2023-06-21 01:38:41
During the 1980s, the Massachusetts Woods Hole Oceanographic Institution organized an expedition to the Gulf of Cádiz, on the southwest coast of the Iberian Peninsula. Those researchers were the first to discover that the waters they were much more enriched in metals, mainly zinc, cadmium, arsenic and copper, than other coastal waters of the world.
Initially, after analyzing the main Iberian rivers that drain this part of the region (essentially the Guadalquivir river), the scientists ruled out the fluvial origin of the pollutants and explained this enrichment by the rise of water from deep areas of the ocean and the sequestration of metals in the Gulf of Cádiz. But they were wrong.
Researchers from a group from the University of Montpellier later verified that the metals came from the Red and Odiel rivers. The finding was confirmed a year later by the same authors who detected the anomaly in the ocean.
Today we know that the pollution plume circulates and dominates the chemical composition of the Atlantic entrance to the Mediterranean Sea through the Strait of Gibraltar.
Map of the Gulf of Cádiz, Strait of Gibraltar and the Mediterranean Sea showing the plume of pollution from the Tinto and Odiel rivers in the ocean and detail of the Ría de Huelva estuary. Author provided
two toxic rivers
The mining residues rich in sulphides, the result of some 5,000 years of mining activity in the Iberian pyritic belt they produce a leachate known as acid mine drainage, highly acidic water loaded with metals that contaminates the Tinto and Odiel rivers.
The union of the mouths of both rivers defines an estuarine system known as the Huelva estuary. The estuary associated with each river joins in a common channel, known as holy father channelwhich goes from the convergence of both estuaries to the Atlantic Ocean.
The transfer of acidity and toxic metals to the Huelva estuary has been the target of numerous research. Pollution levels are so extreme that both rivers and the estuary are considered one of the most polluted aquatic systems in the world.
Changes between river and sea water
The Huelva estuary represents a transition medium between rivers and the ocean, where the mixture of continental waters affected by acid mine drainage and marine waters with greater alkalinity occurs, which triggers the neutralization of acidity and a series of geochemical processes that determine which pollutants reach the Gulf of Cádiz and in what quantity.
in a recent studywe have evaluated the behavior of metals during the mixing between the Tinto and Odiel rivers and the seawater in the estuary by collecting samples by boat through the Huelva estuary.
Fluvial waters discharge various pollutants of mining originsome majorities, such as the hierrohe aluminumhe copper and the zincand other minorities, such as the arseniche manganesehe nickel and the cobalt.
Location of the sampling points in the Huelva estuary: O refers to the points in the Odiel river estuary, TR to the points in the Tinto river estuary and C to the points in the common channel, which leads to the ocean Atlantic. Author provided
What about pollutants in the estuary?
During the transit through the estuary to reach the Atlantic Ocean, the different pollutants behave in different ways.
Iron, aluminum and, to a lesser extent, copper are removed from water by mineral precipitation during acid drainage neutralization. That is, they tend to form part of the particulate matter that settles in the estuary.
The behavior of iron is remarkable, since it is the main metal of mining origin that both rivers discharge in greater concentration into the estuary. The total retention of iron during the mixing of the waters is produced by the precipitation of schwertmannite, an ocher-colored mineral with an iron composition.
The iron concentration is so high in the Tinto River that the mixing zone of its estuary is stained with ocher colors due to the abundant presence of this mineral in suspension.
Mixing zone in the Tinto River estuary. High amounts of particulate matter in suspension of ocher colors are observed, which correspond to the precipitation of schwertmannite. Author provided
For his part, zinc, cadmium, manganese, nickel and cobalt are mainly kept dissolved in water throughout the transit through the estuary, thus reaching the Atlantic Ocean.
Arsenic is originally retained on the sword manita which is generated by iron precipitation and is later released back into the water, also reaching the ocean.
The surface of the schwertmannite during its formation at acidic pH is positively charged, which means that it has a high potential for attracting negatively charged ions (anions) such as arsenic, which is why it retains it.
However, when the pH reaches values close to neutrality in the ocean, the polarity of the schwertmannite surface reverses, becoming negatively charged, which causes a repulsion of previously retained anions, causing their release into the ocean. water.
This hold-release behavior also affects other elements such as chromium, vanadium, molybdenum, and antimony.
Contribution to the oceans
When comparing the pollutant load values of the Tinto and Odiel rivers during a long study period (1995 to 2006) with data on the transfer of dissolved metals from all the world’s rivers to the seas and oceans, it can be seen that the amounts contributed by these rivers during the study period represent very high percentages.
Thus, two small rivers in the province of Huelva transport 14% of the copper and 47% of the dissolved zinc that reaches all the world’s oceans.
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