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

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The Role of Science in Managing Yucatan’s Groundwater Luis E. Marín, Universidad Nacional Autonoma de Mexico In this report, three issues are addressed, first, the importance of groundwater for Mexico; second, the hydrogeology of the Yucatan is described, and finally, the role of science for the Yucatan is discussed within the framework of the regional hydrogeology. Importance of groundwater for Mexico Brief overviews of the hydrogeology of Mexico have been reported elsewhere (Marín, 2002; Marín, in press; and Arreguín-Cortés and López-Pérez , this volume). Thus, the aim here is only to highlight the importance of groundwater for Mexico. According to the Comisión Nacional del Agua (CNA; National Water Commission), there are 653 aquifers in Mexico. Of the total, only 200 have been studied to one degree or another. Marín (in press) reports that in 1975 there were 35 aquifers that were over-exploited. In the year 2000, the National Water Commission reported over 100 aquifers that were being over-exploited (CNA, 2001). To study and manage the groundwater resources of Mexico, with an area of approximately two million square kilometers and a population in excess of one hundred million, as of 2002 there were only 24 people with PhD’s in hydrogeology in the country. Prior to 1990, there were no hydrogeologists with doctoral degrees. In 1990, Mexico had its first Ph.D. in hydrogeology. In 1992, the number grew 100%, to two hydrogeologists. Today, in addition to the 24 PhD’s, there are 40 people with an M.S. in hydrogeology. The Universidad Nacional Autónoma de México has graduated 15 M.S. in hydrogeology, and six PhD’s in the period 1990-2002 (Figure 1). In addition, some Mexican nationals have received hydrogeology degrees in other countries, including the United States. Although the numbers of people trained in hydrogeology in Mexico are growing, the country still urgently needs to train more hydrogeologists. As a comparison, there are more than 10,000 hydrogeologists in the United States. 50 40 UNAM 30 2002 20 1992 10 0 MS PhD Figure 1 Trained hydrogeologists in Mexico. A question closely related to the lack of trained hydrogeologists in Mexico, is the availability and quality of information. Until June, 2003, it was practically impossible to 52 

have access to any technical reports that were contracted out by the CNA. However, the present administration led by President Fox, recently passed a Freedom of Information Act, and now, the public has a federal right that calls for public access to this information. Stronger communication among the academic community, government agencies, and the private sector could help facilitate even more effective water resource management. Mexico has been divided into basins for the purpose of water resource management. Figure 2 shows a figure that was part of the document that was circulated to the interested parties when the Comisión Nacional del Agua started to organize the Basin Council of the Peninsula of Yucatan. Although M. Villasuso (oral com.) reports that the groundwater flow lines are the result of a numerical model, it is clear that a very simplistic model for the hydrogeology of the Yucatan was assumed. Information that would facilitate the development of more sophisticated models is available in the peer reviewed literature (Perry et al., 1989, 1990; Marin et al., 2003; Steinich and Marin, 1997). With greater collaboration between the authors of the peer reviewed literature and the Comision Nacional del Agua, even better models could be developed for the basis of future decision-making. Figure 2 Groundwater flow lines for the Peninsula of Yucatan (Prado Roque, 2000). Hydrogeology of the Yucatan Peninsula The hydrogeology of the Peninsula of Yucatan can be divided artificially into three areas (in terms of the attention each region has received): the Northwest, the Northeast, and the Central portion. Thus, a brief overview of the regional hydrogeology of Yucatan is presented here, followed by short descriptions of the Northwest, Northeast, and central areas. Figure 3 below offers a satellite image of the Yucatan peninsula. 53 

