Proceedings of the International Conference on Mining and Environment, Metals & Energy Recovery

Size: px

Start display at page:

Download "Proceedings of the International Conference on Mining and Environment, Metals & Energy Recovery"

Constance Hall
6 years ago
Views:

1 The Tintillo acidic river (Rio Tinto mines, Huelva, Spain): an example of extreme environmental impact of pyritic mine wastes on the environment or an exceptional site to study acid-sulphate mine drainage systems? Javier Sánchez España*, Enrique López Pamo, Esther Santofimia Pastor, Osvaldo Aduvire Pataca, Jesús Reyes Andrés, Juan Antonio Martín Rubí Área de Ingeniería Geoambiental Dirección de Recursos Minerales y Geoambiente Instituto Geológico y Minero de España (IGME) Rios Rosas, 23, 28003, MADRID, SPAIN Paper presented to the International Conference on Mining and the Environment, Metals and Energy Recovery, SECURING THE FUTURE, June 27-July 1, Skellefteä, Sweden. Suggested reference for this paper is: Sánchez España, F.J., López Pamo, E., Santofimia, E., Aduvire, O., Reyes, J., Martín Rubí, J.A. (2005). The Tintillo acidic river (Rio Tinto mines, Huelva, Spain): an example of extreme environmental impact of pyritic mine wastes on the environment or an exceptional site to study acid-sulphate mine drainage systems?. Proceedings Volume of the Securing the Future International Conference on Mining, Metals and the Environment,, 27 June-1 July 2005, Vol I, pp Sánchez España et al., 2005, June 27-July 1, Volume I 278

2 The Tintillo acidic river (Rio Tinto mines, Huelva, Spain): an example of extreme environmental impact of pyritic mine wastes on the environment or an exceptional site to study acid-sulphate mine drainage systems? Javier Sánchez España*, Enrique López Pamo, Esther Santofimia Pastor, Osvaldo Aduvire Pataca, Jesús Reyes Andrés, Juan Antonio Martín Rubí Área de Ingeniería Geoambiental Dirección de Recursos Minerales y Geoambiente Instituto Geológico y Minero de España (IGME) Rios Rosas, 23, 28003, MADRID, SPAIN * Corresponding author: Dr. Javier Sánchez España (j.sanchez@igme.es) Abstract The Tintillo acid river represents an exceptional example of environmental impact of historically intensive metal mining on the water quality of an hydrological basin. This river is exclusively formed by leachates of acidic water (ph ) emerging from the base of huge waste-rock piles and tailings ponds situated in the surroundings of the Corta Atalaya open pit (Rio Tinto mines, Huelva, Spain). The enormous accumulation of these pyrite-bearing mine wastes which took place during the 19 th and 20 th centuries was conducted without any kind of environmental perspective. The spoil heaps and waste piles do not have any cover or liner systems, peripheral drainage channels or revegetated sites. Moreover, all these wastes were originally situated at the head of the valleys adjacent to the mine operations, so that they actually constitute the source areas of several springs which form the Tintillo river. Thus, the waters of this river present, from the same discharge points, extreme physical and chemical conditions (e.g., acidity above 15 g/l CaCO 3 eq., >2 g/l of dissolved Fe(II), >2 g/l of dissolved Al, >30 g/l of dissolved sulphate, in addition to Mn, Cu, Zn, Cd, Co, Ni, etc.) which preclude the presence of most forms of aquatic life. However, this river offers a unique opportunity to study the geochemical, mineralogical and microbiological features of typical acid-sulphate aqueous systems related to mine drainage, including the adaptation of extremophile microorganisms (eukaryotic and prokaryotic) to these extreme environmental conditions. The existing algal and bacterial community shows a strong capacity not only to determine the downstream geochemical evolution of the acidic waters (e.g., oxidizing the dissolved iron, reducing the sulphate and metal loads by ochre precipitation), but also to form unique bioconstructions (Festromatolites) which can only be observed in these acidic environments. After emerging in the mining area, the Tintillo river flows downstream during 10 km and is finally discharged into the Odiel river. At this confluence, several geochemical and mineralogical processes typical of acid mine watercircumneutral water interaction take place. After this interaction, the Odiel river becomes severely polluted with large amounts of acidity and dissolved metals transfered by the Tintillo river. The ph of the Odiel waters drops sharply from circumneutral to around 3, and the river remains acid until it reaches the Atlantic Ocean (70 km downstream) in the coast of Huelva. Keywords: Acid mine waters, iron oxidation, trace metal removal, Tintillo river, Iberian Pyrite Belt. 1. Location and description of the area The Tintillo river (10 km in length) drains an area of about 57 km 2 in the North of the province of Huelva (SW Spain; Figure 1). This river is formed by several springs of acid sulphate waters (ASW) which emanate from the base of a large, sulphide-bearing Sánchez España et al., 2005, June 27-July 1, Volume I 278

