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In this work, we integrate five case studies harboring vulnerable deep-sea benthic habitats in different geological settings from mid latitude NE Atlantic Ocean (24–42° N). Data and images of specific deep-sea habitats were acquired with Remoted Operated Vehicle (ROV) sensors (temperature, salinity, potential density, O2, CO2, and CH4). Besides documenting some key vulnerable deep-sea habitats, this study shows that the distribution of some deep-sea coral aggregations (including scleractinians, gorgonians, and antipatharians), deep-sea sponge aggregations and other deep-sea habitats are influenced by water masses’ properties. Our data support that the distribution of scleractinian reefs and aggregations of other deep-sea corals, from subtropical to north Atlantic could be dependent of the latitudinal extents of the Antarctic Intermediate Waters (AAIW) and the Mediterranean Outflow Waters (MOW). Otherwise, the distribution of some vulnerable deep-sea habitats is influenced, at the local scale, by active hydrocarbon seeps (Gulf of Cádiz) and hydrothermal vents (El Hierro, Canary Island). The co-occurrence of deep-sea corals and chemosynthesis-based communities has been identified in methane seeps of the Gulf of Cádiz. Extensive beds of living deep-sea mussels (Bathymodiolus mauritanicus) and other chemosymbiotic bivalves occur closely to deep-sea coral aggregations (e.g., gorgonians, black corals) that colonize methane-derived authigenic carbonates.

Previous studies have addressed the occurrence of Acid Rock Drainage (ARD) affecting La Silva stream due to the generation of large dumps of Middle Ordovician black shales during the construction of a highway close to El Bierzo (León, Spain). This ARD was characterized by sulphated acid waters with high concentration of heavy metals and anomalies in dissolved thorium (Th) and uranium (U). In the present study, we analyse in depth black shales and water, streambed sediments and precipitates of La Silva stream and its tributaries using different petrographic, mineralogical and geochemical approaches. Black shales, with average Th and U contents of 20 and 3 μg/g respectively contain disseminated detritic micro-grains of high weathering-resistant minerals, such as monazite and xenotime, that present smaller amounts of yttrium and rare earth elements (REY) and other elements as Ca, U, Th, Si and F. Results of the affected waters by ARD show an enrichment in dissolved Th, U and REY of several orders of magnitude with respect to natural waters. Sampled precipitates were mainly schwertmannite (Fe

Extensive beds of the deep-sea mussel Bathymodiolus mauritanicus (currently also known as Gigantidas mauritanicus) linked to active cold seeps related to fissure-like activity on Al Gacel mud volcano, Gulf of Cádiz, were filmed and sampled for the first time during the oceanographic expedition SUBVENT-2 aboard R/V Sarmiento de Gamboa. Al Gacel mud volcano is one of up to 80 fluid venting submarine structures (mud volcanoes and mud volcano/diapir complexes) identified in the Gulf of Cádiz as result of explosive venting of hydrocarbon-enriched fluids sourced from deep seated reservoirs. This mud volcano is a cone-shaped edifice, 107 m high, 944 m in diameter constituted by mud breccias and, partially covered by pavements of seep carbonates. Extensive beds of this deep-sea mussel were detected at the northern flank at 810–815 m water depth associated with bacterial mats around intermittent buoyant vertical bubble methane plumes. High methane concentrations were measured in the water column above living mussel beds. Other chemosymbiotic species (Siboglinum sp., Solemya elarraichensis, Isorropodon sp., Thyasira vulcolutre and Lucinoma asapheus) were also found in different parts of Al Gacel mud volcano. Al Gacel mud volcano may currently represent one of the most active mud volcanoes in the Gulf of Cádiz, delivering significant amounts of thermogenic hydrocarbon fluids which contribute to foster the extensive chemosynthesis-based communities detected. This finding is of paramount importance for linking extremophile bivalve populations along the North Atlantic, including cold seeps of the Gulf of México, hydrothermal vents of the Mid-Atlantic Ridge and now, detailed documented at the Gulf of Cádiz.

