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The role of secondary minerals in controlling the migration of arsenic and metals from high-sulfide wastes (Berikul gold mine, Siberia)
Applied Geochemistry 18 (2003) 1347–1359 www.elsevier.com/locate/apgeochem
The role of secondary minerals in controlling the migration of arsenic and metals from high-sulfide wastes (Berikul gold mine, Siberia) R. Giere´a,*, N.V. Sidenkob, E.V. Lazarevab a
Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1397, USA The United Institute of Geology, Geophysics and Mineralogy, pr. Koptyuga 3, Novosibirsk 630090, Russia
b
Abstract The role of secondary minerals in controlling the migration of As, Cu, Zn, Pb and Cd has been investigated in piles of high-sulfide waste at the Berikul Au mine, Kemerovo region, Russia. These wastes contain 40–45 wt.% sulfides and have been stored for approximately 50 a near the Mokry Berikul river. Sulfide oxidation generates acid pore solutions (pH=1.7) with high concentrations of SO2 4 (190 g/l), Fe (57 g/l), As (22 g/l), Zn (2 g/l), Cu (0.4 g/l), Pb (0.04 g/l), and Cd (0.03 g/l). From these solutions, As is precipitated as amorphous non-stoichiometric Fe-sulfoarsenates in the lower horizons of the waste piles. During precipitation of the Fe-sulfoarsenates, the concentration of Fe in these phases decreases from 34 to 21 wt.%, that of As increases from 11 to 22 wt.%, while the S content remains approximately constant (5.4–5.8 wt.%). Arsenic is also accumulated in jarosite-beudantite solid solutions (up to 8.4 wt.% As), which occur as inclusions in the amorphous Fe-sulfoarsenates. In efflorescent crusts on the surface of the waste pile, As coprecipitates with the Fe(III) sulfates copiapite (0.27 wt.% As) and rhomboclase (0.87 wt.% As). Zinc and Cu are incorporated primarily into Fe(II) sulfates, i.e. melanterite in the interior of the waste pile, and rozenite in the efflorescent crust. The Zn mineral dietrichite is also formed at the surface of the waste pile as a result of evaporation of pore solutions, and is the only Fe(II) sulfate containing detectable amounts of As (0.64 wt.%). Lead is mainly co-precipitated with minerals of the jarosite group, where the Pb content may reach 4.3 wt.%. Co-precipitation of toxic elements with sulfates and sulfoarsenates of Fe is shown to be a significant mechanism in controlling the concentration of heavy metals in pore solutions of high-sulfide mine wastes. Precipitation of secondary phases causes the formation of a hardpan layer with low permeability at a depth of 1–1.5 m below the surface of the waste pile. Rainwater accumulates above the hardpan horizons and slowly drains along these aquicludes to the bottom of the pile. Most of the rainwater evaporates during infiltration. This leads to formation of the described efflorescent sulfate crusts. Dissolution of these crusts during the next rain storm produces highly acidic surface waters (pH=1.1) rich in SO2 4 (30 g/l), Fe (18 g/l), As (0.24 g/l), Zn (0.12 g/l), Cu (0.04 g/l), Pb and Cd (0.002 g/l). During the warm (t> 0 C) period of the year, which lasts about 7 months, these surface waters transport a total of a few tens of kilograms of As and Zn, several kilograms of Cu, and a few hundred grams of Pb and Cd from the waste pile into the Mokry Berikul river. As a result, the concentrations of these metals in the river water increase by an order of magnitude, thus reaching levels close to, or exceeding the maximum values permissible for drinking water. # 2003 Elsevier Science Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (R. Giere´). 0883-2927/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0883-2927(03)00055-6
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1. Introduction Abandoned mine wastes containing high sulfide concentrations are among the most serious sources of environmental pollution. Understanding the geochemical processes which control precipitation and dissolution of secondary minerals in abandoned sulfide mines is crucial for the formulation of models that predict the environmental impact of such sites. Moreover, a better knowledge of these mechanisms will allow remediation of existing problem sites and/or a reduction of the extent of future pollution. Pore waters in and drainage solutions from high-sulfide waste are characterized by low pH values and high concentrations of various heavy metals (Blowes et al., 1991; Nordstrom, 1991). The concentration of heavy metals in the pore solutions is mainly controlled by their precipitation together with Fe hydroxides and/or sulfates (Blowes and Jambor, 1990). Precipitation and dissolution cycles of some secondary minerals are strongly influenced by seasonal wetting and drying cycles (Frau, 2000), and thus it is important to include meteorological parameters in models that simulate environmental impacts. Although mobilization of As from mine waste is discussed in the literature, most reports focus on low-sulfide wastes only (e.g., Al et al., 1994; Leblanc et al., 1996; Roussel et al., 1998; Langmuir et al., 1999; Shuvaeva et al., 2000; Ardau et al., 2001; Gaskova and Bortnikova, 2001). The goal of the present study is to understand the processes controlling the migration and sequestration of Cu, Zn, and Pb and, in particular, the mobility of As in high-sulfide waste piles. To achieve this goal, the authors investigated high-sulfide mine wastes at the Berikul Au mine in Siberia. A specific objective of the present paper is to combine detailed mineralogical investigations of the secondary phases occurring in the waste with both meteorological observations and studies of the water
chemistry of samples collected in various parts of the waste pile and in the nearby river.
2. Description of the waste site The Berikul Au mine is situated in the northern part of the Kemerovo region, Western Siberia, about 450 km NE of Novosibirsk (Fig. 1). The deposit, an Au-sulfidequartz vein with a Au content of 1–5 g/t, was mined exclusively for Au. The mine was in operation between 1942 and 1991, but no other production data were available to the authors. Gold occurred as fine-grained intergrowths with sulfide minerals, mainly pyrite and arsenopyrite. From these sulfides, Au was extracted at the Berikul mill by using the cyanide technique. After removal of the Au-bearing cyanide solutions, the sulfide flotation residues were neutralized by adding Ca(ClO)2 before being dumped on waste piles. The waste studied here has been in a pile since 1952, while waste from other piles was used for road construction when new tailings impoundments were built in 1972. The discarded material contains 40–45 wt.% finegrained sulfides, including pyrite (35–40 wt.%), arsenopyrite (2–5 wt.%), and minor amounts of pyrrhotite, sphalerite, chalcopyrite, and galena. Among the gangue minerals found in the wastes, the predominant phases are quartz (30–35 wt.%), albite (5–10 wt.%), chlorite (5–10 wt.%), muscovite (about 5 wt.%), and calcite (3–5 wt.%). The studied high-sulfide wastes have been deposited on alluvial material, consisting of carbonate boulders, and were stored for about 50 a on the left bank of the Mokry Berikul river. The waste pile has a length of approximately 250 m, a width of 50 m at its base, and a height of 3 m (Fig. 2). The alluvial material around the waste pile is dry, and no springs have been detected near the river bank, suggesting that the river
Fig. 1. Map showing the location of the Berikul Au mine in the Kemerovo region of Southwestern Siberia.
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Fig. 2. Schematic map of the studied waste pile and the surrounding area at the Berikul site showing sample localities, hydrological stations, and excavation pits.
represents the local ground water table. The only ‘‘springs’’ found are drainage water emanations at the base of the waste piles (Plate IA, Fig. 2). The Berikul site is located at an altitude of 850 m above sea level. The average yearly temperature is 1.5 C, and the total precipitation averages 600 mm of rain and snow annually. There is, however, a distinct warm period during which the temperature is above freezing. This warm period lasts from May to October, and most of the precipitation (approximately 400 mm) falls during this time.
3. Methods of investigation Solid samples were collected from both surface outcrops and excavation pits, which were dug with a mechanical excavator in different parts of the waste pile (see Fig. 2). The pits were excavated in order to gain an insight into the stratification of the waste pile, to compare vertical and lateral zoning, and to collect samples from the interior of the pile. The solids were then put
into hermetically sealed polyethylene bags and frozen immediately to preserve the initial characteristics of pore waters. The solid waste material was studied by reflected light microscopy and scanning electron microscopy (Jeol JSM-36). X-ray powder diffraction (XRD) analyses were performed with a DRON-UM diffractometer (Burevestnik, made in Russia) using filtered Cu-Ka radiation. Thermogravimetric analysis (TGA) was carried out with a MOM device (made in Hungary), using a 120 mg sample, which was heated from room temperature to 1000 C, at 12 C/min. The compositions of the secondary phases were determined with a CAMECA-SX50 electron microprobe at the Department of Earth and Atmospheric Sciences of Purdue University. The instrument is equipped with 4 wavelength dispersive spectrometers, and was operated for quantitative analysis at an acceleration potential of 20 kV and a beam current of 80 nA measured on a Faraday cup. Samples and standards were coated with 20 nm of C. Well-characterized minerals and synthetic oxides were used as standards. Data collection time was 20 s for most major elements, and
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Plate I. (A) Drainage water emanates from the waste pile and forms reddish-brown streamlets which eventually enter the Mokry Berikul river. This photograph shows where drainage water sample B-3 was collected (see Fig. 2). (B) Photomicrograph displaying sequence of alteration zones around leaching cavities within the hardpan layer (horizon 2 in Fig. 3). The leaching cavities appear as open space (white) in the upper part of the figure. Zone 1: sulfides + gypsum. Note large euhedral secondary gypsum crystals; Zone 2: hydronium jarosite + gypsum +sulfides; Zone 3: amorphous Fe-sulfoarsenates (reddish-yellow, see arrow). Note that these Fe-sulfoarsenates encrust the walls of the leaching cavity. Plane-polarized light. (C). Photomicrographs showing the different morphologies of jarosite. Left: spherical aggregates of hydronium jarosite (arrows) in horizon 4 (see Fig. 3); Right: oolites of jarosite in a crack in horizon 4 (see Fig. 3). Both pictures taken in plane-polarized light. (D). Backscattered electron image (BSE1) and element distributions maps (for As, Fe, Al, K and Pb) showing matrix and inclusions in the amorphous Fe-sulfoarsenates of horizon 4 (see Fig. 3). Color coding: elemental concentrations increase from blue to green, to yellow, to red.
