According to the CSIC report When freshwater becomes salty (Cuando el agua dulce se vuelve salada, in Spanish), it is estimated that globally, one in three freshwater ecosystems is salinised. In Spain, official data provided by the Hydrographic Confederations indicate that 28% of rivers are salinised. Additionally, in urban areas, wastewater treatment plants only remove 20% of the salt generated through domestic use.
These figures reflect a concerning reality: freshwater salinisation is a global environmental problem that affects ecosystems and their biodiversity.
Salt as an environmental pollutant
Salts are natural compounds that originate from the dissolution of rocks and other geological substrates. Their concentration varies across ecosystems: ≈0.5 g/L in inland waters, ≈35 g/L in the sea, and >300 g/L in brines. Among the latter are salt flats, natural deposits of mineral salts where brines filling the pores and cavities of their aquifers constitute a key source of lithium for batteries and pharmaceuticals. At the opposite extreme, freshwater bodies have the lowest proportion of salts, though they always contain a certain amount in solution. While salt flats are naturally hypersaline ecosystems, in other environments, excessive salt accumulation results from human activities.
Lagoon ecosystems in the marginal zone of the Atacama Salt Flat, the largest lithium salt deposit in the world. | Miguel Ángel Marazuela
The fact that salt is a natural compound has led to the denial of its environmental impact, a phenomenon that researcher Santiago Gorostiza from the Autonomous University of Barcelona calls the ‘naturalisation of pollution’.
“Salt itself is not a pollutant, but its excess is because it disrupts the ecosystem’s balance. Through activities such as agriculture and mining, we are increasing salt concentrations and altering the natural balance of ions in the water,” explains Miguel Cañedo-Argüelles, a researcher at IDAEA-CSIC.
Salinisation, like nutrient pollution (from fertilisers), has been underestimated because its compounds exist naturally. Elements such as phosphorus, for example, are naturally present in soils and water. However, scientific evidence has demonstrated that over-fertilisation disrupts ecosystems, causing massive algal blooms and fish die-offs, among other consequences, leading to regulations limiting its use. Similarly, managing salinity in freshwater ecosystems requires greater attention and mitigation measures.
Causes of aquatic ecosystem salinisation
The human activities that contribute most to water salinisation in Spain and globally are:
- Agriculture
Intensive irrigation crops contribute significantly, as irrigation water evaporates, leaving salts in the soil that later infiltrate rivers and aquifers. Additionally, fertilisers add ions such as sodium, potassium, sulphate, and chloride, further increasing salinity.
The anthropic salt cycle. | Freepress Coop
- Mining
The extraction of coal and other materials exposes natural salts in rocks, which are then washed into freshwater ecosystems. A well-known case is the potash mines in Bages, Catalonia, where extraction has created a massive salt pile towering above the surrounding mountains. These salts, stored without isolation, dissolve with rain and humidity, contaminating groundwater and surface water bodies. The Llobregat River is a good example of this.
- Desalination plants
By removing salt from water, they produce liquid waste with high salt concentrations (brines), which are discharged into the sea or rivers, affecting entire ecosystems. Additionally, they consume vast amounts of energy. According to Miguel Cañedo-Argüelles, desalination plants “should only be used in emergency situations, such as droughts or supply shortages, but they are not a medium- or long-term solution due to their potential ecological impact and high energy demand.”
- Road de-icing salts
In winter, common salt (sodium chloride) is spread on roads to prevent ice formation and ensure traffic safety. However, this salt is washed away by rain into rivers, lakes, or groundwater systems. In the US and Canada, this issue is well-documented, and alternatives and regulations are currently being explored.
- Urban wastewater
Various household and industrial products, such as detergents, hygiene products, and dishwasher salt, contain salts. The problem is that wastewater treatment plants only remove 20% of the salts, meaning their discharge significantly increases water salinity.
- Climate change
Overexploitation of aquifers and rising sea levels allow saltwater to penetrate inland, a process known as marine intrusion. Additionally, droughts reduce river flow, decreasing the capacity to dilute salts and increasing vulnerability to saline intrusion.
Current measures to address salinisation
Given the diverse sources contributing to salinisation, solutions must be multi-faceted.
Firstly, current limits should be reviewed and set systematically, robustly, transparently, and based on scientific evidence, which is not currently the case. This would require combining field ecological studies with controlled laboratory experiments. The results would establish ecological thresholds for salinisation, informing public policy. Despite two decades of research on salinisation, public administrations have yet to implement effective measures to tackle the problem.
