Tidal rivers, which are a source of water for drinking and irrigation, are increasingly vulnerable to saltwater intrusion, according to the findings of an international research team, including Penn State scientists. Saltwater entering these freshwater supplies can tip the salinity scales, making the water unsuitable for human and animal consumption or for irrigating crops.
In a global perspective paper, published in Environmental Science & Technology Letters, the team highlighted how a combination of climate change impacts - including prolonged drought and rapid sea-level rise - along with localized human activities, are intensifying the increase in salt in vital freshwater sources. For example, in 2023, the United States Army Corps of Engineers had to bring in millions of gallons of freshwater by barge to New Orleans to decrease the salinity of drinking water supplies caused by ocean saltwater moving up the Mississippi River.
Despite the widespread saltwater intrusion, most of the available scientific literature focuses on site-specific studies. To better understand the global impact, the researchers synthesized studies from around the world, identified major themes and recommended future research directions.
The synthesized studies, including some of their own, were on oceanographic and hydrological processes that promote saltwater intrusion, as well as processes in watersheds that lead to increased erosion and weathering and thus to increased salt transport into rivers.
"The transition from salt to fresh water fluctuates quite dramatically with the tides, with the seasons, with sea level and with stream flow." Najjar said. "Saltwater contamination is an issue that's coming from two different directions. To help assess the impacts of salinization, we needed to identify and characterize the water intakes along tidal rivers."
As part of a project in the Chesapeake Bay, which was included in the literature review for this study, Najjar and his team identified tidal water withdrawal sites.
"We were interested in determining where water was being withdrawn from tidal rivers and by whom," Najjar said. "This was not a trivial task since there is no national database with that information. A few sources do exist, like the United States Geological Survey's data on surface water and groundwater withdrawals and their uses, which are compiled by state. However, the data doesn't separate tidal from non-tidal sources, so you don't know which intakes are threatened by saltwater intrusion."
Najjar worked with water agencies in Maryland and Virginia, the two states that cover most of the bay's shores, to identify tidal intakes.
"We basically identified all of the water withdrawal sites and then had to determine what sites were considered tidal based on the tidal shoreline maps," Najjar said. "Tidal rivers are influenced by both river flows from land and tides from the ocean."
Key scientific literature findings from the paper include:
- Climate change as a major driver: Accelerated relative sea-level rise, altered drought and river flow, and extreme weather events are significantly increasing saltwater intrusion.
- Human impacts: Local human activities, such as channel deepening in estuaries and excessive use of salt on roadways, have historically contributed to and continue to exacerbate salinity issues.
- Widespread impact: Saltwater intrusion is affecting rivers across all continents, impacting diverse regions, from semi-arid climates to precipitation-rich temperate zones, and causing damage to infrastructure.
- Broader consequences: Freshwater salinization can lead to secondary effects, including the exacerbation of hypoxia (low-oxygen zones) and the mobilization of contaminants such as nutrients and metals, which further stress water systems and their associated infrastructure.
The researchers concluded their work with recommendations for future investigations, including:
- Ion-specific measurements: Enhance the monitoring and measurement of major salt ions - electrically charged atoms and molecules - to better understand their sources, transport and fate in watersheds and tidal rivers, aiding in the protection of infrastructure.
- Ion-specific hydrological-hydrodynamic models: Develop models that can simulate the transport of individual salt ions to accurately predict and assess risks to infrastructure.
- Human-centered decision support tools: Design decision support tools with a human-centered approach so that stakeholders can better predict and manage salt contamination, with a focus on infrastructure resilience.
"This critical research underscores the urgent need to bring together every stakeholder to protect freshwater supplies and the vital infrastructure that delivers them from the escalating threat of salt contamination," Najjar said, noting that there isn't a federal drinking water standard for salinity in the United States. "I think the research that we're doing can really help with planning for the future as we face a changing climate."
Other Penn State co-authors included Alfonso Mejia, professor of civil and environmental engineering, and Antonia Hadjimichael, assistant professor of geosciences and Wilson Faculty Fellow in the College of Earth and Mineral Sciences. D'Andre Tillman, who earned an undergraduate degree in meteorology and atmospheric science at Penn State and is now a graduate student at the University of Wisconsin, and Maria Herrmann, associate research professor in meteorology and atmospheric science, analyzed the data about water intakes in Maryland and Virginia.
Other co-authors include Sujay Kaushal from the University of Maryland, Robert Chant from Rutgers University, David Ralston from Woods Hole Oceanographic Institution, Hans Burchard from Germany's Leibniz Institute for Baltic Sea Research, Allison Lassiter from the University of Pennsylvania, and Xiaohong Wang from Salisbury University.
Funding was provided by the U.S. National Science Foundation's Convergence Accelerator Program.
