Acid Drainage Mine (AMD)

The oxidation of the sulfides (1)  

Sulfides are rare minerals in the earth's crust. However, in certain geological situations abundance of these minerals increases, rising to become the majority. The high content of sulfides on Au, Ag, Fe, Cu, Zn and Pb or obtaining sulfuric acid have led to its exploitation and utilization by man throughout history. Pyrite (FeS 2) is the most common sulfide in nature, found in hydrothermal formations, igneous and sedimentary deposits.


Accumulation and massive sulphide detailed very fine grained, very characteristic of the Iberian Pyrite Belt. Mine Almagrera, Calanas.(Source: Amaya)

The sulfides are stable and highly insoluble under the reducing conditions prevailing in the subsurface. But exposure to atmospheric conditions these minerals destabilizing its structure through oxidation reactions. The oxidizing agent for this process, it may by the oxygen (O 2) or ferric iron (Fe 3 +) . Furthermore, depending on the absence or presence of microorganisms which catalyze reactions discussing weathering abiotic and biotic, respectively oxidation.

Upon contact with the atmosphere (abiotic oxidation of pyrite) (1)

In contact with air and water in the presence of the direct oxidation of pyrite occurs:

Ie occurs acidity and Fe sulfates and released, together with other accessory elements ( As , Cd , Co , Ni , Pb , etc.) to a greater or lesser extent, are part of pyrite. Oxygen availability is therefore essential for the oxidation of sulfides.

When circulating leachate surface ferrous iron is oxidized to ferric quickly, producing precipitates which give the water its characteristic reddish (source: Amaya)  

Example of a mass of sulfides exposed to atmospheric conditions artificially, by opening a trench exploitation. Soldier Mine (municipality of Aracena) (source: Amaya)

An endless cycle (biotic oxidation of pyrite) (1)

The abiotic oxidation of pyrite is a very slow process and therefore would not produce a serious environmental problem. However, in nature it is found that the rate of these reactions is much greater, due to the operation of certain chemolithotrophic bacteria (those that are "fed" to an inorganic substrate). These microorganisms, such asAcidithiobacillus ferrooxidans, Leptospirillum ferrooxidans, and Acidithiobacillus thiooxidans, have optimum growth conditions of pH acid catalyzing oxidation reactions and obtaining energy in this process.

Detail of bacteria in AMD development. The mining complex Viñas, Calanas. (Source: Amaya)  

Especially important is the increased rate of oxidation of Fe 2 + to Fe 3 + , which was the limiting factor of the whole process of abiotically. Acidithiobacillus ferrooxidans bacteria can accelerate this reaction up to 100,000 times. By increasing the concentration of Fe 3 + increased indirect oxidation of pyrite, which generates most produced Fe 2 + and again oxidized to Fe 3 + , so that these reactions are fed back in an endless cycle.

Natural and anthropogenic causes (1)

The oxidation of pyrite and other sulfide occurs naturally in the part of the sulphide deposits that outcrop at the surface, in this case being called acid rock drainage (ARD or initials in English Acid Rock Drainage).

Short overview of San Miguel. At the top and bottom is observed with a marked red gossan development (ARD product) that was in relation to the position of the water table as evidenced by its horizontal contact with the level immediately below that in the recumbent case consists of a stockwork of sulfide between volcanic rocks. (Source: Amaya)

On the other hand, rarely metals typically found in pure form in nature, but are part of the structure of the minerals.Mining activity begins with the extraction of the mineral, which must then be concentrated and purified for commercial use by processing or ore beneficiation and extractive metallurgy.

  Cut Mine Design, Tharsis and dumps in old industrial plant mine in Calanas Almagrera (source: Amaya)

All these processes greatly increase the processes of oxidation of pyrite, in this case being referred to acid mine drainage (AMD or Acid Mine Drainage English). Acid mine drainage produces the same result as the acid rock drainage: acidity, sulphates and toxic metals and metalloids, but in much larger quantities due to the creation of large extractive waste dumps, creating miles of tunnels and galleries by atmospheric oxygen entering, operating methods and treatments used historically (as the weavers), digging large open pit short, smelters and tailings ponds from hydrometallurgy activity. (3).

 Pictures of leachate that arise in Mina Esperanza (tunnels and galleries), with a high content of Fe 2 + (left) and the discharge from the Odiel river Mina Poderosa (mainly short and tailings), with more oxidizing conditions and high proportion of Fe 3 + (source: Amaya)

An environment that does not help (natural attenuation mechanisms) (1)

The high acidity of mining leachate attack causes the minerals in the gangue (material that is discarded from a mineral deposit) or materials that constitute the course of rivers affected by AMD. The dissolution of these minerals consume acid, in the event that it in the middle carbonate materials protons released during the oxidation of sulphides, becoming a mechanism for natural attenuation of pollution are neutralized. The abundance of these minerals in the vicinity of the sulfides and their ability to neutralize the resulting leachate determine whether acidic or neutral conditions presented. For example Linares area where there has also been an important mining sulfides (mainly galena) acidification no problems due to the existence of a carbonate gangue. However, the neutralization capacity of the materials of the Iberian Pyrite Belt is very low.

