Sustainable Management of Groundwater in Mexico: Proceedings of a Workshop (2007)

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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. 62 

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 63 

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. 64 

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. 65 

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