The occurrence of hydrothermal emissions implies the existence of heat sources related to magma reservoirs both in convergent margins (Bransfield-South Shetland) and in mid-ocean ridge and intra-plate settings (Azores). The importance of these systems lies in (a) producing important mineralizations, (b) favouring extremophilic ecosystems, (c) being precursors of underwater volcanic eruptions, (d) playing a major role they play in the matter and energy exchange between the geosphere and the hydrosphere and (d) their impact on the geochemistry of the oceans. In subduction margins, rifts, transforming faults or volcanic buildings in hot spots, emissions of hot fluids related to magmas and/or circulation in hydrothermal systems can occur. The fluids associated with magmas are fundamentally gases (CO2, H2O, H2, SO2, H2S, He, etc.). Hydrothermal fluids constitute a complex system where seawater percolates through fissures and fractures in sediments and rocks at different depths and heats up upon contact with magmas and hot volcanic rocks, leaching a large amount of chemical elements. The identification of acoustic plumes in the water column is the first step in the exploration of unknown underwater emissions. The new acoustic detection technologies, which operate with a wide frequency range, are one of the most innovative tools for detecting gas plumes and other fluids in the water column, especially in deep waters. Once detected, physical-chemical parameters (temperature, salinity, turbidity, cations, anions, dissolved gases, isotopic signature, etc.) that allow their characterization and classification will be determined. This type of studies is particularly useful when it is not possible to collect free gases, fumarolic and/or bubbling gases, as in the case of submarine activity. In this work, we show the results obtained regarding the chemical composition of dissolved gases (He, H2, CO2 (aq), O2, N2, CH4 and He) and isotopic signature of the dissolved CO2 (δ13C-CO2) in sea water sampled in sites of hydrothermal interest. With this purpose, we carried out two oceanographic surveys (EXPLOSEA1 and EXPLOSEA2) in 2019: the first in Antarctica aboard the Spanish Research Vessel (RV) Hespérides and the second in North Atlantic Ocean aboard the Spanish RV Sarmiento de Gamboa. To do so, 13 and 10 water vertical profiles were studied in the RV Hespérides and the RV Sarmiento de Gamboa, respectively, using a SBE 911plus CTD system where there was evidence of acoustic plumes or where appropriate, emission buildings of fluids were present. Water samples were kept in glass bottles for subsequent analysis. The establishment of the physicochemical characteristics of volcanic hydrothermal fluids and the characterization of the nature and origin of the different types of fluid emissions will help to classify the hydrothermal fluids in order to understand the phenomena that take place in them and their surroundings.

Hydrothermal iron (Fe)-rich sediments were recovered from the Tagoro underwater volcano (Central Atlantic) that formed during the 2011–2012 volcanic event. Cruises in 2012 and 2014 enabled the monitoring and sampling of the early-stage establishment of a hydrothermal system. Degassing vents produced acoustic flares imaged on echo-sounders in June 2012, four months after the eruption. A novel hydrothermal vent system was discovered and sampled in 2014 during a ROV dive. The system is characterized by hornito-like structures and chimneys showing active CO2 degassing and anomalous temperatures at 120–89 m water depth, and along the SE flank at 215-185 m water depth associated with secondary cones. Iron- and silica-rich gelatinous deposits pooled over and between basanite in the hornitos, brecciated lavas, and lapilli. The low temperature, shallow-water hydrothermal system was discovered by the venting of Fe-rich fluids that produced a seafloor draped by extensive Fe-flocculate deposits precipitated from the neutrally buoyant plumes located along the oxic/photic zone at 50-70 m water depths. The basanite is capped by mm- to cm-thick hydrothermally derived Fe-oxyhydroxide sediment, and contains micro-cracks and degasification vesicles filled by sulfides (mostly pyrite). Mineralogically, the Fe-oxyhydroxide sediment consists of proto-ferrihydrite and ferrihydrite with scarce pyrite at their base. The Fe-rich endmember contains low concentrations of most trace elements in comparison with hydrogenetic ferromanganese deposits, and the sediments show some dilution of the Fe oxyhydroxide by volcanic ash. The Fe-oxyhydroxide phase, with a mean particle size of 3–4 nm, low average La/Fe ratios of the mineralized deposits from the various sampling sites, and the positive Eu anomalies indicate rapid deposition of the Fe oxyhydroxide near the hydrothermal vents. Electron microprobe studies show the presence of various organomineral structures, mainly twisted stalks and sheaths covered by iron-silica deposits within the mineralized samples, reflecting microbial iron-oxidation from the hydrothermal fluids. Sequencing of 16 s rRNA genes also reveals the presence of other microorganisms involved in sulfur and methane cycles. Samples collected from hornito chimneys contain silicified microorganisms coated by Fe-rich precipitates. The rapid silicification may have been indirectly promoted by microorganisms acting as nucleation sites. We suggest that this type of hydrothermal deposit might be more frequent than presently reported to occur in submarine volcanoes. On a geological scale, these volcanic eruptions and low-temperature hydrothermal vents might contribute to increased dissolved metals in seawater, and generate considerable Fe-oxyhydroxide deposits as identified in older hot-spot seamounts.