“Most likely, measures will come from the European Union, through an update of the Water Framework Directive, which individual countries will then adapt to their specific cases,” says Miguel Cañedo-Argüelles. However, he acknowledges there is still much progress to be made, as most European countries have yet to adopt freshwater ecosystem salinisation management measures.
The involvement of countries varies widely, depending on whether they have experienced severe salinisation issues. Australia leads global regulation after suffering severe soil and river basin salinisation (e.g., the Murray-Darling Basin) due to replacing tree vegetation with crops, which raised the water table and dissolved soil salts. In response, Australia implemented a credit system limiting the amount of salt discharged into rivers, helping maintain salt concentrations within reasonable limits.
In Europe, recent incidents have prompted action. A striking case is Poland’s Oder River, where, in the summer of 2022, tens of tonnes of fish died due to cyanotoxin blooms linked to salinisation. This alarming event has led to discussions about modifying existing regulations.
These examples demonstrate that much work remains to be done to address salinisation and protect our freshwater ecosystems.
IDAEA’s research on salinisation
At IDAEA, we approach salinisation from two fields of study:
- Hydrogeology: We develop numerical models to understand key processes such as saline intrusion and the distribution of underground freshwater reserves in islands and coastal aquifers or the impact of lithium mining in the Andean salt flats. One strategy under investigation is reinjecting brines generated during lithium extraction.
“Although this means reintroducing saline water into the environment, these injections can help maintain the brine-salt balance in the subsurface, thus conserving these hypersaline ecosystems increasingly threatened by intensive brine exploitation due to rising lithium demand,” explains Miguel Ángel Marazuela, a researcher at IDAEA-CSIC.
- Ecology: We study the effects of salinisation on aquatic ecosystems such as rivers, lakes, and wetlands. Additionally, we conduct toxicological studies on model organisms like Daphnia to assess its impact on biodiversity. Recently, researchers Carlos Barata Martí and Miguel Cañedo-Argüelles participated in a global study on the effect of salinity on Daphnia pilicaria populations from North America, Europe, and Asia. The results showed that salinity has a significant impact on these populations: the most salt-tolerant populations were those living in the saltiest habitats.
The SALBIA project, led by Miguel Cañedo-Argüelles, studies the impact of salinization on the Gambia River. | Laura Carrau
Despite the environmental problem posed by salinisation, there are still few research groups in Spain working on this issue. One of them is the REMOTE project, a collaboration between the IMDEA Water Institute and IDAEA-CSIC, which aims to study the fate, impact, and mitigation of particles released from tyres and salts from winter de-icing agents in rivers and streams of High Mountain National Parks. The use of de-icing agents on roads in these parks could be increasing the salinity of adjacent ecosystems. Additionally, the mechanical abrasion of tyres against the road surface continuously releases rubber particles containing a wide range of additives, such as heavy metals or polycyclic aromatic hydrocarbons, which could also reach aquatic ecosystems through runoff water.
‘The project’s results will allow us to identify rivers and streams affected by increasing salinity and the presence of tyre-derived particles and associated additives, as well as to pinpoint sensitive taxa as potential indicators of these pollutants entering the environment,’ explains Paula Redondo Hasselerharm, a researcher at IMDEA Water and the project’s principal investigator.
Individual actions against water salinisation
Research is key to understanding and addressing salinisation, but we must also consider what we can do as a society. Since salinisation has not yet gained strong recognition in public administration as a major regulatory issue, personal alternatives remain limited. Everyday products that generate saline waste, such as detergents or dishwasher salt, currently have few salt-free alternatives available on the market.
“What we can do is organise at a local and citizen level to pressure and demand solutions from the industries responsible for salinisation,” concludes Miguel Cañedo-Argüelles.
In Catalonia, platforms such as Boicot ICL, Prou SAL, and MontSalat monitor river salinity and take legal action against polluting industries. More general organisations focused on river and aquatic ecosystem health include Taula del Llobregat, Projecte Rius, and the Fundación Nueva Cultura del Agua.
As regulation advances, research and citizen action will remain essential in curbing salinisation and protecting our freshwater ecosystems.
Alicia S. Arroyo
Communication and Outreach | IDAEA
