"Acuíferos Costeros: ¿Intrusión de Agua Salina?"

"Coastal Aquifers: Saline Water Intrusion?"

Water is a vital resource for life on Earth, and although the planet has around 1,400 million cubic km of water, of which 2.5% is fresh water, it is located mainly in rivers, lakes, glaciers, ice sheets and underground aquifers.

Image 1. Water in the World.

According to INEGI, the uses of water in Mexico are: 76% agriculture, 15% urban public supply, 5% industry and 4% electricity generation. It is important to note that currently more than 30% of the drinking water supply and nearly 43% of the water used for irrigation and food production comes from underground sources, according to the International Groundwater Resources Assessment Center (IGRAC).

Image 1. Water Uses in Mexico (SINA, 2020)

Groundwater reserves, such as aquifers, play a crucial role in everyday life, the economy and environmental preservation. However, a lack of detailed information about their functioning and inadequate management can lead to serious problems, such as saline water intrusion into coastal aquifers, which can result in permanent aquifer unusability if not detected in time.

Mexico has approximately 0.1% of the total fresh water available worldwide.

In Mexico, the sector that wastes the most water is the one that consumes the most: the agricultural sector (farming and livestock). Estimates from the National Water Commission indicate that 57% of the water it consumes is lost due to evaporation but, above all, due to inefficient, poorly maintained or obsolete irrigation infrastructure. The irrigated surface area is 6.3 million hectares and provides 42% of national agricultural production. Losses due to infiltration and evaporation account for more than 60% of the water stored and distributed for agricultural purposes.

What is Salt Intrusion?

Saline water intrusion is a process by which seawater seeps into underground freshwater aquifers, contaminating them and reducing their quality. This contaminates freshwater and can be stopped only when freshwater pressure increases enough to balance the salt intrusion.

Illustration 2. Conceptual model of possible sources of contamination in coastal aquifers.

Factors contributing to saline water intrusion

Excessive extraction of fresh water from coastal aquifers is a major cause of saline water intrusion. When more water is extracted than is naturally recharged, a pressure gradient is created that allows saltwater from the sea to infiltrate into the aquifer. In addition, rising sea levels due to climate change also exacerbate this problem by pushing saltwater inland. Saline water intrusion can also occur due to overexploitation of water resources and a lack of physical barriers protecting aquifers, as well as changes in recharge mechanisms. Furthermore, saline water intrusion can damage coastal ecosystems, affecting local vegetation and fauna. This, in turn, can have economic consequences by reducing agricultural productivity and the availability of drinking water.

Similarly, salinization of the coastal aquifer can also be attributed to marine sediments partially washed into or adjacent to the aquifer, displacement of salt water trapped in deep formations, infiltration of brackish water from estuaries or surface sources, among others.

Mitigation and prevention techniques

To combat saline water intrusion, several mitigation and prevention techniques have been developed. One such technique is the construction of freshwater barriers, which help protect coastal aquifers by preventing saltwater from infiltrating. Another important measure is artificial aquifer recharge, which involves injecting freshwater into aquifers to maintain an appropriate balance between freshwater and saltwater. In addition, sustainable water resource management is critical to ensure proper and equitable use of water.

Continuous monitoring of coastal systems can assist in remediation, but policies must also be enacted to protect these systems. In addition to controlling industrial and residential development near the coast, the construction of new and deeper wells must be stopped. Overexploited regions must be constantly monitored to stop groundwater extraction from deeper aquifers where seawater intrusion into the system is threatened.

Since salt water cannot be used for some production processes, irrigating crops or being consumed by people, saltwater intrusion can be very problematic for coastal communities that rely on groundwater supplies for their livelihoods.

A mixture of fresh water with 2% salt water makes it unfit for human consumption and the proximity of the wells to the coastline increases the probability of salinization by sea water.

The negative effects of saltwater intrusion include unusable aquifer areas due to high salinity, the abandonment of groundwater extraction points, and the need to find new water sources. In addition, it can cause water tables to rise, resulting in flooding problems in basements of buildings near the coast, among other problems.

