Mapping bar-sand coastal aquifer structure and saltwater intrusion mechanisms using Electrical Resistivity Tomography: a case study fromBinh Thuan coast, south-central Vietnam

Along the edge of many tropical countries, the ground holds a precious, invisible resource: a lens of fresh water floating on top of heavier, salty seawater. This natural separation is fragile. When people pump too much water out, or when the sea rises, the salt pushes inland, turning the fresh supply undrinkable. In sandy coastal areas, this boundary is not a sharp line but a mixing zone that shifts with the tides and the seasons. Understanding exactly where this saltwater sits and how it moves is critical for communities that rely on these underground reservoirs for their daily survival. Scientists have long known that saltwater conducts electricity much better than fresh water, a simple physical fact that allows researchers to "see" underground without digging a single hole. By measuring how easily electricity flows through the ground, they can map the hidden layers of sand and water, distinguishing the fresh from the salty.

In the south-central coast of Vietnam, a team of researchers set out to map these hidden layers across a 95-kilometer stretch of shoreline known as the Binh Thuan coast. This region is built from ancient sand dunes and beach ridges that sit atop older volcanic rock. While the surface looks like a continuous belt of sand, the underground structure is complex, and the threat of saltwater contamination is high. The researchers walked along the beach and dunes with a specialized instrument that sends electrical currents into the ground. They used a method called electrical resistivity tomography, which creates a cross-sectional image of the earth, similar to a medical scan but for the landscape. They laid out eight long lines of electrodes, stretching a total of nearly two kilometers, and measured the resistance of the soil at depths up to 57 meters. The data revealed a clear picture of the underground world: a top layer of dry or fresh sand, a middle transition zone, and a deep, conductive layer of salty water.

The maps produced by this survey told a story of four distinct ways salt is invading the fresh water. In some places, the intrusion is natural and predictable. At the very edge of the ocean and at river mouths, the saltwater pushes inland in a wedge shape, rising higher as it moves deeper underground, following a classic pattern known to hydrogeologists. This is the direct result of the density difference between fresh and salt water, where the heavier saltwater slides underneath the lighter fresh water. However, the study found that human activity is creating other, more chaotic patterns of contamination. In one area, the researchers found a pocket of highly salty water trapped in a depression left behind by old mining operations. Decades ago, miners had dug into the dunes to extract titanium, using seawater to wash the ore. That seawater, now trapped in the low ground, has slowly leaked into the surrounding sand, creating a relic plume of salt that persists long after the mining stopped.

In other locations, the source of the salt is active and ongoing. The survey identified low-resistivity zones directly beneath and near shrimp farms. These farms, common along the coast, use seawater to raise their stock. The researchers observed that saltwater is seeping out of the farm ponds and flowing into the ground, or being discharged through open channels that drain directly into the shallow aquifer. In one specific case, a line of measurement ran parallel to the shore, with both ends equidistant from the sea. One end showed fresh water, while the other showed a deep, salty anomaly. The only difference was that the salty end sat directly above a shrimp farm's drainage system. This proved that the contamination was not coming from the ocean, but from the farm itself, infiltrating the ground from above.

The most complex picture emerged at the mouth of the Song Phan river. Here, the entire underground section appeared highly conductive, indicating a mix of salt and fresh water. The researchers determined this was not due to a single cause, but a combination of three factors working together. First, the river flow is low during the dry season, allowing the tide to push saltwater far upstream. Second, the ground in this specific spot is made of clay and silt rather than clean sand, which naturally conducts electricity better and lowers the resistance reading regardless of the water's saltiness. Third, active shrimp farms nearby are discharging saline water into the same area. The result is a compound effect where natural tides, human drainage, and the type of soil combine to create a zone of severe contamination that is difficult to untangle.

The study also confirmed the shape of the fresh water lens itself. In areas where the sand dunes are thick and high, the fresh water sits deep and stable, protected by a heavy cap of dry sand. In these spots, the saltwater wedge is pushed down to the bottom of the aquifer, keeping the upper layers safe for drinking. The researchers found that the thickness of this fresh water layer varies significantly along the coast, depending on the height of the dunes and the distance from the river mouths. By comparing their electrical maps with water samples taken from nearby wells, they verified that their method was accurate. The water samples showed that where the ground resistance was high, the water was fresh, and where the resistance was low, the water was salty.

This work provides a clear, non-invasive way to see the invisible battle between fresh and salt water. It shows that while the natural laws of physics dictate the general shape of the saltwater wedge, human activities like mining and shrimp farming are creating local pockets of contamination that follow different rules. The findings suggest that managing these resources requires more than just watching the tide; it demands understanding the specific history of the land, from old mine pits to modern drainage ditches. The electrical maps serve as a baseline, a snapshot of the current state of the aquifer that can be used to monitor changes over time. For the communities living on this coast, knowing exactly where the salt is and how it got there is the first step toward protecting the fresh water they depend on.