The generation of acid rock drainage (ARD) was observed after the oxidation dissolution of pyrite-rich black shales, which were excavated during the construction of a highway in León (Spain). ARDs are characterized by the presence of high concentrations of sulfate and metals (Al, Fe, Mn, Zn, Cu, Co, Ni, Th, and U) that affect the La Silva stream. Dissolved element concentrations showed values between one and four orders of magnitude higher than those of natural waters of this area. A passive treatment system was constructed; the aim of which was to improve the quality of the water of the stream. This work provides a hydrochemical characterization of the La Silva stream after its transit through the different elements that constitute the passive treatment system (open limestone channel (OLC), small ponds, and a wetland), during its first year of operation. The passive treatment system has two sections separated by a tunnel 230 m long. The first section, which stretches between the highway and the tunnel entrance, is an OLC 350 m long with a slope of 16 %. The second section, which stretches from the tunnel exit to the end wetland, has a length of 700 m and a slope of 6 %; it is in this section where six small ponds are located. In the first section of this passive treatment system, the OLC was effectively increasing the pH from 3 to 4-4.5 and eliminating all of the dissolved Fe and the partially dissolved Al. These elements, after hydrolysis at a pH 3-3.5 and 4-4.5, respectively, had precipitated as schwertmannite and hydrobasaluminite, while other dissolved metals were removed totally or partially for adsorption by the precipitates and/or by coprecipitation. The second section receives different inputs of water such as ARDs and natural waters. After exiting the treatment system, the stream is buffered by Al at a pH of 4-4.3, showing high Al concentrations (19-101 mg/L) but with a complete removal of dissolved Fe. Unfortunately, the outflow shows similar or higher acidity than the inflow into the system due to the discharge of ARDs (mainly from the tunnel) that is received and to the existence of a natural stream, which is affected by a waste-rock pile. The predictions and calculations necessary for the design of any remediation/attenuation techniques are quite difficult. Despite the fact that the selected design is the most adequate one for this valley and type of passive treatment system (including adequate slopes), we must admit that the physicochemical characteristics of the ARD were not the most appropriate according to the literature. Moreover, during the design, engineers were unaware of the existence of the inflow from two highly polluting sources, which have rendered the passive treatment system ineffective and which therefore suggest that certain improvement measures could be considered.

The generation of acid rock drainage (ARD) was observed in an area of Nevado Pastoruri as a result of the oxidative dissolution of pyrite-rich lutites and sandstones. These ARDs are generated as abundant pyrite becomes exposed to atmospheric conditions as a result of glacier retreat. The proglacial zone contains lagoons, springs, streams and wetlands, scant vegetation, and intense fluvioglacial erosion. This work reports a comprehensive identification and the results of sampling of the lagoons and springs belonging to the microbasin, which is the headwaters of the Pachacoto River, as well as mapping results based on the hydrochemical data obtained in our study. The physical properties and water chemistry of 12 springs and 22 lagoons from the proglacial zone are also presented. Water springs are far from being chemically uniform, with pH and EC values ranging between 2.55–6.42 and 23–1110 μS/cm respectively, which suggests a strong geologic control on water chemistry. Fe-SO4 −2 concentrations confirm the intense process of pyrite oxidative dissolution. Many of the lagoons are affected by ARD, with low pH (~ 3), and high EC (256–1092 μS/cm) values when compared with unaffected lagoons (EC between 7 and 59 μS/cm), indicating a high degree of mineralization. The affected lagoons show higher concentrations of SO4 2− and SiO2, and elements as Fe, Al, Mg, Mn, Zn, Co, and Ni, which are related to the alteration of pyrite and the dissolution of aluminosilicate minerals. Schwertmannite-goethite appears to be the most important mineral phases controlling the Fe solubility at a pH of 2–3.5. Moreover, they act as a sorbent of trace elements (As, Sb, V, Pb, Zn, Cr), which is an efficient mechanism of natural attenuation. Despite of this, the water flowing out from the basin is acid (pH 3.1) and contains significant concentrations of Fe (0.98 mg/L) and Al (3.76 mg/L) that confer mineral acidity to water. The Pachacoto River located 5.5 km downstream from this point showed a strong natural attenuation, with a pH of 6.9 and low concentration of metals. This mitigating process is possible due to (i) the formation of precipitates that retain toxic elements and (ii) the mixing with natural waters that promote dilution, which favor the increase of pH until circumneutral conditions., Instituto Geológico y Minero de España, España, Departamento Académico de Ciencias Ambientales, Universidad Nacional Agraria de la Selva, Perú, Centro de Astrobiología, Instituto Nacional de Técnica Aeroespacial, España, Centro de Astrobiología, Consejo Superior de Investigaciones Científicas, España

