macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 29

Los residuos mineros distribuidos sobre grandes áreas son importantes peligros para el medioambiente. La enorme superficie que ocupan proporciona acceso a la erosión incontrolable por el viento y la lluvia, la infiltración del agua y el intercambio de aire. La conta- minación de la materia gaseosa, disuelta, coloidal, particulada e incluso orgánica afecta a zonas pequeñas o, en el peor de los casos, a enormes áreas, dependiendo del tamaño de grano, modo de deposición y forma del material depositado. Centrándonos en el estu- dio de los procesos que tienen lugar dentro del propio material acumulado, es necesario un conocimiento básico respecto a la histo- ria de deposición, roca madre, morfología, clima y vegetación. Es crucial elucidar el estatus quo del residuo minero depositado – su química, mineralogía, tamaño de grano, laminación, conductividad hidráulica, modelo de drenaje, química de las soluciones, actividad microbiológica, la relación ácido-base, los frentes de reacción y la formación de hardpan. La interacción de todos los parámetros con- trola el impacto medioambiental en un cierto intervalo de tiempo así como para toda la vida de una escombrera. La atenuación natural de contaminantes puede ser observada en el origen, a lo largo del camino del drenaje, así como debido a la mezcla y dilución con aguas no contaminadas. Un número de aspectos parecen ser relevantes para los procesos de atenuación en el origen. Basado en información del fondo químico, el peor escenario puede ser modelado. Esto puede ser modificado por el potencial de neu- tralización del propio material a corto y largo plazo. Teniendo en cuenta parámetros adicionales tales como la distribución del tamaño de grano, laminación, conductividad hidráulica, accesibilidad de fases reactivas, superficie específica expuesta y la morfología de la escombrera, podría hacerse una extrapolación del avance del frente de alteración así como del desarrollo de drenaje ácido de minas a largo plazo. Debido a que la mayoría de las escombreras no son monitoreadas para su impacto medioambiental, la información obte- nida sólo proporciona datos de un segmento de tiempo. Un aspecto, que difícilmente puede ser incorporado a los modelos, es la reor- ganización interna de una escombrera. Después de la deposición, una escombrera sufre reorganización específica debido a procesos inducidos por meteorización y diagénesis, que básicamente afectan a los caminos del agua. En el marco de reorganización, el aumen- to del contraste físico-químico actúa como una capa límite que separa las porciones de las escombreras accesibles y alteradas, de las que están protegidas e inalteradas. Desafortunadamente, sólo en casos extremadamente raros, una cuantificación del balance de agua puede ser realizada debido a la pérdida de información básica sobre la precipitación, evaporación, retención, incorporación mineral o drenaje a ríos o aguas subterráneas. Mining residues distributed over large areas are a major thread to the environment. Their enormous surface area provides access to uncontrollable erosion by wind and rainwater, to water infiltration and air exchange. Contamination of gas, dissolved, colloidal, particu- late and even organic matter affects narrow zones or, in a worst case scenario, huge areas, depending on the grain size, deposition mode and shape of the deposited material. Focusing on the study of processes taking place within the dumped material itself, basic knowledge is needed regarding depositional history, bedrock, morphology, climate and vegetation. It is crucial to elucidate the status quo of the deposited mine residue – its chemistry, mineralogy, grain size, layering, hydraulic conductivity, drainage pattern, solution che- mistry, microbial activity, acid base accounting, reaction fronts and hardpan formation. The interaction of all parameters controls the environmental impact at a certain time interval as well as for the lifetime of a dump. Natural attenuation of contaminants can be obser- ved at the source, along the drainage path, as well as due to mixing and dilution with uncontaminated waters. A number of aspects appear to be relevant for attenuating processes at the source. Based on chemical background information, a worst case scenario can be modelled. This may be modified by the short and long term neutralization potential of the material itself. Taking into account addi- tional parameters such as grain size distribution, layering, hydraulic conductivity, accessibility of reactive phases, exposed surface areas and morphology of the heap, an extrapolation of the advancement of the alteration front as well as the long term development of acid mine drainage could be possible. Since most heaps are not monitored for their environmental impact, the information obtained only provides data of a time segment. One aspect, which hardly can be incorporated into models, is the internal reorganization of a heap. After deposition a heap undergoes specific reorganization due to processes induced by weathering and diagenesis, which basically affect the water pathways. In the frame of reorganisation, the increasing physico-chemical contrast acts as a boundary layer separa- ting accessible altered from protected unaltered portions of the heap. Unfortunately, only in extremely rare cases a quantification of the water balance can be done because of missing basic information either on precipitation, evaporation, retention, mineral incorporation or drainage to rivers or groundwater. Acid Mine Drainage Remediation Starts at the Source / DIETER RAMMLMAIR (1) / CHRISTOPH GRISSEMANN (1) / TORSTEN GRAUPNER (1) / JEANNET A. MEIMA (1), / ANTONIO ROMERO BAENA (2)