3 waste-rock pile ( 1 km long, 40 m high) situated in the surroundings of Corta Atalaya, a vaste open pit (1,200 m long x 900 m wide x 365 m deep) exploited from 1907 to 1991 by the company Minas de Rio Tinto. Despite the Tintillo river recieves three additional inputs of ASW from the Barrizal (ph 2.6), Gangosa (ph 2.6) and Escorial (ph 4.5) creeks (all emerging from waste piles and/or spoil heaps of the Rio Tinto mine district), its waters show a ph between 2.6 and 2.9, which remains fairly constant along the entire stream course from the mine area to the confluence with the Odiel river (Figure 2). The acid mine waters that form the Gangosa and Barrizal creeks also drain large waste piles, whereas the Escorial creek is mainly formed by the outflow of a big mine dam (Embalse del Cobre) situated near the Rio Tinto tailings dump (4 km 2 ; Figure 3). The geologic substrate of the area is dominated by volcanic-silceous rocks (rhyolitic to dacitic tuffs, breccias and lava flows, tuffites), greywakes and shales, with an absolute lack of carbonate or alkaline materials (Sánchez-España, 2000). Therefore, the capacity to neutralize the acidity of the acidic streams is very limited. 2. Chemical composition of the acidic waters The water chemistry of the Tintillo river has been thoroughly studied during an entire hydrological year (March 2003-March 2004; Table 1). The typical composition of these acidic waters at the source point (main spring, sample station T-1 in Tables 1 and 2) during the summer may include, as an example, 30 g/l SO = 4, 2 g/l Fe(II), 2 g/l Al, 2.8 g/l Mg, 400 mg/l Mn, 600 mg/l Zn, 200 mg/l Cu and 46 mg/l Co, in addition to 8,500 µg/l Cd, 5,300 µg/l Ni or 1,100 µg/l U. This composition, however, is rather variable temporally (1) and spatially (2). These chemical variations are caused by (1) hydrological changes provoked by the alternations of long and extremely dry summers (higher concentrations of acidity, sulphate and metals) with sporadic rainfall episodes (which dilute both sulphate and metal concentrations), and (2) the formation of Feoxyhydroxysulphate compounds (mostly schwertmannite), which represents a form of natural attenuation that reduces the sulphate and iron loads and, by sorption, a noticeable decrease in trace elements such as As, Co or Cd. Sánchez España et al., 2005, June 27-July 1, Volume I 279

4 Figure 2 Figure 1. Location of the study area (see Figure 2) within the Odiel river basin (Huelva, Spain), with the location of some of the most important mines of the Iberian Pyrite Belt. Dissolved Fe(II) is rapidly oxidized to Fe(III) at rates of between 5.5x10-6 and 4x10-7 mol L -1 s -1, which are characteristic of bacterially catalyzed oxidation and provoke the apeareance of both dissolved and particulate Fe(III) which turns the acidic waters into a deep red ( tinto ) colour. The ph of the water, close to 3 throughout most of the year, favours the hydrolysis and precipitation of Fe(III) ions, which takes place usually in the Sánchez España et al., 2005, June 27-July 1, Volume I 280

form of very fine-grained schwertmannite (with traces of jarosite). These minerals are mineralogically meta-stable and tend to be transformed to goethite. Odiel river (ph 7.5) Escorial creek (ph 4.

