Details are reported in a study led by the ICM-CSIC, which reveals the existence of a polluting underground flow that was previously unknown but could have significant environmental implications.
Researchers from the Institut de Ciències del Mar (ICM-CSIC) have identified a new and previously overlooked source of mercury in the Mar Menor (Region of Murcia, Spain): groundwater flowing beneath the surface, which releases this metal and its most toxic form, methylmercury, at levels comparable to atmospheric deposition and far higher than those from the Albujón River, the only permanent stream that flows into the lagoon.
The finding, published in the journal Environmental Science & Technology, shows that this submarine groundwater discharge not only transports mercury but also creates favorable conditions for the formation of methylmercury upon reaching the coastal zone — a compound that accumulates and biomagnifies in fish and can affect both ecosystem and human health.
“What happens beneath the surface can be as decisive as what we see,” explains Céline Lavergne, researcher at ICM-CSIC and lead author of the study. “Groundwater flow acts as an invisible connection that transports pollutants, nutrients, and microorganisms from land to coastal ecosystems. Our results show that, to protect the Mar Menor, we must also look underground.”
An Invisible but Significant Source
For the first time, the research team quantified the fluxes of mercury and methylmercury entering the Mar Menor through groundwater. The results are striking: this pathway contributes around 1 kilogram of mercury per year — an amount equivalent to that delivered by the atmosphere and about 70 times greater than that from the Albujón River.
They also discovered that much of this mercury was emitted in the past. This is what scientists call “legacy mercury”: residues accumulated over decades in sediments due to mining and agricultural activities in the area, now being released back into the water through groundwater flow.
“A significant portion of the mercury we measured originates from past activities. It’s an environmental legacy that remains active decades later,” notes Andrea G. Bravo, co-author of the study and researcher at ICM-CSIC. “This demonstrates that even if current emissions are reduced, pollutants stored in sediments can continue to be released for a long time.”
The Formation of Methylmercury
The study reveals that the mixing of groundwater and seawater in the coastal areas of the lagoon creates ideal conditions for mercury to transform into methylmercury. These areas show low oxygen levels and high concentrations of organic matter, which promote the activity of microorganisms capable of carrying out this conversion.
“We detected high concentrations of methylmercury in nearshore waters,” Lavergne explains. “This means that areas where groundwater discharges into the sea can be true hotspots for methylmercury production.”
Although current mercury levels in Mar Menor waters are not alarming, the team warns that the process could intensify with rising temperatures and oxygen depletion — effects linked to climate change that increase the vulnerability of Mediterranean coastal lagoons. Higher methylmercury concentrations in the Mar Menor could lead to greater accumulation of this contaminant in food webs.
Rethinking Pollution Pathways
The ICM-CSIC study improves understanding of how mercury reaches the Mar Menor and similar ecosystems. Until now, conservation strategies have focused on visible sources — the atmosphere and rivers — but the research demonstrates that submarine groundwater discharge (SGD) is also a key route for pollutant input.
“Our results expand current knowledge and open a new line of research showing that mercury reach the sea through more pathways than previously considered, such as air or rivers,” Bravo explains. “Groundwater, in addition to serving as a conduit for contaminants between land and sea, can also be a site of mercury transformation— especially methylmercury — depending on its composition.”