Salinization of aquifers: what it is and why it occurs

The salinization of aquifers can be defined as the process by which the concentration of salts and minerals in groundwater increases, deteriorating its quality parameters.

This process is considered a type of soil and water pollution that affects coastal aquifers to a greater extent . Under normal conditions, in these geological formations the fresh water reserves remain in contact with the salty sea water, both water masses keeping a certain equilibrium relationship. However, there are numerous causes, of natural or man-made origin, that can trigger or accelerate salinization processes.

Due to the tourist, agricultural, urban and population pressures that support coastal ecosystems, the Spanish Mediterranean coast can be taken as an example, as it presents serious problems of salinization of the aquifers . In fact, it is estimated that of a total of 95 existing underground water bodies (MASb), around 56 have salinization problems.

As indicated in the previous section, the reasons for the salinization of the water in the aquifers can be of natural or anthropic origin.

Natural causes of aquifer salinization

• If we take into account that one of the most important sources of water entry into aquifers is rainfall , it is reasonable to think that irregularities in rainfall patterns may favor the salinization of their reserves. For example, in periods of drought or in regions with dry climates, where evapotanspiration is higher than annual precipitation, the accumulation of salts and minerals in the water can increase.
• Salinization can also be favored by the lithology and the physical-chemical characteristics of the land (texture, porosity, permeability, capacity to retain moisture and cation exchange). In this sense, when rainwater runs through saline soils, it is capable of dragging and transporting salts and minerals that end up being incorporated into the mineralogical composition of underground water reserves.
• Other natural factors that can favor saline intrusion include: the presence of saltwater aquifers, the proximity of lowlands to the coast, the presence of swamps, coastal lakes and mines.

Anthropic causes of aquifer salinization

• In the first place, among the causes of contamination of groundwater reserves by salts, the overexploitation of aquifers stands out as the great trigger for this phenomenon. With the capture of large volumes of groundwater , the water reserves are reduced, which causes a decrease in the water table and, consequently, the lateral displacement of the saline wedge inland, which gains space in depth. Here we talk more about the Overexploitation of water: causes, consequences and solutions .
• Climate change can also be pointed out as a causal factor. In this sense, global warming accelerated by humans has caused the melting of glaciers, translating this into a rise in sea level that, with each passing year, increases the degree of threat to the state of vulnerability of coastal aquifers.
• Likewise, to the regime of low rainfall typical of the Mediterranean are added the anomalies in rainfall, the result of the current climate crisis that we are experiencing. It could be said that, in general terms, it rains less and less and, when it rains, important episodes of torrential rains take place, during which a large part of the volume of water, due to surface runoff, flows into the sea without reaching the aquifers. As a result, the lack of fresh water inputs does not compensate for the resource’s exit from the aquifer (by catchment or because it flows into the sea) and marine intrusion into an aquifer or groundwater increases. Furthermore, this is aggravated by the sealing of the soil for the creation of urban zones and roads, which seriously reduces the infiltration capacity and, therefore, the water recharge of the aquifers.
• Brine discharges, a product of seawater desalination activities, generate serious effects on aquifers and wetlands. Two examples of areas highly affected by this problem, as pointed out by Custodio Gimena (2017), are: the Campo de Cartagena-Mar Menor and the East of Gran Canaria ^[1] .
• Finally, salinization phenomena can take on greater speed in those cases in which there is a mass of salty water below the fresh one, either because it was already there (saltwater aquifer) or because it has penetrated from the sea in the form of a saline wedge . Thus, when water is extracted, the hydraulic potential decreases and a vertical hydraulic gradient is generated that favors the rise of salty waters.

We advise you to expand the information with this other article on Groundwater Pollution: causes and consequences .

Once we know the causes of the salinization of the aquifers, it is worth wondering about the impacts derived from this phenomenon. Below are the negative consequences of the salinization of aquifers that turn out to be more significant:

• The worsening of the quality of the water generates health, social and industrial and domestic infrastructures affections, since the salt acts as a corrosive agent.
• The saline stress caused in the vegetation leads to physiological and biochemical changes that compromise its survival and that of other living beings for which it is a source of food.
• In relation to the latter, agriculture is so threatened that 40% of the European supply of winter vegetables is at risk ^[2] . However, agriculture is not an exception, there are also many difficulties that can intervene in the normal development of other socio-economic activities.

If you want to learn more about underground water bodies, we encourage you to read this other article about underground rivers: what they are and how they are formed .