Extreme Carbon Dioxide Concentrations in Acidic Pit Lakes Provoked

Article pubs.acs.org/est

Extreme Carbon Dioxide Concentrations in Acidic Pit Lakes Provoked by Water/Rock Interaction Javier Sánchez-España,*,†,∥ Bertram Boehrer,‡,∥ and Iñaki Yusta§,∥ †

Geological Survey of Spain (IGME), c/Calera, 1, 28760 Tres Cantos, Madrid, Spain Helmholtz Center for Environmental Research−UFZ, Brueckstrasse 3a, D-39114 Magdeburg, Germany § Department of Mineralogy and Petrology, University of the Basque Country (UPV/EHU), Apdo. 644, 48080 Bilbao, Biscay, Spain ‡

S Supporting Information *

ABSTRACT: We quantify the gas pressure and concentration of a gas-charged acidic pit lake in SW Spain. We measured total dissolved gas pressure, carbon dioxide (CO2) concentration, major ion concentration, isotopic composition of dissolved inorganic carbon (δ13CDIC), and other physicochemical parameters. CO2 is the dominant dissolved gas in this lake and results mainly from carbonate dissolution during the interaction of acidic water with wall rocks, followed by diffusive and advective transport through the water column. The δ13CDIC values suggest that the biological contribution is comparatively small. Maximum CO2 concentrations higher than 0.1 M (∼5000 mg/L) have been measured, which are only comparable to those found in volcanic crater lakes. The corresponding gas pressures of CO2 alone (pCO2 ∼3.6 bar) imply 60% saturation relative to local pressure at 50 m depth. High CO2 concentrations have been observed in other pit lakes of the region. We recommend gas-specific monitoring in acidic pit lakes and, if necessary, the design of feasible degassing strategies.

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The Guadiana pit lake in Herrerı ́as mine is a small (17 000 m2) meromictic APL situated in the Iberian Pyrite Belt (IPB) in SW Spain. This lake shows high gas content at depth, which was first observed in exploratory studies carried out between 2008 and 2010. Elevated gas pressure and gas bubble formation were evidenced in sampling bottles and sediment cores taken at depth. Although with minor intensity, high gas contents have also been detected in other APL of the IPB, including Cueva de la Mora (high CO2 contents near the lake bottom12) and Filón Centro in Tharsis mine (highly gaseous water at depths of 30− 40 m; unpublished work). An important number of pit lakes that are either not accessible (for legal or physical reasons) or poorly studied could potentially present a similar problem of gas accumulation. This work aims at defining the nature and sources of the gas present in Guadiana pit lake, as well as the implications of such high gas concentration. The observations and conclusions of our study are worth considering in other pit lakes and flooded mines with similar geological characteristics.

INTRODUCTION

Acidic pit lakes (APL) formed by flooding of abandoned coal and metal-mine pits are potential sources of acidity and metal pollution for nearby aquifers and surface water courses.1−3 Many studies on APL are therefore focused on metal transport through the water column and underlying sediments. The concentration of dissolved gases (e.g., CO2, H2S, CH4) is normally low and restricted to isolated environments. The possibility of gas accumulation in APL has been vaguely suggested,2,4 though it has never been demonstrated. However, based on prior knowledge, the dissolution of carbonates from wall rocks in a stratified water body may favor the accumulation of CO2 at depth. The metabolic activity of microorganisms (e.g., FeIII and SO42− reducers, methanogens) can also release byproducts such as dissolved inorganic carbon (DIC), H2S, or CH4 that may accumulate in the deep waters.1,2,5 Unlike most natural lakes (where HCO3− dominates), CO2 is the dominant DIC species at low pH.6,7 Some dissolved gases (N2, O2) contribute negatively to the water density of lakes, while others (CO2) cause a positive effect on density and thus help to stabilize the lake stratification.8 However, dissolved gas accumulation in deep waters of meromictic (i.e., permanently stratified) lakes may have catastrophic consequences. This was dramatically shown during the explosive limnic eruptions of the volcanic lakes Nyos and Monoun, Cameroon, in 1984 and 1986.9−11 © 2014 American Chemical Society