Partial view of the mining complex of the Vines, where the removal of waste was carried out (as part of the restoration work of this space degraded by metal mining, conducted by Amaya) has exposed substrate materials and You can see that not only low neutralizing capacity but there is still a high potential for the generation of AMD. (Source: Amaya)

Similarly, in the basins of the rivers Tinto and Odiel , there is little carbonate materials and alkalinity of water flowing in streams and rivers is very low, so it has very little capacity to neutralize the acidity generated by the process oxidation of sulphides.

Environmental impact (2 and 3)

Acid mine (English AMD, Acid Mine Drainage) drainage is the appearance of water with pH typically between 2 and 4 and high concentrations of toxic metals as a result of subaerial exposure during metal sulfide mining.

In the photograph the canyon stream arriving at Tintillo Escorial, is observed as the contact with acidic water margin vegetation and macrophytes (source: disappears

AMD processes are one of the most serious types of water pollution, by its nature, extent and difficulty of resolution, as well as the economic costs of traditional remediation. The rivers affected by this pollution are characterized by their acidity, as well as the high content of sulfates and heavy metals from the water and the metal content of the sediments. Damage range from sublethal alterations to some individuals of affected ecosystems in the cases of very weak pollution, with associated problems of bioaccumulation and biomagnification until the disappearance of the river fauna and loss of water resources to become unusable the water for human, agricultural or industrial use.

Impact on large hydraulic works (1 and 4)

An important aspect of pollution from acid mine drainage is the condition on large hydraulic works. So with the acidic discharges from São Domingos mine (Portugal) affecting dam Chanza, or from the Blacksmith and Lagunazo mines reaching the reservoir Andévalo, these being the largest reserves of potable water for supply Huelva. Another important problem is the bottom sediments represent these reservoirs which act as sinks of pollution due to precipitation and coprecipitation / adsorption of toxic elements.

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Orthophotography in which the acidic discharge from São Domingos mine (Portugal) to Chanza reservoir (source: google earth) is observed

In addition, the acid mine drainage affects not only current major hydraulic works in the FPI but mortgage future. In this sense, the estimates made ​​on the water quality of the reservoir future Alcolea, emphasizes that this water could not be used directly in agriculture or for any other purpose.

Alcolea Bridge in the lower reaches of the river Odiel, the dam of the reservoir Alcolea future will be located in the vicinity (source:

Impact on the estuary of the Ria de Huelva (1)

Similarly, the environmental impact on the estuary of the Ria de Huelva caused by pollutants transported by rivers Tinto and Odiel until it is very important. Thus the Odiel River is the main source of metals and sulfates to the Ria de Huelva, with over 80% of the contribution of manganese and nickel, and between 70 and 80% of the contribution of sulfates, aluminum, cobalt and zinc. Red River just beyond the Odiel in the toxic load of lead and especially iron (69% of total).

Most of the toxic elements precipitate when acid waters of the rivers Tinto and Odiel mix in the estuary of the Ria de Huelva with seawater. This is an improvement to water quality, however these toxic elements are retained in sediments producing pollution. Depending on the environmental conditions ( pH , salinity, redox potential, etc..) some of these elements are bioavailable and pose a major environmental problem to enter the food chain.

Union of the rivers Tinto and Odiel estuary in Huelva estuary (source:

Contribution to the seas and oceans (1)

However, some toxic elements more mobile in these conditions, such as zinc, reaching the Gulf of Cádiz in significant concentrations causing contamination of coastal sediments and even penetrate at certain times in the Mediterranean.

The magnitude of mining pollution of the rivers Tinto and Odiel is clearly manifested if we compare these figures with global flows all the world's rivers carry to the ocean (GESAMP, 1987), the quantities transported by the rivers Tinto and Odiel account for approximately 15% copper and 47% zinc. That is, these two small rivers in the province of Huelva carry almost half the zinc contributed by all the world's rivers to the seas and oceans.


(1) Miguel-Sarmiento, A., Nieto-Líñán, J.M., Olías-Álvarez, M., Ruiz-Cánovas, C. 2010. La Contaminación Minera de los Ríos Tinto y Odiel.

Disponible en: Junta de Andalucía

(2) Impacto y Riesgo Ambiental de las Actividades Minero-Metalúrgicas.

(3) J.A. Grande, T. Gómez, M.L. de la Torre, T. Valente, C. Barranco. V. Domínguez, J. Graiño. 2010. Drenaje ácido de mina y metales disueltos: del impacto ambiental al patrimonio minero. Aplicación al caso de mina herrerías en la faja pirítica ibérica. Patrimonio geológico y minero. Una apuesta por el desarrollo sostenible, 87-98.

(4) Canovas, C.R., Galván, L., Nieto, J.M., Olías, M., Sarmiento, A.M. 2007. Sobre la calidad del agua del futuro embalse de Alcolea (Cuenca del río Odiel, Huelva). GEOGACETA, 42: 59-62

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