Figure 3 The Basins of the Yucatan Peninsula (taken from NASA). The upper hundreds of meters of the rocks of the Yucatan consist of limestone and evaporites. As a result of the dissolution of the calcium carbonate, a mature karstic system exists throughout the Peninsula. Due to the absence of terrigenous material, there is a thin to non-existent soil cover. The aquifer is a thin, fresh water lens that floats on top of saline water. Underneath the city of Mérida, which is the largest city in the Peninsula with a population of approximately 800,000 inhabitants, the thickness of the freshwater lens measures only 45 meters (Marin, 1990). Steinich and Marin (1996) have mapped the presence of the salt water more than 110 kilometers from the coast. Perry and others (1995) have shown that the origin of the salt water is two-fold: salt water intrusion near the coast, and dissolution of evaporites. The aquifer is used both for disposal of domestic and industrial liquid waste and as a major source of drinking water. Northwest Yucatan Northwest Yucatan has received the most attention in the last decade, primarily as a result of the discovery of the Chicxulub Impact Structure (Penfield and Camargo, 1981; Sharpton et al., 1992). The Chicxulub Impact Crater, with a potential diameter on the order of 300 km (Sharpton et al., 1993), has been shown to have an age of 65 million years (Sharpton et al., 1992; Marín et al., 2001). Other authors (Pope et al., 1991; Perry et al., 1995; Connors et al., 1996; Pope et al., 1996), however, have suggested that the size of the crater is on the order of 180-200 km in diameter. Regardless of its size, it has been proposed that Chicxulub caused the fifth mass extinction that occurred in our planet 65 million years ago including the demise of the dinosaurs. As part of the exploration of the Chicxulub Impact Crater, Marín led a shallow drilling program between 1994-1996 that recovered more than 2,000 meters of core including impact materials. As a result, there are three cores that penetrate the Tertiary column and can be used to constrain the hydrogeology of the Yucatan. Figure 4 and 5 show the map view of the Chicxulub Impact Structure, and the gravity image of the structure. 54 

Figure 4 Map view of the Chicxulub Impact Structure showing the four rings as proposed by Sharpton et al., (1993). Figure 5 Gravity image of the Chicxulub Impact Structure (Sharpton et al., 1993). Marin (1990), Steinich and Marin (1996), Steinich and others (1996) and Perry and others (1995), have shown that the Ring of Cenotes functions as an underground river. Groundwater that flows south to north is intercepted by the Ring of Cenotes and it discharges at either intersection of the Ring with the sea. At both intersections of the Ring of Cenotes, there are nature reserves, to the west is Celestún, and to the east is Dzilam de Bravo. 55 

Groundwater contamination with metals and organic compounds of this fragile system has been documented by Marín and Perry (1994) and Marin and others, (2001); Pacheco and others (2000 and 2001) have documented biological and inorganic contamination in different areas of northwest Yucatan. Perhaps one of the main issues that threatens the inhabitants of northwest Yucatan, and in particular those that live in Merida, and downgradient from Merida, is the lack of a municipal sewage collection and treatment system. Sewage is deposited directly into the aquifer. Northeast Yucatan Although the northeast portion of the Yucatan Peninsula received a lot of attention in the 1970´s, primarily by geologists and hydrogeologists from the United States (Ward and others, 1985; Back and Hanshaw, 1970, etc.), this research interest was not sustained. Recently, there has been some activity from the University of Bristol (U.K.) and now from the Universidad Nacional Autónoma de México. Perhaps of greater impact is the attention to this area, which has the largest collection of subaquatic cave systems in the world, by cave divers. The caves are being systematically mapped by members of the Quintana Roo Speleological Survey. Central Yucatan This area is the least understood, and is one that merits more attention. For example, the regional groundwater divide between the northwest and northeast is found in this area. However, as of today, it has not been mapped. This information is very important in terms of trying to quantify how much water drains towards the eastern and western portions of the Peninsula, contributing substantially to our understanding of the hydrogeologic regime of the area and our understanding of contaminant transport. Role of Science in managing Yucatan’s groundwater resources The geology of Yucatan has given rise to an extensive karstic system. Currently, more than 500 km of subaquatic cave systems have been mapped in the area, and the mapping efforts continue to this day. This mapping has shown that the conduits are very large, indicating that contaminant transport may be unusually fast and efficient. Any contaminant that may reach the water table can quickly reach the sea. For example, at Ox Bel Ha (south of Tulum), which is the largest subaquatic cave system of the world with more than 140 kilometers of mapped passageway, it is possible for a diver to enter a cenote nine kilometers from the coast, and come out at sea without ever seeing daylight (Meacham, oral communication).1 1 There is no official tracking system for cave lengths. However, one often-cited source is Bob Gulden of Odenten, Maryland, who maintains the website “Worlds Longest Caves” http://www.caverbob/wlong.htm . As of September 19, 2006, this website listed the Ox Bel Ha system as the 9th longest cave system in the world and the longest underwater cave system with a recorded length of 88.765 miles (142854 meters) and maximum depth of 110 feet (33.5 meters). 56 