30–60 s for minor elements. Data collection on background positions on either side of each peak was half the time of data collection on the corresponding peak position. The raw data were corrected on-line by the PAP correction procedure (Pouchou and Pichoir, 1984). Detection limits for different components varied from 0.01 to 0.05 wt.%, and analytical precision was approximately 2% for most major elements, and better than 10% for most of the trace elements. Element distribution maps were generated at 20 kV and 100 nA, and the data were collected for 100 ms/pixel in wavelength dispersive mode. In water-soluble phases, the concentrations of As, Cu, Zn, and Pb were determined by X-ray fluorescence (XRF) analysis. For this purpose, less than 1 mg of single-crystal particles and monomineralic aggregates (about 100 mm across) were selected, identified by XRD, and then mounted on polyethylene film, which was inserted into a Russian-made IRIS-3 spectrometer. The
sample was analyzed using the method described by Kolmogorov and Trounova (2002). Detection limits of the XRF analysis were 0.05 wt.% for As, and 0.01 wt.% for Cu, Zn, and Pb, and the analytical precision was within 20%. Water samples from the surface of the waste pile were collected during rain events using special catchers, resembling gutters and connected to a tank, to estimate the volume of water flowing down the slopes of the waste pile during each rain event. The amount of rain was measured with a pluviometer. Samples of drainage solutions were collected from streamlets that emanate at the bottom of the waste pile (Plate IA, Fig. 2). Water samples of the Mokry Berikul river were collected at 3 sites: upstream (200 m away from the waste pile), downstream (200 m away), and near the bank, where the waste is stored (see Fig. 2). Pore solutions were extracted from the moist silt material by fluid expulsion at a pressure of 100 Pa after the samples had been
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thawed in the laboratory. With the exception of the pore solutions, the Eh and the pH of all water samples were measured in the field immediately after sampling by using an INFRASPAK potentiometer (made in Russia), whereby the precision of a single measurement was 10 mV for Eh, and 0.08 for pH. The Eh and pH measurements for the pore solutions were carried out only after the samples had been thawed and the solutions extracted in the laboratory. Approximately 100 ml of solution were collected by a syringe through a filter (0.45 mm) into glassware, and were subsequently divided into two aliquots. One of the aliquots was then acidified with HNO3 to preserve the metal concentration, whereas the other aliquot was preserved in its initial state. All water samples were stored about 1 month at a temperature 4–6 C in a refrigerator. The concentrations in the acidified solutions of As, Ca, Cd, Cu, Fe, K, Mg, Na, Pb, and Zn were determined by flame atomicabsorption spectrometry (AAS; Perkin-Elmer equipment, model 3030E equipped with an HGA-600 graphite furnace) and by thermal-electric AAS (PueUnikam equipment, model SP-9). The contents of 2 NH+ were deter4 , Cl , F , NO3 , HCO3 , and SO4 mined in the second aliquot by ion chromatography using a Russian-made MILIXROM chromatograph.
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4. Results and discussion 4.1. The waste pile in cross-section During the fifty 50 a of storage in the pile, the waste was subjected to supergene alteration, which led to a characteristic layering. In cross-section, 5 main horizons can be distinguished in the waste pile (Fig. 3). These are distinct in terms of mineral content, color, porosity and hardness, and comprise, from bottom to top: 4.1.1. Horizon 1 This zone consists of friable, only slightly altered sulfide wastes of gray-green color. It contains 3–5 wt.% calcite, and is approximately 1–1.5 m thick. 4.1.2. Horizon 2 Overlying horizon 1, this layer consists of lithified waste containing gypsum as binding material. It is a hardpan layer with a thickness of approximately 1 m. Leaching cavities are observed in this zone (shown in black in Fig. 3), and these are often filled by jarosite and amorphous Fe-sulfoarsenates (Plate IB). As a rule, such hardpan layers are characterized by low permeability, which prevents penetration of solutions and gases into the deeper horizons of waste piles (Blowes et al., 1991). At the studied site, the hardpan layer contains trace amounts of calcite, i.e. approximately 1 wt.%, but all overlying horizons are devoid of calcite. 4.1.3. Horizon 3 Overlying the hardpan layer is a tobacco-colored, 0.5–1 m thick horizon, which consists of very moist and fine-grained wastes of silt size. This zone, referred to as the melanterite zone, contains leached relics of hardpan material (Fig. 3), which are overgrown by melanterite aggregates (up to 10 cm across). The presence of such relics of lithified material in horizons above the actual hardpan layer documents that the lithified waste is dissolving as a result of decreasing pH in the pore solutions during sulfide weathering. The hardpan material, however, decomposes only slowly, because of its low permeability. 4.1.4. Horizon 4 This thin intermediate zone has a gray color, and is composed of quartz, jarosite and sulfides. It separates the melanterite zone from the overlying horizon 5.
Fig. 3. Supergene weathering profile through the waste pile at the Berikul site. This profile displays the situation encountered in excavation pit B-2/99 in the eastern part of the waste pile (see Fig. 2), but it is representative of the entire site. The numbered horizons are described in the text.
4.1.5. Horizon 5 The uppermost horizon is distinctly yellow, is up to 0.5 m thick, and contains some strongly oxidized relics of hardpan material. The predominant phase is jarosite; hence, this horizon is referred to below as the jarosite zone.
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The second jarosite variety, jarosite sensu stricto, occurs as oolites of about 5 mm in diameter along cracks (Plate IC) in horizon 4 (see Fig. 3). It also forms rims around hydronium jarosite, showing clearly that it precipitated after hydronium jarosite. Jarosite is considerably richer in Cu and Zn than hydronium jarosite (Table 1). Moreover, jarosite is characterized by significantly higher contents of both As (1.7 wt.%) and Pb (4.3 wt.%). Joint incorporation of As and Pb into jarosite is consistent with the possible occurrence of a solid solution between jarosite [KFe3+ 3 (SO4)2(OH)6] and beudantite [PbFe3+ (AsO ,SO )(OH) 3 4 4 6; see also Jambor and Dutrizac, 1983; Rattray et al., 1996].
4.2. Secondary minerals 4.2.1. Gypsum (CaSO42H2O) Gypsum is the earliest secondary mineral found, and it occurs even in the slightly altered sulfide wastes of zone 1. In the hardpan layer, gypsum crystals are up to 2 mm across and form a framework that binds the finegrained sulfide phases (Plate IB). The concentrations of As, Cu, Zn, and Pb in gypsum are all below detection limits. 4.2.2. Jarosite solid solutions Two main varieties of jarosite-group phases were detected, and these can be distinguished on the basis of the predominance of alkali-site ions (Table 1). The first variety is hydronium jarosite (H3O,K,Na)Fe3+ 3 (SO4)2(OH)6. The second variety is jarosite sensu stricto, (K,H3O,Na)Fe3+ 3 (SO4)2(OH)6, with K concentrations that are considerably higher (3.1 wt.%) than in hydronium jarosite. Average Na concentrations are low in both varieties, namely 0.3 and 0.5 wt.% in hydronium jarosite and jarosite, respectively. Hydronium jarosite forms spherical aggregates up to 20 mm in size, which were observed in all horizons (Plate IC). Extensive precipitation of hydronium jarosite must have started after formation of gypsum in the hardpan layers, as documented by the mineralogical zoning observed around leaching cavities in the hardpan (typical zoning shown in Plate IB). The zones more distant from the open cavity are less altered and contain the earliest secondary phase, i.e. gypsum (Zone 1 in Plate IB). In the more weathered material, which forms the walls of cavities (Zone 2 in Plate IB), hydronium jarosite begins to appear. Sulfoarsenates are observed closest to and as incrustations of the cavities (Zone 3 in Plate IB). From these observations, it is concluded that during the weathering of the waste material, the secondary minerals crystallized in the following sequence: gypsum ! hydronium jarosite ! sulfoarsenates.
4.2.3. Amorphous iron sulfoarsenates (AISA) Sulfoarsenates of Fe are precipitated in the leaching cavities observed in the hardpan layer (Plate IB), as well as in the lithified hardpan relics in the waste of the overlying horizons 3 and 5 (Fig. 3). These phases are Xray amorphous, and exhibit a reddish-brown to reddishorange color. As outlined below, there are 3 varieties of AISA, which can be distinguished on the basis of their chemical composition. Since the authors were unable to select enough material of a specific type of AISA based on its physical appearance, a sample of non-distinct AISA had to be used for infrared (IR) spectroscopy, TGA and XRD. The IR spectrum of this material is similar to that of sarmientite, Fe3+ 2 (AsO4)(SO4)OH5H2O, and thus qualitatively points to a similar composition (Fig. 4a). The thermal properties of these two phases are also similar, as documented by the weight loss curves shown in Fig. 4b. The weight loss between 100 and 685 C, representing water content, is 30.2% for the amorphous phase, and 23.0% for sarmientite. The weight losses between 685 and 980 C, representing the SO3 content, are 4.8 and 16.4% for the amorphous phase and sarmientite, respectively. To explore which phases would crystallize from the amorphous substance if it were heated, the AISA sample was heated to 220 C and kept at that temperature for 6 h.
Table 1 Average elemental concentrations (with standard deviations) in sulfoarsenates and sulfates of Fe (wt.%), as determined by electron probe microanalysis
Hydronium jarosite from horizon 4 Hydronium jarosite from horizon 5 Jarosite from horizon 4 AISA, matrix of group I AISA, matrix of group II AISA, group III Inclusions in matrix of group I Inclusions in matrix of group II
n
Al
As
Cu
Fe
K
Pb
7 24 7 12 33 82 6 16
0.11 0.07 0.12 0.08 0.06 0.02 0.30 0.11 1.2 0.3 1.9 0.4 0.20 0.10 2.9 0.9
0.73 0.10 0.35 0.13 1.7 0.2 11.0 1.0 14.0 1.3 21.6 1.4 8.1 2.3 8.4 2.5
0.010.01 0.030.02 0.240.08 0.040.03 0.030.03 0.090.04 0.040.03 0.030.03
28.81.3 27.22.3 29.90.6 34.31.4 28.81.0 20.91.7 32.01.1 25.22.6
0.970.26 1.60.3 3.10.4 0.070.08 0.020.03 0.030.10 1.00.5 1.81.5
0.310.08 11.8 0.3 0.01 0.02 0.360.24 10.8 0.7 0.02 0.02 4.30.8 12.5 0.3 0.17 0.09 0.070.03 5.4 0.7 0.08 0.05 0.090.05 5.9 0.4 0.26 0.10 0.890.35 5.5 1.0 0.29 0.07 0.340.26 7.8 1.4 0.06 0.04 1.71.1 8.0 1.2 0.24 0.10
n=Number of analyses; AISA is amorphous Fe-sulfoarsenates.
S
Zn
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Fig. 4. Properties of an amorphous Fe-sulfoarsenate sample. (a) Infrared (IR) absorption spectra showing the absorption bands corresponding to molecules of water, sulfate and arsenate. Solid line=spectrum for amorphous Fe-sulfoarsenate (this study); dotted line=spectrum for sarmientite (from Abeledo and Benyacar, 1968.); (b) thermogravimetric data for amorphous Fe sulfoarsenate (solid line) and sarmientite (dotted line); (c) powder XRD pattern obtained after heating the amorphous Fe-sulfoarsenate material. The pattern reveals a mixture of beudantite (Bd), jarosite (Jr) and goethite (Gt).
