David Sedlak—
World Water Day normally includes discussions about the struggle to provide water to meet the needs of people and the environment. In the coming decades, crises related to water scarcity is going to be an even greater concern. A less appreciated problem—namely the accumulation of salt in water and soil—is likely to cause another set of crises that may turn out to be just as problematic.
As we have experienced over the past few years, climate change means a lot more than just a gradual increase in average temperatures. Global warming is intensifying the circulation of air in the Hadley cells that determine the locations of world’s deserts great deserts. As a result, by midcentury the climate of Southern Mediterranean will be as dry today’s North African region, while much of the American Southwest will feel like the current Sonoran Desert. Aridification is not confined to places on the edges of deserts; water is evaporating faster just about everywhere because a warmer atmosphere holds more water and plants living under warmer conditions transpire more water. This means that the water applied to irrigated fields and landscaped areas evaporates faster. Desertification and enhanced evapotranspiration rates mean that a greater quantity of salt accumulates wherever water flows. A portion of the salt remains behind in the soil while the remainder flows into reservoirs, rivers, and lakes.
Salt levels in groundwater also are increasing in many places for other reasons. Growing cities and the intensification of agriculture result in the release of more salt, especially in coastal regions where people tend to congregate. Due to the connection between coastal aquifers and the sea, over-pumping of aquifers allows seawater to migrate inland, leading to higher salt levels in groundwater. The problem posed by the intrusion of seawater into coastal aquifers is exacerbated by sea level rise, which drives salty groundwater inland.
The buildup of salt in soil, surface waters and groundwater pose numerous threats to humanity. Salinization and waterlogging—problems related to salt accumulation in topsoil—already affect about a quarter of the irrigated farmland in the world. Expansion of irrigation to feed growing populations and to satisfy the demand of increasingly wealthier populations that hunger for an animal protein-rich diet is likely to worsen the problem. Elevated concentrations of salts like sodium in the water supply can contribute to high blood pressure, hypertension and other diseases. The salts that build up in water often include toxic elements, like arsenic, chromium, and selenium. Exposure to these elements can damage ecosystems and pose health risks to humans who are exposed to contaminated water and dust produced as terminal lakes dry up.
Luckily, we have many effective strategies for avoiding the worst impacts of salt buildup. First and foremost, we can pay more attention to activities that result in higher salt levels. Regulations or user fees can be employed to discourage excessive use of salts by industry and farms. Water use on irrigated lands susceptible to salinization can be managed in a manner that ensures that salts are periodically flushed out of the soil.
When source control is impractical, water supplies can be desalinated. Removing salt from groundwater is cheaper than ever. As a result, cities and towns facing water stress are turning groundwater, that a generation ago was considered too salty for human use, into reliable water supplies. As costs continue to drop, farmers in areas where the water supply has been salinized are eyeing investments in desalinating irrigation water to carry them through dry periods and to lower salt levels so they can grow higher value salt-sensitive crops.
Better management of salt sources and use of desalination will not solve all the problems associated with salt. Water flowing out of arid lands will always contain more salt than waterways in wetter places. Humanity cannot eliminate its need for salt in industry, agriculture, and other facets of modern life. Desalination remains expensive and produces brines that need to be managed. Nonetheless, by paying more attention to salt and making smart investments in salt removal technologies the people involved in managing the world’s water can help us navigate a saltier future.
David Sedlak is the Plato Malozemoff Professor of Civil and Environmental Engineering at the University of California, Berkeley, and director of the Berkeley Water Center. He is author of the award-winning Water 4.0: The Past, Present, and Future of the World’s Most Vital Resource and Water for All: Global Solutions for a Changing Climate. He lives in Berkeley, CA.
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