The Water under Our Feet. Karst, Infrastructure and Water Vulnerability

The Yucatan Peninsula is home to one of the most extensive and unique groundwater systems on the planet. Unlike other regions, water here does not flow in visible rivers, but through a complex network of caves, cenotes, and karst conduits that make up the so-called Mayan Aquifer. This natural infrastructure has sustained human societies for millennia, but its very hydrological efficiency makes it an extraordinarily vulnerable system.

This article explores how the peninsula’s karst system was formed and how it works, why its high connectivity makes it particularly sensitive to pollution, and how human activities have progressively transformed a natural system into a karst system with strong anthropogenic influence. Based on recent scientific evidence, the effects of urban growth, tourism, agriculture, livestock activities, and inadequate wastewater management on groundwater quality are analyzed, as well as the regional scale of these impacts.

In this context, the Tren Maya project is critically examined, not as an isolated work, but as an intervention inserted in an already stressed territory. The risks associated with construction and induced territorial reorganization are discussed, emphasizing those impacts that are not visible from the surface or immediate in time, but that can durably compromise the stability of the subsoil and the quality of the aquifer.

Figure 1. Yucatan Peninsula aquifer vulnerability

The route of the Tren Maya is shown, along with the regions identified as highly vulnerable and with evidence of pollution.

 Elaboración propia basada en el Índice de la Vulnerabilidad del Acuífero Kárstico Yucateco a la contaminación.

Finally, the role of climate change is addressed as a factor that amplifies all existing risks by modifying rainfall patterns, favoring saline intrusion, and reducing the resilience capacity of the system. Understanding the invisible water that circulates under the Yucatan Peninsula is not just a scientific exercise: it is an indispensable condition for making informed decisions about the water, environmental and social future of the region.

WATER WE DO NOT SEE: A PARADOX IN THE YUCATAN PENINSULA
In the Yucatan Peninsula, water defines the territory, although it is rarely seen streaming. Unlike other regions of Mexico, there are no surface rivers here, nor mountains structuring the landscape. The limestone plain appears dry and uniform, but beneath that surface flows one of the most extensive and dynamic groundwater systems on the planet: a hidden world of flooded caves, underground rivers, and cenotes, that has sustained human and natural life for thousands of years.

This condition is not fortuitous. The history of water in Yucatan is written in rock. The progressive dissolution of limestone by rainwater gave rise to a highly permeable karst landscape, where water infiltrates quickly and circulates almost unhindered. Cenotes are just the visible manifestations of a complex and deeply interconnected underground network. This efficiency explains the abundance of the resource and, at the same time, its extreme fragility.

In a karst aquifer, what happens on the surface is transmitted almost immediately to the subsoil. There are no thick soils that filter pollutants or long residence times that attenuate impacts. Urbanization, tourism, agriculture, livestock activities, inadequate wastewater management, and infrastructure are directly reflected in groundwater quality. The effects of climate change put even more pressure on an already strained system.

Figure 2. Conceptual diagram of a coastal karst aquifer

Its main components are the freshwater lens, the saltwater intrusion wedge, the interface between them known as the mixing zone or halocline, and submerged conduits, passages, and galleries. Systems where freshwater and saltwater coexist are known as anchialine systems.

 Elaborated by the author, based in Van Hengstum et al, 2011

Understanding this invisible world is essential to assess today’s challenges. Before talking about impacts and conservation, it is necessary to explain how the karst system that supports the peninsula was formed and how it works. To understand why water here is as abundant as it is vulnerable, it is necessary to descend (conceptually) to the subsoil. The next section explores the formation of karst systems step by step: how water shapes the rock, how caves and cenotes are born, and why this natural architecture influences everything that happens today on the surface.

THE WATER FACTORY OF NATURE: WHAT IS A KARST SYSTEM?
To understand the functioning of water in the Yucatan Peninsula, it is necessary to abandon the classic image of runoff and visible rivers and adopt another way of thinking about the landscape. In karst systems, water does not organize the landscape 
at the surface: it infiltrates, dissolves the rock, and slowly builds up a complex underground network. This “natural water factory” operates on geological scales but directly conditions daily life.

The process begins with rain. When mixed with carbon dioxide in the atmosphere and soil—the surface layer of the Earth’s crust, which supports vegetation—water becomes slightly acidic and reacts with limestone, composed mainly of calcium carbonate, dissolving it. Over millions of years, this reaction widens microscopic fractures that evolve into ducts, galleries, and extensive cave systems.