Following heat treatment, an XRD pattern was generated, revealing peaks that correspond to the d-spacings of the strongest lines of jarosite, beudantite, and goethite (Fig. 4c). The crystalline equivalent of the studied AISA sample, thus, is a phase mixture, which displays an overall similarity in IR and thermal properties to the Fe-sulfoarsenate sarmientite. This crystalline phase mixture, however, does not correspond exactly to the amorphous starting material, because the heat treatment must also have changed the water content (see Fig. 4b).
Among the amorphous Fe-sulfoarsenates, up to 3 types may be distinguished by chemical composition and microstructure. The two earliest varieties, adjoining the walls of the cavities, are reddish-brown in color and consist of a matrix containing microscopic spherical inclusions (Plate ID), which will be discussed below. Farthest away from the cavity walls, a third zone occurs which is reddish-orange in color, and does not contain any inclusions. The As content in the matrix increases from the earliest variety (AISA, group I, Table 1) to the
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Fig. 5. Ternary diagram (in mol%) for the system As2O5–SO3–Fe2O3–H2O, showing the ideal composition (star symbols) of selected minerals in the system, and the analyzed compositions of jarosites and amorphous Fe-sulfoarsenates (AISA) from the Berikul site.
latest one (group III) from 11 to 22 wt.%, while the Fe content decreases in the same order, from 34 to 21 wt.%. The heavy metal contents increase to a lesser extent, namely Zn from 0.08 to 0.29 wt.%, Cu from 0.04 to 0.09 wt.%, and Pb from 0.07 to 0.9 wt.%. This chemical evolution from group I to group III takes place at nearly constant S contents (Table 1). In the ternary diagram shown in Fig. 5, the matrix of group II plots close to the ideal composition of beudantite (PbFe3+ 3 (AsO4,SO4)(OH)6), i.e., it is significantly richer in Fe than stoichiometric bukovskyite (Fe3+ 2 (AsO4)(SO4)OH7H2O) and sarmientite. Bukovskyite and beudantite have been identified at the Carnoules Pb-(Zn) mine, Gard, France where they are associated with scorodite (Fe3+AsO42H2O) and angelellite (Fe3+ As O ), 2 11 which were precipitated from acidic mine 4 waters (Leblanc et al., 1996). Group-III AISA plot near the ideal composition of zykaite (Fe3+ 4 (AsO4)3(SO4)OH15H2O) in Fig. 5, but they are richer in SO3. As displayed in Fig. 5, the studied amorphous sulfoarsenates exhibit a wide compositional variation, and their average compositions do not correspond to the stochiometry of the discussed minerals. The inclusions in group I and II contain more Al, K, Pb and less As in comparison to the matrix (Plate ID, Table 1). On the ternary As2O5–SO3–Fe2O3 diagram, the compositions of these
inclusions form distinct trends between jarosite and beudantite, suggesting that they represent solid solutions between these two phases. In summary, the chemical analyses and the X-ray powder pattern of the heated amorphous substance indicate that the inclusions may represent poorly crystallized jarosite (KFe3+ 3 (SO4)2(OH)6)–beudantite (PbFe3+ (AsO ,SO )(OH) 3 4 4 6) solid solutions. The average contents of SO3 and As2O5 in jarosite–beudantite are similar in both group I and group II inclusions, but the group I inclusions are richer in Fe (Table 1, Fig. 5). Additionally, the second group is characterized by higher concentrations of Al and Zn. The latter is substituting for Fe in the jarosite structure. The overall sequential increase in Al, As, Zn and Pb observed for both the matrix and the jarosite - beudantite inclusions from earliest to latest varieties suggests that the concentrations of these elements in the pore solution increased during formation of these phases. Such an increase in concentration in solutions with simultaneous deposition of solids is possible only if water evaporates. 4.2.4. Melanterite (Fe2+SO47H2O) Melanterite occurs as green crystals (up to 2 cm across) in the interior parts of the waste pile. This is in contrast to other soluble Fe-sulfates, which have been
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Table 2 Average elemental concentrations (with standard deviations) of As, Zn, Cu and Pb in the soluble sulfate phases (wt.%)
Melanterite, Fe2+SO47H2O Rozenite, Fe2+SO44H2O Copiapite, Fe2+Fe3+ 4 (SO4)6(OH)220H2O Rhomboclase, HFe3+(SO4)24H2O Dietrichite, (Zn,Fe2+)Al2(SO4)422H2O
n
As
Zn
Cu
Pb
3 1 3 3 3
< 0.05 < 0.05 0.27 0.04 0.87 0.36 0.64 0.27
0.340.28 0.5n.a. 0.130.09 0.080.03 3.251.73
0.240.2 0.16n.a. 0.180.17 0.120.04 0.080.01
< 0.01 0.05 n.a. < 0.01 0.05 0.05 < 0.01
n=Number of analyses; n.a.=not enough analyses to calculate standard deviation.
found only on the surface of the waste pile. Melanterite is observed to overgrow hardpan relics, which contain jarosite solid solutions and amorphous Fe-sulfoarsenates, and therefore is clearly younger than both the jarosite solid solutions and the amorphous Fe-sulfoarsenates. In the uppermost portions of the waste pile, however, melanterite gives way to jarosite (horizons 4 and 5, Fig. 3), probably as a result of the more oxidizing conditions near the surface of the waste pile. Thus, sulfates and sulfoarsenates of Fe3+ are precipitated during the earliest and last stages of weathering, whereas sulfates of Fe2+ (i.e. melanterite) are formed during the intermediate stage. This unusual development of the weathering process, is at present not understood. Melanterite contains considerable amounts of Zn and Cu (Table 2). Pisanites, i.e. Cu–Zn varieties of melanterite, are common in wastes of sulfide ore deposits in the Ural region (Emlin, 1991), and Zn-bearing melanterite has been reported from Iron Mountain, California, USA (Alpers et al., 1994). Recently, melanterite has also been described as an environmentally important secondary phase formed as a result of pyrite oxidation at an abandoned mine in Sardinia, Italy (Frau, 2000). 4.2.5. Iron sulfates in efflorescent crust On the slopes of the waste pile, efflorescent crusts consisting of Fe-sulfates were observed. In these crusts, formed through evaporation of pore solutions, the following Fe-sulfates were identified: rozenite (Fe2+ SO44H2O), copiapite (Fe2+Fe3+ 4 (SO4)6(OH)220H2O), and rhomboclase (HFe3+(SO4)24H2O). The efflorescent crusts further contain the Zn mineral dietrichite, (Zn,Fe2+)Al2(SO4)422H2O, which was also identified by powder XRD and which contains equal amounts of Zn and Fe (3.3 wt.%). The Fe2+/Fe3+ ratio decreases from rozenite to copiapite, to rhomboclase (see the formulae above), in parallel to the decrease in the contents of Zn and Cu (Table 2). At the Berikul site, thus, Fe2+sulfates seem to be richer in Cu and Zn than Fe3+-sulfates. On the other hand, the highest As content is observed in rhomboclase, suggesting that As is captured when Fe(III) sulfates precipitate.
4.3. Water characteristics During this study, 4 types of water were distinguished on the basis of their occurrence: pore waters, drainage (infiltrating) waters, surface waters, and water of the Mokry Berikul river. Each type of water is a link in the pathway of element migration into the environment (Fig. 6). The pore solutions are generated through interaction of sulfides, rainwater and atmospheric O2. They are accumulated above the low-permeable hardpan layer, mainly in the melanterite zone (horizon 3, Fig. 3). It is assumed that only a small part of the solutions could penetrate through the hardpan layer, because the slightly altered wastes in horizon 1 remained dry even
Table 3 Average chemical compositions (with standard deviations) of the different types of water at the Berikul site. Concentrations are in mg/l Pore waters x s
Drainage waters Surface waters n x s
pH 1.70.1 6 2.3 0.1 Eh (mV) 62129 6 649 n.a. As 22,25014,549 6 976 544 Ca 32145 6 218 n.a. Cd 2719 6 11 14 780n.a. 1 184 156 Cl CN n.d. – n.d. Cu 410183 6 74 51 F 200n.a. 1 93 53 Fe 57,33320,166 6 18,0949154 HCO2 <0.05 1 <0.05 3 K 15.57.5 6 5.4 2.8 Mg 63171294 6 519 n.a. Na 6969 6 8.9 5.9 NO <0.1 6 <0.1 3 O2 dissolv. n.d. – 0.30 n.a. Pb 369 6 17.1526.06 SO2 187,50020,065 6 19,7521256 4 Zn 19371314 6 616 804
n x s 8 2 7 2 8 3 – 8 3 8 8 6 2 5 1 2 8 3 8
n
1.1n.a. 2 635 6 4 235 64 4 247 n.a. 1 1.71.1 4 88n.a. 1 0.5n.a. 1 3913 4 65n.a. 1 17,8064297 4 <0.05 1 37n.a. 1 245 n.a. 1 17n.a. 1 <0.1 1 0.30 0.28 3 1.70.1 4 30,240n.a. 1 119 73 4
x=Average; s=standard deviations; n=number of analyses; n.d.=no data; n.a.=not enough data to calculate standard deviation.
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Fig. 6. Schematic diagram showing the sampling locations for the various water types: 1—pore water; 2—drainage water; 3—surface water; 4—river water. Arrows represent the pathway of elemental migration from the Berikul waste pile to the river, whereby the width of the arrows corresponds to the quantity of elements. Note that the waste pile rests on alluvial material made of carbonate boulders.
