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Drainage waters affected by pyrite oxidation in a coal mine in Galicia (NW Spain): Composition and mineral stability
The Science of the Total Environment 216 Ž1998. 121]132
Drainage waters affected by pyrite oxidation in a coal mine in Galicia Ž NW Spain. : Composition and mineral stability C. MonterrosoU , F. Macıas ´ Department of Edafologıa Agrıcola, Facultad de Biologıa, ´ y Quımica ´ ´ ´ Uni¨ersidad de Santiago, 15706 Santiago de Compostela, Spain Received 1 December 1997; accepted 9 February 1998
Abstract The quality of the drainage waters from the As Puentes lignite mine dump ŽGalicia, Spain. was evaluated along with the geochemical processes which determine its composition. Analysis of water from different areas of the dump was carried out at monthly intervals over a period of 2 years. During this period, pH values ranged between 2.1 and 8.0 and redox potential values between 150 and 750 mV, although most of the water samples were strongly acidic ŽpH 2.5]3.5. and oxidising ŽEh 600]750 mV.; conditions which are usually found in mine drainage systems where pyrite is present. In general, the water samples were characterised by the presence of elevated concentrations of Fe, SO42y and Hq, liberated from the oxidation of pyrite, and of Si, Al, Ca and Mg derived from the accelerated mineral hydrolysis occurring under these conditions. At the same time, very high concentrations of elements, in particular Mn, Zn, Ni and Co, which were liberated from both processes, were recorded. The best water quality was found in the most recently constructed areas of the dump, comprising pre-selected material low in sulfides. Q 1998 Elsevier Science B.V. Keywords: Acid mine drainage; Land reclamation; Mineral stability; Pyrite oxidation
1. Introduction Acid drainage represents one of the largest environmental problems associated with surface mining, due to oxidation of the sulfides found in many spoils. The mine waters affected by this
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process may show great variability in their physicochemical characteristics, but are usually highly acidic and have elevated concentrations of SO42y, Fe and some heavy metals ŽMills, 1985; Caruccio et al., 1989; Winland et al., 1991; Calvo de Anta and Macıas, ´ 1992; Evangelou and Zhang, 1995; Bigham et al., 1996.. The damage produced in aquatic ecosystems by acidic mine effluents has been widely studied all over the world since the beginning of the 1960s ŽKinney, 1964; ARC, 1969;
0048-9697r98r$19.00 Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00149-1
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Letterman and Mitsch, 1978; Kempe, 1982; Glover, 1982; Johnson and Thornton, 1987; Herlihy et al., 1990; Short et al., 1990, amongst others.. In Galicia ŽNW Spain., the impacts made on the river Brandelos, tributary of the Ulla, the Xestosa and the Eume ŽAlvarez et al., 1993; Macıas ´ and Calvo de Anta, 1993; Calvo de Anta and Perez ´ Otero, 1994; Macıas, ´ 1994., have been exhaustively studied. Although a large number of studies exist, the oxidation of sulfides is a complex subject and its effects vary enormously between different sites and under different conditions. The river Eume receives the effluents generated at the As Puentes lignite mine ŽGalicia, NW Spain.. The generation of acid drainage becomes problematic because of two main factors } the high rainfall and the existence of a large surface area where streams intercept and are exposed, both of which favour the oxidation of sulfides. Today, the impact on the receiving catchment area is avoided by physicochemical treatment of the water generated during excavation of the mine. However, the production and circulation of acidic streams in the dump area creates a problem for land reclamation as it impedes the establishment of vegetation and even causes the disappearance of already well established vegetation ŽGil Bueno et al., 1990.. To minimize these effects, a reclamation programme has been developed whose objectives are to reduce the surfaces affected by the water flows, slow down the kinetics of the oxidation of sulfides and increase the pH of the rhizosphere. The programme includes selective management of spoils, use of soil correctors and drainage work and the results are evaluated by following the quality of the circulating water and studying the soil solutions. In the present study, analysis of the composition of the drainage water of the As Puentes lignite mine dump was made, the geochemical processes which give rise to its character were identified and the effects of the selective spoil management on its characteristics were evaluated. 2. Materials and methods The climate of As Pontes is characterised by high rainfall Žaverage annual rainfall: 1684 mm.
