Groundwaters in Mexico. Situation, Challenges and Opportunities

Groundwater represents the main source of fresh water on the surface of the Earth (Freeze & Cherry, 1979). In Mexico, 97 percent of water is groundwater in the different subsoil stones and materials and only three percent is superficial in lakes, dams, rivers, and streams (Carrillo Rivera et al., 2016). In 2017, the concessioned volume of groundwater was used in agriculture (76.3 percent), public supply (14.4 percent) and industry (9.3 percent), with increases in the period 2001-2017 of 32.8 percent in public supply, 18.3 percent in agriculture, and 26.9 percent in industry (see Informe del Medio Ambiente of Mexican Government in https://apps1.semarnat.gob.mx:8443/dgeia/informe18/tema/cap6.html#tema1).

The map of aquifers in Mexico illustrates the distribution of 653 administrative aquifers—not corresponding to their natural extension, but to artificial limits proposed by the National Water Commission (CONAGUA)—of which 114 (17.4 percent; marked red in the map), are considered overexploited based on the extraction-recharge ratio (see the “Aquifers” section in the National System of Water Information, https://sinav30.conagua.gob.mx:8080/Estadistico/#/acuiferos), in addition to registering aquifers with saline intrusion: 11 in Baja California, five in Sonora, and two in the Yucatan Peninsula. Transboundary aquifers shared with United States, Belize, and Guatemala are also included in the map.

 Carrillo Rivera et al., 2016

Management of groundwater and surface water is carried out by the Secretariat for the Environment and Natural Resources (SEMARNAT) through the CONAGUA. The groundwater situation in Mexico is undoubtedly a complex issue, but it can be summarized by the map, which shows that the aquifers considered overexploited are mainly located in the north and center of the country, regions where arid and semi-arid conditions dominate and groundwater represents practically the only source for the different uses. The situation, nevertheless, could be much more critical. 

Two main axes of groundwater management by CONAGUA are the administrative delimitation of aquifers and an equation of availability to grant concessions. Both axes can be questioned and have had important repercussions on the quantity and quality of groundwater, as will be shown below. To do this, we use a fundamental concept (although there are several evaluation factors): the age of water or the residence time of groundwater in the subsoil from the areas where it comes in (recharge zones) to the areas where it is incorporated into ecosystems (rivers, lakes, springs, wetlands), the discharge zones, and at the points where it is extracted through wells, which involve different ages. 

THE AGE OF GROUNDWATER 
The framework for a comprehensive understanding of groundwater and its relationship with the other components of the hydrological cycle is the Gravitational Systems of Groundwater Flow (GSGF) proposed since the first half of the 
20^th century (Tóth, 2016). Due to the great capacity for explanation of its integrative methodology, it is still in use and continuously updated by science around the world. The International Association of Hydrogeologists (IAH) has a working group focused on in this important issue (see https://regionalgwflow.iah.org/). Mexico has different examples of application of these GSGF since the end of the 1980s. 

Groundwater moves in a three-dimensional way. Figure 1 shows a view across a hydrological basin, with its lateral boundaries, the land surface from its upper to its lower parts, and the depth, which can reach hundreds of meters. With this figure, Tóth summarizes different physical (movement of water associated with topography and hydraulic loads), chemical (reactions with minerals in subsoil rocks and atmospheric gases), and biological processes (ecosystems) that occur in the geological environment in which groundwater moves and stores in its interaction with the atmosphere (climate), soil, and surface water bodies (rivers, lakes). The interaction of all these elements allows us to quantitatively define the residence time and characteristics of this system with important vertical components, in local, intermediate, and regional flows. “Topography,” Tóth says, “has a ubiquitous effect on these patterns, causing them to move to greater depths. Local systems have residence times of months to a few years; intermediate ones, hundreds of years, and regional ones, up to thousands of years.”

 Tóth, 2016

This evaluation must be quantitative, using tools from physics, chemistry, and biology through numerical schemes that allow their instrumentation, analysis, evolution, prediction, and constant feedback. Human intervention in these systems is a central component and has been incorporated recently into environmental and sustainability areas with the introduction of socio-ecosystems (Maass, 2018).

ADMINISTRATIVE DELIMITATION OF AQUIFERS AND AVAILABILITY EQUATION 
As mentioned above, groundwater management in Mexico is based on two fundamental criteria. The first is the delimitation of aquifers based on municipal and state boundaries, where the natural extent of an aquifer is divided into four or six administrative sectors that are evaluated independently and disconnected from each other. This causes the areas of greatest exploitation to capture water from adjacent administrative aquifers. On this issue, it should be noted that the administrative aquifers do not coincide with their natural extension and this generates important mistakes.

The second criterion is that the equation used to calculate availability is erroneous (SEMARNAT, 2015), since it considers that the water in the aquifer enters from rainfall infiltration in periods between one and 15 years, but it has been determined that the age of water in different aquifers, under current conditions, can be between 5000 and 35 000 thousand years, that is, water that entered the aquifer as part of the hydrological cycle, but in the Ice Age. Younger water in these aquifers has already been depleted or is in the process of being depleted, with the consequent disappearance of numerous springs and ecosystems that depended on it (Ortega-Guerrero, 2009; 2011).