after abundant rain fall (2 mm/3 h), whereas the silty melanterite zone (horizon 3) was wet. Therefore, it was possible to extract pore solutions only from horizons 3–5, i.e. from waste lying above the hardpan layer. These pore solutions are characterized by a pH of 1.7, an Eh of 621 mV, and a high salinity, with total salt contents of about 270 g/l (Table 3). Major components of the pore waters are SO2 4 (188 g/l), Fe (57 g/l), As (22 g/l) and Mg (6.3 g/l). Such extraordinary high As concentrations in low-temperature drainage solutions have not been reported previously; the highest As concentration described so far in the literature is 340 mg/l, determined for acid waters (pH=1.0) draining from sulfide waste at Iron Mountain, California, USA (Nordstrom, 1991). The studied pore solutions also contain relatively high concentrations of Zn (1.9 g/l) and Cl (0.8 g/l). Among the different water types occurring at the Berikul site, the highest contents of heavy metals are detected in these pore waters (Table 3). The pore water is evaporated during the infiltration process, as indicated by the presence of extensive efflorescent crusts on the surface of the waste pile. The drainage waters slowly move along the hardpan layers to the bottom of the pile, from where they emanate to form puddles and streamlets with a reddishbrown color (Plate IA). The drainage waters almost certainly do not contain a groundwater component, since the alluvial material around the waste pile is dry, and no springs occur on the river bank. The salinity of the studied drainage water samples is about 40 g/l. The solutions are acid (pH=2.3) and have an Eh of 649 mV. Major components of these drainage solutions are SO2 4 (20 g/l) and Fe (18 g/l). Relative to the pore solutions described above, the drainage waters have significantly lower heavy metal contents (by 2–6 times). Moreover, the As concentration is 20 times lower than in the pore solutions. These observations suggest that the metal content
of the drainage waters has decreased relative to that of the pore solutions, most likely due to precipitation of secondary phases in the interior parts of the waste pile, as well as outside the waste, where the drainage waters have been in contact with atmospheric O2 before being sampled. It is of note that the Ca/Mg ratio increases from about 1:20 in the pore waters to about 1:2 in the drainage waters (Table 3), suggesting that a Mg-rich solid has perhaps been precipitated from the solutions. No Mg-phase was observed, however, even though the preliminary thermodynamic calculations [using WATEQ4F (Ball et al., 1987) and the data shown in Table 3] indicate that the pore water is supersaturated with respect to epsomite (MgSO47H2O) at 20 C (Sidenko, 2001). Chlorite, which accounts for 5–10 wt.% of the waste, might contain significant amounts of Mg, but this element was not analyzed in any phase since the study was focused on other metals. The surface waters form as a result of dissolution of the efflorescent crusts by rainwater. The surface solutions, sampled during rain storms, are characterized by the lowest pH values (1.1), an Eh of 635 mV, and by a total salinity of 50 g/l (Table 3). Major components of these solutions are SO2 4 (30 g/l) and Fe (18 g/l ). The average contents of As, Cu, Zn, Pb and Cd are several times lower than those in the drainage solutions (Table 3). The quantity of As, Cu, Zn, Pb and Cd removed from the waste pile by these surface waters was estimated on the basis of meteorological, chemical and hydrological measurements taken during two consecutive rain storms, which lasted about 3 h each. The data in Table 4 document that both precipitation and discharge during the second rain storm were similar to those measured during the first storm. Most of the metal concentrations in the surface waters, however, were considerably lower in the second event, which took place only two days after the first one. This indicates
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Table 4 Results of individual meteorological, chemical and hydrological measurements during two consecutive rainstorms in August 2000: precipitation, elemental concentrations in surface waters (in mg/l), and volume of surface water (in l) discharged into the river for each measuring station (see Fig. 2) Precipitation
Sampling station
Zn
Cd
Pb
Cu
As
Volume
1.93 mm (20.08.2000)
ST-1/1 ST-1/2 ST-2/1 ST-2/2
200 160 55 60
2.6 2.6 0.7 0.8
1.9 1.6 1.7 1.7
52 47 26 30
300 280 170 190
446 500 423 551
1.96 mm (22.08.2000)
The data shown in Table 3 for the surface waters represent the average of all 4 measurements listed here.
that dissolution of surface minerals was much more pronounced during the first rain storm, probably because the intermittent dry period between the storms was too short to build up a significant amount of secondary surface minerals in the efflorescent crusts. Taking the average metal concentrations in the surface waters (listed in Table 3) and the average discharge volume for these two events (480 l), it was calculated that each storm removed, on average, approximately 113 g As, 57 g Zn, 19 g Cu, 0.8 g Pb, and 0.8 g Cd from the slope of the waste pile facing the Mokry Berikul river, and discharged the metals into the river. According to these measurements, and based on average meteorological data (400 mm of rain during the warm season), it has been calculated that during the warm season, a few tens of kilograms of As and Zn, sev-
eral kilograms of Cu, and a few hundred grams of Pb and Cd might be transported into the Mokry Berikul river. Some of the dissolved metals will be transformed into suspended solid precipitates as soon as the acid surface streams mix with the nearly neutral river water. The above calculation assumes that the rainstorms which were sampled are typical of other rainstorms during the warm season, and thus the authors are unable to anticipate the effects of much heavier or much lighter rains. The river water exhibits a pH that ranges from 7.6 (upstream, sample B-4) to 6.5 (downstream, sample B-11), reflecting the input of acid water from the waste pile (Table 5). The river water samples have an average Eh of +370 mV, and their total salt content is approximately 120 mg/l (Table 5). This water is a SO24-enriched (12.5 mg/l on average) HCO2 3 –Ca type, with average
Table 5 Water analyses of samples collected from the Mokry Berikul River (see Fig. 2). B-4: upstream; B-10: near waste pile; B-11: downstream Sample #
B-4
B-10
B-11
MCL (US)
MCL (Russia)
pH Eh (mV) As Ca Cd Cl CN Cu F Fe HCO2 3 K Mg Na NO 3 O2 dissolv. Pb SO2 4 Zn
7.6 389 <0.05 18.5 0.0001 0.16 n.d. 0.001 <0.05 <0.03 83.42 1.0 3.3 1.7 <0.1 6.8 0.004 11.9 0.038
7.1 359 0.3 17.8 0.0044 0.21 n.d. 0.005 <0.05 <0.03 81.48 1.3 3.1 1.5 <0.1 11 0.004 13.5 0.560
6.5 346 0.05 17.9 0.0119 0.37 n.d. 0.010 <0.05 <0.03 n.d. 1.4 1.9 2.1 <0.1 8.9 0.009 12.1 0.813
– – 0.05 – 0.005 – 0.2 1.3 4.0 – – – – – 10 * – 0.015 – –
– – 0.03 – 0.001 – – 1 – – – – – – – – 0.03 – 1
Data represent individual analyses in mg/l. n.d.=Not determined; * measured as N; MCL (US)=maximum contaminant level (in mg/l) allowed in drinking water according to the US Environmental Protection Agency (EPA, 2001); MCL (Russia)=maximum contaminant level (in mg/l) allowed in drinking water in Russia (Eremeev, 1990), respectively.
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contents of 18 mg/l Ca and 82.5 mg/l HCO2 3 . Compared to the upstream river water (sample B-4), the concentrations of As, Cu, Zn, Pb and Cd are significantly higher in the water near the waste pile (sample B-10), and generally even higher further downstream (sample B-11). For example, the downstream concentration of Cd is higher by two orders of magnitude than the upstream concentration. The concentration of As in the river near the waste pile (sample B-10) is much higher than that in the upstream water, and specifically, it is one order of magnitude higher than the maximum contaminant level allowed in drinking water in the US (EPA, 2001). A similar observation can be made for the downstream concentration of Cd. For Zn, the concentrations are close to the maximum concentrations permitted in drinking water in Russia (Eremeev, 1990). The data clearly document that the waters emanating from and running off the waste pile have a serious impact on the water quality of the Mokry Berikul river. This environmental impact is particularly dramatic during the warm season, when most of the precipitation is recorded and evaporation is most intense. During the major part of the cold season, the waste pile is covered by snow, which reduces the evaporation as well as the amount of runoff.
5. Conclusions Acid solutions containing high concentrations of SO2 4 , Fe, As and heavy metals are generated during sulfide oxidation in the Berikul waste pile. The mobility of As, Cu, Zn and Pb within the waste pile is controlled by precipitation of these metals as secondary phases. Arsenic is precipitated in the form of amorphous Fesulfoarsenates, which form a matrix with inclusions of jarosite–beudantite. These inclusions show lower concentrations of As than the matrix. On the surface of the waste pile, As co-precipitates with sulfates containing trivalent Fe. Copper and Zn are precipitated together with melanterite and rozenite, because these metals substitute for Fe2+ in sulfates. Within the efflorescent crusts, Zn is accumulated in dietrichite. Most of the Pb is captured by jarosite–beudantite solid solutions. A sequential increase in Al, As, Zn and Pb in both amorphous sulfoarsenates and jarosite–beudantite inclusions suggests that the concentrations of these elements increase in the pore solution when the solid phases precipitate, a feature that can be explained by slow water evaporation. This study has established that the main pathway of element migration is via surface drainage during and after rainstorms. In dissolved form, a few tens of kilograms of As and Zn, several kilograms of Cu, and subkilogram quantities of Pb and Cd are removed by surface waters and transported into the Mokry Berikul river during the warm season of the year. This discharge results in a dramatic increase in the As, Cu, Zn, Pb and
Cd concentration in the river water, where some of these metals reach concentrations near or in excess of the maximum levels permitted for safe drinking water.
Acknowledgements The authors are grateful to Dr. Richard Wanty, Dr. Pierfranco Lattanzi, and an anonymous reviewer for providing constructive reviews. Their valuable suggestions and thoughtful criticism of an earlier version helped us to improve the quality of this paper. We further would like to thank Carl Hager for his assistance at the Purdue University electron microprobe.
References Abeledo, M.E.J., Benyacar, M.A.R., 1968. New data on sarmientite. Am. Mineral. 53, 2077–2082. Al, T.A., Blowes, D.W., Jambor, J.L., 1994. A geochemical study of the main tailings impoundment at the Falconbrige Limited, Kidd Creek Division Metalurgical Site, Timmins, Ontario. In: Jambor, J.L., Blowes, D.W. (Eds.), Short Course Handbook on Environmental Geochemistry of Sulfide Mine Waste. Mineral. Assoc. Can, Waterloo, pp. 59– 102. Alpers, C.N., Nordstrom, D.K., Jambor, J.L., 1994. Secondary minerals and acid mine-water chemistry. In: Jambor, J.L., Blowes, D.W. (Eds.), Short Course Handbook on Environmental Geochemistry of Sulfide Mine Waste. Mineral. Assoc. Can, Waterloo, pp. 247–270. Ardau, C., Frau, F., Dadea, C., Mattusch, J., Wennerich, R., Titze K., 2001. Solid- state of arsenic in waste materials and stream sediments from abandoned mine area of Baccu Locci (Sardinia, Italy). In: Proc. Int. Conf. WRI - 10. Villasimius, Italy, pp. 1173–1176. Ball, J.W., Nordstrom, D.K., Zachmann, D.W., 1987. WATEQ4F—a personal computer FORTRAN translation of the geochemical model WATEQ2 with revised data base: US Geol. Surv. Open-File Rep. 87-50. Blowes, D.W., Jambor, J.L., 1990. The pore-water geochemistry and the mineralogy of the vadose zone of sulfide tailings, Waite Amulet, Quebec, Canada. Appl. Geochem. 5, 327–346. Blowes, D.W., Reardon, E.J., Jambor, J.L., Cherry, J.A., 1991. The formation and potential importance of cemented layers in inactive sulfide mine tailings. Geochim. Cosmochim. Acta 55, 965–978. Emlin, E.F., 1991. Technogenesis of pyrite deposits of Ural. Publication of Ural State University Sverdlovsk (in Russian). EPA, 2001. United States Environmental Protection Agency, Office of Water. Available from: www.epa.gov/safewater. Eremeev, A.N. (Ed.), 1990. Handbook of Geochemical Exploration of Mineral Resources. Nedra, Moscow. Frau, F., 2000. The formation-dissolution-precipitation cycle of melanterite at the abandoned pyrite mine of Genna Luas in Sardinia, Italy: environmental implications. Mineral. Mag. 64, 995–1006.