(1) Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, Germany. (2) Dpto. Cristalografía, Mineralogía y Química Agrícola, Facultad de Química, Universidad de Sevilla, Apdo. 553, 41071 Sevilla, Spain. INTRODUCTION deposited close to the adits, on slopes modified treatment techniques, enor- and into valleys, or was used for buil- mous volumes of waste rock and treat- Mining residues are in general characte- dings, roads or to stabilize the mining ment residues and slags were produ- rized by area consumption. Medieval area. Area consumption was minimal ced. This changed the morphology of mining was focussed on high grade ore with locally high degree of contamina- the country side significantly. The distri- generally associated to veins and she- tion potential. bution of mining residues over tens of ars - consequently, minimal waste rock, square kilometres makes long term low grade ore, and fines were produced. With changing mining technologies from remediation almost impossible (e.g. lig- In general, the waste material was high to low grade, but high tonnage and nite mining in Germany; porphyry cop- palabras clave: Drenaje ácido de mina, atenuación natural, hardpan, key words: Acid mine drainage, natural attenuation, hardpan, internal estructura interna, contaminantes structure, contaminants

Conferencia invitada: Rammlmair et al., Macla 10 (2008) 29-33 *corresponding author: [email protected] Conferencia 30 Acid Mine Drainage Remediation Starts at the Source

per mines in Chile, Iberian Pyrite Belt, horizontal top might cause focussed Spain). water transport, due to a reduced infil- tration capacity. The stability of some Mining residues have been in the environ- tailings dams is based on this sub-hori- mental focus for several decades. Most zontal drainage. Blocky heaps are detailed investigation focussed on the open to infiltration. To inhibit rain water processes generating acid mine drainage infiltration fines have to lock the mega (AMD), on the estimation of the worst pore system to enhance the water case scenario. This scenario is meliora- retention capacity and enable horizon- ted by the short and long time buffering tal rerouting (Fig.3). capacity of the gangue minerals, by chan- fig 2. a) Scheme of the depositional sequence at Peña del Hierro, showing grainsize sorting at the slope . Water transport ging accessibility of acid producing pha- (black) is observed at sinkholes, in surface parallel channels Natural attenuation and on the flanks. b) detail of a heap. Lithological changes at ses due to precipitating crystalline and the heap from slate and gossan in the center (dark grey) to pyri- amorphous secondary phases clogging te rich tuffs in the E and W, c) Fotograph of grain size fractio- Natural attenuation is a welcome attri- pores and forming occasionally hard- nation due to deposition. bute to large sources of contamination, pans, by accumulation of heavy metals in residues rarely can be followed up in since remediation efforts would be storage minerals in the heap and along detail. Besides accessible informa- enormous. Natural attenuation takes the drainage path. Finally dilution of the tion for recently active mines provided advantage of all available ingredients, contaminants by mixing with uncontami- by mining companies or by former to achieve equilibrium with the surroun- nated rivers or groundwater (Fig.1) employees of the mine, little data dings. Since materials from the mining exist about historical mine sites. industry provide enhanced surface Some information is provided by areas and good accessibility, processes aerial photograph time series, which, taking place within this material are however, generally do not cover the much faster then similar processes whole depositional history. Detailed taking place over thousands of years in reconstruction of the depositional the source rock, e.g. gossan forma- process regarding changes of the tions. Within the treated material, material itself as well as the mode of accessibility is optimized due to grain deposition of the material with conse- size reduction. Therefore, high amounts quent fractionation of grains and of dissolved material are made availa- minerals (Fig 2) is needed for further ble to the environment within a short investigation. time period. This is the source of AMD with high loads of contaminants, which Both unsorted blocky materials with is related to the immediate availability limited amounts of fines as well as sli- of the reacting phases. mes, contribute to AMD. The amount of contaminants mobilized and removed Taking into account the volume of sour- from the source can vary enormously ce material and the contaminants, enor- even when the material and climatic mous amounts of eventually toxic com- premises fall into the same range. The pounds would be released. mode of deposition plays a key role. Homogeneously deposited material The interactions at the gas-fluid-rock often provides better access to fluids interfaces and the kinetics of the pro- compared to chaotically deposited or cesses taking place at this interfaces fig 1. Scheme of natural attenuation of potential contaminants within the source it self, along the seepage, and finally by layered material (Rammlmair, 2002). have a major influence on the dissolu- mixing with uncontaminated waters. The fractions of neutrali- Sorting in the course of deposition of tion, absorption, reaction, and precipi- zation , accessibility, fixation and acid mine drainage (AMD) over lifetime is highly dependent on deposition mode, material, the material might be responsible for tation processes taking place. climate and age. reactive mineral enrichment in layers, Contrasting micro-environments might Investigation of mining residues is an forming quasi impermeable thin layers even enhance these processes. extremely complex issue, since little is due to alteration and sub- documented about the process of depo- sequent clogging of pores sition, the material deposited, the treat- by substantial amounts of ment etc.. Deciphering of the depositio- secondary amorphous and nal history and the processes taking crystalline phases. place, requires intensive investigation on site and in the laboratory. Another crucial point is the depositional slope. The aim of this paper is to show how Steep flanks contrast natural attenuation is affected by struc- with sub-horizontal tops. tural reorganization, and the possible Flanks are often subject influence by acid mine drainage. to severe erosion, contri- buting fines to the rivers RESULTS AND DISCUSSION and, therefore, contami- nating the river itself. On Depositional history the other hand, even small morphological fig 3. Heap at Peña del Hierro showing lithologies and the drainage channels draining The depositional history of mining expressions at the sub- into the heap by sinkholes or off heap by horizontal rerouting and slope channels.

