A new study reveals that centuries of intensive mining have turned the Mar Menor, Europe’s largest saltwater lagoon, into a massive reservoir for toxic metals. Researchers found that sediments on the lagoon floor are laden with hazardous materials, including lead, arsenic, and mercury, at levels that surpass established toxicity thresholds and concentrations found in similar coastal ecosystems worldwide.
The investigation, which reconstructs the history of contamination over the 20th and 21st centuries, identifies the primary source as the mining industry in the nearby Cartagena-La Unión mountain range. This industry was heavily active from the late 19th century through the mid-20th century. Even though the mines were shuttered in the 1990s, the study warns that the vast quantities of metals trapped in the sediment pose a latent but significant threat. Future environmental shifts, such as those driven by climate change or pollution-induced algae blooms, could remobilize these toxins and endanger the lagoon’s fragile biodiversity.
A Long Industrial Shadow
The historical heart of the contamination lies in the extensive mining operations of the Cartagena-La Unión district, a region with a history of mineral extraction dating back to Roman times. The most intensive phase of industrial mining occurred between the late 1800s and the mid-1900s, fundamentally altering the landscape and the adjacent marine environment. During this period, waste materials and refining byproducts containing high concentrations of heavy metals were discharged with little to no environmental controls. These discharges flowed directly into the Mar Menor lagoon through ephemeral streams and channels, leading to a rapid accumulation of pollutants in the marine sediments.
The study highlights that the peak of metal concentration in the lagoon’s sediment occurred in the mid-20th century. A ban on direct mining discharges was enacted in 1955, which led to a decrease in the concentration of some metals. However, the contamination did not cease. The vast, unprotected waste heaps and tailings left behind by the mining operations continued to be a source of pollution. Rainfall and surface runoff eroded these waste piles, carrying a persistent flow of toxic metals into the lagoon for decades after the mines officially closed in the 1990s. This ongoing, non-point source pollution has ensured that the lagoon remains a sink for these hazardous materials.
Cataloging the Contaminants
The research provides a stark inventory of the toxic metals now residing in the Mar Menor’s sediments. The study estimates that the lagoon holds thousands of tons of metals, including approximately 9,200 tons of lead (Pb), 10,000 tons of zinc (Zn), 450 tons of arsenic (As), 270 tons of copper (Cu), 1.6 tons of mercury (Hg), and 12 tons of silver (Ag). These figures paint a picture of historical pollution on an immense scale, transforming the lagoon bed into a hazardous waste repository.
The highest concentrations of these metals are found in the southern sector of the lagoon, the area closest to the mouths of the ephemeral streams that served as conduits for the mining waste. Analysis of sediment cores shows that concentrations for some metals reached alarming levels during the industrial peak, with lead measured as high as 3,400 milligrams per kilogram (mg/kg) and zinc at 3,700 mg/kg. Researchers note that current surface concentrations of some metals have decreased compared to the mid-20th century highs. However, levels of lead, arsenic, and zinc still frequently exceed the Probable Effect Level (PEL), a guideline indicating that adverse effects on bottom-dwelling organisms are likely to be observed.
The Modern Environmental Threat
While the direct discharge from active mining has ended, the lagoon faces persistent and evolving environmental threats. Metal flows continue to impact the Mar Menor, particularly during heavy rainfall events that wash over the old mining sites. Furthermore, new sources of pollution have emerged over the past several decades, contributing to the lagoon’s ecological stress. These include runoff from agricultural fertilizers and pesticides, wastewater from urban expansion, and chemicals from specialized boat paints used in marinas. These modern pollutants introduce their own complex mix of chemicals and nutrients, which can interact with the legacy metals buried in the sediment.
The study expresses significant concern that climate change could exacerbate the existing contamination. More extreme weather events, such as intense storms, could physically disturb the bottom sediments, a process known as resuspension. This churning of the lagoon floor could reintroduce buried metals into the water column, making them bioavailable to marine organisms. This would effectively reactivate the dormant threat, potentially triggering widespread toxic effects throughout the ecosystem that have so far been limited by the metals’ relative stability within the sediment.
A Latent Risk to Marine Life
Currently, the immense store of toxic metals appears to be largely contained within the sediments, limiting its direct impact on organisms living in the water column. Researchers have found a relatively low bioconcentration factor in most of the lagoon’s organisms, suggesting that the metals are not being effectively transferred up the food chain at present. However, this stability is not guaranteed to last.
According to Irene Alorda, a researcher at ICTA-UAB and the UAB Department of Physics, the situation could change dramatically. “The impact of these accumulations of metals, which at the moment are not affecting living organisms, could become greater in the future due to interactions with other pressures derived from human activities,” she stated. One of the most significant pressures is eutrophication—a condition caused by excess nutrient runoff (often from agriculture) that leads to severe algal blooms, or “anoxia” events, which deplete oxygen in the water.
These anoxic conditions can alter the chemical environment of the sediments, changing the form of the trapped metals and increasing their solubility and bioavailability. If this occurs, organisms living in and above the sediment, such as worms, mollusks, and crustaceans, would be exposed to much higher levels of toxicity. This could lead to impaired growth, reduced reproduction, and increased mortality in these foundational species. As these smaller organisms are consumed by fish, birds, and other predators, the toxic metals could then move up the food web, posing a significant risk to the entire ecosystem’s health and biodiversity.