R. Giere´ et al. / Applied Geochemistry 18 (2003) 1347–1359 Gaskova, O.L., Bortnikova E.P., 2001. Experimental modeling of trace element leaching from As-bearing tailings impoundments. In: Proc. Int. Conf. WRI - 10. Villasimius, Italy, pp. 1233–1236. Jambor, J.L., Dutrizac, J.E., 1983. Beaverite – plumbojarosite solid solutions. Can. Mineral. 21, 101–113. Kolmogorov, Y., Trounova, V., 2002. Analytical potential of EDXRF using toroidal focusing systems of highly oriented pyrolytic graphite (HOPG). X-ray Spectrom. 31, 432–436. Langmuir, D., Mahoney, J., MacDonald, A., Rowson, J., 1999. Predicting arsenic concentrations in the pore waters of buried uranium mill tailings. Geochim. Cosmochim. Acta 63, 3379– 3394. Leblanc, M., Achard, B., Ben Othman, D., Bertrand-Sarfati, J., Personne´, J.Ch., 1996. Accumulation of arsenic from mine waters by ferruginous bacterial accretions (stromatolites). Appl. Geochem. 11, 541–554. Nordstrom, D.K., 1991. Chemical modeling of acid mine waters in the Western United States. Meeting Proc. USGS Water Resour. Invest. Rep. # 91-4034, pp. 534–538.
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Pouchou, J.L., Pichoir, F., 1984. Un nouveau mode`le de calcul pour la microanalyse quantitative par spectrome´trie de rayons X. Partie I: application a` l’analyse d’e´chantillons homoge`nes. Recherches Ae´rospatiales 3, 167–192. Rattray, K.J., Taylor, M.R., Bevan, D.J.M., Pring, A., 1996. Compositional segregation and solid solution in the leaddominant alunite-type minerals from Broken Hill, N.S.W. Mineral. Mag. 60, 779–785. Roussel, Ch., Bril, H., Fernandez, A., 1998. Hydrogeochemical survey and mobility of As and heavy metals on the site of a former gold mine (Saint-Yrieix mining district, France). Hydroge´ologie 1, 3–12. Shuvaeva, O.V., Bortnikova, S.B., Korda, T.M., Lazareva, E.V., 2000. Arsenic speciation in a contaminated gold processing tailings dam. Geostand. Newslett. 24, 247–252. Sidenko, N., 2001. Arsenic and Heavy Metal Migration in Weathering Zone of Berikul Sulfide Waste. PhD thesis, Institute of Geology SB RAS. Novosibirsk, Russia (in Russian).
The role of secondary minerals in controlling the migration of arsenic and metals from high-sulfide wastes (Berikul gold mine, Siberia) R. Giere´a,*, N.V. Sidenkob, E.V. Lazarevab a
Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1397, USA The United Institute of Geology, Geophysics and Mineralogy, pr. Koptyuga 3, Novosibirsk 630090, Russia
b
Abstract The role of secondary minerals in controlling the migration of As, Cu, Zn, Pb and Cd has been investigated in piles of high-sulfide waste at the Berikul Au mine, Kemerovo region, Russia. These wastes contain 40–45 wt.% sulfides and have been stored for approximately 50 a near the Mokry Berikul river. Sulfide oxidation generates acid pore solutions (pH=1.7) with high concentrations of SO2 4 (190 g/l), Fe (57 g/l), As (22 g/l), Zn (2 g/l), Cu (0.4 g/l), Pb (0.04 g/l), and Cd (0.03 g/l). From these solutions, As is precipitated as amorphous non-stoichiometric Fe-sulfoarsenates in the lower horizons of the waste piles. During precipitation of the Fe-sulfoarsenates, the concentration of Fe in these phases decreases from 34 to 21 wt.%, that of As increases from 11 to 22 wt.%, while the S content remains approximately constant (5.4–5.8 wt.%). Arsenic is also accumulated in jarosite-beudantite solid solutions (up to 8.4 wt.% As), which occur as inclusions in the amorphous Fe-sulfoarsenates. In efflorescent crusts on the surface of the waste pile, As coprecipitates with the Fe(III) sulfates copiapite (0.27 wt.% As) and rhomboclase (0.87 wt.% As). Zinc and Cu are incorporated primarily into Fe(II) sulfates, i.e. melanterite in the interior of the waste pile, and rozenite in the efflorescent crust. The Zn mineral dietrichite is also formed at the surface of the waste pile as a result of evaporation of pore solutions, and is the only Fe(II) sulfate containing detectable amounts of As (0.64 wt.%). Lead is mainly co-precipitated with minerals of the jarosite group, where the Pb content may reach 4.3 wt.%. Co-precipitation of toxic elements with sulfates and sulfoarsenates of Fe is shown to be a significant mechanism in controlling the concentration of heavy metals in pore solutions of high-sulfide mine wastes. Precipitation of secondary phases causes the formation of a hardpan layer with low permeability at a depth of 1–1.5 m below the surface of the waste pile. Rainwater accumulates above the hardpan horizons and slowly drains along these aquicludes to the bottom of the pile. Most of the rainwater evaporates during infiltration. This leads to formation of the described efflorescent sulfate crusts. Dissolution of these crusts during the next rain storm produces highly acidic surface waters (pH=1.1) rich in SO2 4 (30 g/l), Fe (18 g/l), As (0.24 g/l), Zn (0.12 g/l), Cu (0.04 g/l), Pb and Cd (0.002 g/l). During the warm (t> 0 C) period of the year, which lasts about 7 months, these surface waters transport a total of a few tens of kilograms of As and Zn, several kilograms of Cu, and a few hundred grams of Pb and Cd from the waste pile into the Mokry Berikul river. As a result, the concentrations of these metals in the river water increase by an order of magnitude, thus reaching levels close to, or exceeding the maximum values permissible for drinking water. # 2003 Elsevier Science Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (R. Giere´). 0883-2927/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0883-2927(03)00055-6
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1. Introduction Abandoned mine wastes containing high sulfide concentrations are among the most serious sources of environmental pollution. Understanding the geochemical processes which control precipitation and dissolution of secondary minerals in abandoned sulfide mines is crucial for the formulation of models that predict the environmental impact of such sites. Moreover, a better knowledge of these mechanisms will allow remediation of existing problem sites and/or a reduction of the extent of future pollution. Pore waters in and drainage solutions from high-sulfide waste are characterized by low pH values and high concentrations of various heavy metals (Blowes et al., 1991; Nordstrom, 1991). The concentration of heavy metals in the pore solutions is mainly controlled by their precipitation together with Fe hydroxides and/or sulfates (Blowes and Jambor, 1990). Precipitation and dissolution cycles of some secondary minerals are strongly influenced by seasonal wetting and drying cycles (Frau, 2000), and thus it is important to include meteorological parameters in models that simulate environmental impacts. Although mobilization of As from mine waste is discussed in the literature, most reports focus on low-sulfide wastes only (e.g., Al et al., 1994; Leblanc et al., 1996; Roussel et al., 1998; Langmuir et al., 1999; Shuvaeva et al., 2000; Ardau et al., 2001; Gaskova and Bortnikova, 2001). The goal of the present study is to understand the processes controlling the migration and sequestration of Cu, Zn, and Pb and, in particular, the mobility of As in high-sulfide waste piles. To achieve this goal, the authors investigated high-sulfide mine wastes at the Berikul Au mine in Siberia. A specific objective of the present paper is to combine detailed mineralogical investigations of the secondary phases occurring in the waste with both meteorological observations and studies of the water
chemistry of samples collected in various parts of the waste pile and in the nearby river.
2. Description of the waste site The Berikul Au mine is situated in the northern part of the Kemerovo region, Western Siberia, about 450 km NE of Novosibirsk (Fig. 1). The deposit, an Au-sulfidequartz vein with a Au content of 1–5 g/t, was mined exclusively for Au. The mine was in operation between 1942 and 1991, but no other production data were available to the authors. Gold occurred as fine-grained intergrowths with sulfide minerals, mainly pyrite and arsenopyrite. From these sulfides, Au was extracted at the Berikul mill by using the cyanide technique. After removal of the Au-bearing cyanide solutions, the sulfide flotation residues were neutralized by adding Ca(ClO)2 before being dumped on waste piles. The waste studied here has been in a pile since 1952, while waste from other piles was used for road construction when new tailings impoundments were built in 1972. The discarded material contains 40–45 wt.% finegrained sulfides, including pyrite (35–40 wt.%), arsenopyrite (2–5 wt.%), and minor amounts of pyrrhotite, sphalerite, chalcopyrite, and galena. Among the gangue minerals found in the wastes, the predominant phases are quartz (30–35 wt.%), albite (5–10 wt.%), chlorite (5–10 wt.%), muscovite (about 5 wt.%), and calcite (3–5 wt.%). The studied high-sulfide wastes have been deposited on alluvial material, consisting of carbonate boulders, and were stored for about 50 a on the left bank of the Mokry Berikul river. The waste pile has a length of approximately 250 m, a width of 50 m at its base, and a height of 3 m (Fig. 2). The alluvial material around the waste pile is dry, and no springs have been detected near the river bank, suggesting that the river
Fig. 1. Map showing the location of the Berikul Au mine in the Kemerovo region of Southwestern Siberia.
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Fig. 2. Schematic map of the studied waste pile and the surrounding area at the Berikul site showing sample localities, hydrological stations, and excavation pits.
represents the local ground water table. The only ‘‘springs’’ found are drainage water emanations at the base of the waste piles (Plate IA, Fig. 2). The Berikul site is located at an altitude of 850 m above sea level. The average yearly temperature is 1.5 C, and the total precipitation averages 600 mm of rain and snow annually. There is, however, a distinct warm period during which the temperature is above freezing. This warm period lasts from May to October, and most of the precipitation (approximately 400 mm) falls during this time.
