A hidden underground river carrying mercury has been discovered beneath Mar Menor, a well-known coastal lagoon in southeastern Spain. Researchers found that this flow of contaminated groundwater quietly delivers about one kilogram of mercury into the lagoon each year. The finding raises new questions about how safe and resilient this tourist hotspot really is.
The work, led by marine scientist Céline Lavergne at the Institut de Ciències del Mar in Barcelona, shows that this underground source brings in mercury at levels similar to those from the air and around seventy times more than the Albujón River, the only permanent stream feeding the lagoon. To a large extent, it turns the seafloor into a hidden pipeline for pollution. At the end of the day, that is what makes this discovery so unsettling for people who swim, sail, and fish there.
What scientists found under Mar Menor
Instead of a surface river, the team tracked groundwater moving beneath the lagoon floor through what experts call submarine groundwater discharge, which simply means water flowing from coastal aquifers into the sea out of sight.
By sampling lagoon water, sediments near the beaches, boreholes on land, and the Albujón River, they could map how this invisible flow moves. Their measurements showed that groundwater recirculating through the sediments carries especially high amounts of dissolved mercury.
When the researchers added up those measurements, they estimated that this hidden river sends roughly one kilogram of mercury every year into Mar Menor, which is about 2.2 pounds. That load is similar to the mercury that falls directly from the atmosphere and far higher than the amount brought in by the Albujón River. In simple terms, the main drip of mercury now seems to come from below rather than from rain or surface runoff.
Much of the mercury is what scientists call legacy pollution that was left behind in soils and sediments by decades of mining and farm activity in the region. Co-author Andrea G. Bravo notes that “a significant portion of the mercury we measured originates from past activities” and is still moving through the system. For the most part, that means old contamination is being slowly flushed back into the lagoon throughgroundwater instead of staying buried.
From hidden metal to toxic methylmercury
Mercury itself is a metal that can harm the brain, kidneys, and nervous system when people are exposed to high levels over time. The most dangerous form in water is methylmercury, an organic version that easily builds up in fish and other animals.
According to the World Health Organization, exposure to methylmercury is especially risky for fetuses and young children because their developing nervous systems are more vulnerable.