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MATERIALS AND METHODS Study Site. The Guadiana open pit at Herrerı ́as mine is located in the province of Huelva (UTM coordinates 121081, Received: Revised: Accepted: Published: 4273

April 24, 2013 March 13, 2014 March 16, 2014 March 16, 2014 dx.doi.org/10.1021/es5006797 | Environ. Sci. Technol. 2014, 48, 4273−4281

Environmental Science & Technology

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Figure 1. Schematic cross section of the Guadiana open pit mine with the associated subhorizontal galleries and vertical shafts (modified with permission from 13; copyright 1963 Summa). The second floor, situated at 90 m depth (55 m below the lake surface) is the most probable source for the CO2 present in the lake. The dashed line in the pit lake represents the chemocline and separates the upper mixolimnion from the lower monimolimnion.

Total Dissolved Gas Pressure. Total dissolved gas (TDG) pressure was measured on-site in Guadiana and other nearby pit lakes with a sensor installed in a submersible probe (Hach). The sensor consists of a sensitive pressure transducer attached to a coiled tubing of gas-permeable membrane.16 The sensor measures partial pressure of gases in the range of 400−2100 mmHg (0.5−2.8 bar), with resolution of 1.0 mmHg (0.0013 bar) and accuracy of ±0.1%.16 The sensor was calibrated on-site with a zero and upscale from a known pressure source (atmospheric pressure). In Guadiana pit lake, the sensor reached its maximum range at depths greater than 40 m, so this instrument could not directly measure gas pressure below that depth. However, due to the long response time of the sensor (20−30 min), an approximation of the final value could be followed. Numerical extrapolation (Supporting Information S4) indicated values close to absolute pressure, which raised the question about the danger of spontaneous ebullition. CO2 Partial Pressure and Concentration. The concentration of O2 and H2S in the deep waters was always below detection. Other gases (e.g., CH4, N2) were limited by absolute pressure and the small Bunsen coefficient compared to CO2.17 Hence, we implemented a special gas sampling method for a reliable and accurate quantification of dissolved CO 2 (Supporting Information S5). Sampling and Chemical Analyses of Waters. Water samples for chemical analyses were taken from different depths with a Van Dorn sampling bottle (KC Denmark A/S). All samples were filtered on-site (0.45 μm, Millipore), stored in polyethylene bottles (60−250 mL), acidified with HNO3, and cool-preserved during transport. Water samples were analyzed by AAS (Mg, Ca, Fe) and ICP-AES (S, given as SO42−) using Varian SpectrAA 220 FS and Varian Vista MPX instruments. DIC (expressed as mg/L CO2) was measured on-site in gastight vials using a portable UV−VIS DR2800 spectrophotometer (Hach) following Hach method LCK 388.16 Neutralization Experiments. To evaluate the possible inorganic sources of CO2 in the lake, we conducted several neutralization experiments with different rock types taken from the mine site and nearby localities (details are given as Supporting Information). These rocks were examined by

4172018; Supporting Information S1 and S2). The mine was intermittently exploited from 1895 to 1989 for the extraction of pyrite (for sulfuric acid production) and base metals (Cu, Zn, Pb).13 The pit was connected with several subhorizontal galleries13 (Figure 1). The pit lake resulted from the natural flooding of the mine pit and associated galleries. The incoming mine water soon became acidic and metal-rich due to the oxidative dissolution of pyrite and other sulfides. This acidic mine water started entering the pit around 1995. The pit still receives groundwater inflow, and the water level gradually increases (Supporting Information S3). Extrapolation of the current flooding rate suggests that the lake will not reach hydrological equilibrium before 2018. The pit lake is strongly stratified and shows sharp vertical gradients of chemical composition.14,15 The chemistry of the deepest waters (50− 60 m) includes extreme concentrations of sulfate and metals (e.g., 23−27 g/L SO42− 6600−7200 mg/L FeII, 355−426 mg/L Zn, 444−494 mg/L Mn),12,14,15 as well as many toxic elements such as Co (11 700 μg/L), Ni (8500 μg/L), As (1600 μg/L), Cd (600 μg/L), Pb (150 μg/L), Cr (90 μg/L), or Se (200 μg/ L). Bathymetry. The bathymetric study was conducted with a portable eco-sounder (GARMIN, Ltd., Olathe, KS) and a geographic positioning system (GARMIN). Depth and positioning data were subsequently processed with SURFER (Golden Software, Golden, CO). Because of the high gas content below 40 m depth, greater depths were recorded with submersible depth sensors. Physicochemical Parameters. Vertical profiles of temperature (T), specific conductance (corrected for 25 °C, κ25), pH, redox potential (ORP) and dissolved oxygen (DO) were obtained with submersible probes (Hydrolab, Hach Company, Loveland, CO) operated from a rubber boat. All sensors were calibrated with commercial standards (pH, ORP, κ25) or watersaturated air (DO). H2S concentration was measured with a H2S-microsensor (Sea & Sun Technology, Trappenkamp, Germany). Parameters were recorded at depth intervals of 0.2−0.5 m in the transitional zones (thermocline, chemocline) and 1 m for the rest of the water column. 4274