3. Methods of investigation Solid samples were collected from both surface outcrops and excavation pits, which were dug with a mechanical excavator in different parts of the waste pile (see Fig. 2). The pits were excavated in order to gain an insight into the stratification of the waste pile, to compare vertical and lateral zoning, and to collect samples from the interior of the pile. The solids were then put
into hermetically sealed polyethylene bags and frozen immediately to preserve the initial characteristics of pore waters. The solid waste material was studied by reflected light microscopy and scanning electron microscopy (Jeol JSM-36). X-ray powder diffraction (XRD) analyses were performed with a DRON-UM diffractometer (Burevestnik, made in Russia) using filtered Cu-Ka radiation. Thermogravimetric analysis (TGA) was carried out with a MOM device (made in Hungary), using a 120 mg sample, which was heated from room temperature to 1000 C, at 12 C/min. The compositions of the secondary phases were determined with a CAMECA-SX50 electron microprobe at the Department of Earth and Atmospheric Sciences of Purdue University. The instrument is equipped with 4 wavelength dispersive spectrometers, and was operated for quantitative analysis at an acceleration potential of 20 kV and a beam current of 80 nA measured on a Faraday cup. Samples and standards were coated with 20 nm of C. Well-characterized minerals and synthetic oxides were used as standards. Data collection time was 20 s for most major elements, and
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Plate I. (A) Drainage water emanates from the waste pile and forms reddish-brown streamlets which eventually enter the Mokry Berikul river. This photograph shows where drainage water sample B-3 was collected (see Fig. 2). (B) Photomicrograph displaying sequence of alteration zones around leaching cavities within the hardpan layer (horizon 2 in Fig. 3). The leaching cavities appear as open space (white) in the upper part of the figure. Zone 1: sulfides + gypsum. Note large euhedral secondary gypsum crystals; Zone 2: hydronium jarosite + gypsum +sulfides; Zone 3: amorphous Fe-sulfoarsenates (reddish-yellow, see arrow). Note that these Fe-sulfoarsenates encrust the walls of the leaching cavity. Plane-polarized light. (C). Photomicrographs showing the different morphologies of jarosite. Left: spherical aggregates of hydronium jarosite (arrows) in horizon 4 (see Fig. 3); Right: oolites of jarosite in a crack in horizon 4 (see Fig. 3). Both pictures taken in plane-polarized light. (D). Backscattered electron image (BSE1) and element distributions maps (for As, Fe, Al, K and Pb) showing matrix and inclusions in the amorphous Fe-sulfoarsenates of horizon 4 (see Fig. 3). Color coding: elemental concentrations increase from blue to green, to yellow, to red.
30–60 s for minor elements. Data collection on background positions on either side of each peak was half the time of data collection on the corresponding peak position. The raw data were corrected on-line by the PAP correction procedure (Pouchou and Pichoir, 1984). Detection limits for different components varied from 0.01 to 0.05 wt.%, and analytical precision was approximately 2% for most major elements, and better than 10% for most of the trace elements. Element distribution maps were generated at 20 kV and 100 nA, and the data were collected for 100 ms/pixel in wavelength dispersive mode. In water-soluble phases, the concentrations of As, Cu, Zn, and Pb were determined by X-ray fluorescence (XRF) analysis. For this purpose, less than 1 mg of single-crystal particles and monomineralic aggregates (about 100 mm across) were selected, identified by XRD, and then mounted on polyethylene film, which was inserted into a Russian-made IRIS-3 spectrometer. The
sample was analyzed using the method described by Kolmogorov and Trounova (2002). Detection limits of the XRF analysis were 0.05 wt.% for As, and 0.01 wt.% for Cu, Zn, and Pb, and the analytical precision was within 20%. Water samples from the surface of the waste pile were collected during rain events using special catchers, resembling gutters and connected to a tank, to estimate the volume of water flowing down the slopes of the waste pile during each rain event. The amount of rain was measured with a pluviometer. Samples of drainage solutions were collected from streamlets that emanate at the bottom of the waste pile (Plate IA, Fig. 2). Water samples of the Mokry Berikul river were collected at 3 sites: upstream (200 m away from the waste pile), downstream (200 m away), and near the bank, where the waste is stored (see Fig. 2). Pore solutions were extracted from the moist silt material by fluid expulsion at a pressure of 100 Pa after the samples had been
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thawed in the laboratory. With the exception of the pore solutions, the Eh and the pH of all water samples were measured in the field immediately after sampling by using an INFRASPAK potentiometer (made in Russia), whereby the precision of a single measurement was 10 mV for Eh, and 0.08 for pH. The Eh and pH measurements for the pore solutions were carried out only after the samples had been thawed and the solutions extracted in the laboratory. Approximately 100 ml of solution were collected by a syringe through a filter (0.45 mm) into glassware, and were subsequently divided into two aliquots. One of the aliquots was then acidified with HNO3 to preserve the metal concentration, whereas the other aliquot was preserved in its initial state. All water samples were stored about 1 month at a temperature 4–6 C in a refrigerator. The concentrations in the acidified solutions of As, Ca, Cd, Cu, Fe, K, Mg, Na, Pb, and Zn were determined by flame atomicabsorption spectrometry (AAS; Perkin-Elmer equipment, model 3030E equipped with an HGA-600 graphite furnace) and by thermal-electric AAS (PueUnikam equipment, model SP-9). The contents of 2 NH+ were deter4 , Cl , F , NO3 , HCO3 , and SO4 mined in the second aliquot by ion chromatography using a Russian-made MILIXROM chromatograph.
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4. Results and discussion 4.1. The waste pile in cross-section During the fifty 50 a of storage in the pile, the waste was subjected to supergene alteration, which led to a characteristic layering. In cross-section, 5 main horizons can be distinguished in the waste pile (Fig. 3). These are distinct in terms of mineral content, color, porosity and hardness, and comprise, from bottom to top: 4.1.1. Horizon 1 This zone consists of friable, only slightly altered sulfide wastes of gray-green color. It contains 3–5 wt.% calcite, and is approximately 1–1.5 m thick. 4.1.2. Horizon 2 Overlying horizon 1, this layer consists of lithified waste containing gypsum as binding material. It is a hardpan layer with a thickness of approximately 1 m. Leaching cavities are observed in this zone (shown in black in Fig. 3), and these are often filled by jarosite and amorphous Fe-sulfoarsenates (Plate IB). As a rule, such hardpan layers are characterized by low permeability, which prevents penetration of solutions and gases into the deeper horizons of waste piles (Blowes et al., 1991). At the studied site, the hardpan layer contains trace amounts of calcite, i.e. approximately 1 wt.%, but all overlying horizons are devoid of calcite. 4.1.3. Horizon 3 Overlying the hardpan layer is a tobacco-colored, 0.5–1 m thick horizon, which consists of very moist and fine-grained wastes of silt size. This zone, referred to as the melanterite zone, contains leached relics of hardpan material (Fig. 3), which are overgrown by melanterite aggregates (up to 10 cm across). The presence of such relics of lithified material in horizons above the actual hardpan layer documents that the lithified waste is dissolving as a result of decreasing pH in the pore solutions during sulfide weathering. The hardpan material, however, decomposes only slowly, because of its low permeability. 4.1.4. Horizon 4 This thin intermediate zone has a gray color, and is composed of quartz, jarosite and sulfides. It separates the melanterite zone from the overlying horizon 5.
Fig. 3. Supergene weathering profile through the waste pile at the Berikul site. This profile displays the situation encountered in excavation pit B-2/99 in the eastern part of the waste pile (see Fig. 2), but it is representative of the entire site. The numbered horizons are described in the text.
4.1.5. Horizon 5 The uppermost horizon is distinctly yellow, is up to 0.5 m thick, and contains some strongly oxidized relics of hardpan material. The predominant phase is jarosite; hence, this horizon is referred to below as the jarosite zone.
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The second jarosite variety, jarosite sensu stricto, occurs as oolites of about 5 mm in diameter along cracks (Plate IC) in horizon 4 (see Fig. 3). It also forms rims around hydronium jarosite, showing clearly that it precipitated after hydronium jarosite. Jarosite is considerably richer in Cu and Zn than hydronium jarosite (Table 1). Moreover, jarosite is characterized by significantly higher contents of both As (1.7 wt.%) and Pb (4.3 wt.%). Joint incorporation of As and Pb into jarosite is consistent with the possible occurrence of a solid solution between jarosite [KFe3+ 3 (SO4)2(OH)6] and beudantite [PbFe3+ (AsO ,SO )(OH) 3 4 4 6; see also Jambor and Dutrizac, 1983; Rattray et al., 1996].
4.2. Secondary minerals 4.2.1. Gypsum (CaSO42H2O) Gypsum is the earliest secondary mineral found, and it occurs even in the slightly altered sulfide wastes of zone 1. In the hardpan layer, gypsum crystals are up to 2 mm across and form a framework that binds the finegrained sulfide phases (Plate IB). The concentrations of As, Cu, Zn, and Pb in gypsum are all below detection limits. 4.2.2. Jarosite solid solutions Two main varieties of jarosite-group phases were detected, and these can be distinguished on the basis of the predominance of alkali-site ions (Table 1). The first variety is hydronium jarosite (H3O,K,Na)Fe3+ 3 (SO4)2(OH)6. The second variety is jarosite sensu stricto, (K,H3O,Na)Fe3+ 3 (SO4)2(OH)6, with K concentrations that are considerably higher (3.1 wt.%) than in hydronium jarosite. Average Na concentrations are low in both varieties, namely 0.3 and 0.5 wt.% in hydronium jarosite and jarosite, respectively. Hydronium jarosite forms spherical aggregates up to 20 mm in size, which were observed in all horizons (Plate IC). Extensive precipitation of hydronium jarosite must have started after formation of gypsum in the hardpan layers, as documented by the mineralogical zoning observed around leaching cavities in the hardpan (typical zoning shown in Plate IB). The zones more distant from the open cavity are less altered and contain the earliest secondary phase, i.e. gypsum (Zone 1 in Plate IB). In the more weathered material, which forms the walls of cavities (Zone 2 in Plate IB), hydronium jarosite begins to appear. Sulfoarsenates are observed closest to and as incrustations of the cavities (Zone 3 in Plate IB). From these observations, it is concluded that during the weathering of the waste material, the secondary minerals crystallized in the following sequence: gypsum ! hydronium jarosite ! sulfoarsenates.
4.2.3. Amorphous iron sulfoarsenates (AISA) Sulfoarsenates of Fe are precipitated in the leaching cavities observed in the hardpan layer (Plate IB), as well as in the lithified hardpan relics in the waste of the overlying horizons 3 and 5 (Fig. 3). These phases are Xray amorphous, and exhibit a reddish-brown to reddishorange color. As outlined below, there are 3 varieties of AISA, which can be distinguished on the basis of their chemical composition. Since the authors were unable to select enough material of a specific type of AISA based on its physical appearance, a sample of non-distinct AISA had to be used for infrared (IR) spectroscopy, TGA and XRD. The IR spectrum of this material is similar to that of sarmientite, Fe3+ 2 (AsO4)(SO4)OH5H2O, and thus qualitatively points to a similar composition (Fig. 4a). The thermal properties of these two phases are also similar, as documented by the weight loss curves shown in Fig. 4b. The weight loss between 100 and 685 C, representing water content, is 30.2% for the amorphous phase, and 23.0% for sarmientite. The weight losses between 685 and 980 C, representing the SO3 content, are 4.8 and 16.4% for the amorphous phase and sarmientite, respectively. To explore which phases would crystallize from the amorphous substance if it were heated, the AISA sample was heated to 220 C and kept at that temperature for 6 h.