Perhaps the biggest threat that this area faces, is the explosive growth of the tourist industry. Although the government is trying to regulate the growth, the reality is that little planning is taking place. There are several issues that need to be addressed in this area in order to achieve sustainable development. They are: 1. Estimating the groundwater resources of the Peninsula 2. Mapping of the extensive subaerial and subaquatic cave systems 3. Lack of collection and treatment of waste waters 4. A master plan for the collection, and treatment of solid residues Three water balances that have been proposed for the Peninsula of Yucatan have been discussed in Beddows (2003). There are important differences in the volumes that are reported by the three different authors, and Beddows (2003) suggests that the water balances have taken an overly simplistic view (for example, they assume that the aquifer behaves a porous media, and the karstic nature is not taken into account), and thus, they are probably flawed. Although the Peninsula of Yucatan is considered as a single aquifer system for administrative purposes, the aquifer system should be divided into sub-basins. To cite two examples, one has the Merida Basin, which is the area found within the Ring of Cenotes. The groundwater flow regime is different within this basin than outside (Marin, 1990; Perry et al., 1995). The paper in this volume by S. Meacham describes a complex system of subaquatic caves. To date, more than 500 km of underground cave systems have been mapped. Additionally there are also important subaereal cave systems. The subaquatic cave systems provide important pathways for groundwater circulation. These conduits may also allow contaminants to travel through them. Thus, it is important to map them and to use this information for land-use zoning. Throughout this area, there are few systems for the collection and treatment of waste waters. Typically in the Yucatan Peninsula, sewage is disposed of through cesspools. The hotels and tourist resorts, have to have waste water collection and treatment facilities. However, these services are not available to the local population. As a result, untreated waste waters are disposed of directly into the aquifer system. Each municipality has its own landfill. Due to large amounts of waste that are being generated, on the order of 200 tons of waste per day are generated daily at Municipio de Solidaridad (Meacham, this volume) we proposed that a regional plan should be implemented, and that geologic and hydrogeologic studies (including the mapping of subaerial and subaquatic cave systems) be carried out before new landfills are built. Currently, there are approximately 25,000 hotel rooms in the Riviera Maya. Estimates vary between 10-18 as to the number of persons needed as support personnel per hotel room. This number includes the hotel support staff, as well as the indirect support staff (mechanics, staff at the supermarket, etc.). What is lacking is the carrying capacity estimate in terms of the number of the “hotel” rooms that the area can accommodate in a sustainable fashion. Although this area is developing fast, basic information with regards to groundwater is missing. Fundamental questions such as what is the thickness of the freshwater lens, what is the nature of the interface, and what are the groundwater flow 57 

directions and velocities, are lacking. Information on the spatial distribution of water quality is also lacking. The hotels and tourist resorts have advanced state-of-the-art water purification and water treatment facilities. However, the support settlements that typically grow up in support of these facilities lack basic services, such as piped potable water, or sewage collection and treatment systems. As a result, what is typically done, is that the inhabitants drill a shallow well (water table is less than five or six meters deep), and dispose of their domestic waste by dumping it into cenotes (sinkholes) if available, or excavating another shallow well. Thus, although no study is available on a regional basis, there is reason to suspect that groundwater quality, especially in the larger cities, has a significant bacteriological water quality issue. Finally, solid waste collection and final disposal is another major issue that negatively impacts the water quality. Current practice calls for using abandoned quarries as “sanitary landfills”. Obviously, this practice is not a healthy one. What is needed is a master plan for the Peninsula of Yucatan that considers at least three issues: providing clean water to the inhabitants of the area, a waste collection and treatment system, and a solid waste collection and disposal program. 58 

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