Freshwater Salinization: The Anthropogenic Salt Cycle and Current Concerns

 

     While freshwater does normally contain some dissolved mineral salts, human activities are driving dramatic changes in freshwater salinity. These activities include road salt application, water softening, mining and oil production, commercial and industrial processes, weathering of concrete, sea level rise, and fertilizer application. Studies show that especially in water bodies around urban areas, increases in salinity are happening all over the U.S. and globally. These increases can harm aquatic life. Salts are of many different kinds, the main ones being sodium, chloride, potassium, calcium, and magnesium salts. Increased salinity can degrade drinking water and lead to increased piping and pump damage. Higher salinity water can leach more metals out of soils, including radioactive minerals, mobilizing more of them which later leads to higher concentrations in groundwater and surface water. It can also mobilize more nutrients, exacerbating nutrient pollution and the potential for harmful algae blooms. The problem has been dubbed freshwater salinization syndrome.

     Salts have chemical effects. They can corrode metals and increase metal contamination in drinking water, increase nutrient and heavy metal contamination in streams and lakes, and can stress sensitive species. Salts are chemically reactive and their buildup in water bodies can create “chemical cocktails” that are difficult to remediate.

     The EPA developed a five-stage analysis of salinity dangers to freshwater quality as shown below:

 

Stage 0: Highest water quality, minimally disturbed

Stage I: Abnormally elevated concentrations of one or more salt ions across one season

Stage II: Chronically elevated concentrations of salts ions across multiple seasons

Stage III: Formation of harmful chemical cocktails exceeding water quality thresholds

Stage IV: Systems-level failures in infrastructure and ecosystem functions and services (e.g. drinking water and biodiversity)

 

     EPA also developed five strategies to deal with freshwater salinization syndrome. These are reducing agricultural and industrial sources, managing road salt applications and alternatives, evaluating risk, increasing public education to spread awareness of the issue, and federal and state protections. Major agricultural and industrial sources include fertilizers, water softeners, industry, and food processing. The EPA recommends technology and equipment advances, process changes, product choice, and best practices to alleviate the problem. For reducing road salt the EPA recommends “upgrading salt application equipment and technologies, employing web-based decision-support systems, conducting regular training for vehicle operators on optimal techniques to apply salt most efficiently and effectively, and implementing product alternatives where appropriate.” Evaluating risk involves monitoring and assessment by scientists and watershed managers. Lakes, streams, watersheds, and groundwater all need to be monitored and assessed and remediation strategies pursued. Education can be very important in understanding the risks of increased salinization. Salty water can impact infrastructure like bridges, accelerating oxidation and rust damage. It can impact drinking water infrastructure as well, including by leaching more heavy metals, including lead, from pipes into drinking water. Federal and state protections are led by the EPA’s Clean Water Act which requires water quality standards to be met for drinking water and for water that is home to aquatic organisms. Total maximum daily loads (TMDLs) for different contaminants can be established for specific water bodies, including TMDLs for chloride.

The Anthropogenic Salt Cycle

     The natural salt cycle is driven by slow geological processes where changes in salt concentrations (salinity) are gradual. Human activities have vastly accelerated changes in salinity. Around 300 million tons of sodium chloride (NaCl) are produced annually in the world. More recently, an anthropogenic salt cycle has been recognized and proposed. According to a 2023 paper in Nature which defined the anthropogenic salt cycle:

“There is a need to identify environmental limits and thresholds for salt ions and reduce salinization before planetary boundaries are exceeded, causing serious or irreversible damage across Earth systems.”

Differences in the natural salt cycle and the anthropogenic salt cycle are explored in the graphic below from the paper.

     Salts of calcium, potassium, magnesium and sulfate ions are often brought from underground to the surface through mining and land development. This hyper-accelerates geologic processes. Salts related to limestone, gypsum and calcium sulfate are being added to freshwater at alarming rates and affecting 2.5 billion acres of soil around the world, say the authors. Thus, not only the water bodies but land itself is becoming saltier. Increases in salt production and consumption are shown below.