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Figure 2. Vertical profiles of temperature (T, in °C), specific conductance (κ25 in mS/cm), pH, and oxidoreduction potential (ORP, in mV) measured in Guadiana pit lake in several seasons.

Information S3). Direct precipitation only contributes a small portion, as deduced from local weather data series. The vertical evolution of κ25 between 21 and 48 m shows a continuous increase of total dissolved solids (TDS) with depth, and a constant TDS in the last 15 m of the water column (Figure 2). The slope break in the κ25 profiles approximately corresponds to the position of the connection between a mine gallery and the open pit and points to an important control of the mine structure on the lake stratification. The pH fluctuates between 2.1 and 3.2 in the mixolimnion (strongly influenced by FeIII buffering and schwertmannite precipitation20), and between 3.5 and 4.3 in the monimolimnion (which is more typical of FeII-dominated waters buffered by Al precipitation20). The ORP indicates oxidizing conditions near the lake surface and moderately reducing conditions in the anoxic monimolimnion, in agreement with the vertical evolution of O2 concentration and iron speciation.12,14 The ORP values near the lake bottom are higher than those measured in other pit lakes where sulfate reduction has been detected.17,21 Total Dissolved Gas Pressure. The TDG measurements obtained in April and September 2011 and December 2013 are shown in Figure 3. The vertical evolution of total dissolved gas pressure discloses three different layers. A first part corresponds to the mixolimnion with TDG readings around 1 bar, which indicates equilibrium with the atmosphere. This background value of TDG was only disrupted by photosynthetically produced O2 peaks (approaching 160% sat.) which are produced by phytoplanktonic algae.12 Once in the monimolimnion, TDG increased continuously with depth and reached values around 2 bar at 40 m depth. Gas pressure approached the TDG sensor limit of 2.7 bar between 40 and 50 m depth and was far beyond this value at greater depths. Extrapolated measurements at 50 and 55 m suggested gas pressures close to the absolute pressure (i.e., hydrostatic plus atmospheric pressure) at these depths. Gas pressures higher than the absolute pressure would result in the formation of gas bubbles and hence would not be observed. A smooth input of further gas would cause continuous ebullition. Abrupt changes such as vertical excursions of water parcels by internal waves or convectively driven flows could result in a sudden release of a larger gas volume.

optical microscopy and analyzed by X-ray diffraction and fluorescence (XRD, XRF) at SGIker facilities (UPV/EHU). Carbon Isotopic Analyses (δ13CDIC). The δ13C composition of DIC in the lake, nearby creeks, and final solutions of neutralization experiments was measured at the Stable Isotopes Laboratory of the Universidad Autónoma de Madrid (UAM), Madrid, Spain. δ13C data are given in parts per thousand (‰) and relative to Pee Dee Belemnite (PDB). For comparison with the pit lake waters, the δ13C composition of diverse plant species typical of the area was measured at the Scottish Universities Environmental Research Center (SUERC), East Kilbride, Scotland, U.K.

