Saltwater intrusion threatens coastal groundwater supplies worldwide

Saltwater intrusion, a process where seawater infiltrates freshwater aquifers near coastlines, poses a growing threat to water supplies, ecosystems, and infrastructure. 

According to a study led by researchers at NASA’s Jet Propulsion Laboratory (JPL), by the year 2100, three-quarters of the world’s coastal areas are likely to experience saltwater intrusion.

This is due to the combined effects of climate change, particularly sea level rise and reduced groundwater recharge.

Disruption of the natural balance

Under normal circumstances, coastal aquifers maintain a balance between freshwater flowing from land and seawater pressing inland. Rainfall replenishes these freshwater reserves, pushing the saltwater back toward the ocean, while ocean pressure exerts a counterforce. 

In the transition zone where the two meet, a mix of freshwater and saltwater exists, but the equilibrium keeps saltwater from encroaching further inland.

Climate change is altering this delicate balance. Sea level rise, driven by global warming, is causing coastlines to shift inland, increasing the pressure of seawater on freshwater aquifers. 

At the same time, reduced rainfall and warmer weather patterns are slowing the recharge of freshwater, weakening its ability to counteract the saltwater.

Projected impact of saltwater intrusion 

The study, published in Geophysical Research Letters, analyzed over 60,000 coastal watersheds worldwide to assess how saltwater intrusion might evolve by the end of the century. 

The researchers considered two primary drivers of the phenomenon – sea level rise and reduced groundwater recharge – both individually and in combination.

Spurred by planetary warming, sea level rise is making coastlines to migrate inland and increase the force pushing salt water landward. Moreover, slower groundwater recharge due to less rainfall and warmer weather is weakening the force moving the underground fresh water in some regions.

Saltwater intrusion into coastal watersheds 

When examined separately, sea level rise is expected to cause saltwater to encroach in 82% of coastal watersheds. 

In these areas, the transition zone between fresh and saltwater is projected to move up to 656 feet (200 meters) inland, particularly affecting low-lying regions such as Southeast Asia, the Gulf of Mexico, and the U.S. Eastern Seaboard.

In 45% of the watersheds studied, slower groundwater recharge will independently drive saltwater intrusion farther inland, with the transition zone advancing as much as three-quarters of a mile (1,200 meters) in some places. 

Regions most impacted by reduced recharge include the Arabian Peninsula, Western Australia, and Mexico’s Baja California peninsula.

Combining these effects, the study predicts that 77% of coastal watersheds will experience saltwater intrusion by 2100.

Factors influencing saltwater intrusion 

According to study lead author Kyra Adams, a groundwater scientist at JPL, lower rates of groundwater recharge will drive how far saltwater intrudes inland, while sea level rise will have a major effect on how widespread it is around the world. 

Adams emphasized that local management strategies will need to be tailored to the dominant factor in each region. For instance, if low recharge is the main reason intrusion is happening in one region, officials could address it by protecting groundwater resources. 

However, if the greater concern is that sea level rise will oversaturate aquifers, officials might choose to divert groundwater.

The impacts of sea level rise

The study is part of a broader effort to evaluate how sea level rise will impact coastal regions, co-funded by NASA and the U.S. Department of Defense. It used data from HydroSHEDS, a World Wildlife Fund-managed database based on elevation observations from NASA’s Shuttle Radar Topography Mission. 

The researchers employed a model that incorporated variables such as groundwater recharge rates, water table changes, and sea level rise to estimate saltwater intrusion distances.

According to co-author Ben Hamlington, a climate scientist at JPL, a globally consistent framework which captures localized climate impacts is essential for countries that don’t have the necessary expertise to generate one on their own. 

“Those that have the fewest resources are the ones most affected by sea level rise and climate change, so this kind of approach can go a long way,” he said.

Hamlington likened the issue of saltwater intrusion to the risks posed by coastal flooding. 

“As sea levels rise, there’s an increased risk of flooding everywhere. With saltwater intrusion, we’re seeing that sea level rise is raising the baseline risk for changes in groundwater recharge to become a serious factor,” he added.

Preparing for a new reality

The findings underscore the urgency of proactive measures to manage saltwater intrusion and protect freshwater supplies. 

From safeguarding drinking water to preserving ecosystems and infrastructure, governments and local officials must act swiftly to address this growing threat. 

With a clear and consistent model provided by this study, communities around the globe can better understand and mitigate the long-term impacts of saltwater intrusion.

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