and moderate temperatures Žaverage annual temperature: 11.68C.. The lignite mine dump is designed to store 800 M m3 of spoils and has a forecasted final height of 220 m and a final surface area of 1300 ha, of which 370 ha are presently undergoing restoration. Most of the spoils are slates and Tertiary sediments containing different amounts of carbonaceous material, with pyrite often being present Žbiogenic pyrite in the carbonaceous sediments and geologic pyrite in the slates .. To control the run-off, the banks have been subdivided into partial banks and berms and water is conducted and evacuated through a system of channels and ditches operating by gravity. The contact between spoils of different permeabilities allows the frequent outflow of subsuperficial water from the banks which are interconnected with the general circulation. The soil restoration work includes using correctors, such as lime or ashes from the lignite combustion, inorganic and organic fertilisers and on some occasions, the spreading of a layer of topsoil. The different kinds of spoils dumped and the procedures used have given rise to a wide variety of physicochemical conditions at the dump surface. To ensure that all types of water emerging and circulating were represented, a total of 21 water quality control points were selected from the different restoration areas. These were termed, in chronological order: CS, E1, E2, EH and EF. The presence of sulfide rich materials is common in the oldest areas ŽCS, E1., whereas the more recent areas have been constructed from material selected for having low or zero pyrite content. Sampling was carried out monthly over a period of 2 years and a total of approx. 450 water samples were obtained. Measurements of pH, Eh and electrical conductivity were made in the field and duplicate solution samples were collected and filtered through 0.45-m m Nucleopore membranes. One of the subsamples was acidified with ultrapure HNO3 for measurement of dissolved metals. Total concentrations of Ca, Mg, Na, K, Fe, Al, Si, Mn, Zn, Ni and Co were measured by ICP-AES with a Thermo Jarrel Ash Mod. Atom Scan 25 equipment. The emission intensity, detection limits and potential spectral interferences were taken in
C. Monterroso, F. Macıas ´ r The Science of the Total En¨ironment 216 (1998) 121]132
consideration for selecting a wavelength for each element; the instrument operating parameters were improved for each element and background correction was applied ŽTable 1.. Sample rate was 1 ml miny1 , nebulizer pressure was 30 psi, Ar flow was 15 l miny1 for the plasma and 1 l miny1 as auxiliary, Ar optics flush was on or below 200 nm. The ICP was calibrated by using two-point standardization and multielement standards were prepared in 2% Žvrv. HNO3 from 1000 m g mly1 stock standard ŽMerk KGaA, Germany.. Trace concentrations of Cu, Cd and Pb were determined by means of differential pulse anodic stripping voltammetry ŽDPASV., using a Metrohm 646VA Stand with three electrodes: a hanging mercury drop electrode, an AgClrAg reference electrode and a Pt counter electrode. To avoid the problems derived from the matrix of the samples an automatized standard-addition method was used, by making two or three successive additions. Sulfate and Cly were measured by ion chromatography ŽDionex 2000., Fy, NOy and 3 Ž NHq 4 , with ion selective electrodes Orion 94-09, Orion 93-07 and Orion 95-10, respectively., PO43y, by visible spectrophotometry with molybdate reagent, as well as Fe 2q using o-phenanthroline reagent. The concentration of Fe 3q was calculated from the difference between total dissolved Fe determined by ICP-AES and Fe 2q determinations. At least 20% of samples were analyzed in duplicate and for each batch of samples analyzed two spiked samples were also analyzed. The CV for replicate analyses was F 5%. Ion activities were calculated with the SOLMINEQ88 geochemical computer programme ŽKharaka et al., 1989.. The stabilities of various mineral phases were evaluated by using Saturation Indices ŽSI., calculated by comparing measured ion activities with those expected from the solubility product of a given mineral phase. The measured activities, expressed as an ion activity product ŽIAP. were divided by the equilibrium constant of each mineral phase ŽK eq .: SI s log ŽIAPrK eq .. Positive, negative and zero SI values indicate, respectively, that the system is oversaturated, unsaturated or at equilibrium with respect to a particular mineral phase.