Image 2. Diagram of saline water intrusion into the aquifer and how it affects soils

Determining the saline water mass in a coastal aquifer is extremely complex due to the need for hydrogeological studies, as well as specific measurements of salinity levels at different points in the aquifer. Similarly, pollution processes induced by overexploitation of coastal aquifers or by rising sea levels require specific studies involving field work (sampling and drilling) and the use of numerical models as a decision-making tool.

This will not only allow for the analysis of the current situation and understanding of the phenomenon, but also the evaluation of possible future scenarios that incorporate anthropogenic variables and natural resources, such as climate change, population growth and tourism development.

All of this could definitely contribute to the design of sustainable strategies to mitigate and control saline intrusion processes.

and how are we?

Mexico's aquifers face serious problems of overexploitation and salinization. The National Water Commission (CONAGUA) has divided Mexico into 13 hydrological-administrative regions (with 653 aquifers), with the XII Yucatan Peninsula having the largest amount of groundwater, with only 4 aquifers, two of which suffer from marine intrusion. In contrast, the VI Río Bravo region has 102 aquifers, of which 46 are overexploited and eight suffer from soil salinization and brackish groundwater.

In 2003, 17 coastal aquifers with saline intrusion problems were identified in the states of Baja California, Baja California Sur, Colima, Sonora and Veracruz (CONAGUA, 2004); this condition continued until 2007 (CONAGUA, 2005-2008). In 2008-2009, the problem occurred in 16 aquifers: Ensenada, Maneadero, Camalú, Colonia Vicente Guerrero and San Quintín in Baja California; Santo Domingo, Los Planes, La Paz and Mulegé in Baja California Sur; Sonoyta-Puerto Peñasco, Caborca, Costa de Hermosillo, Valle de Guaymas and San José de Guaymas in Sonora; and finally the Coast of Veracruz and the Coast of Coatzacoalcos in Veracruz (CONAGUA, 2010, 2011). By the end of 2012 and until 2014, 15 aquifers with the same problem were identified in Baja California, Baja California Sur and Sonora (CONAGUA 2013-2015). Finally, from 2015 to 2018, 18 coastal aquifers with the same problem were identified in the states of Baja California, Baja California Sur, Sonora, Quintana Roo and Yucatan (CONAGUA, 2016-2018).

Image 4. Overexploited aquifers, 2019 and 2020.

The Baja California peninsula has 88 aquifers, however, 23 are overexploited and 11 have saline intrusion, being a serious problem for a semiarid region where the 70% of water is used for agricultural activities.

Table 1. Hydrological regions of Mexico (CONAGUA, 2018)

On the other hand, we have the Sinaloa River with saline intrusion, being one of the main irrigation districts in northern Mexico along with Guasave, Sinaloa; these are the districts that use more than 50% of the water for agriculture in the state. In Veracruz, 33% of the country's fresh water is concentrated and they already have poor management of the resource due to salinization, so Xalapa is supplied with water from Puebla, a state with 2.57% of the country's water (CONAGUA, 2015).

Image 5. Three-dimensional model of the salt intrusion wedge in the Sinaloa River Aquifer.
Image 6. Intersection of the intrusion along the coastline with depth variations of 15-60m.

The results of the saline intrusion model show that the saline wedge penetrates the aquifer along the coastline at a depth that varies between 10 and 60 m. The saturated thickness of the aquifer reported by CONAGUA is 100 m, for this depth, the advance of the wedge on the surface varies between 2.5 and 3.5 km from the shoreline . Although the static levels measured in the sampling wells do not exceed 10 m in depth, there are records that some wells belonging to the Municipal Water Board have levels of up to more than 30 m, which could increase the risk of the wedge advancing in the event of overpumping. The results obtained indicate a significant risk in the future.

Currently, there is a high demand for water for human consumption in metropolitan areas and large cities nationwide, which also face high levels of pollution and scarcity. The situation is especially worrying in northern Mexico due to the increasing periods of drought and scarcity, as seen in the supply problems in Baja California.

Image 7. Germen Energy biostimulant, a solution to water and salt stress.

Overexploitation processes exceed the possibilities and attempts at recovery; however, it is possible to maintain the stable functioning of aquifers by developing policies for the protection of water resources, such as the implementation of recharge programs. A comprehensive understanding of current problems is necessary in order to propose possible solutions that allow measures to be taken for the preservation and sustainable use of water.

References

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