a very high hydraulic conductivity. Marin (1990) concluded that the average
1
Calcarenites are sand sized particles of limestone. Coquinas are sedimentary rocks composed of mineral
calcites and often some phosphate.
2
Caliche is a hardened deposit of calcium carbonate the cements with other materials such as gravel, sand
clay and silt.
3
Karst is terrain with distinctive hydrology and landforms arising from a combination of high rock
solubility and well developed tertiary porosity.
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value of the hydraulic conductivity for Northwest Yucatan was on the order of 10 cm/s
based on numerical modeling of the area. The depth of the unsaturated zone varies from
approximately 30 meters just below the Sierrita de Ticul to less than a meter parallel to
the coast (SARH, 1988).
Regional Hydrogeological Characteristics
The Sierrita de Ticul is a small chain of rolling hills running from the north to southeast
for about 110 km, with elevations varying between 50 and 100 meters above the coastal
plain and reaching an elevation of about 150 meters above sea level near Tekax., The
Sierrita de Ticul is in sharp contrast with the flat topography of the peninsula and
constitutes its main topographic characteristic, since it separates the karstic coastal plain
located to the north from the ridges and small valleys located south of the Sierrita. The
axis of the Sierrita corresponds to a normal fault, (Marin, et al., 2004). The Northeast
slope forms a short escarpment with steep topography, whereas the opposite side has a
gentle slope and gives origin to the undulating terrain. One of the most important features
of the Sierrita is the presence of caverns and cave passages, such as the Cavern of
Yaatlin, with a development of 450 meters of galleries located 30 meters beneath mean
sea level and the cavern of Loltum has 2,400 m of underground passages (Thomas, et al.,
1997).
Another one of the more important geomorphologic characteristics of the peninsula is the
ring of cenotes4 (Pope and Duller, 1989) which is approximately 5 km wide and
approximately 90 km wide, centered at the Port of Chicxulub (Marin, 1990, Marin et al,
1990). The density of cenotes (sinkholes) in this ring varies from several cenotes per
kilometer to several kilometers between cenotes. It is a zone of high permeability that
isolates hydrogeologically the Mérida Block from the rest of the Peninsula (Marin, 1990;
Perry; Perry et al., 1995).
The ring of cenotes, as a zone of high permeability, acts as an underground river
intercepting groundwater and discharging it to the sea (Marin, 1990). Groundwater, rich
in sulfates due to the dissolution of evaporates, are intercepted and channeled by the Ring
of Cenotes, and finally discharge to the sea, near Celestun (Perry et al, 2002). Recent
work has shown that the ring of cenotes does not intercept groundwater flow along the
entire ring. One of these areas is located along the southern portion of the ring, where
Steinich (1996) and Steinich and Marin (1997) have described a highly variable zone
including a groundwater divide. Escolero et al (in review) have identified an area west of
the ring of cenotes where there is mixing of waters both from outside and inside of the
ring of cenotes.
Another important geomorphologic characteristic is the Holbox Fracture System, located
in the northwest part of the peninsula. It consists of a series of fractures running NNE-
SSW. It is 100 km long and 50 km wide, running parallel to the coast. The fracture
extends from Cabo Catoche north, to Playa del Carmen. Large volumes of groundwater
4
Freshwater filled limestone sinkholes
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flow through this fracture system which discharges into the Lagoon of Conil (Thomas et
al., 1997).
The Conil Lagoon or Yalahu Lagoon, is the most important geomorphologic feature of
the northeast portion of the peninsula. The lagoon is more than 30 km in length and 10
km in width. The River Conil or River Yalikin discharges to the lagoon between 30 and
40 m3/s in high tide, and the Rio Vista Alegre with a discharge on the order of several
tens of liters per second (Thomas, et al., 1997). Also, other groundwater discharge zones
have been identified north of the zone of fractures of Holbox; Cenote Yalahu has a spring
associated with it whose discharge is greater than one m3/s (Thomas, et al., 1997).
The most important geomorphologic feature in the southern portion of the peninsula is
the Hondo River Fracture System. This system runs NNW-SSE, similar to the Holbox
Fracture System. The Hondo River Fracture system runs in parallel strips for 200 km in
length and is approximately 50 km wide, starting north of Belize reaching the Caleta of
Xel-Ha. It is likely that groundwater discharges into the Bacalar Lagoon, and thus gives
rise to the bays of Chetumal, Espiritu Santo and Ascención.
The most important geomorphologic features in the eastern portion of the peninsula are
the extensive systems of submarine caves, interconnected by dissolution conduits. One of
the cave systems, Ox Bel-Ha, has more than 100 kilometers of explored passageways.
These cave systems serve as conduits for groundwater flow as it discharges into the
ocean. Some of these cave systems form large underground rivers where fresh and salt
water mixing occurs and dissolution of the carbonate rocks occur, resulting in the
formation of beautiful caletas5 , such as Xel-Ha.
Groundwater quality
Due to the mature karstic system, this aquifer is highly vulnerable to contamination
(Marin and Perry, 1994). Two distinct processes have been identified which may lead to
the degradation of the available drinking water: 1) the freshwater lens is underlain with
saltwater, thus, mixing of the salt and fresh water may degrade water quality (Steinich,
1996; Steinich and Marin, 1996; Escolero et al., in review), and 2) anthropogenic
activities such as improper construction of landfills (i.e. lack of collection of leachates),
septic tank leaks, and other industrial residues may also degrade the water quality
(Pacheco and Cabrera, 1997; Pacheco et al., 1997; Pacheco et al., 2000; Pacheco et al.,
2001; Graniel et al., 1999; Marin et al., 2000). To offset these impacts, scientists have
proposed the creation of a hydrogeological reserve zone and improved waste
management practices for the city of Merida (Escolero et al., 1994; Escolero et al., 2000:
Escolero et al 2002).
5
Caletas: these are coastal geomorphic patterns, often circular in nature, that are formed as a
result of the dissolution of the carbonate rock. In some cases, such as in Xel-Ha, the roofs of
underground rivers have collapsed, forming the caletas.
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In the Southern portion of the peninsula, one can find the Xpujil Formation, made up
primarily of gypsum. This rock dissolves when it comes into contact with rainwater, thus,
increasing the total dissolved solids (TDS) in the groundwater. The TDS concentration is
so high that it can’t be used a drinking water source. As a result, although there is an
abundance of water in this area, water must be imported to provide drinking water to the
local population.
The Dynamics of the Fresh Water - Salt Water Interphase
Almost every year the Yucatan Peninsula is struck by hurricanes originating in the
Caribbean Sea. These hurricanes result in large amounts of rain and the aquifer receives
unusually high recharge that alters the natural quality of the groundwater by introducing a
number of surface pollutants that were previously deposited on the surface of the land.
Additionally, the mixing of the fresh and saltwater lens occurs because of the increase in
fresh water (Escolero et al., in review). The position of the fresh/salt water lens may also
be altered due to the injection of treated and untreated sewage. A suitable control of the
geochemical processes that may occur as a result of the mixing processes is needed. Thus
research that may lead to establish new design and construction criteria of the injection
wells is needed in order to adequately manage the groundwater resources of the Yucatan
Peninsula.
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References
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Escolero, O. A., Méndez-Rámos, R., and Vidal-López, J. 1994. Plan de Accion para el
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Representative terms from entire chapter:
northwestern yucatan