In mining areas in which sulfur-containing ores are exposed, the oxidation of sulfides leads to the formation of acidic mine drainage (AMD) waters. Both traditional and molecular microbial studies have shown that chemolithotrophic sulfur- and iron-oxidizing bacteria are responsible for this activity. The Iberian Pyrite Belt (IPB), located in southwestern Spain represents one of the world’s largest accumulations of mine wastes and AMD waters. Mineralogical and textural characteristics of the IPB ores favor the oxidation and dissolution of pyrite and the subsequent formation of AMD waters. Acidic pit lakes in the IPB are a hotspot for this activity. These pit lakes are former open pit mines where ores have been exposed leading the formation of AMD waters. At present, there are more than 25 of these pit lakes between the provinces of Huelva and Seville (SW Spain). This work reports the physical properties, hydrogeochemical characteristics, and microbial diversity of two pit lakes located in the IPB Nuestra Señora del Carmen (NSC), and Concepción (CN). Both pit lakes are acid (pH 2-4) and showed chemical and thermal stratification with well defined chemoclines. One particular characteristic of NSC is that it has developed a chemocline very close to the surface (2 m depth). Microbial community composition of the water column was analyzed by 16S and 18S rRNA gene cloning and sequencing. The microorganisms detected in NSC are characteristic of acid mine drainage (AMD), including iron oxidizing bacteria (Leptospirillum) and facultative iron reducing bacteria and archaea (Acidithiobacillus ferrooxidans, Actinobacteria, Acidimicrobiales, Ferroplasma) detected in the bottom layer. Diversity in CN was higher than in NSC. Microorganisms known from AMD systems (Acidiphilium, Acidobacteria and Ferrovum) and microorganisms never reported from AMD systems were both identified.

Nuestra Señora del Carmen (NSC) mine pit located in the northwest of Huelva province in the Iberian Pyrite Belt has been entirely flooded for several decades. The pit lake dimensions are 110 m×80 m, with a depth of 34 m and a volume of 79,500 m3. Lake water is acidic (average pH 2.7) with high concentrations of sulfate (8.5 g/L), magnesium (1.0 g/L) and metals (Fe 760 mg/L, Al 230 mg/L, Mn 88 mg/L and Cu 29 mg/L). As with the majority of mining lakes of the Iberian Pyrite Belt, this lake develops chemical stratification (meromictic lake) but unlike the rest, it has occasionally showed circulation throughout the entire water column at the beginning of some winters, displaying total homogenization (holomictic lake). The lake regains chemical stratification during periods of high precipitation due to freshwater runoff contribution from the basin (ectogenic meromixis). Furthermore, another unusual period of mixing was observed in the summer period. Evapoconcentration of the warmer upper layer has been identified as the reason for the chemical destabilization observed in the water column. This work presents the processes (evapoconcentration), parameters (temperature and electric conductivity) and climatic factors (runoff, precipitation and wind) that influence the development of chemical stratification or mixing of the water column in the lake, Instituto Geológico y Minero de España, España

The Arcas pond complex in Cuenca (Spain) develops within an interstratified karst system that affects gypsum deposits of the Cretaceous-Tertiary transition period. The formation of this complex is influenced by fracturing, which acts both as the preferential pathway for groundwater movement and the connecting and upward flow mechanism across aquifer formations. Such fracturing also favors the development of karst morphologies in the form of collapse dolines that are connected to the aquifer and which display a permanent character. Hydrochemical studies have allowed the identification of four types of ponds that show varying degrees of behavior and connection with the confined gypsum aquifer. Evaporation, oversaturation and gypsum and limestone mud precipitation processes take place in the more disconnected ponds. The study of the annual evolution of the water column in two of the ponds that show differing degrees of connection with the groundwater allows for an evaluation of the manner in which groundwater influences the stratification/mixing processes that take place in such ponds, and of the physico-chemical characteristics governed by these processes.