On the other hand, to quantify the volume of water extracted from the aquifer, it is necessary to know the total number of wells and the flows extracted by each of them through flow meters; most agricultural wells do not have this device. There is evidence that the number of wells in an aquifer is greater than the number of wells under concession and, of course, the extraction volumes exceed those concessioned by up to 59.3 percent. This is the case of the administrative aquifer 0523 in Comarca Lagunera, where the volume concessioned is of 645.5 million cubic meters per year, against 1088.5 extracted according to field measurements (CONAGUA, 2024). This means that many of the existing wells are illegal.

The other components of the availability equation are not adequately measured as they quantify ancient water in the current recharge component.

GROUNDWATER CONTAMINATION
In addition to the problem of mismanagement that guides decision-making in Mexico, there is the problem of groundwater contamination on a regional scale—due to excessive extraction caused by the migration of ancient water—generally from regional flow systems, rich in arsenic, fluoride, and other elements harmful to health and the environment (Ortega-Guerrero, 2009; 2017). To this regional problem, which can cover areas of hundreds of square kilometers, industrial and agricultural pollution adds; industrial pollution with the irregular disposal of organic pollutants, many of them heavier than water and whose concentration limits for human consumption are parts per billion, while in the agro-industrial sector the application of fertilizers and pesticides with a great impact on health and the environment. Pollution of groundwater by urban waste, both liquid and solid, is also widespread in Mexico. These pollutants require, together with regional ones, to be classified in the GSGF context: charge and discharge zones and residence time. 

POLLUTION OF GROUNDWATER BY URBAN WASTE, BOTH LIQUID AND SOLID, IS ALSO WIDESPREAD IN MEXICO. THESE POLLUTANTS REQUIRE, TOGETHER WITH REGIONAL ONES, TO BE CLASSIFIED IN THE GSGF CONTEXT: CHARGE AND DISCHARGE ZONES AND RESIDENCE TIME

The norm NOM-127-SSA1-2021 sets the permissible limits of water for human consumption but, in practice, they are not applied in areas of the country where with major health problems associated with groundwater contamination by any of the causes described above, leaving the population unprotected. Nor the SEMARNAT and the CONAGUA, nor the Secretariat of Health recognize groundwater contamination problems. 

CHALLENGES AND OPPORTUNITIES
There are clear challenges and opportunities in the identification of sustainability objectives and work must be done to achieve them within a new paradigm of groundwater management that involves the broad quantitative implementation of GSGF. 

The case of La Laguna region is emblematic of the groundwater situation in Mexico. Several civil associations in this region won the amparo or constitutional relief proceeding (similar to habeas corpus in other countries’ law) 543/2022, in the Supreme Court of Justice of the Nation, which obliges SEMARNAT and CONAGUA to develop actions for the recovery of the functions of the aquifer in the states of Coahuila and Durango (see https://desc.scjn.gob.mx/amparo-en-revision-5432022). The implementation of the verdict requires a paradigm shift towards GSGF not only for this aquifer, but for those of the whole country, in terms of understanding and quantifying groundwater functioning and its interaction in time and space with the other elements of the hydrological cycle. This involves scientific, technical, educational, and legal challenges that must be faced urgently, and generates endless opportunities in every knowledge area and for the participation of citizens.

REQUIRES A PARADIGM SHIFT TOWARDS GSGF NOT ONLY FOR THIS AQUIFER, BUT FOR THOSE OF THE WHOLE COUNTRY, IN TERMS OF UNDERSTANDING AND QUANTIFYING GROUNDWATER FUNCTIONING AND ITS INTERACTION IN TIME AND SPACE WITH THE OTHER ELEMENTS OF THE HYDROLOGICAL CYCLE

To summarize, the intensive exploitation of groundwater in La Laguna began in the 1920s and by the 1980s there were almost 3000 wells that extracted more than one billion cubic meters per year. Coupled with the construction of dams on the Nazas and Aguanaval rivers—which reduced the recharge of the main aquifer—this caused piezometric (pressure) levels to drop rapidly, at a rate of at least two to three meters per year (CONAGUA, 2024) with an age of water in the aquifer of between 5000 and 30 000 years (González de Hita et al., 1994; Brouste et al., 1997), although availability reports consider that water is recent, from the last 15 years (CONAGUA, 2024).

The extraction of groundwater has exceeded the safe capacity of the aquifer for several decades, reversing the direction of groundwater flow, so it is to be expected that the invasion of water more saline than seawater—product of evaporation over thousands of years, with high concentrations of arsenic and other toxic elements—increases in the future, causing significant damage to the quality of groundwater in this region, perhaps irreversible on a human scale if the necessary urgent actions are not taken (Ortega-Guerrero, 2017).

An important step in developing a new management system for this precious element is to adopt an appropriate scientific model that allows us to understand the water cycle as a whole and in a systemic way. GSGF are extremely useful for mapping the short- and long-term consequences of human actions on the water cycle and for proposing future scenarios towards sustainability. Its incorporation into research, education, and legislation will correct management errors, so the challenges and opportunities in all areas of knowledge for this paradigm shift are very broad. If urgent action is not taken in the management of groundwater in Mexico, there is a risk of continuing to deplete and pollute many of the country’s aquifers with negative effects preventing to guarantee the human right to water and a healthy environment.

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