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RESULTS AND DISCUSSION Lake Morphology. The basin morphology of the Guadiana pit lake is shown in the Supporting Information (S6). The pit lake basin shows a f unnel-type geometry, which is typical of most pit lakes in the IPB, with steep slopes composed of hard rock and two small, deep sub-basins in the lake center. In December 2013, these sub-basins locally reached maximum depths of 68 m. This geometry usually hinders the development of wind-driven convection cells, thus favoring meromixis.2,8 Lake Stratification. The vertical evolution of T, κ25, pH, and Eh observed in several seasons (June 2010 to Dec 2013) is provided in Figure 2. The physical structure of the lake includes an upper, 21 m thick mixolimnion and a lower, 40 m thick monimolimnion. The mixolimnion is seasonally stratified into a warmer epilimnion and a cooler hypolimnion. The epilimnion shows a high thermal variability (12 °C in winter vs 27 °C in summer), while the monimolimnion does not. Water temperature increases from 13−15 °C below the chemocline to up to 26 °C near the pit lake bottom (Figure 2). Such temperature gradient is notably more pronounced than in most pit lakes of the IPB, which commonly show colder temperatures comprised between 12 and 16 °C near the lake bottom at depths down to 100 m.14 Therefore, the geothermal heat flux probably does not suffice to introduce enough heat.18 The water chemistry (described below) suggests extensive pyrite and carbonate dissolution reactions, which are both exothermic.7,19 Groundwater represents the dominant inflow, as evidenced by the continuous volume increase over recent years (Supporting 4275



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estimated in 2011. Part of this difference could be contributed by other gases (e.g., N2). This is well illustrated by the pCO2 calculated for depths of 0 and 20 m (pCO2 ≈ 0; Figure 3). The corresponding TDG values for these depths (≈1 bar) can only be explained by the presence of other gases such as O2 and N2 (see below). Alternatively, gas pressure could have changed over the meantime by mixing and dilution. The latter is supported by an increase of ∼2.5 m in the lake water level, which represents a volume increase of around 7 to 8%. In this period, we observed a slight change in some parameters in the deep part (e.g., κ25, from 18 800 to 18 200 μS/cm; T, from 26.2 to 25.7 °C; pH, from 4.2 to 3.7). The TDG pressure also decreased slightly in the 30−45 m depth interval during the same period (Figure 3). The lake had probably received the input of less saline and colder groundwater at depth, which had provoked dilution and homogenization of the deep layer. A dilution of ∼6−10% was also deduced from decreases in major ion concentrations (see below). CO2 Concentration. The concentrations of several gases (O2, H2S, CO2) measured in the pit lake by several methods, along with the DIC isotopic composition (δ13CDIC), are given in Table 1. Oxygen concentration was only significant near the Table 1. Measured and/or Calculated Concentration of Several Gases (O2, H2S, CO2) and Isotopic Composition of Dissolved Inorganic Carbon (δ13CDIC) in the Guadiana Pit Lake at Herrerı ́as Mine, Huelva, Spaina

Figure 3. Profiles of total dissolved gas pressure (TDG, in bars) in the Guadiana pit lake, Herrerı ́as mine, in different periods. Measurements obtained in December 2013 with gas sampling bags (black triangles) refer to partial pressure portion of CO2 only (pCO2). The red line indicates the absolute pressure (hydrostatic plus atmospheric pressure). Gas pressures beyond this line would result in gas exolution and the formation of gas bubbles. Gas pressure of potentially residual CO2 from dissolved carbonates based on Mg−Mn concentration (open triangles) is included for comparison (see text).

The extremely high gas pressure in this lake is evidenced in the sampling devices deployed at depth, which degas spontaneously when recovered to the lake surface (Supporting Information S7). TDG pressures around 6 to 7 bar (representing 97% gas saturation) were also measured between 1992 and 2003 at 60−90 m depth in Lake Monoun, Cameroon, which led to a controlled degassing of this lake in 2004 to prevent a new limnic eruption.11 High gas pressures have been observed in other African volcanic lakes (CO2 and CH4 in Lake Kivu, CO2 in Lake Nyos22,23). As we had no method to countercheck the extrapolated values directly, we decided to confirm at least the leading CO2 part in December 2013. Gas-tight sampling bags were filled in situ at depths 0.1, 20, 35, 40, 45, 50, and 55 m. Under controlled temperature conditions and atmospheric pressure, gas bubbles formed in the sampling bags (Supporting Information S5). From measurements of volume and mass, the dissolved CO2 concentration could be evaluated, and the partial pressure of CO2 was calculated (Figure 3). The highest value corresponded to a volume of 2.5 L of dissolved CO2 (under normal pressure) per liter of lake water. This is a higher gas load than in Lake Nyos and Lake Monoun at similar depths (35−50 m, even during the 1990s9−11). Only Nyos showed much higher gas pressures in a bottom layer around 200 m depth. Resulting partial CO2 gas pressures in Guadiana pit lake are between 1.7 and 3.6 bar. In conclusion, most of the gas pressure could be contributed to dissolved CO2. Despite the enormous volume of dissolved CO2, however, a significant difference still exists with respect to the TDG values