5 form of very fine-grained schwertmannite (with traces of jarosite). These minerals are mineralogically meta-stable and tend to be transformed to goethite. Odiel river (ph 7.5) Escorial creek (ph 4.5) Tailings ponds Tintillo river (ph 2.7) Gangosa creek (ph 2.6) Waste piles Tintillo river (ph 2.7) Figure 2. Aereal photograph showing the hydrological framework of the Tintillo river basin (Rio Tinto mines, Huelva, Spain) with the location of sampling points (circles). 3. Redox chemistry The acidic leachates which feed the Tintillo river are virtually anoxic in origin due to a strong oxygen demand for the bacterially mediated oxidation of dissolved Fe(II). The equilibration with atmospheric oxygen causes a relative increase in the oxygen content to around 4-5 mg/l O 2 (50% sat.), although this O 2 subsaturation is maintained during 4-5 kms due to consumption by Fe(II) oxidation. The Eh value, which is basically governed by the Fe(II) to Fe(III) oxidation rate, varies from mv (typical of Fe(II)-rich waters) to values of around 500 mv (characteristic of more oxidized aqueous environments). When the initial Fe(II) is mostly oxidized (at a distance of about 6-7 km from the discharge points), the aqueous acidic solutions become O 2 -saturated and the Eh value is estabilized. Sánchez España et al., 2005, June 27-July 1, Volume I 281

A B C D E F Figure 3. Several views of the Tintillo river basin.

usually colonized by blooms of deep green algal biofilms which are specially adapted to these acidic environments.

laminated, organosedimentary, iron-formations.

(±jarosite±goethite) with interlayered algal biofilms and plant debris.

5 which is buffered by the hydrolysis and precipitation of dissolved Al (in the form of amorphous Al gels or basaluminite).

6 A B C D E F Figure 3. Several views of the Tintillo river basin. (A) sulphide-bearing waste-rock pile near Corta Atalaya (Rio Tinto), where several leachates of acid mine water emanate and feed the acidic river. (B) Detail of (A) showing one of the discharge points of AMD recognized at the waste-rock pile; these discharge points are usually colonized by blooms of deep green algal biofilms which are specially adapted to these acidic environments. (C) the Tintillo river, between sampling stations T-2 and T-3, showing the characteristic red colour and the spectacular laminated, organosedimentary, iron-formations. (D) detail of (C) showing the laminated nature of these bioconstructions, which are basically composed of schwertmannite (±jarosite±goethite) with interlayered algal biofilms and plant debris. (E) The Escorial creek shows an excellent example of AMD with a ph of around 4.5 which is buffered by the hydrolysis and precipitation of dissolved Al (in the form of amorphous Al gels or basaluminite). (F) Confluence of the Tintillo river (left) and the Odiel river (right), where a strong gradient in the water ph causes the formation of several bands of precipitating iron (ph 3, red) and Al (ph 4.5, white). After recieving the acidity and metal loads of the Tintillo river, the Odiel becomes a severely polluted river that remains acidic (ph 3±0.5) for the rest of its course to the Atlantic Ocean. Sánchez España et al., 2005, June 27-July 1, Volume I 282

7 However, the iron speciation can be also modified by redox processes other than oxidation. Specifically, the reduction of some amounts (around 6-8%) of previously formed Fe(III) to Fe(II) in the final course of the Tintillo river has been detected and quantified. This Fe(II) reduction has been observed to be highly dependent of light intensity, although there is no definitive evidence to ascertain whether this process is a photoreduction or if, on the other hand, it consists in a bacterially induced reduction. 4. The chemical buffering of Fe and Al The hydrolysis of Fe 3+ and Al 3+ ions takes place at well defined ph values (around 3 and 4.5, respectively) and provokes the formation of colloidal precipitates (schwertmannite and amorphous Al compounds; Figure 3E-F) which are transported downstream as suspended matter or deposited on the stream bedrock (Sánchez-España et al., 2004). Additionally, both hydrolysis reactions release free H + ions which decrease the ph, and thus represent a strong chemical buffering of the aqueous solutions, which tend to reach a steady-state balanced by the competing effects of H + consumption (for example, by oxidation of Fe(II), reaction with aluminosilicates or dilution) and H + gain (resulting from the hydrolysis reactions). 5. Organosedimentary, laminated iron formations (Fe-sulphate-stromatolites) The laminated terrace iron deposits formed across the main channel of the Tintillo stream course are, without doubt, among the most spectacular features that can be observed in the area (Figure 3C-D). These finely laminated formations (so-called Fe-sulphate-stromatolites ) are characteristic of many other AMD systems studied in the Iberian Pyrite Belt and world wide (e.g., Leblanc et al., 1996; Brake et al., 2002), although they show an exceptional development in the Tintillo river. They are mainly composed of a mixture of ochre colloids (mostly schwertmannite), biofilms composed of green algal mats (probably Euglena mutabilis) and plant debris. These iron-sulphate rich deposits are considered to be organosedimentary structures in the sense that they are formed by the sucessive alternation of biologically derived laminae and schwertmannite-rich layers. This lamination is considered to record hydrological (dry-rainy) cycles with differential rates of both algal growth and schwertmannite precipitation/deposition. Sánchez España et al., 2005, June 27-July 1, Volume I 283