THE PENINSULA WORKS LIKE A HUGE STONY SPONGE: WATER INFILTRATES QUICKLY, DESCENDS UNDERGROUND AND CIRCULATES THROUGH A NETWORK OF INTERCONNECTED PORES, CRACKS, AND TUNNELS

In Yucatan, where limestone rock predominates and rainfall is seasonal but intense, this process is amplified. The peninsula works like a huge stony sponge: water infiltrates quickly, descends underground and circulates through a network of interconnected pores, cracks, and tunnels. Unlike granular aquifers where the flow is slow and diffuse, in karst, water can move relatively quickly through well-developed ducts, traveling great distances in a short time.

Cenotes are the visible expression of this hidden architecture. They form when the ceiling of a cave collapses and opens a direct connection to the surface. For the Mayan civilization they were vital sources and sacred spaces. For hydrogeology, they function as windows to the aquifer, points where groundwater interacts directly with the outside.

This direct connection explains the vulnerability of the system. In a karst aquifer, the soil is thin or non-existent, and the fractured rock offers little retention capacity. Water—and everything it carries—can be incorporated into the subsoil quickly. Nutrients, microorganisms, and chemical contaminants can spread on a regional scale.

Karst is also not static. The water table responds to rainfall, droughts, and variations in sea level. During glacial periods, when the sea receded, many caves that are now flooded were formed in aerial conditions; with the postglacial rise they were submerged and incorporated into the current aquifer system.

Thus, the karst system of the Yucatan Peninsula is not just a set of spectacular caves. It is a highly efficient natural infrastructure for storing and distributing water, but extraordinarily sensitive to disturbances. Understanding its origin and functioning is essential to evaluate contemporary challenges.

THE MAYAN AQUIFER: DIMENSION, FUNCTIONING, AND FRAGILITY
Beneath forests, cities, and roads of the Yucatan Peninsula lies a regional-scale body of fresh water, the Maya Aquifer. It is not a uniform underground lake, but a heterogeneous system of conduits, flooded caverns, and interconnected storage areas ​(Bauer-Gottwein et al., 2011; Beddows et al., 2007).

Figure 3. Structure of a karstic aquifer

Karst aquifers exhibit three types of porosity (matrix, fractures, and conduits) at different scales and rates that control how groundwater is stored and circulated. This makes them highly vulnerable to contamination and difficult to model.

 Elaborated by the auhor based on Palmer et al., 1999

Unlike aquifers confined by impermeable layers, the Yucatan aquifer is directly exposed to what happens on the surface. Rain can turn into groundwater in hours or days, with little physical or chemical attenuation. This dynamic guarantees availability but also implies high sensitivity to any disturbance.​ (Gondwe et al., 2010; Perry et al., 2009).​

The underground flow combines slow paths through the rock matrix with rapid circulation through well-developed conduits ​(Kambesis & Coke, 2016; Smart et al., 2006)​. This coexistence of diffuse and concentrated flow allows large volumes to be stored and, at the same time, solutes and contaminants to be transported over long distances, reducing the natural filtration capacity.

The relationship with the sea adds another dimension of fragility. Surrounded by the Gulf of Mexico and the Caribbean Sea, the peninsula maintains a delicate balance between continental freshwater and marine saltwater. Freshwater, less dense, floats on top of saltwater forming a relatively thin lens, according to the Ghyben–Herzberg principle and regional studies (Beddows et al., 2007). This balance depends on constant recharge and moderate extraction.

Salinization is one of the most serious risks associated with both climate change and human activity. The rise in sea level and overexploitation reduces the pressure of fresh water and favors the advance of the saline wedge inland, which affects wells, cenotes, and coastal ecosystems.

Fragility is not only physical, but also chemical and biological. The presence of nutrients, bacteria, and compounds, linked to urban growth, tourism, and agriculture has been documented (Leal-Bautista et al., 2013; Metcalfe et al., 2011). Reliance on septic tanks and insufficient wastewater treatment facilitates the direct incorporation of pollutants.

Although the aquifer is still abundant in quantity, its quality shows signs of stress. Scientific evidence indicates that its degradation can be rapid and difficult to reverse (Metcalfe et al., 2011; Moreno-Pérez, Hernández-Téllez & Bautista Gálvez, 2021). Its high hydraulic connectivity makes it particularly sensitive to current territorial and environmental transformations.

