Toxic Underground River Discovered Leaking Mercury into Spain's Mar Menor Lagoon

New Source of Mercury Pollution Identified in Mar Menor

A team of scientists has uncovered a significant and previously undetected source of mercury pollution flowing into the Mar Menor lagoon in southeastern Spain. An underground river, part of a network of submarine groundwater discharge, is estimated to be leaking approximately 1 kilogram (2.2 pounds) of mercury into the lagoon annually. This discovery was recently reported in November and December 2025.

The research, led by Dr. Céline Lavergne of the Institute of Marine Sciences (ICM-CSIC) in Barcelona, Spain, highlights a critical, overlooked pathway for contaminants. Co-author Andrea G. Bravo also contributed to the study.

Legacy Contamination and Methylmercury Formation

The mercury originates primarily from 'legacy mercury' – contamination left behind in soils and sediments from decades of mining and farming activities in the region. This historical pollution, stored underground, is now being mobilized and transported into the lagoon via groundwater flow.

A significant concern is that the mixing of this mercury-laden groundwater with the lagoon's salty water near the shore creates ideal conditions for the formation of methylmercury. Methylmercury is the most toxic and bioaccumulative form of the metal, meaning it can build up in aquatic food chains. Researchers found high concentrations of methylmercury in nearshore waters, indicating these areas are 'hotspots' for its production.

Environmental and Health Implications

While current mercury levels in Mar Menor's water and fish are not yet considered 'alarming,' scientists warn that the continuous influx of methylmercury could change this picture. Methylmercury can damage the brain and nervous system, particularly during fetal development and early childhood, according to the World Health Organization (WHO).

The Mar Menor, Europe's largest saltwater lagoon, is already under immense environmental stress. It has suffered repeated algal blooms and murky water due to extensive nutrient runoff, primarily nitrates, from agricultural practices. These nutrient pulses have led to significant ecological disruptions, including low-oxygen events that have caused widespread seagrass and fish mortality in recent years.

The discovery of this hidden mercury source adds another layer of complexity to the lagoon's ecological challenges. Scientists also caution that climate change, leading to warmer coastal waters and decreased oxygen levels, could accelerate the conversion of inorganic mercury into methylmercury, further intensifying the problem.

Broader Significance

This finding underscores the critical role of underground water flows in coastal contamination and broadens the understanding of mercury pollution pathways beyond traditional riverine and atmospheric sources. The study suggests that many other coastal lagoons worldwide could be experiencing similar, unquantified mercury inputs from subterranean sources. Addressing this 'legacy mercury' will require patient local efforts in addition to international regulations on current emissions.