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macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 31

Carbonates and some aluminosilica- on the local premises. It is the most phases, the stability under changing pH- tes such as biotite, hornblende and effective natural remediation measu- conditions, the accessibility of even plagioclase are able to neutralize part re, since the directly accessible sour- soluble phases due to reduced porosity of the acidity released due to oxida- ce might be reduced by more then or enhanced water retention in the tion of sulfides, and Fe(II) to Fe(III) 50% (Rammlmair, 2002; Rammlmair uppermost layers. transformation. Besides K, Ca, Al, Si et al., 2008). Unfortunately, a direct and other elements go into solution, approval is not easy to be made. Besides the materials chemical, physi- and even getting pH-conditions norma- cal, and mineralogical background, the lized, the load of Al, Si could get Few geophysical methods can be climatic influence has to be considered. enhanced. Precipitation of gypsum, applied to obtain information on the Another critical aspect is the mineral jarosite, goethite and of gels of highly heap internal structure. Single shot and grain size fractionation during depo- variable compositions might occur. electrical resistivity profiles provide sition of the material. This can cause ambiguous information. The interac- preferential pathways, capillary The system “heap” shows a number of tion of water content, ion concentra- barriers, or even critical enrichment of extremes and transitions. At zones of tion, conductive minerals, grain size, reactive phases being responsible for defined physical contrasts, such as and porosity makes interpretation of the development of hardpans and saturated/unsaturated zones, pH , the results difficult. cemented layers (Graupner et al., redox, T, and gas composition might 2007). show significant differences. Loss of Tomographic information can be obtai- water due to evaporation, changing pH ned by parallel profiles, which outline Acid mine drainage conditions or mixing of fluids, might bodies of the same resistivity range, result in precipitation of secondary pha- and provide information on the bedrock The head waters of the Rio Tinto ses and a collapse of colloids to gels. morphology and eventually watersheds. (Spain) consist of a number of small In combination with surface observa- creeks. Besides pH-neutral waters and Some positions in the heap favour tions, one can derive zones of material a natural contribution of acid mine these dramatic changes. The most of related resistivities (pyrite rich tuffs, drainage, the basic source of the river significant changes can be observed gossan, slate). is low pH seepage draining the valleys at the capillary fringe, were oxidation filled with mining residues. This see- is prominent, and capillary driven Using differential imaging of geoelectri- page contains substantial amounts of transport with hyper-saturated fluids cal resistivity measurements prior and contaminants, such as Fe, As, S, Pb, provides the environment for precipi- after rainfalls helps understanding Ba, Mo, Cu, and Al. The chemical load tating phases and gels due to evapo- water migration within the heap and the pH of the seepage water is ration. At an oscillating redox front at (Grissemann et al., 2007; Rammlmair much dependent on the deposited the boundary saturated/unsaturated et al., 2008). source rock type, the water retention zone, e.g. in a tailings system, capacity of fines in the heap’s interior, cementation could lock the pores for- Fixation of contaminants on surfaces of the sampling season, and the sam- ming thin consolidated layers primary and secondary phases, in pling point (Romero Baena, 2005.). (Graupner et al., 2007). Similar, but amorphous and crystalline storage probably less pronounced contrasts minerals, and within plants is a com- Basically, five rock types were deposi- could be found at capillary barriers, at monly observed feature. The duration of ted in heap slope parallel sheets, transitions between matrix and the fixation is much dependent on the namely slate, gossan, pyrite ashes, macro pore channels. desorption ability, the solubility of the pyrite poor and rich rhyolitic tuffs. The