THE KARST-ANTHROPOGENIC SYSTEM: WHEN DEVELOPMENT PENETRATES THE SUBSOIL
Understanding the Mayan Aquifer forces us to look beyond geology. The karst of the Yucatan Peninsula is no longer an exclusively natural system: it is a system profoundly modified by human action. Cities, tourist developments, agricultural and livestock areas, and infrastructure are located on a highly permeable subsoil, configuring what can be called a karst-anthropogenic system (Lebedeva, Mikhalev and Nekrasova, 2017), where natural processes and human activities interact inseparably.

THE CONVERSION OF KARST INTO A KARSTANTHROPOGENIC SYSTEM IS NOT IN ITSELF NEGATIVE; THE PROBLEM ARISES WHEN DEVELOPMENT IGNORES THE FRAGILITY OF THE SUBSOIL AND LACKS PLANNING BASED ON SCIENTIFIC EVIDENCE

In other regions, impacts may be softened by deep soils or less connected aquifers. In Yucatan, on the other hand, the footprint of development is transmitted almost unfiltered to groundwater. Urban and tourist growth has often occurred without adequate sanitation infrastructure for the vulnerability of the system. In many locations, treatment depends on septic tanks and absorption wells that discharge directly into the subsoil, facilitating the entry of nutrients, microorganisms, and chemical compounds into the aquifer.​ Hernández-Terrones et al., 2011; Kantún Manzano et al., 2018).

Figure 4. Structure of a karstic aquifer

Changes in sea level at the end of the Pleistocene, which began 2.6 million years ago, and with it, the modern glacial periods. As sea level changed, the position of the halocline also shifted, and the cave systems we dive in today began to form and expand along it.

 Modified from González González et al., 2008.

Tourism intensifies this pressure. Hotels, recreational complexes, and real estate developments demand large volumes of water and generate significant amounts of wastewater. Although there are treatment plants in some areas, their coverage and efficiency do not always correspond to the rate of regional growth.​ (Metcalfe et al., 2011).​

Agriculture and livestock also contribute to this transformation. The use of fertilizers and pesticides, as well as the disposal of organic waste, introduce substances that can travel long distances through the karst network. Studies have documented high concentrations of nitrates and microbiological contamination in different parts of the peninsula, evidencing cumulative impacts on a regional scale.

This scenario is compounded by a misperception: the idea that water “disappears” when it infiltrates. The truth is that it circulates and reappears. The conversion of karst into a karst-anthropogenic system is not in itself negative; the problem arises when development ignores the fragility of the subsoil and lacks planning based on scientific evidence. The result is a silent deterioration that compromises water supplies, ecosystems, and public health. Given this fragility, large projects such as Tren Maya take on special importance: they are not developed in an intact environment, but in a system under signs of environmental stress, where any intervention can increase the risks for the essential water resource of the peninsula. In such a highly connected system already under increasing pressure, the insertion of large-scale infrastructure requires a particularly rigorous assessment of its hydrogeological implications.

TREN MAYA AND THE SUBSOIL: IMPACTS NOT VISIBLE FROM THE TRACK
Among big projects, the Tren Maya is the most visible example of this new stage of territorial intervention, as it is developed in a hydrologically and geologically exceptional, but as we have said, highly vulnerable, territory. It is not only a railway that crosses the jungle, but a large-scale intervention inserted in a karst-anthropogenic system already subjected to multiple stress sources. Assessing its impacts requires considering not only the visible trace, but also the underground architecture on which it is built. 

From the karst hydrogeology point of view, the infrastructure is not located on a homogeneous medium, but on a heterogeneous substrate where primary porosity, fractures, and well-developed conduits coexist with variable rock thicknesses (Ford & Williams, 2007; White, 2002). This complexity influences both geotechnical stability and water and pollutants transportation.

The illustration shows impacts of public works on a karstic system.

 Illustration courtesy of the author

One of the main challenges of the project has been the proper characterization of the subsoil. It is not a homogeneous block of rock, but a complex network of cavities, conduits, and galleries whose distribution is not random; it responds to structural controls and paleowater tables (Ford & Williams, 2007). The absence of cavities detected at a point does not guarantee their non-existence on a local scale, which requires detailed 3D geophysical and hydrogeological studies. Various technical observations to the Environmental Impact Assessment (Ayala Azcárraga et al., 2022; INECOL, 2022) have pointed to limitations in the identification and comprehensive evaluation of the underground system in some sections.

Construction on karst terrain entails documented physical risks, such as subsidence and collapses associated with alterations in surface loads, excavations, or changes in infiltration patterns (Ford & Williams, 2007). These processes can manifest years after the intervention due to the progressive dynamics of weakening of the cavities’ ceilings (Waltham, 2008). The vibration produced by rail traffic can also induce rearrangements in partially filled cavities.