depth (m)

O2 (mg/L)b

H2S (mg/L)c

CO2 (mg/L)d

CO2 (mg/L)e

δ13CDIC (‰)f

0 5 10 15 20 30 35 40 45 50 55 60

9.3 10.4 0.5 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0

b.d. b.d. 0.3 0.2 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.

71 83 78 924 1100 2300 2980 n.a. >3160 n.a. n.a. n.a.

b.d. n.c. n.c. n.c. n.c. n.c. 2659 2589 3289 4924 4962 n.c.

−6.9 n.a. −10.0 −11.0 −11.1 −10.6 n.a. n.a. n.a. −12.3 n.a. −18.1

a

Abbreviations: b.d., below detection limit; n.c., not calculated; n.a., not analyzed (degassed sample). bMeasured with a luminiscence dissolved oxygen (LDO) sensor in September 2011. cMeasured on-site with a microsensor in September 2011. dCarbon dioxide concentration measured in June 2010 by UV−VIS spectrophotometry. eCarbon dioxide concentration obtained by gas volume measurement in gas sampling bags in December 2013. fδ13C data for dissolved inorganic carbon (∼CO2) relative to PDB, as measured on March 2012.

surface, and it drastically dropped below detection values at 10 m depth. Traces of H2S (0.2−0.3 mg/L) were measured between 10 and 15 m, but the concentration of this gas was below detection at all other depths. There is no available data on nitrogen concentration, but equilibrium concentration with the atmosphere would correspond to 15 mg/L of N2. Concentration of dissolved CO2 has been obtained by two independent methods: UV−VIS spectrophotometry and gas volume measurement in gas-sampling bags. These separate data sets are complementary and indicate an increasing concentration of CO2 with depth (Table 1). The spectrophotometric measurements in the upper 10 m yield CO2 concentrations of 4276



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Figure 4. Vertical evolution of major ion concentration (SO42−, Fe, Mg, Ca) measured in the Guadiana pit lake in several seasons (2008−2011). Two punctual analyses conducted in December 2013 are also plotted for comparison. The iron content represents total iron; for depths greater than 20 m, Fet ≈ FeII.

Figure 5. Vertical evolution of Fe/S (a) and Ca/Mg (c) molar ratios (as deduced from the chemical data of Figure 4) and binary plots of Fe−SO42− (b) and Ca−Mg (d) in the Guadiana pit lake. Linear trends between elements in (b)−(d) are clearly differentiated at both sides of the chemocline (empty circles, mixolimnion, above 20 m depth; solid circles, monimolimnion, below 20 m depth).