Internal reorganization

The effect of the physical changes taking place in the heap is twofold. On the one hand, high amounts of conta- minants could be absorbed, precipita- ted and fixed in secondary amorphous and crystalline phases. On the other hand, the volumetric growth of hydra- ted phases locks up pores and agglu- tinates particles. These changes have major effects on the erosion, water migration, and air exchange. Due to the inhibition of infiltration by, horizon- tal rerouting, focused transport occurs on the surface or through the heap. Zones of the heap will be partially or even totally withdrawn from direct accessibility, and therefore from fur- ther reaction. The size of such cells varies from cm to hundreds of meters in diameter. The development of such cells is a continuous process, lasting fig 4. Watersheds based on geoelectrical resistivity investigation outline the bedrock drainage. The formation of hardpans at the top of the heap in a channel-rip system reroute the water. This causes a shift of the watershed. This has influence on the fraction a few to hundreds of years depending of the different lithologies being source of AMD within individual segments.

depósito legal: M-38920-2004 • ISSN: 1885-7264 Conferencia 32 Acid Mine Drainage Remediation Starts at the Source

changes. Besides the changes in quan- tity, quality and load regarding the fast drainage after rainfall related to macro pore systems and the even in the dry season persistent slow matrix flow (Romero Baena, 2005), changes can be observed along the drainage path, too. Unfortunately reactions within the heap cannot be monitored, but locally attribu- ted to changes in the source, from dominantly slate, gossan or pyritic tuffs (Rammlmair et al, 2008). The physico- chemical premises of the seepage are influenced by oxygen availability via air, enhanced by cascades based on the river bed morphology, by temperature decrease or increase depending on the season, by reaction with and absorbtion by minerals of the riverbed, by colloid formation, coagulation and precipitation of amorphous FeH-films. The dissolved ions and the pH might show significant changes.

Most of the analyzed waters of Peña del Hierro show hyper-saturation of goethite in the course of oxidation, protons will be liberated when goethite precipitates. fig 5. The outflow “D” shows significant changes of conductivity, redox, pH, oxygen, temperature, and morphology along a 110 m Other phases which would precipitate profile. The loss of temperature, conductivity and the increase in pH and oxygen is significant. are small amounts of gypsum, anglesite central part, dominated by gossan and Drainage path and SiO2 (am). Critical for the pH-chan- slate is surrounded by rhyolitic tuffs, ges is the kinetics of Fe(II)-oxidation which are in part highly enriched in Along the drainage path within and out- (tendency: pH increase), the precipita- pyrite. side the heap, a number of physicoche- tion kinetics of iron oxyhydroxides and mical changes occur. Potential contami- jarosite (tendency: pH decrease), and Since the drainage system of the nants might absorb on crystalline and the kinetics of minerals from the river- investigated heaps is influenced by a amorphous mineral surfaces or coagu- bed tendency: pH increase). number of factors such as horizontal late as colloids or precipitate as gels or drainage, slope erosion and sinkho- individual crystalline phases. Changing The observed slight increase in pH les, drainage areas do not corres- Redox condition, pH, temperature drop, along the seepage D over some 100 meters, with significant drop in conduc- pond to the natural watershed O2, bacteria, and evaporation might system. (Fig 4) (Rammlmair et al. cause this. Seapage from mining tivity and temperature (Fig.5) can be 2008). dumps undergoes significant seasonal modelled on the base of Fe(II) oxidation

fig 6. Geochemical modelling of ionic strength and pH for oxidation of Fe(II) only, and combined with precipitation of amorphous Fe-phases. The model in part explains the development of Fig.5.