Mineralogical characterization of the precipitates developed in passive systems treating mine-polluted waters is an essential tool to fully understand and control the removal processes taking place in these systems. In 2008, after five years of operation, a section of the permeable reactive barrier (PRB) at Shilbottle, Northumberland, was subjected to a low intrusive/non-destructive solid sampling. These solid samples were mineralogically characterized by XRD, ESEM-EDS, and sequential extractions. In addition to the solid sampling, 44 water samples obtained in the PRB from January 2004 to August 2009 were used to study the mineral stability of some selected phases in these waters. It was observed that the main iron phases in the PRB were those associated with mineral phases typically developed in non-reducing environments (schwertmannite and goethite), while the presence of a significant amount of pyrite was also observed. The low residence time of the water within the PRB (from 10 to 40 h) appears to be the reason for the absence of a more reducing and less acidic environment in the reactive substrate. An increase of residence time in the PRB, by increasing reactive mixture porosity and resizing the PRB, changes in the reactive material employed (smaller limestone grain size and inclusion of zerovalent iron) and changes in the PRB design (isolating top layer and forced homogeneous flow upward through all the reactive material) are proposed for future reconditioning of the system.

A marked vertical trend of increasing temperature and dissolved metal concentrations is observed in the monimolimnia of some meromictic pit lakes of the Iberian Pyrite Belt (IPB) in SW Spain. Temperature differences between the chemocline and the pit lake bottom can be as high as 15°C (e.g. Herrerias), and the respective concentration of some metals (e.g. Fe) and metalloids (e.g. As) can increase by several orders of magnitude (e.g. Cueva de la Mora). The redox conditions also change drastically from the upper and oxygenated mixolimnion (strongly oxidizing) to the lower and anoxic monimolimnion (moderately reducing). Processes such as the inflow of metal–sulphate laden ground water from flooded shafts and galleries, and other factors such as the pit geometry or the relative depth of the lakes, must be considered to account for the observed stratification pattern. The vertical profiles of physico-chemical parameters and water chemistry obtained in Cueva de la Mora and other meromictic pit lakes of the IPB are also compatible with a reactive bottom in which several geochemical and microbial reactions (including reductive dissolution of Fe3+ minerals, bacterial reduction of Fe3+ and SO4 2− in pore waters within the sediments, and decomposition of organic matter) could be taking place.

This work describes recent research carried out in an extremely acidic (pH 0.61–0.82) and hypersaline (e.g., 134 g/L SO4 2-, 74 g/L Fe, 7.5 g/L Al, 3 g/L Mg, 2 g/L Cu, 1 g/L Zn) leachate which seeps from a pyrite pile in San Telmo mine (Huelva, SW Spain) and forms evaporative pools of ultra-concentrated water in which attractive crystals of Zn-rich melanterite (FeIISO4 7H2O) are formed. Geochemical modeling with the Pitzer method indicates that the acidic brine was near saturation with respect to melanterite (SIMel = 0 ± 0.2). The microbiological investigation has revealed a surprisingly high biomass (1.4 × 106 cells mL−1) and an exotic ecosystem composed of acidophilic, Fe-oxidizing archaea (mainly Ferroplasma spp., representing 52% of the microbial population), and minor numbers of acidophilic bacteria (including Leptospirillum spp. (3.2%), Acidithiobacillus spp. (1.6%), and Alphaproteobacteria (2.8%)). The microbial production of FeIII allows the oxidative dissolution of pyrite and other sulphides, which results in additional inputs of FeII, SO4 2- and acidity to the system. The surfaces of the pyrite crystals show a typical etch-pitted texture, as well as blobs of elemental sulphur, which are both compatible with this indirect, microbially mediated oxidation mechanism. The composition of the acidic leachate seems to result from the combination of several processes which include: (1) formation of melanterite within the pile during relatively dry seasons, (2) subsequent dissolution of melanterite during rainy episodes, (3) microbial oxidation of FeII, (4) sulphide oxidation mediated by FeIII, (5) dissolution of chlorite and other aluminosilicates present in the pile, and (6) cooling and/or evaporation of seepage from the pile and consequent melanterite precipitation.