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area,25,26 while the latter is less common and usually associated with very acidic AMD. A good correlation between these two cations exists in the upper 20 m of the water column, whereas no correlation is observed in the lower part of the lake (Figure 5d). Such discrepancy might be related to the different solubility of these two elements. In acid-sulfate solutions, Mg is highly soluble and tends to be conservative, while Ca is less soluble and its aqueous concentration is normally controlled by gypsum solubility.27,28 This possibility is supported by geochemical calculations that suggest gypsum solubility equilibrium at all depths and under-saturation for Mg-sulfate species (exemplified by epsomite; Supporting Information S8). The plateau observed in Figure 5d likely derives from solubility equilibrium of Ca. The maximum concentration of this element (600 mg/L) roughly equals the theoretical gypsum solubility limit at the sulfate concentrations found in the lake.28 Gypsum crystals have been observed in the suspended particulate matter of the deep waters. Thus, the continuous precipitation of gypsum during years would have caused a strong depletion of Ca, while Mg would have remained unaffected. The most probable source for Mg and Ca is the dissolution of carbonates, which are usually present in variable proportions in the IPB mineralizations13,29 and specially abundant in Herrerı ́as mine. Both calcite (CaCO3) and dolomite (CaMg(CO3)2) were common gangue minerals in this mine,30,31 while malachite (Cu2(CO3)(OH)2) and azurite (Cu3(CO3)2(OH)2) were the ores mined in the nearby Santa Bárbara pit32 (Supporting Information S1). Neutralization experiments (Figure 6; see also Supporting Information S9) indicate that even a small amount of highly reactive carbonate in the host rocks may provoke (by a differential dissolution effect due to a faster reaction kinetics with respect to silicates and sulphides) a rapid acidity neutralization along with prompt releases of CO2(g) to the solutions. Among the rock types present in this mine site, the altered (spilitic) basaltic rocks are especially rich in carbonates (magnesian calcite; Supporting Information S10), and thus, their neutralization potential is considerably higher than other carbonate-deficient lithologies like shales or other volcanic rocks (Figure 6). Carbonates may be also present in the pyrite mineralizations,29−32 thus releasing high amounts of CO2 to the acidic mine waters flooding the deep mine workings (Supporting Information S9). Mass Balance Calculations. To check whether the CO2 present in the lake can entirely originate from carbonate dissolution, we have followed a geochemical approach based on the observed water chemistry. Because Ca2+ is not conservative, this cation cannot be used for the calculations. Dissolved Mg2+ in groundwater and mine waters is usually ascribed to the dissolution of carbonates (dolomite, magnesian calcite, ankerite).28 Some contribution from the silicate (e.g., chlorite) dissolution cannot be discounted, although the latter is usually minor due to their comparatively slow dissolution kinetics.28 Our experimental results (Supporting Information S9) support this statement. Thus, if we assume that Mg mostly derives from the dissolution of carbonates with dolomitic composition (reaction 1):

71−83 mg/L, which are very similar to those found in other pit lakes in the area.12 The values obtained spectrophotometrically for the 15−30 m depth interval indicate increasing concentrations between 1000 mg/L at 15 m and 2300 mg/L CO2 at 30 m. These concentrations are also comparable to those found in the nearby pit lake of Cueva de la Mora12,17 at the same depths. CO2 concentrations in the 35−45 m depth interval range between 2600 mg/L (35 m) and 3600 mg/L (45 m; Table 1). The content of CO2 then abruptly increases in the deeper layer and far exceeds the concentrations observed in any other lake, river, or groundwater of the area. When oversaturated (gas pressure >1 bar) samples are recovered from these deep waters, gas escapes by violent ebullition, and thus, CO2 concentration cannot be quantified by conventional methods. Dissolved CO2 content of these deep waters could only be calculated by gas volume measurement in gas bags sampled in 2013 (Supporting Information S5). Comparable CO2 concentrations have only been found in the deeper layers of the Cameroonian volcanic crater lakes Nyos and Monoun.9−11,22−24 Major Ion Chemistry. The vertical variation of major ions (SO42−, Fe, Mg, Ca) is shown in Figure 4. The depth profiles of these elements approximately follow the pattern described for κ25, with a steep concentration gradient below the chemocline (21−48 m depth) and nearly constant values in the bottommost layer. Dissolved iron is fully oxidized (Fet ≈ FeIII) in the upper 20 m, while only ferrous iron is present in the monimolimnion. The mixolimnetic concentrations of SO42− (2−6 g/L), Fet (25−600 mg/L), and Mg (175−550 mg/L) are similar to those measured in other pit lakes and mine waters of the province.25,26 However, the values measured near the lake bottom (21−27 g/L SO42−, 6200−7200 mg/L Fet ≈ FeII, 2700 mg/L Mg) are among the highest measured in any pit lake.1−3 The concentration of Mg is exceptionally high compared to most other mine waters of the area.25,26 Calcium concentration (300−600 mg/L Ca) is comparable to that of other pit lakes and AMD systems of the IPB and follows a reverse trend (Figure 4). A gradual temporal decrease of salt concentration in the deep part is suggested by the vertical profiles from 2008 to 2013 (Figure 4), which is coherent with the aforesaid dilution process by groundwater inflow. The Fe/S molar ratio increases from 0.05−0.1 near the surface to around 0.5 at depth (Figure 5a). The latter value matches the stoichiometry of pyrite (FeS2). Both the Fe/S profile and the Fe−SO42− binary plot (Figure 5b) strongly suggest increasing water/rock interaction with depth. Extensive pyrite dissolution would have occurred in pit walls, galleries, and shafts connected to the pit lake. In addition to deeply affecting the pit lake chemistry, pyrite dissolution is likely responsible for the sharp temperature gradient observed in the lake. The water chemistry suggests the dissolution of around 0.12 mol of pyrite/L of water. Considering the enthalpy change of the pyrite oxidation reaction (−1409 kJ/mol) and the specific heat of water (4186 kJ/(kg °C)), the heat released during the oxidation of this amount of pyrite (≈163 kJ) would theoretically account for a temperature increase (ΔT) close to 39 °C/L of water in a closed system. This simple estimation does not consider the subsequent heat loss due to conduction and convection but demonstrates the potential of pyrite oxidation as a heat source in these mines. The Ca/Mg ratio varies from 1.0−1.2 at near-surface conditions to around 0.2 at depth (Figure 5c). The former value is typical of natural surface and ground waters in the