Table 1 Average elemental concentrations (with standard deviations) in sulfoarsenates and sulfates of Fe (wt.%), as determined by electron probe microanalysis
Hydronium jarosite from horizon 4 Hydronium jarosite from horizon 5 Jarosite from horizon 4 AISA, matrix of group I AISA, matrix of group II AISA, group III Inclusions in matrix of group I Inclusions in matrix of group II
n
Al
As
Cu
Fe
K
Pb
7 24 7 12 33 82 6 16
0.11 0.07 0.12 0.08 0.06 0.02 0.30 0.11 1.2 0.3 1.9 0.4 0.20 0.10 2.9 0.9
0.73 0.10 0.35 0.13 1.7 0.2 11.0 1.0 14.0 1.3 21.6 1.4 8.1 2.3 8.4 2.5
0.010.01 0.030.02 0.240.08 0.040.03 0.030.03 0.090.04 0.040.03 0.030.03
28.81.3 27.22.3 29.90.6 34.31.4 28.81.0 20.91.7 32.01.1 25.22.6
0.970.26 1.60.3 3.10.4 0.070.08 0.020.03 0.030.10 1.00.5 1.81.5
0.310.08 11.8 0.3 0.01 0.02 0.360.24 10.8 0.7 0.02 0.02 4.30.8 12.5 0.3 0.17 0.09 0.070.03 5.4 0.7 0.08 0.05 0.090.05 5.9 0.4 0.26 0.10 0.890.35 5.5 1.0 0.29 0.07 0.340.26 7.8 1.4 0.06 0.04 1.71.1 8.0 1.2 0.24 0.10
n=Number of analyses; AISA is amorphous Fe-sulfoarsenates.
S
Zn
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Fig. 4. Properties of an amorphous Fe-sulfoarsenate sample. (a) Infrared (IR) absorption spectra showing the absorption bands corresponding to molecules of water, sulfate and arsenate. Solid line=spectrum for amorphous Fe-sulfoarsenate (this study); dotted line=spectrum for sarmientite (from Abeledo and Benyacar, 1968.); (b) thermogravimetric data for amorphous Fe sulfoarsenate (solid line) and sarmientite (dotted line); (c) powder XRD pattern obtained after heating the amorphous Fe-sulfoarsenate material. The pattern reveals a mixture of beudantite (Bd), jarosite (Jr) and goethite (Gt).
Following heat treatment, an XRD pattern was generated, revealing peaks that correspond to the d-spacings of the strongest lines of jarosite, beudantite, and goethite (Fig. 4c). The crystalline equivalent of the studied AISA sample, thus, is a phase mixture, which displays an overall similarity in IR and thermal properties to the Fe-sulfoarsenate sarmientite. This crystalline phase mixture, however, does not correspond exactly to the amorphous starting material, because the heat treatment must also have changed the water content (see Fig. 4b).
Among the amorphous Fe-sulfoarsenates, up to 3 types may be distinguished by chemical composition and microstructure. The two earliest varieties, adjoining the walls of the cavities, are reddish-brown in color and consist of a matrix containing microscopic spherical inclusions (Plate ID), which will be discussed below. Farthest away from the cavity walls, a third zone occurs which is reddish-orange in color, and does not contain any inclusions. The As content in the matrix increases from the earliest variety (AISA, group I, Table 1) to the
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Fig. 5. Ternary diagram (in mol%) for the system As2O5–SO3–Fe2O3–H2O, showing the ideal composition (star symbols) of selected minerals in the system, and the analyzed compositions of jarosites and amorphous Fe-sulfoarsenates (AISA) from the Berikul site.
latest one (group III) from 11 to 22 wt.%, while the Fe content decreases in the same order, from 34 to 21 wt.%. The heavy metal contents increase to a lesser extent, namely Zn from 0.08 to 0.29 wt.%, Cu from 0.04 to 0.09 wt.%, and Pb from 0.07 to 0.9 wt.%. This chemical evolution from group I to group III takes place at nearly constant S contents (Table 1). In the ternary diagram shown in Fig. 5, the matrix of group II plots close to the ideal composition of beudantite (PbFe3+ 3 (AsO4,SO4)(OH)6), i.e., it is significantly richer in Fe than stoichiometric bukovskyite (Fe3+ 2 (AsO4)(SO4)OH7H2O) and sarmientite. Bukovskyite and beudantite have been identified at the Carnoules Pb-(Zn) mine, Gard, France where they are associated with scorodite (Fe3+AsO42H2O) and angelellite (Fe3+ As O ), 2 11 which were precipitated from acidic mine 4 waters (Leblanc et al., 1996). Group-III AISA plot near the ideal composition of zykaite (Fe3+ 4 (AsO4)3(SO4)OH15H2O) in Fig. 5, but they are richer in SO3. As displayed in Fig. 5, the studied amorphous sulfoarsenates exhibit a wide compositional variation, and their average compositions do not correspond to the stochiometry of the discussed minerals. The inclusions in group I and II contain more Al, K, Pb and less As in comparison to the matrix (Plate ID, Table 1). On the ternary As2O5–SO3–Fe2O3 diagram, the compositions of these
inclusions form distinct trends between jarosite and beudantite, suggesting that they represent solid solutions between these two phases. In summary, the chemical analyses and the X-ray powder pattern of the heated amorphous substance indicate that the inclusions may represent poorly crystallized jarosite (KFe3+ 3 (SO4)2(OH)6)–beudantite (PbFe3+ (AsO ,SO )(OH) 3 4 4 6) solid solutions. The average contents of SO3 and As2O5 in jarosite–beudantite are similar in both group I and group II inclusions, but the group I inclusions are richer in Fe (Table 1, Fig. 5). Additionally, the second group is characterized by higher concentrations of Al and Zn. The latter is substituting for Fe in the jarosite structure. The overall sequential increase in Al, As, Zn and Pb observed for both the matrix and the jarosite - beudantite inclusions from earliest to latest varieties suggests that the concentrations of these elements in the pore solution increased during formation of these phases. Such an increase in concentration in solutions with simultaneous deposition of solids is possible only if water evaporates. 4.2.4. Melanterite (Fe2+SO47H2O) Melanterite occurs as green crystals (up to 2 cm across) in the interior parts of the waste pile. This is in contrast to other soluble Fe-sulfates, which have been
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Table 2 Average elemental concentrations (with standard deviations) of As, Zn, Cu and Pb in the soluble sulfate phases (wt.%)
Melanterite, Fe2+SO47H2O Rozenite, Fe2+SO44H2O Copiapite, Fe2+Fe3+ 4 (SO4)6(OH)220H2O Rhomboclase, HFe3+(SO4)24H2O Dietrichite, (Zn,Fe2+)Al2(SO4)422H2O
n
As
Zn
Cu
Pb
3 1 3 3 3
< 0.05 < 0.05 0.27 0.04 0.87 0.36 0.64 0.27
0.340.28 0.5n.a. 0.130.09 0.080.03 3.251.73
0.240.2 0.16n.a. 0.180.17 0.120.04 0.080.01
< 0.01 0.05 n.a. < 0.01 0.05 0.05 < 0.01
n=Number of analyses; n.a.=not enough analyses to calculate standard deviation.
found only on the surface of the waste pile. Melanterite is observed to overgrow hardpan relics, which contain jarosite solid solutions and amorphous Fe-sulfoarsenates, and therefore is clearly younger than both the jarosite solid solutions and the amorphous Fe-sulfoarsenates. In the uppermost portions of the waste pile, however, melanterite gives way to jarosite (horizons 4 and 5, Fig. 3), probably as a result of the more oxidizing conditions near the surface of the waste pile. Thus, sulfates and sulfoarsenates of Fe3+ are precipitated during the earliest and last stages of weathering, whereas sulfates of Fe2+ (i.e. melanterite) are formed during the intermediate stage. This unusual development of the weathering process, is at present not understood. Melanterite contains considerable amounts of Zn and Cu (Table 2). Pisanites, i.e. Cu–Zn varieties of melanterite, are common in wastes of sulfide ore deposits in the Ural region (Emlin, 1991), and Zn-bearing melanterite has been reported from Iron Mountain, California, USA (Alpers et al., 1994). Recently, melanterite has also been described as an environmentally important secondary phase formed as a result of pyrite oxidation at an abandoned mine in Sardinia, Italy (Frau, 2000). 4.2.5. Iron sulfates in efflorescent crust On the slopes of the waste pile, efflorescent crusts consisting of Fe-sulfates were observed. In these crusts, formed through evaporation of pore solutions, the following Fe-sulfates were identified: rozenite (Fe2+ SO44H2O), copiapite (Fe2+Fe3+ 4 (SO4)6(OH)220H2O), and rhomboclase (HFe3+(SO4)24H2O). The efflorescent crusts further contain the Zn mineral dietrichite, (Zn,Fe2+)Al2(SO4)422H2O, which was also identified by powder XRD and which contains equal amounts of Zn and Fe (3.3 wt.%). The Fe2+/Fe3+ ratio decreases from rozenite to copiapite, to rhomboclase (see the formulae above), in parallel to the decrease in the contents of Zn and Cu (Table 2). At the Berikul site, thus, Fe2+sulfates seem to be richer in Cu and Zn than Fe3+-sulfates. On the other hand, the highest As content is observed in rhomboclase, suggesting that As is captured when Fe(III) sulfates precipitate.
4.3. Water characteristics During this study, 4 types of water were distinguished on the basis of their occurrence: pore waters, drainage (infiltrating) waters, surface waters, and water of the Mokry Berikul river. Each type of water is a link in the pathway of element migration into the environment (Fig. 6). The pore solutions are generated through interaction of sulfides, rainwater and atmospheric O2. They are accumulated above the low-permeable hardpan layer, mainly in the melanterite zone (horizon 3, Fig. 3). It is assumed that only a small part of the solutions could penetrate through the hardpan layer, because the slightly altered wastes in horizon 1 remained dry even
Table 3 Average chemical compositions (with standard deviations) of the different types of water at the Berikul site. Concentrations are in mg/l Pore waters x s
Drainage waters Surface waters n x s
pH 1.70.1 6 2.3 0.1 Eh (mV) 62129 6 649 n.a. As 22,25014,549 6 976 544 Ca 32145 6 218 n.a. Cd 2719 6 11 14 780n.a. 1 184 156 Cl CN n.d. – n.d. Cu 410183 6 74 51 F 200n.a. 1 93 53 Fe 57,33320,166 6 18,0949154 HCO2 <0.05 1 <0.05 3 K 15.57.5 6 5.4 2.8 Mg 63171294 6 519 n.a. Na 6969 6 8.9 5.9 NO <0.1 6 <0.1 3 O2 dissolv. n.d. – 0.30 n.a. Pb 369 6 17.1526.06 SO2 187,50020,065 6 19,7521256 4 Zn 19371314 6 616 804
n x s 8 2 7 2 8 3 – 8 3 8 8 6 2 5 1 2 8 3 8
n
1.1n.a. 2 635 6 4 235 64 4 247 n.a. 1 1.71.1 4 88n.a. 1 0.5n.a. 1 3913 4 65n.a. 1 17,8064297 4 <0.05 1 37n.a. 1 245 n.a. 1 17n.a. 1 <0.1 1 0.30 0.28 3 1.70.1 4 30,240n.a. 1 119 73 4
x=Average; s=standard deviations; n=number of analyses; n.d.=no data; n.a.=not enough data to calculate standard deviation.
