Risk of saltwater intrusion into coastal groundwater spans the globe: Study

Globally, about half of drinking water and a quarter of irrigation water comes from under the ground. Yet many coastal sites throughout the world are seeing notable declines in their groundwater levels, putting them at risk of saltwater intrusion, a new study says.

The study, published April 14 in the journal Nature Water, found that more than 10% of monitored locations showed a significant years-long decline in groundwater levels, indicating a susceptibility to saltwater intrusion, which can render water unusable.

Annika Nolte, a data scientist at the University of Bremen in Germany and lead author of the study, said the results amounted to a “warning” and the work offered a “broad global look at the existing risks” while also identifying “specific regions where we should prioritize management and monitoring.”

Groundwater’s role as a key source of freshwater makes it essential for human existence, according to co-author Robert Reinecke, a professor of earth sciences at Johannes Gutenberg University Mainz in Germany.

“Generally speaking, the availability of drinking water is a prerequisite for people to be able to live anywhere, grow food, and for us to have healthy ecosystems,” Reinecke told German news program Tagesschau.

The insidious creep of saltwater isn’t always easy for people to see, said Nolte, who undertook the study as part of her PhD work at the University of Hamburg in Germany.

“One point about groundwater is for sure that it’s hidden in the ground so it’s not directly visible, the problem,” Nolte told Mongabay. “[And] salinity in particular is not visible. So it’s hard to see the problem, in a double sense.”

Groundwater level change and seawater intrusion have been studied academically since at least the 19th century. However, most studies done at a continental or global scale have been model-based.

Nolte and her co-authors, on the other hand, used observations from about 480,000 coastal locations from 1990-2024, compiling a data set from more than 20 sources, mostly Global North institutions such as the European Environment Agency and the International Groundwater Resources Assessment Centre, a United Nations-affiliated research institute. Nolte said one limitation of the study was that there were less data for Global South locations.

All of the observations were at wells no deeper than 100 meters (about 330 feet) and within 100 kilometers (60 miles) of the coast.

Because many of the sites didn’t have full records, the researchers looked at nine-year and, where possible, 19-year windows for more than 50,000 of the locations. They found that 24% showed significant change over time; of those, 54% were trends of declining groundwater, while the remainder were of rising groundwater. Some locations have seen drops of more than 50 centimeters (20 inches) per year.

Declining groundwater is often caused by people overdrawing wells and reduced recharge in drier climates, often due to limited rain or snowmelt. Higher groundwater levels relative to sea level tend to create an outward pressure that blocks the intrusion of saltwater. When groundwater levels dip, danger awaits.

“Saltwater intrusion is a very serious threat in coastal areas,” Holly Michael, a professor of Earth sciences at the University of Delaware in the U.S., told Mongabay in an email. Michael has published other papers with the study authors before but wasn’t involved in this study. “Only 1% seawater is enough to ruin drinking water supply, and so this is a major concern for water managers in coastal regions.”

Global sea-level rise caused by climate change is another important factor. It affects groundwater differently from place to place, but generally it changes the land-sea gradient so as to reduce the outward pressure of groundwater needed to keep salinization at bay, adding to the risks posed by overdrawing and reduced recharge.

The locations at greatest risk of saltwater intrusion were those with a flat or landward gradient — that is, a low groundwater level relative to sea level — and more arid climates, the authors found.

Nolte named a few examples of areas that seem to be highly susceptible to saltwater intrusion based on the data in the study: the Kerala-Karnataka coast in southwest India, as well as the Gulf of Khambhat farther up the coast; certain areas near Perth and Melbourne in Australia; the Algarve coast in Portugal, the Murcia-Alicante coast in Spain, and the Bordeaux region in France; and Los Angeles, San Francisco, Houston and southeastern Florida in the United States. She named these areas in response to questions from Mongabay, and cautioned that the study wasn’t designed to assess saltwater intrusion susceptibility with high certainty at specific locations. For that, local hydrological assessments with good depth and salinity data are needed, she said.

Declining groundwater is not the only trend the study identifies; in nearly as many locations, groundwater levels increased. One factor in such increases is sea-level rise.

“As sea level rises, the base hydrologic level rises too, which means groundwater levels will increase (if all else is the same),” Michael said.

This trend, just like its opposite, poses potential problems. For example, rising groundwater levels can cause groundwater flooding, Michael said. She said the frequency of rising groundwater trends stood out as she read the study.

“I thought it was interesting that increasing and decreasing trends were approximately equally prevalent,” Michael said.