The Rio Tinto has always been red, they say, so I begin to wonder about the origins. Research brings a strange result: none of the Roman (Pliny, Avenius) or Greek (Diodorus Siculus, Strabo) sources, even those who wrote extensively about the area and named the river, the mines and their products, mention its colour. After the Romans withdrew, there seems to have been a significant drop in mining activity until the sixteenth century. It is not until a report to King Philip II by Diego Delgado in 1556 relating to potential mining opportunities that something solid emerges. He writes: We also visited another cave which was full of water and from under which sprang a river said to be the Rio Tinto. The reason why it is known as the red river is because it springs from vitriol, which in another part is known as copperas , used for ink. The geology is complicated. As I understand it, the main ore is iron pyrite, which explains the colour. Within the ore however, there is also copper, lead, tin and gold. The pyrites are sulphides created at the bottom of an ancient sea that had no oxygen. The exposure to oxygen in the presence of water, with the help of bacteria, produces sulphuric acid and the metals ... you can see the chemical history: the yellow sulphur, the red iron deposits on the banks of the Rio Tinto. The Odiel has a different acidity level due to more alkali tributaries which result in a different reaction. At the higher pH the precipitate mineralogy is dominated by schwertmannite (a pale yellow-orange iron ohyhdroxysulfate) rather than the deeper red geothite hematite that colours Rio
Tinto. Odiel’s metal load ids proportionally richer in copper, zinc, aluminum and magnesium relative to Rio Tinto’s iron. Perhaps Delgado was not that familiar with chemistry as we know it, however he did mention that nothing can live in the river, which he tested by throwing a frog into it. It died. Most of the writers who mention it are concerned with the wealth the mines generated, rather than the physical properties of the stream itself. Modern, more economic methods of open pit mining, rather than the long history of shafts and galleries, use water in processing, affecting its colour. Crushing ore allows water to come in contact with a much larger surface area of pyrite (iron sulphide), initiating a process of oxidisation driven largely by acidophilic bacteria such as Acidithiobacillus . The slurry is held in huge tailing ponds behind dams, affecting surrounding ground water which percolates into the river more directly, deepening its colour and accentuating its acidic properties and heavy metal content (it is, in fact, dilute sulphuric acid coloured by iron). All of these rivers, the Tinto, the Odiel and the Guadiana exhibit these characteristics to a greater or lesser degree, part of the legacy of both defunct and continuing mining. How much is natural and how much is caused by mining is a contentious issue— it should not be though that I’m only describing the past and the physical legacy of that past. The Minas Los Frailes catastrophe in 1998 is a case in point. The retaining dam of a tailings pond ruptured sending six million cubic metres of toxic sludge (containing cadmium, nickel, zinc, arsenic, mercury and lead) into the Agrio River, a tributary of the Guadiamar River
Tim Sharp
The Rio Tinto: Iron-tinted acidic water, near the Tharsis mine.
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on site review 49 :: shorelines
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