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Fig. 6. Schematic diagram showing the sampling locations for the various water types: 1—pore water; 2—drainage water; 3—surface water; 4—river water. Arrows represent the pathway of elemental migration from the Berikul waste pile to the river, whereby the width of the arrows corresponds to the quantity of elements. Note that the waste pile rests on alluvial material made of carbonate boulders.
after abundant rain fall (2 mm/3 h), whereas the silty melanterite zone (horizon 3) was wet. Therefore, it was possible to extract pore solutions only from horizons 3–5, i.e. from waste lying above the hardpan layer. These pore solutions are characterized by a pH of 1.7, an Eh of 621 mV, and a high salinity, with total salt contents of about 270 g/l (Table 3). Major components of the pore waters are SO2 4 (188 g/l), Fe (57 g/l), As (22 g/l) and Mg (6.3 g/l). Such extraordinary high As concentrations in low-temperature drainage solutions have not been reported previously; the highest As concentration described so far in the literature is 340 mg/l, determined for acid waters (pH=1.0) draining from sulfide waste at Iron Mountain, California, USA (Nordstrom, 1991). The studied pore solutions also contain relatively high concentrations of Zn (1.9 g/l) and Cl (0.8 g/l). Among the different water types occurring at the Berikul site, the highest contents of heavy metals are detected in these pore waters (Table 3). The pore water is evaporated during the infiltration process, as indicated by the presence of extensive efflorescent crusts on the surface of the waste pile. The drainage waters slowly move along the hardpan layers to the bottom of the pile, from where they emanate to form puddles and streamlets with a reddishbrown color (Plate IA). The drainage waters almost certainly do not contain a groundwater component, since the alluvial material around the waste pile is dry, and no springs occur on the river bank. The salinity of the studied drainage water samples is about 40 g/l. The solutions are acid (pH=2.3) and have an Eh of 649 mV. Major components of these drainage solutions are SO2 4 (20 g/l) and Fe (18 g/l). Relative to the pore solutions described above, the drainage waters have significantly lower heavy metal contents (by 2–6 times). Moreover, the As concentration is 20 times lower than in the pore solutions. These observations suggest that the metal content
of the drainage waters has decreased relative to that of the pore solutions, most likely due to precipitation of secondary phases in the interior parts of the waste pile, as well as outside the waste, where the drainage waters have been in contact with atmospheric O2 before being sampled. It is of note that the Ca/Mg ratio increases from about 1:20 in the pore waters to about 1:2 in the drainage waters (Table 3), suggesting that a Mg-rich solid has perhaps been precipitated from the solutions. No Mg-phase was observed, however, even though the preliminary thermodynamic calculations [using WATEQ4F (Ball et al., 1987) and the data shown in Table 3] indicate that the pore water is supersaturated with respect to epsomite (MgSO47H2O) at 20 C (Sidenko, 2001). Chlorite, which accounts for 5–10 wt.% of the waste, might contain significant amounts of Mg, but this element was not analyzed in any phase since the study was focused on other metals. The surface waters form as a result of dissolution of the efflorescent crusts by rainwater. The surface solutions, sampled during rain storms, are characterized by the lowest pH values (1.1), an Eh of 635 mV, and by a total salinity of 50 g/l (Table 3). Major components of these solutions are SO2 4 (30 g/l) and Fe (18 g/l ). The average contents of As, Cu, Zn, Pb and Cd are several times lower than those in the drainage solutions (Table 3). The quantity of As, Cu, Zn, Pb and Cd removed from the waste pile by these surface waters was estimated on the basis of meteorological, chemical and hydrological measurements taken during two consecutive rain storms, which lasted about 3 h each. The data in Table 4 document that both precipitation and discharge during the second rain storm were similar to those measured during the first storm. Most of the metal concentrations in the surface waters, however, were considerably lower in the second event, which took place only two days after the first one. This indicates
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Table 4 Results of individual meteorological, chemical and hydrological measurements during two consecutive rainstorms in August 2000: precipitation, elemental concentrations in surface waters (in mg/l), and volume of surface water (in l) discharged into the river for each measuring station (see Fig. 2) Precipitation
Sampling station
Zn
Cd
Pb
Cu
As
Volume
1.93 mm (20.08.2000)
ST-1/1 ST-1/2 ST-2/1 ST-2/2
200 160 55 60
2.6 2.6 0.7 0.8
1.9 1.6 1.7 1.7
52 47 26 30
300 280 170 190
446 500 423 551
1.96 mm (22.08.2000)
The data shown in Table 3 for the surface waters represent the average of all 4 measurements listed here.
that dissolution of surface minerals was much more pronounced during the first rain storm, probably because the intermittent dry period between the storms was too short to build up a significant amount of secondary surface minerals in the efflorescent crusts. Taking the average metal concentrations in the surface waters (listed in Table 3) and the average discharge volume for these two events (480 l), it was calculated that each storm removed, on average, approximately 113 g As, 57 g Zn, 19 g Cu, 0.8 g Pb, and 0.8 g Cd from the slope of the waste pile facing the Mokry Berikul river, and discharged the metals into the river. According to these measurements, and based on average meteorological data (400 mm of rain during the warm season), it has been calculated that during the warm season, a few tens of kilograms of As and Zn, sev-
eral kilograms of Cu, and a few hundred grams of Pb and Cd might be transported into the Mokry Berikul river. Some of the dissolved metals will be transformed into suspended solid precipitates as soon as the acid surface streams mix with the nearly neutral river water. The above calculation assumes that the rainstorms which were sampled are typical of other rainstorms during the warm season, and thus the authors are unable to anticipate the effects of much heavier or much lighter rains. The river water exhibits a pH that ranges from 7.6 (upstream, sample B-4) to 6.5 (downstream, sample B-11), reflecting the input of acid water from the waste pile (Table 5). The river water samples have an average Eh of +370 mV, and their total salt content is approximately 120 mg/l (Table 5). This water is a SO24-enriched (12.5 mg/l on average) HCO2 3 –Ca type, with average
Table 5 Water analyses of samples collected from the Mokry Berikul River (see Fig. 2). B-4: upstream; B-10: near waste pile; B-11: downstream Sample #
B-4
B-10
B-11
MCL (US)
MCL (Russia)
pH Eh (mV) As Ca Cd Cl CN Cu F Fe HCO2 3 K Mg Na NO 3 O2 dissolv. Pb SO2 4 Zn
7.6 389 <0.05 18.5 0.0001 0.16 n.d. 0.001 <0.05 <0.03 83.42 1.0 3.3 1.7 <0.1 6.8 0.004 11.9 0.038
7.1 359 0.3 17.8 0.0044 0.21 n.d. 0.005 <0.05 <0.03 81.48 1.3 3.1 1.5 <0.1 11 0.004 13.5 0.560
6.5 346 0.05 17.9 0.0119 0.37 n.d. 0.010 <0.05 <0.03 n.d. 1.4 1.9 2.1 <0.1 8.9 0.009 12.1 0.813
– – 0.05 – 0.005 – 0.2 1.3 4.0 – – – – – 10 * – 0.015 – –
– – 0.03 – 0.001 – – 1 – – – – – – – – 0.03 – 1
Data represent individual analyses in mg/l. n.d.=Not determined; * measured as N; MCL (US)=maximum contaminant level (in mg/l) allowed in drinking water according to the US Environmental Protection Agency (EPA, 2001); MCL (Russia)=maximum contaminant level (in mg/l) allowed in drinking water in Russia (Eremeev, 1990), respectively.
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contents of 18 mg/l Ca and 82.5 mg/l HCO2 3 . Compared to the upstream river water (sample B-4), the concentrations of As, Cu, Zn, Pb and Cd are significantly higher in the water near the waste pile (sample B-10), and generally even higher further downstream (sample B-11). For example, the downstream concentration of Cd is higher by two orders of magnitude than the upstream concentration. The concentration of As in the river near the waste pile (sample B-10) is much higher than that in the upstream water, and specifically, it is one order of magnitude higher than the maximum contaminant level allowed in drinking water in the US (EPA, 2001). A similar observation can be made for the downstream concentration of Cd. For Zn, the concentrations are close to the maximum concentrations permitted in drinking water in Russia (Eremeev, 1990). The data clearly document that the waters emanating from and running off the waste pile have a serious impact on the water quality of the Mokry Berikul river. This environmental impact is particularly dramatic during the warm season, when most of the precipitation is recorded and evaporation is most intense. During the major part of the cold season, the waste pile is covered by snow, which reduces the evaporation as well as the amount of runoff.
5. Conclusions Acid solutions containing high concentrations of SO2 4 , Fe, As and heavy metals are generated during sulfide oxidation in the Berikul waste pile. The mobility of As, Cu, Zn and Pb within the waste pile is controlled by precipitation of these metals as secondary phases. Arsenic is precipitated in the form of amorphous Fesulfoarsenates, which form a matrix with inclusions of jarosite–beudantite. These inclusions show lower concentrations of As than the matrix. On the surface of the waste pile, As co-precipitates with sulfates containing trivalent Fe. Copper and Zn are precipitated together with melanterite and rozenite, because these metals substitute for Fe2+ in sulfates. Within the efflorescent crusts, Zn is accumulated in dietrichite. Most of the Pb is captured by jarosite–beudantite solid solutions. A sequential increase in Al, As, Zn and Pb in both amorphous sulfoarsenates and jarosite–beudantite inclusions suggests that the concentrations of these elements increase in the pore solution when the solid phases precipitate, a feature that can be explained by slow water evaporation. This study has established that the main pathway of element migration is via surface drainage during and after rainstorms. In dissolved form, a few tens of kilograms of As and Zn, several kilograms of Cu, and subkilogram quantities of Pb and Cd are removed by surface waters and transported into the Mokry Berikul river during the warm season of the year. This discharge results in a dramatic increase in the As, Cu, Zn, Pb and
Cd concentration in the river water, where some of these metals reach concentrations near or in excess of the maximum levels permitted for safe drinking water.
Acknowledgements The authors are grateful to Dr. Richard Wanty, Dr. Pierfranco Lattanzi, and an anonymous reviewer for providing constructive reviews. Their valuable suggestions and thoughtful criticism of an earlier version helped us to improve the quality of this paper. We further would like to thank Carl Hager for his assistance at the Purdue University electron microprobe.
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