This study examines some relevant limnological and hydrogeochemical characteristics of 22 mine pit lakes of the Iberian Pyrite Belt (IPB). The studied pit lakes include some of the largest and historically most important mines of the IPB (including Corta Atalaya and Cerro Colorado in Riotinto, Filon Norte, Filon Centro and Filon Sur in Tharsis, Aznalcollar, or San Telmo) and many other of minor size. As a whole, these lakes constitute, at present, a large volume of highly acidic and metal-polluted water. Some of these pit lakes are very recent (e.g., Corta Atalaya, 2.5 a; Los Frailes, 6 a) and present a continuous hydrological and geochemical evolution, although many others were abandoned decades ago and show nearly constant water volume. Depth profiles obtained in several pit lakes (e.g., San Telmo, Confesionarios, Cueva de la Mora, Concepcion) indicate that many of them have developed meromixis and show, at present, a permanent thermal and chemical stratification with a well defined chemocline separating an anoxic, Fe(II)-rich monimolimnion, and a well mixed, oxygenated and Fe(III)-rich mixolimnion. In the upper layer, the bacterial oxidation of Fe(II) competes with photoreductive processes which take place in the surface water, thus provoking diel cycles of Fe(II) concentration. The observed water chemistry reflects the oxidation and dissolution of pyrite and other sulphides and gangue aluminosilicates from the country rock. The pit lakes of the IPB cover a wide range of water compositions, from circumneutral and relatively low-metal (e.g., Los Frailes, pH 7.2, 0.07 mg/L Fe, 3.8 mg/L Mn, 30 mg/L Zn), to extremely acidic and metal(loid)-rich (e.g., Corta Atalaya, pH 1.2, 36.7 g/L Fe, 6.7 g/L Zn, 1.3 g/L Cu, 159 mg/L As). Most pit lakes, however, are comprised within the pH range of 2.2–3.6 and appear to be strongly buffered by the hydrolysis and precipitation of Fe(III) in the form of schwertmannite, which forms colloids that can sorb trace elements from the aqueous phase.

The oxidation of Fe(II) in acidic mine waters emerging from the portals of abandoned mines (Lomero, Esperanza, Tharsis and Poderosa) of the Iberian Pyrite Belt (IPB) massive sulphide province, has been investigated. Field and laboratory studies have been performed in mine effluents with different iron concentrations (290 to 1940 mg/L Fe(II)) and pH (1.9–3.1) to calculate: (1) the reaction rates at which dissolved Fe(II) is oxidised to Fe(III) in the mine portals, (2) the hourly variations of these oxidation rates, and (3) the rate at which Fe(III) is subsequently hydrolyzed and precipitated as schwertmannite. The calculated field rates, between 5.5 × 10 − 6 and 4 × 10 − 7 mol L − 1 s − 1 , are characteristic of bacterial oxidation. A marked difference has been recognized between the different mine sites depending on the biomass density. The oxidation rates measured in the mine portals with dense biofilms of acidic slime streamers (Tharsis and Esperanza), are notably faster than those measured in the mine sites where streamers are not present (Lomero mine portal). Among the sites with the highest density of biofilms, pH also appears to control the oxidation rate, with the effluents of lowest pH (Poderosa mine, pH 1.9) showing slower rates than those with higher pH (Tharsis and Esperanza, pH 2.7–3.0). The oxidation kinetics of Fe(II) appears to be zero-order and highly dependent on the water temperature ( T ). Consequently, iron-oxidation rates vary significantly along the day, being minimum at the early morning and sunset (with T ∼ 15–25 °C), and maximum at midday (with T ∼ 25–35 °C). Laboratory oxidation studies performed with samples of these mine effluents have confirmed the influence of T , pH, dissolved oxygen content and biofilm presence/absence on the oxidation rate of Fe(II). Precipitation of Fe(III) takes place at pH 2.7–3.1 and rates ranging from 1.7 × 10 − 6 to 10 − 7 mol L − 1 s − 1 , also showing a daily cycle.

