Coastal regions face a growing peril hidden beneath their surfaces — saltwater intrusion. Over 2.5 billion people rely on aquifers (natural underground water reservoirs) for fresh water, and if enough saltwater infiltrates into aquifers, it can have devastating effects, making the water unsuitable for drinking and agriculture. This salinization is a major global threat, yet it’s rarely discussed.

According to a new study, this risk was underestimated. By 2100, 77% of aquifers in the coastal areas below are expected to experience significant saltwater intrusion, and climate change is a key factor.

If you live in a coastal area, “saltwater intrusion” is a concept you may want to look into. In coastal aquifers, freshwater naturally pushes against seawater, maintaining a boundary known as the freshwater-saltwater interface. When this balance is disrupted — whether by rising seas or reduced groundwater recharge — saltwater infiltrates further inland.

Saltwater intrusion is primarily driven by two processes: recharge decline and sea level rise. Recharge occurs when rainwater or surface water seeps into the ground to replenish aquifers. Climate change reduces this recharge, allowing saltwater to infiltrate further inland. Meanwhile, climate change also affects sea level rise, which in turn, pushes seawater inland, displacing the freshwater boundary.

Low-lying regions, such as those in Southeast Asia, are particularly vulnerable to this encroachment due to their elevation and proximity to the coast, but all coastal aquifers are threatened by these processes.

For the first time, researchers have assessed saltwater intrusion on a global scale, combining sea level rise and recharge projections with the unique characteristics of coastal regions. Using data from over 60,000 coastal watersheds, the study provides a stark forecast: three-quarters of global coastal areas will experience measurable saltwater intrusion by 2100.

How bad is it?

The researchers used information on watersheds collected in HydroSHEDS, a database managed by the World Wildlife Fund that uses elevation observations from the NASA Shuttle Radar Topography Mission. To create a forecast, the team used a model accounting for numerous variables including groundwater recharge, water table rise, fresh- and saltwater densities, and coastal migration from sea level rise.

According to the model 2100, rising sea levels alone will cause saltwater to infiltrate 82% of the coastal watersheds studied, pushing the freshwater-saltwater boundary inland by up to 656 feet (200 meters). In contrast, declining groundwater recharge due to climate change will drive saltwater intrusion in 45% of coastal watersheds, pushing the transition zone even farther — up to three-quarters of a mile (1,200 meters) inland.

If we continue in a “business as usual” scenario, saltwater intrusion could compromise drinking water supplies and agriculture. Almost 3 in 4 aquifers could become contaminated.

Southeast Asia, with its low-lying deltas and dense populations, is highly vulnerable to sea level rise. Even with stable or increasing recharge, rising seas will push saltwater further inland. Countries like Vietnam, Indonesia, and Bangladesh could see widespread salinization of groundwater. This will affect millions of people who rely on these reserves.

In the United States, coastal regions like Florida and the Gulf Coast face a dual threat from declining recharge and rising sea levels. Urban infrastructure, including water supply systems and building foundations, is particularly at risk.

“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,” says co-author Ben Hamlington, a climate scientist at JPL and a co-leader of NASA’s Sea Level Change Team.

Can we do anything about it?

Saltwater intrusion is not a distant threat — it is a present and growing danger for coastal communities worldwide.

Addressing this challenge will require global cooperation, regional planning, and local action. One effective way to combat saltwater intrusion is by increasing groundwater recharge. Managed Aquifer Recharge (MAR) projects involve artificially replenishing aquifers by directing excess rainwater, treated wastewater, or stormwater into recharge basins or injection wells. This helps maintain pressure in the aquifer, pushing back against intruding seawater.

Another approach that can work is managed retreat. As the name implies, it involves taking gradual steps to relocate infrastructure away from vulnerable coastlines. At the same time, you would build physical barriers or natural barriers to protect against sea level rise. Seawalls and levees serve as physical barriers while mangroves and reefs are effective natural barriers. However, this measure is bound to be extremely unpopular — who wants to give up coastal property?

Lastly, we should also simply reduce our use of groundwater. Groundwater depletion is a major challenge to maintaining aquifer health. Over-pumping of groundwater, often for agriculture or urban supply, accelerates saltwater intrusion by lowering aquifer pressure. Implementing sustainable water use policies, such as regulated groundwater extraction, efficient irrigation systems, and water recycling, can help preserve freshwater resources.

Ultimately, a mix of natural, engineered, and policy-driven solutions is needed to safeguard coastal groundwater. Investing in these measures today can protect freshwater supplies and ensure the sustainability of coastal communities for generations to come. This study comes in handy because it quantifies how aquifers around the world will be in trouble. Furthermore, it can also help us to take protective measures in areas that don’t have expertise of their own.

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