8 Table 1. Spatial evolution of the water chemistry in the Tintillo acidic river (June 2003) Sampling point Distance 1 Q* SO 4 Cl Na K Ca Al Fe Mg Mn Cu Zn As Cd Co Cr Ni Pb Th U Units m L/s g/l mg/l µg/l T ,107 1,824 2, ,546 45, < ,107 T ,135 1,847 2, ,075 45, ,142 T-3 1, ,032 1,689 2, ,369 7,197 38, ,093 T-4 1, ,453 1,135 2, ,970 8, T-5 3, , , ,719 7, T-6 3, , , T-7 5, , , T-9 10, , , T-10 12, , * Water flow. 1 Distance from main discharge point. Table 2. Spatial evolution of the water chemistry in the Tintillo acidic river (March 2004) Sampling point Distance 1 Q* SO 4 Cl Na K Ca Al Fe Mg Mn Cu Zn As Cd Co Cr Ni Pb Th U Units m L/s g/l mg/l µg/l T ,990 1,751 2, , ,210 < T ,577 1,762 2, , , T-3 1, ,492 1,567 2, , , T-4 1, ,223 1,248 2, , , T-5 3, , , , , T-6 3, , , , T-7 5, T-8 6, , , T-9 10, , , T-10 12,000 2, * Water flow. 1 Distance from main discharge point. Sánchez España et al., 2005, June 27-July 1, Volume I 284

9 Concentration (mg/l) 3,500 3,000 2,500 2,000 1,500 1, Mg Al Fe A Concentration (mg/l) ,000 4,000 6,000 8,000 10,000 12,000 Zn Mn Ca Cu Distance (m) 0 2,000 4,000 6,000 8,000 10,000 12,000 Distance (m) B Concentration (g/l) TDS SO 4 C 0 0 2,000 4,000 6,000 8,000 10,000 12,000 Distance (m) Figure 4. Downstream variation of metal and sulphate concentrations (and total dissolved solids, TDS) from T-1 to T-10 (June 2003; Table 1). The final drop in sulphate and metal concentration is caused by an abrupt ph increase (from 2.7 to 3.3; Fig. 3F) in the confluence of the Tintillo and Odiel rivers. 6. Retention of metals and natural attenuation Many major and trace metals (including Cu, Zn, Mn, Cd, Co, Ni and Pb) progressively decrease in concentration downstream (Figure 4), as a consequence of two general mechanisms, namely: (1) dilution by inputs of other AMD courses (Barrizal, Gangosa, Escorial) of lesser metallic content, and (2) sorption onto the iron colloids formed in the water column and stream substrate. Additionally, the algal biofilms could also be responsible for some metal retention in the Fe-sulphate stromatolites, although this has not been demonstrated for the moment. These processes represent a form of natural attenuation Sánchez España et al., 2005, June 27-July 1, Volume I 285