Ca 0.5Mg 0.5CO3 + H 2SO4 ↔ 0.5Ca 2 + + 0.5Mg 2 + + SO4 2 − + CO2 (g) + H 2O (1)

Then, the theoretical amount of CO2 released during this reaction approaches 9000 mg/L for depths of 50−55 m. The 4278



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volcanic rock samples (Figure 6b) have yielded values around −15‰, which are also coherent with the δ13C values obtained for CO2 in the lake. In contrast, the obtained δ13CDIC data are very different from those commonly measured in magmatically derived CO2 accumulated in volcanic lakes such as Lake Nyos in Cameroon (≈ −3‰).10 The more negative δ13C value obtained near the lake bottom (−18.1‰ at 60 m) seems to be influenced by anaerobic decomposition of phytoplanktonic organic matter that usually accumulates in lake sediments38 (Supporting Information S12). Overall, the carbon isotopic data suggest that the CO2 present in the lake is mostly derived from carbonate dissolution. Implications for Lake Stability. The contribution of CO2 to the water density at the zone of maximum gas pressure (ρ = 1032 g/L at 55 m depth, as suggested by the RHOMV numerical program39,40) accounts for only ∼3% of the total solute contribution, in good agreement with calculations in other meromictic pit lakes.41 This is small in comparison with the contribution of major ions such as SO42− (52.6%), Fe2+ (25%), or Mg2+ (11.5%). However, when released into the gas phase, the CO2 dissolved in 1 L of lake water would occupy a volume of 2.5 L at surface pressure (1 bar). The gas pressure pCO2 = 3.7 bar explained the violent ebullition of deep water samples when brought to the surface and demonstrates the immense volume of potentially available gas and the destructive energy stored in water samples charged with CO2. All dissolved CO2 could fill 115 000 m3, which corresponds to a 4−8 m layer above the lake surface. Because the studied pit lake is still in its flooding stage, CO2 concentration will largely depend on a delicate equilibrium between CO2-generating processes (carbonate dissolution, microbial respiration) and dilution processes (groundwater inflow, mixing and occasional turnover, diffusion). We believe that carbonate dissolution is mainly taking place in the pit basin (where acidic water is in contact with carbonate-rich rocks in the pit walls) and that dilution basically results from groundwater inflow. CO2 could also be generated in mine voids and could enter the pit lake through interconnecting galleries by diffusive and advective transport. The graphs of Figure 6b show that the kinetics of calcite dissolution is very fast up to pH∼5.5 but still proceeds at a slower rate up to neutral pH. Hence, the acid dissolution of carbonates will continue in the deep, Al3+-buffered pit lake waters (pH 3.7− 4.3) as long as Al3+ is not removed from solution. The physical stability of the lake does not seem to be compromised by internal factors. The higher temperature of the deep water is compensated for a much higher dissolved solids content, so that a marked vertical density gradient exists between the deep, gas-rich layer (ρ = 1032 g/L) and the overlying mixolimnetic water (ρ = 1008 g/L). However, external factors could provide the energy required to provoke a sudden gas ebullition. Landslides and mine collapses are historically frequent in the area13 and represent the most obvious threat. Moderate seismological activity in the form of low intensity earthquakes (3−5 on the Richter scale)42 represents an additional risk. A final threat to consider is a hypothetical mine dewatering (which is being considered in some flooded mines of the IPB with unknown gas charge). Pumping lake water from the surface of a gas-charged pit lake would decrease the hydrostatic pressure at depth, and this could bring the deep water into gas saturation. Dewatering Guadiana pit lake by 25 m would result in spontaneous ebullition from the deep layer. If other gases contribute to the