The geochemical evolution of two acid mine effluents in Tharsis and La Zarza-Perrunal mines (Iberian Pyrite Belt, Huelva, Spain) has been investigated. In origin, these waters present a low pH (2.2 and 3.1) and high concentrations of dissolved sulphate and metals (Fe, Al, Mn, Cu, Zn, As, Cd, Co, Cr, Ni). However, the natural evolution of these acidic waters (which includes the bacterial oxidation of Fe(II) and the subsequent precipitation of Fe(III) minerals) represents an efficient mechanism of attenuation. This self-mitigating process is evidenced by the formation of schwertmannite, which retains most of the iron load and, by sorption, toxic trace elements like As. The later mixing with pristine waters rises the pH and favours the total precipitation of Fe(III) at pH 3.5 and, subsequently, Al compounds at pH 4.5, along with the sorption of trace metals (Mn, Zn, Cu, Cd, Co, Ni) until chemical equilibrium at circumneutral conditions is achieved.

This work reports the physical properties and water chemistry of 64 AMD discharges from 25 different mines in the IPB draining to the Odiel river watershed, which have been investigated during the hydrologic year 2003–2004. These AMD solutions vary largely in flow rate and chemical composition both spatially (between the different mine sites, suggesting a strong geologic control on AMD chemistry) and seasonally (due to marked hydrologic variations), and include cases with very low pH (mostly in the range 1.4–4), and extreme sulphate (up to 44 g/L SO 4 2 - ) and metal content (e.g., up to 7.7 g/L Fe, 2.6 g/L Al, or 1.4 g/L Zn). Different hydrogeochemical facies of AMD (namely, Fe(II)/anoxic, Fe(III)/suboxic, and aluminous/oxic) are recognized in the field, as a response to the continuous oxidation and hydrolysis of dissolved Fe. Relevant geochemical aspects of these AMD environments are discussed, including: (i) the redox chemistry of the Fe(II)/Fe(III) couple, (ii) the reaction rates for bacterially catalyzed oxidation of Fe(II) and hydrolysis of Fe(III), (iii) the role played by dissolved Fe and Al in the acidity and chemical buffering of the AMD systems, and (iv) the solubility and trace metal retention capacity of the Fe oxyhydroxysulphate and hydrated sulphate minerals commonly associated with AMD. In addition, the mineralogy and chemistry of the Fe precipitates (schwertmannite, jarosite, goethite, ferrihydrite), Al phases (e.g., basaluminite) and Mg–Fe–Al efflorescent SO4 salts (e.g., epsomite, hexahydrite, copiapite, halotrichite, rozenite, coquimbite) present in the AMD-generating mine sites, have also been studied. The mineralogy of the Fe precipitates is well correlated with the water pH (with jarosite at pH ∼ 2, schwertmannite at pH 2–4, basaluminite at pH 4–5, and ferrihydrite at pH > 6). Schwertmannite appears to be the most important mineral phase, both in controlling the Fe solubility at pH 2–4, and as sorbent of trace elements (As, Cu, Zn), which favours natural attenuation. Finally, a basin-scale environmental perspective is given in order to evaluate the impact of AMD on the water quality, including calculation of metal loadings transported by AMD from the most important mine districts in the province.

The exposure of fresh sulfide-rich lithologies by the retracement of the Nevado Pastoruri glacier (Central Andes, Peru) is increasing the presence of heavy metals in the water as well as decreasing the pH, producing an acid rock drainage (ARD) process in the area. We describe the microbial communities of an extreme ARD site in Huascaran National Park as well as their correlation with the water physicochemistry. Microbial biodiversity was analyzed by FLX 454 sequencing of the 16S rRNA gene. The suggested geomicrobiological model of the area distinguishes three different zones. The proglacial zone is located in the upper part of the valley, where the ARD process is not evident yet. Most of the OTUs detected in this area were related to sequences associated with cold environments (i.e., psychrotolerant species of Cyanobacteria or Bacteroidetes). After the proglacial area, an ARD-influenced zone appeared, characterized by the presence of phylotypes related to acidophiles (Acidiphilium) as well as other species related to acidic and cold environments (i.e., acidophilic species of Chloroflexi, Clostridium and Verrumicrobia). Sulfur- and iron-oxidizing acidophilic bacteria (Acidithiobacillus) were also identified. The post-ARD area was characterized by the presence of OTUs related to microorganisms detected in soils, permafrost, high mountain environments, and deglaciation areas (Sphingomonadales, Caulobacter or Comamonadaceae).