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Table 1 Some operating parameters for each element analyzed by ICP-AES
Ca Mg Na K Fe Al Si Mn Zn Ni Co
Wavelength Žnm.
Sensitivity Ž m g ly1 .
RF power ŽW.
Observation height Žmm.
317.933 279.553 589.590 766.490 259.940 167.081 251.612 257.610 213.856 231.604 228.616
1 1 10 100 2 2 10 2 2 5 3
1350 1350 950 950 1150 1750 1150 1350 1350 1350 1350
15 15 10 20 12 12 12 12 10 10 10
3. Results and discussion 3.1. Water quality
The results obtained from the analysis of the water showed high variability in Eh]pH and ionic concentrations which illustrates the great diversity of geochemical conditions in the drainage systems ŽTable 2.. However, they also have some characteristics which differ markedly from those of natural waters. These characteristics are determined by the process of sulfide oxidation, which affects, to a greater or lesser extent, all of the waters analysed, as is clear from the high concentration of SO42y Žbetween 500 and 7500 mg ly1 .. The high concentrations of sulfates is important in these type of waters for two reasons; firstly, because of the metabolic requirement of this anion for the oxidation of Fe 2q by chemolithotrophic, acidophilic bacteria, such as T. ferroxidans ŽLazaroff, 1963. and secondly, due to the influence that sulfate appears to have on the thermodynamics and kinetics of the formation of various minerals Žjarosite, schwertmannite, ferrihydrite, goethite,.... from mine drainage systems ŽNordstrom, 1982; Brady et al., 1986; Bigham et al., 1996.. As well as liberation of SO42y, the oxidation of
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Table 2 Composition of streams from the mine dump: average, maximum and minimum values of different parameters Variable
Average S.D.
pH 4.03 1.40 CE ŽmS cmy1 . 3.15 1.25 Eh ŽmV. 547 147 SO42y Žmg ly1 . 2602 1458 NO3y Žmg ly1 . 1.12 3.77 Cly Žmg ly1 . 22.89 12.14 Fy Žmg ly1 . 1.00 1.21 PO43y Žmg ly1 . 0.04 0.07 NH4q Žmg ly1 . 1.38 2.30 Ca Žmg ly1 . 352.6 114.1 Mg Žmg ly1 . 140.0 70.6 Na Žmg ly1 . 18.2 6.6 K Žmg ly1 . 5.7 2.2 Si Žmg ly1 . 13.2 10.4 Al Žmg ly1 . 43.00 62.01 Fe Žmg ly1 . 395.50 432.37 FeŽII. Žmg ly1 . 327.08 375.75 Mn Žmg ly1 . 49.62 30.47 Ni Žmg ly1 . 2.38 1.42 Co Žmg ly1 . 2.09 1.24 Zn Žmg ly1 . 3.18 2.95 Cu Žmg ly1 . 0.33 0.55 Pb Ž m g ly1 . 2.8 5.2 Cd Ž m g ly1 . 16.3 22.1
Minimum Maximum 2.23 0.82 143 424 0.03 6.25 - 0.01 - 0.01 0.04 50.7 14.6 4.6 0.7 1.6 - 0.05 - 0.01 - 0.01 1.30 - 0.01 - 0.01 - 0.01 - 0.01 - 0.5 - 0.5
7.99 6.51 754 7404 43.99 96.40 5.26 0.643 12.48 602.5 370.0 53.2 19.0 54.0 450.00 1592.00 1463.00 148.70 8.60 6.60 17.00 5.610 92.3 141.0
sulfides gives rise to large quantities of iron and Hq. The concentrations of dissolved Fe in the water samples under study varied greatly; from 0.1 to 1600 mg ly1 , most of which Žbetween 70 and 100%. was in the reduced form. However, if we take into account the ratio of SO42y:Fe theoretically obtained from the oxidation of pyrite Ž2:1., the concentration of dissolved Fe in most of the water samples is lower than that expected, indicating that part of the dissolved Fe has precipitated through oxidationrhydrolysis reactions ŽFig. 1a.. High concentrations of dissolved Fe 3q were found only in the most acidic streams ŽpH3.. The relationship found between the concentration of SO42y and the pH of the water samples from the mine dump is far from being linear ŽFig. 1b., since the processes of oxidation and of neutralisation of the acid liberated occur simultaneously. Thus, one group of water samples, which despite being highly affected by the oxidation of sulfides Žand having a concentration of SO42y of
Fig. 1. Relationship between the concentration of SO42y and Ža. dissolved iron, Žb. pH in the streams under study. Line in Ža. indicates the theoretical molar ratio SO42y:Fe derived from the oxidation of pyrite.