Figure 6. Temporal evolution of pH during acid digestion experiments with different rock types from the study area. (a) Shales from Herrerı ́as mine and felsic/basic volcanic rocks from other sites in the IPB. (b) Basic volcanic rocks from Herrerı ́as mine. Note the different temporal scales in (a) and (b). The experimental conditions are explained in more detail in the Supporting Information (S9).

resulting partial pressure (5.8 bar) would be very close to the absolute pressure (Figure 3). Similar values are obtained if Mn2+ (not shown) is used to estimate carbonate dissolution. This estimation would be further increased by considering a Mg-rich calcite as dissolving carbonate. The observed cation concentrations are therefore sufficient to account for the CO2 present in the lake without contribution of additional (geothermal, biological) sources. According to this calculation, one-third of the CO2 has already left the monimolimnion, while two-thirds still remains (Figure 3). Stable Isotopes. The δ13CDIC isotopic analyses conducted in March 2012 show a decreasing vertical trend (Table 1; Supporting Information S11). The δ13CDIC value of the lake surface (−6.9‰) is coherent with water equilibrated with atmospheric CO2 (presently around −7 to −8‰).33 The δ13C values in the 10−50 m depth interval (−10.0 to −12.3‰) match the DIC isotopic data obtained for local creeks and rivers of the area (−10.2 to −14.6‰; Supporting Information S12). These isotopic values are typical of DIC in surface and ground waters equilibrated with edaphic CO2 and/or with modern meteoric carbonates.34−37 Isotopic analyses of DIC in the solutions obtained after neutralization of carbonate-containing 4279



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gas pressure, even a correspondingly smaller water level drop would trigger this process. We conclude that the geological, hydrological, and geochemical features of APL may favor the accumulation of dissolved CO2 at depth. In particular, the combination of a low pH, a permanent stratification, a high water depth (∼hydrostatic pressure), a DIC source (via carbonate dissolution and/or intense microbial respiration), and long residence times (often decades) may lead to significant CO2 entrapment. Such conditions are found in many other pit lakes of the IPB, and could also exist in other mine districts where acidic water reacts with carbonates at depth. The obtained results strongly support the inclusion of on-site measurements of TDG pressure in water quality monitoring programs in pit lakes and flooded mines.

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ASSOCIATED CONTENT

S Supporting Information *

Satellite and panoramic images of Herrerı ́as mine site, depth evolution of Guadiana pit lake, TDG readings, extended methods section, lake bathymetry, deep-water sampling video (.avi), saturation index calculations, results of neutralization experiments, photomicrographs of volcanic wall rocks, depth profile of δ13CDIC, and compilation of δ13C values of diverse carbon pools. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Author Contributions ∥

These authors contributed equally.

Funding Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Spanish Ministry of Science and Innovation through the Plan Nacional de I+D+i (BACCHUS Project, Ref. No. CGL2009−09070) and Gobierno Vasco (IT762-13). Ramón Redondo handled the δ13C isotopic analyses at UAM. Matthias Koschorreck is thanked for the field measurements of H2S and for plant sampling for δ13C analyses. We appreciate the comments of three anonymous reviewers on an earlier draft.

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REFERENCES

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