Beyond local risks, the impact must be assessed in cumulative terms. The train does not operate in isolation: it induces urban growth, tourist expansion, and development of services in areas previously less intervened, in addition to generating waste when carrying out maintenance, which must be disposed of safely and the infrastructure for this does not exist. Thus, the potential effects are not limited to the right-of-way, but extend in a diffuse and regional way, increasing the pressure on the aquifer.

In a highly connected system, any local disruption can reverberate on a larger scale. Filling cavities, modifying recharge points, or changes in hydraulic balance can affect flow patterns that have operated for thousands of years. These effects are difficult to monitor and, in most cases, irreversible.

The impacts do not occur in an environmental vacuum, they add to a context of alteration, pre-existing pollution, overexploitation, and growing climate pressure. The central issue is not only the technical feasibility of the work, but its insertion into a system whose resilience is already limited. Climate change adds an additional layer of complexity to this scenario. Far from acting in isolation, it intensifies and accelerates the processes already described.

CLIMATE CHANGE: THE FACTOR THAT AMPLIFIES ALL RISKS
Until now, the analysis of groundwater in the Yucatan Peninsula has highlighted a naturally fragile system, intensely pressured by human activity and exposed to accelerated territorial transformations. Added to this scenario is a cross-cutting factor that does not create vulnerability, but it does magnify it: climate change. In a karst system such as the Yucatecan, the climate acts as an amplifier of processes already underway, significantly reducing the resilience margins of the aquifer.

One of the most obvious impacts of climate change in the region is the modification of rainfall patterns. Projections for southeastern Mexico indicate greater hydroclimatic variability: longer droughts and more intense rainfall concentrated in short periods of time (Cavazos et al., 2013; IPCC, 2023). Torrential rains favor the rapid infiltration of pollutants; droughts reduce recharge and increase dependence on pumping. The rise in sea level favors saline intrusion, affecting water sources that for decades were fresh and suitable for human consumption​​​​ (Beddows et al., 2007; IPCC, 2023).​​

Large-scale projects such as the Tren Maya face additional risks due to climate change, since infrastructures designed under past climatic conditions can be affected by alterations in recharge, water tables and subsoil stability that can compromise the operational viability of the works in an area where the response of the system is rapid and non-linear.

IN A KARST SYSTEM SUCH AS YUCATAN’S, THE CLIMATE ACTS AS AN AMPLIFIER OF PROCESSES ALREADY UNDERWAY, SIGNIFICANTLY REDUCING THE RESILIENCE MARGINS OF THE AQUIFER

Extreme events such as hurricanes and intense storms increase the likelihood of flooding and overflowing sanitation systems. The combination of pollution, overexploitation, fragmentation, and climate pressures increases the risks. The future of the aquifer requires integrating climate science, karst hydrogeology, territorial planning, and preventive water management, under a vision of sustainability of the region. Recognizing the interaction between climate, territory, and subsoil is key to making responsible decisions about an invisible water system, but essential for the region.

CONCLUSIONS: THE INVISIBLE WATER THAT SUSTAINS THE FUTURE
Throughout this journey through the subsoil of the Yucatan Peninsula, an idea becomes evident: the water that sustains life in the region circulates silently through a network of caves, cenotes, and conduits that make up one of the most extensive and vulnerable karst systems on the planet.

The Yucatecan aquifer is an extraordinarily efficient natural infrastructure. Its ability to capture and distribute water has enabled human development for millennia. However, that same efficiency implies fragility. Rapid infiltration and high hydraulic connectivity make the system respond almost immediately to surface actions.

Abundance does not guarantee water security. Water quality is affected by accelerated urbanization, intensive tourism, agriculture, pork and poultry breeding, as well as inadequate wastewater management. These activities have transformed karst into a karst-anthropogenic system, where impacts are cumulative and regional. Added to this are large-scale projects that reorganize the territory and amplify existing pressures.

Climate change introduces an additional dimension of uncertainty. Variability in recharge, sea level rise, and intensification of extreme events reduce aquifer resilience margins and increase the likelihood of salinization and deterioration of water quality.

Recognizing groundwater as an invisible heritage implies assuming collective responsibility. Their protection requires informed territorial planning, continuous monitoring, and integration of scientific knowledge into decision-making.

The water we do not see sustains everything that happens on the surface. Taking care of it requires learning to look beyond the obvious and accept that, in a territory built on rock and water, the true landscape is under our feet and the sustainable and social future of the peninsula depends on its conservation conditions.

Buceo en cenote.

 Pedro Ibarra

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