Conferencia invitada macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 33

to Fe(III) and by precipitation of small Water that enters the source reacts Graupner, T., Kassahun, A., Rammlmair, D., amounts of goethite or jarosite (Fig.6). with the material, leaches primary and Meima, J.A., Kock, D., Furche, M., Fiege, A., secondary soluble phases. Part of the Schippers, A. & Melcher, F. (2007): Some of the seepage mixes with non con- water will form hydrous compounds, Formation of sequences of cemented layers taminated waters of near neutral pH. part migrates down the heap, part will and hardpans within sulfide-bearing mine tai- lings (mine district Freiberg, Germany). Dilution occurs according to the mixing evaporate and precipitate into soluble Applied Geochemistry 22, 2486-2508. ratio, but additionally some contaminants and insoluble phases close to the sur- will instantaneously flocculate changing face (e.g. hardpans). The seepage lea- Rammlmair, D. (2002): Hard pan formation clear brown waters into turbid yellowish ving the heap after a rainfall might on mining residuals. In Uranium in aquatic for some distance. The particles might show a peak both in concentration and environment 2002, pp. 173-182, (Springer- float or settle on the river bank. water quantity at beginning turning Verlag: Berlin) into a reduced rate after hours. The CONCLUSION high concentration of contaminants is _ , Grissemann, Ch., Furche, M., Noell, U., based on the immediate availability of Graupner, T., Meima, J.A. & Romero-Baena, A. (2008): Evidence of reduced water infiltra- To estimate the acid mine drainage poten- highly soluble secondary phases, tion by micro-hardpans: electrical resistivity tial of a heap, it is basic to characterize accessible pore water and macro pore measurements at Peña del Hierro, Rio Tinto, the status quo of a heap according to the channel system. Each rainfall after a Spain. AusIMM 9th International Congress morphology of the heap itself, the dry period generates a first flush of for Applied Mineralogy (ICAM) (in press) bedrock, the material deposited, the elevated load of dissolved com- mode of deposition, the internal structu- pounds. The intensity of the first flus- Romero Baena, A.J. (2005): Origen del re, and the local climatic conditions. hes might change over time due to Drenaje Ácido de Minas y Problemática Unfortunately, there are few means to wash out of highly soluble effluores- Ambiental de las Escombreras de Peña del obtain information of the interior of a cent phases. It is therefore important Hierro (Faja Pirítica Ibérica), PhD Thesis (unpublished),Univ. de Sevilla, Spain) heap. Long term electrical resistivity mea- to characterize the outflow, and to surements provide a good insight into monitor seepage over a time period to structure and eventually water migration differentiate between first flushes and pathways. The information is not unambi- background seepage. Unfortunately guous, since water saturation and ionic this is not easily performed, but is strength interact. Additional information besides the geophysical investigation has to be obtained by independent chemi- the only non-destructive access to the cal or physical methods, which in general heap interiors. cannot be applied in the field, but at a laboratory scale. Natural attenuation is hardly to be quan- tified, since the interacting processes A number of factors influence the deve- last several years. On the other hand lopment of acid mine drainage directly decision support pro or contra remedia- at the source and along the pathway. tion activities is expected within a short Besides the neutralization potential time period. Some solution can be pro- based on the chemistry of the minerals, vided by modelling. Based on field the long term availability and the observations and laboratory tests, rele- amount of reactants during the process vant parameters can be obtained which of weathering are crucial for the quan- can feed reactive transport models to tity of water getting access to the reac- predict the heap behaviour within the tive phases and for the quality of the next hundreds of years. outflow. Reduction of the accessibility of the material and limiting the amount of ACKNOWLEDGEMENTS water and oxygen infiltration, will have the strongest influence on AMD genera- The investigation was part of a project tion. Besides technical remediation on hardpan formation on mining resi- efforts, natural processes such as clog- dues financed by the German Federal ging of pores, formation of hardpans or Ministry for Education and Research cemented layers, and generation of (BMBF) under the project FKZ 0330523. capillary barriers on the basis of grain Thanks to the Director José María size contrasts, are most effective to Mantecón Jara, of the Fundación Rio withdraw cells of the heap from further Tinto, for the support of the field work weathering. These natural processes on their property. develop over a number of years, much dependent on the material premises REFERENCES and the climatic conditions. Even if the processes are highly effective in limiting Grissemann, Ch., Furche, M., Noell, U., the size of the source, AMD does not Rammlmair, D., Graupner, T., Günther, Th. & stop immediately, but would change in Romero Baena, A J. (2007): Geoelectrical quality and quantity over time. At this observations at the mining dumps of the point, it is crucial to quantify the water Peña de Hierro copper mine in the Rio Tinto mining district / Spain, in Proceedings 10th household of a heap. The attempt to International congress of the Brazilian quantify it is in general hindered by Geophysical Society 2007 pp. (SBGF- incomplete sets of data. Sociedade de Geofisica)

depósito legal: M-38920-2004 • ISSN: 1885-7264