Groundwater stored near the world’s coastlines supports drinking water supplies for huge numbers of people. A new study shows that this resource is coming under increasing pressure.
Many coastal areas are seeing major shifts in groundwater levels that could allow seawater to move inland and contaminate freshwater reserves.
The findings come from a team led by Professor Robert Reinecke from the Institute of Geography at Johannes Gutenberg University Mainz and Annika Nolte from the Climate Service Center Germany in Hamburg.
According to the researchers, the combined effect of excessive groundwater pumping and rising sea levels is creating a dangerous situation for coastal regions around the globe.
“Between 1990 and 2024, more than 20 percent of the coastal areas we studied showed significant changes in groundwater levels. In some cases, levels have dropped by more than 50 centimeters (about 20 inches) per year,” Reinecke said.
“This points to over-abstraction and, consequently, the potential intrusion of seawater and associated salinization.”
Mapping coastal groundwater worldwide
The study stands out because of the scale of the data behind it. The researchers gathered information from about 480,000 wells located in different countries.
This is the largest global collection of coastal groundwater measurements assembled so far. The broad dataset allowed the team to compare groundwater conditions across very different regions in a consistent way.
Instead of looking only at isolated local studies, they were able to build a wider picture of what is happening along coastlines worldwide.
“Our study makes three key contributions,” Reinecke said. “First, it translates available measurement data from different locations into globally comparable metrics, enabling large-scale assessment for the first time.
“Second, it identifies areas at particular risk and highlights the changes occurring there. Third, it provides indicators that can be used to model developments along previously unmonitored coastlines.”
Declines are becoming more common
The results show that groundwater levels are not moving in just one direction everywhere. In some places, they have gone up, while in others, they have fallen.
But the overall pattern has become more worrying in recent years. The researchers found that since 2016, declining groundwater levels have become more widespread.
That shift matters because falling freshwater levels can make coastal aquifers more vulnerable to seawater intrusion.
The study also shows that these changes do not always happen evenly across a region. Some nearby areas may show very different trends.
“The extent of groundwater-level change varies significantly, even on a small scale within many regions,” Reinecke said.
This means that broad regional averages may hide local trouble spots. A coastline may appear relatively stable at first glance, while certain stretches are already under serious pressure.
The groundwater loss hotspots
The strongest declines were observed mainly along the coasts of the United States and Central America, around the Mediterranean, in South Africa, India, and southern Australia.
These are the places where the warning signs are already especially clear. The concern is not only that groundwater levels are changing, but what those changes can trigger.
When freshwater levels fall too low, the balance between land-based groundwater and the sea becomes easier to disrupt. In that situation, saltwater can begin to move into underground freshwater reserves.
The study describes this interaction as especially serious because it is being driven by two forces at once.
Heavy groundwater extraction weakens the system from one side, while climate change, through rising sea levels, increases pressure from the other.
Many coastal zones are high-risk
The researchers also looked at which coastal zones are especially exposed to saltwater intrusion. Their analysis points to two major risk factors.
One is a groundwater table that already lies close to sea level. In such places, even relatively small changes may be enough to upset the balance and let seawater move further inland.
The other is strong dependence on groundwater in dry regions. In arid areas, communities often rely heavily on underground water because other freshwater sources are limited, making any loss of groundwater quality a much more serious problem.
“Coastal areas where the groundwater table is close to sea level are especially at risk, as are arid regions where populations rely heavily on groundwater,” Reinecke said.
“Our study provides global evidence that coastal groundwater is threatened by salinization and must be prioritized for monitoring and management.”
A threat to food and ecosystems
The warning extends far beyond the wells themselves. If coastal groundwater becomes too salty, the effects could spread through daily life, farming, and natural habitats.
The danger could reach all coastal regions over the coming decades. That is a striking warning, especially because more than 30 percent of the world’s population lives in coastal areas.
Any long-term damage to these water reserves could affect not only household supplies but also food production and coastal ecosystems that depend on stable freshwater conditions.
Coastal groundwater is a critical resource, but it is under growing strain. Without closer monitoring and better management, many regions may find that one of their most important freshwater sources is becoming steadily less reliable.
The study is published in the journal Nature Water.
NOTE – This article was originally published in Earth and can be viewed here
