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A toxic underground river of groundwater carrying mercury has been found seeping into Spain’s Mar Menor lagoon. This hidden flow sends roughly 2.2 pounds of mercury into the lagoon every year.
The work was led by Céline Lavergne, Ph.D., at the Institute of Marine Sciences, in Barcelona, Spain. Her research focuses on how trace metals move through coastal waters and affect nearby ecosystems and communities.
Mar Menor is a shallow, hypersaline coastal lagoon in southeastern Spain, separated from the Mediterranean by a narrow sandbar.
It covers about fifty-two square miles, making it the largest coastal lagoon in the western Mediterranean region.
Toxic mercury at Mar Menor
The team traced a network of submarine groundwater discharge, groundwater that flows from coastal aquifers into the sea, beneath the lagoon floor. This unseen flow brings mercury at levels similar to the air and seventy times the Albujón River.
Much of that mercury is legacy mercury, which is contamination left in soil and sediments by past mining and farming.
When this groundwater mixes with salty lagoon water near the shore, conditions favor forming methylmercury, which is a mercury form that builds in food webs.
These mixing zones are low in oxygen and rich in organic matter, supporting microbes that convert inorganic mercury into methylmercury.
Researchers reported finding high concentrations of methylmercury in nearshore waters, indicating that coastal areas where groundwater enters the sea can become hotspots for methylmercury production.
Seafood, mercury, and Mar Menor
Methylmercury moves up aquatic food chains, so larger predatory fish usually carry higher concentrations than the smaller creatures they eat.
According to the World Health Organization (WHO), methylmercury can damage the brain and nervous system, especially during fetal development and early childhood.
For most people in the United States, the main route of mercury exposure is eating contaminated fish and shellfish.
Current mercury levels in Mar Menor water and fish are not considered alarming, yet scientists warn that extra methylmercury could change that picture.
Over recent decades, Mar Menor has suffered repeated algal blooms and murky water driven by nutrient runoff from farms, towns, and livestock.
These nutrient pulses have triggered major ecological disruptions, including low-oxygen events that killed seagrass and fish in several recent years.
The discovery of a toxic underground mercury source adds another layer of stress for an ecosystem already struggling with multiple pressures.
Warmer summers and more frequent heatwaves are likely to intensify low oxygen conditions, giving mercury converting microbes more chances to thrive.
Tracking invisible groundwater flow
The team sampled lagoon water, porewater from beach sediments, groundwater from boreholes, and the Albujón River itself.
They measured concentrations of dissolved mercury and methylmercury alongside radium isotopes, which are radioactive tracers that show how quickly groundwater is entering the lagoon.
By combining those tracer measurements with a hydrogeological model, they estimated how much water each groundwater component contributes through the year.
The calculations showed that long-scale recirculation of lagoon water through sediments dominates the mercury load, while fresh groundwater adds a smaller share.
Global coastal regions
Along the California coast, researchers have shown that submarine groundwater discharge can deliver mercury to the ocean at river-like rates.
Studies in Asia and northern Europe have found that groundwater inputs sometimes rival or exceed known mercury sources to coastal seas.
Until now, no one had quantified how important groundwater might be for mercury cycling in a coastal lagoon like Mar Menor.
The new work closes that gap and suggests many lagoons worldwide could be quietly receiving mercury from below.
Minamata Convention on Mercury
The picture that emerges is one where industrial and mining practices keep leaking contaminants long after factories and pits have closed.
The Minamata Convention on Mercury is a global treaty that commits countries to control mercury supply, uses, emissions, and contaminated sites.
By highlighting legacy mercury stored in sediments and moved by groundwater, the Mar Menor study points to a challenge that rules cannot fix.
Cleaning up buried contamination and cutting hidden leaks will demand patient local work in addition to international rules on current emissions.
Climate stress and pollution
Climate change is already warming coastal waters and making extreme heat more common, trends that can push shallow lagoons toward oxygen loss.
Low oxygen conditions, combined with plentiful organic matter and available inorganic mercury, are exactly the recipe that helps microbes manufacture more methylmercury.
That means a future with hotter summers and intense storms could accelerate the transfer of old mercury stores into food webs.
For communities around Mar Menor, this possibility makes it more important to reduce mercury inputs and track how the lagoon responds over time.
Lessons from Mar Menor mercury
Local and regional authorities can begin by tightening controls on remaining mercury sources, including abandoned mine sites, industrial areas, and contaminated sediments.
Efforts to cut nutrient runoff from agriculture and wastewater will help by reducing algal growth and limiting oxygen conditions that favor methylmercury production.
Monitoring groundwater wells, coastal springs, and porewater in beach sediments can give managers a better sense of mercury entering from below.
Identifying coastal hotspots where groundwater emerges will help prioritize wetland restoration, land-use changes, and engineered barriers to slow pollutant flows.
The study is published in Environmental Science & Technology.
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