10 which is especially important for the most toxic elements (As, Cd, Co and Cr are significantly reduced in concentration during the 10 km-long course of the Tintillo river). This metal attenuation detected in the water chemistry is well correlated with the sediment chemistry of ochre deposits, which always show high trace metal contents (especially As) A Iron phases Aluminium phases ph=2.7 Schw Hem 10 5 K-Jar Na-Jar Goet SI Distance (km) Ferr Gib Basal 10 5 Tintillo river B SI 0-5 Escorial creek mixing Basal Jur ph=4.5 ph=2.7 Gib Alun Distance (km) Figure 5. Geochemical profiles showing the downstream evolution of the saturation index (SI) of several Fe and Al minerals along (A) the entire course of the Tintillo acidic river (ph 2.7), and (B) the Escorial aluminous creek (ph 4.5) and its confluence with the Tintillo river at 8 km from the source (see Fig. 2). Sánchez España et al., 2005, June 27-July 1, Volume I 286

11 7. Geochemical modelling of mineral solubility Geochemical calculations of solubility performed with the PHREEQC 2.7 software predict the oversaturation of the acidic Tintillo river (ph 2.7) with respect to several Fe compounds (schwertmannite, jarosite, goethite) which are known to control the Fe activity in AMD systems (Nordstrom and Alpers, 1999; Figure 5A). On the other hand, the PHREEQC code predicts the undersaturation (and thus, the tendency for dissolution) with respect to ferrihydrite and Al phases such as gibbsite, alunite or basaluminite. In the case of the Escorial creek (ph 4.5), the geochemical modelling suggests saturation of the aqueous solution with respect to Al phases such as basaluminite, gibbsite and alunite until the confluence with the Tintillo river (ph 2.7), where these phases are rapidly redissolved (Fig. 5B). 8. Concluding remarks The Tintillo watershed represents a classic example of the enormous environmental impact that an intensive metal mining can inflict to a natural landscape. At present, this acidic river is exclusively fed by leachates of acid mine water emerging from waste piles and tailings ponds, and this has led to an extremely acidic and polluted fluvial system in which the most common forms of aquatic life hardly occur. On the other hand, this area constitutes a unique opportunity for hydrogeochemists and microbiologists to study macro- and micro-scale processes typical of acid-sulphate mine drainage systems, including the interaction between the aqueous extremophile biota (bacteria, algae and funghi) and the environment. 9. References Brake, S.S., Hasiotis, S.T., Dannelly, H.K., Connors, K.A., Eukaryotic stromatolite builders in acid mine drainage: Implications for Precambrian iron and oxygenation of the atmosphere?. Geology, v. 30, Leblanc, M., Achard, B., Ben Othman, D., Luck, J.M., Bertrand-Sarfati, J., Personné, J. Ch., Accumulation of arsenic from acidic mine waters by ferruginous bacterial accretions (stromatolites). Applied Geochemistry, v. 11, Nordstrom, D.K., Alpers, C.N., Geochemistry of acid mine waters. In: Plumlee, G.S., and Logsdon, M.J. (eds.), The Environmental Geochemistry of Mineral Deposits, Part A. Processes, Techniques, and Health Issues: Society of Economic Geologists, Reviews in Economic Geology, v. 6A, Sánchez España, F.J., Mineralogy and geochemistry of the massive sulphide deposits of the Northern area of the Iberian Pyrite Belt (San Telmo-San Miguel-Peña del Hierro), Huelva, Spain. PhD Thesis, Univ. País Vasco, Bilbao. Sánchez España, F.J., López Pamo, E., Santofimia, E., Aduvire, O., Reyes, J., Barettino, D., Geochemistry and Mineralogy of AMD in the Iberian Pyrite Belt (Huelva, Spain). In: Jarvis, A.P., Dudgeon, B.A. and Younger, P.L. (eds.), Proceedings of the MINE WATER 2004 Symposium, September 2004, Newcastle Upon Tyne, England, UK, Vol. I, pp Sánchez España et al., 2005, June 27-July 1, Volume I 287

Chris Gammons, Montana Tech. Rio Tinto, Spain

Chris Gammons, Montana Tech. Rio Tinto, Spain Diel processes in low-ph streams Chris Gammons, Montana Tech Rio Tinto, Spain Diel processes in different ph regimes low-ph streams Small diel changes in ph Fe redox cycles high-ph streams Large diel changes