 

     A 2021 paper in Science of the Total Environment noted:

“Agro-ecosystems are very sensitive to salinization; after a certain threshold is reached, yields and food quality start to deteriorate sharply. Additionally, salinity often coincides with numerous other environmental constrains (drought, waterlogging, pollution, acidity, nutrient deficiency, etc.) that progressively aggravate the threat to food security and general ecosystem resilience. Some well-proven, widely-used and cost-effective traditional ameliorative strategies (e.g. conservation agriculture, application of natural conditioners) help against salinity and other constraints, especially in developing countries.”

In fact, the effects of increased salinity on agriculture have been wreaking havoc for millennia in places like Mesopotamia where salt levels in soils increased through the centuries, causing severe food shortages. Arid, semi-arid, and coastal regions are most sensitive to salinization. The graphic below shows some of the salinization processes. The second map is from the 2023 Nature paper and shows global heterogeneity in salinity. Also, according to that paper:

“Excess salt propagates along the anthropogenic salt cycle, causing freshwater salinization syndrome to extend beyond freshwater supplies and affect food and energy production, air quality, human health and infrastructure.”

 

Sci Show does a great job of explaining the newly discovered salt cycle in the video below. 

     They note that the natural salt cycle moves about 200 megatonnes of salt by dissolving it in water and through the wind when picking it up over dried-out, once-salty lakes. They also note that humans add another 80 megatonnes to the salt cycle, a 40% increase. Nearly half of the anthropogenic salt added comes from road salt. Some potential future impacts are salty drinking water and salty soils, both detrimental to health and the environment.

We Discovered a New Natural Cycle! | Watch (msn.com)

References:

Earth's salt cycle is swinging out of balance, posing yet another “existential threat," study finds. Matthew Rozsa, Salon, October 31, 2023. Earth's salt cycle is swinging out of balance, posing yet another “existential threat," study finds (msn.com)

The anthropogenic salt cycle. Sujay S. Kaushal, Gene E. Likens, Paul M. Mayer, Ruth R. Shatkay, Sydney A. Shelton, Stanley B. Grant, Ryan M. Utz, Alexis M. Yaculak, Carly M. Maas, Jenna E. Reimer, Shantanu V. Bhide, Joseph T. Malin & Megan A. Rippy. Nature Reviews Earth & Environment volume 4, pages770–784 (2023). October 31, 2023. The anthropogenic salt cycle | Nature Reviews Earth & Environment

EPA Researching the Impacts of Freshwater Salinization Syndrome. U.S. EPA. November 29, 2022. EPA Researching the Impacts of Freshwater Salinization Syndrome | US EPA

The Anthropogenic Salt Cycle, Risk Assessment Portal. U.S. EPA. The Anthropogenic Salt Cycle | Risk Assessment Portal | US EPA

People are disrupting natural 'salt cycle' on a global scale, new study shows. National Science Foundation. January 4, 2024. People are disrupting natural 'salt cycle' on a global scale, new study shows | NSF - National Science Foundation

Humans Are Disrupting Natural ‘Salt Cycle’ on a Global Scale, New Study Shows. University of Maryland. October 31, 2023. Humans Are Disrupting Natural ‘Salt Cycle’ on a Global Scale, New Study Shows | College of Computer, Mathematical, and Natural Sciences | University of Maryland (umd.edu)

We’re Disrupting Another Major Earth Cycle, And No One’s Talking About It. Science News. November 5, 2023. We're Disrupting Another Major Earth Cycle, And No One's Talking About It - Science News (sciencenewslab.com)

Environmental salinization processes: Detection, implications & solutions. Gabrijel Ondrasek and Zed Rengel. Science of The Total Environment. Volume 754, 1 February 2021, 142432. Environmental salinization processes: Detection, implications & solutions - ScienceDirect

We Discovered a New Natural Cycle! Sci Show. We Discovered a New Natural Cycle! (msn.com)