The Iberian Pyrite Belt, located in Southwestern Spain, represents one of the world's largest accumulations of mine wastes and acid mine drainages. This study reports the comparative microbial ecology of the water column of Nuestra Señora del Carmen acid pit lake with the extreme acidic Río Tinto basin. The canonical correspondence analysis identified members of the Leptospirillum, Acidiphilium, Metallibacterium, Acidithiobacillus, Ferrimicrobium and Acidisphaera genera as the most representative microorganisms of both ecosystems. The presence of archaeal members is scarce in both systems. Only sequences clustering with the Thermoplasmata have been retrieved in the bottom layer of Nuestra Señora del Carmen and one station of Río Tinto. Although the photosynthetically active radiation values measured in this lake upper layer were low, they were sufficient to activate photosynthesis in acidophilic microorganisms. All identified photosynthetic microorganisms in Nuestra Señora del Carmen (members of the Chlamydomonas, Zygnemopsis and Klebsormidium genera) are major members of the photosynthetic eukaryotic community characterized in Río Tinto basin. This study demonstrates a close relationship between the microbial diversity of Nuestra Señora del Carmen pit lake and the diversity detected in the Río Tinto basin, which underlain the influence of the shared mineral substrates in the microbial ecology of these ecosystems.

The interpolation of points by means of Information Technology programs appears as a technical tool of some relevancy in the hydrogeology in general and in the study of the humid zones in particular. Our approach has beenthe determination of the 3-D geometry of the humid zones of major depth of the Rabasa Lakes. To estimate the topography of the lake bed, we proceed to acquire information in the field by means of sonar and GPS equipment. A total of 335 points were measured both on the perimeter and in the lake bed. In a second stage, this information was used in a kriging program to obtain the bathymetry of the wetland. This methodology is demonstrated as one of the most reliable and cost-efficient for the 3-D analysis of this type of water masses. The bathymetric study of the zone allows us to characterize the mid- and long-term hydrological evolution of the lakes by means of depth-area-volume curves.

The Arcas and River Moscas lake complexes, protected natural areas in Cuenca province, are located in interstratified karst developed in Cretaceous-Tertiary boundary gypsum. These sites are in the central part of the province, near the town of Cuenca, each covering an area of some 3 km2 and distributed between the municipalities of Arcas del Villar, Cuenca, Fuentes, Valdetortola and Villar de Olalla. This paper presents certain geomorphological aspects established in the framework of a broader study on the hydrogeology of the two complexes, fed chiefly by groundwater discharge from this gypsiferous aquifer, whose hydrodynamic behaviour is determined by its fractures and faults.

Iron terraces that form in acidic mine drainage settings are unique and extreme geomicrobiological systems that can provide highly relevant information about the interaction between microbes and their surrounding aqueous environments. These singular systems can represent, additionally, potential models for the study of ancient geological formations (e.g., banded iron formations, stromatolites) and/or for the cycling of iron on Mars. This work describes geochemical, mineralogical, morphological, and microbiological evidence obtained in the highly acidic and Fe-rich Tintillo River (Riotinto mines, Huelva, SW Spain), which can be used to speculate about the origin and nature of the terraced iron formations (TIFs) that are being currently formed in acid mine drainage environments. The size (up to 36 m long and 1 m thick) and continuity (strong development over 3.5 km) of the iron terraces offer a unique opportunity to study the different organic (mainly microbial) and inorganic processes involved in the construction of these characteristic, travertine-like, sedimentary structures. Evidence presented in this study suggests that both types of processes appear to be controlling factors in the formation and internal arrangement of the TIFs, although no defi nitive evidence has been found to support the prevalence of any of these mechanisms with respect to another. The photosynthetic production of dissolved oxygen by eukaryotic microorganisms (green algae, euglenophytes, and diatoms) and the Fe-oxidizing metabolism of acidophilic prokaryotes are critical factors for the formation of TIFs, whereas abiotic parameters, such as water composition, fl ow rate and velocity, or stream channel geometry, also appear to be essential variables.