between 1500 and 3000 mg ly1 ., has effectively neutralised the acid liberated, principally by the dissolution of large quantities of Ca and Mg, leading to pH values of between 5 and 8. However, most of the water samples are very acidic with pH values lying within a very narrow range, between 2.5 and 3.5, independent of SO42y concentration. This suggests that most of the dump streams are buffered to this range of pH. Miller Ž1979. suggests that the mine dumps containing pyrite may be in equilibrium with jarosite and ferrihydrite, forming a buffering system with a pH value of around 3.19 Ž"0.17.. Most of the streams in this study would thus fall into the buffering range estimated by this author.
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The redox potential values also varied widely ŽEh s 150]750 mV., although the most common range was between 600 and 750 mV. There was a slight relationship between pH and Eh Ž r s y0.84, P- 0.001.. The Eh]pH conditions most commonly found in the dump waters were, therefore, acidic and oxidising ŽTable 2.. The association between high acidity and high redox potential values is mainly due to the presence of significant quantities of dissolved Fe 3q, which acts as a more effective oxidizing agent than O 2 , in the most acidic environments. The high acidity generated during the oxidising process gives rise to accelerated hydrolysis of the other minerals in the spoil materials, causing large quantities of the constituent elements to be made soluble. Concentrations of Ca and Mg were especially high Žthe average values being 352 and 140 mg ly1 , respectively. as were those of Si Ž13.2 mg ly1 . and Al Ž43.0 mg ly1 .. The strong correlation between Si and Al Ž r s 0.71, P- 0.001. reflects the origin of Al, which is principally from the alteration of silicates. Using Si as a tracer of mineral hydrolysis, a strong correlation with Fy Ž r s 0.85 and 0.75, P- 0.001, respecand NHq 4 tively. was found. These are elements which can reach concentrations much higher than those found in natural waters Žup to 5.3 and 12.5 mg
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ly1 , respectively.. Minerals rich in Fy are scarce in slates and are very resistant, which gives some idea of the extraordinary aggressiveness of some of these stream waters to be able to occasionally produce such high concentrations of Fy. NHq 4 may originate from the retention of slate in silicate meshes and from organic binding in carbonaceous sediments. Žusually - 1 mg ly1 . Concentrations of NOy 3 3y Ž and PO4 usually - 100 m g ly1 ., in general lower than those in natural waters, were found. The low concentrations of PO43y may be related to processes of immobilisation by adsorption of Fe hydroxides precipitated in the water circulation channels andror the precipitation of Fe insoluble phosphates. The high acidity of most of the Žusually streams favours the presence of NHq 4 0.5]2 mg ly1 . and makes the appearance of NOy 3 more unlikely. The processes of oxidation of sulfides and of mineral hydrolysis have given rise to the mobilisation of heavy metals, causing abnormally high concentrations of these in the streams. This was especially true for Mn, for which values of up to 140 mg ly1 were seen. Mn, Zn, Ni and Co were strongly correlated with each other and with the concentration of SO42y Žcoefficients of correlation between 0.63 and 0.87, P- 0.001..
Table 3 Range of average values recorded over a period of 2 years for the parameters analysed in water samples from the four different types of streams identified Variable
pH CE ŽmS cmy1 . Eh ŽmV. SO42y Žmg ly1 . NO3y Žmg ly1 . NH4q Žmg ly1 . Ca Žmg ly1 . Mg Žmg ly1 . Na Žmg ly1 . K Žmg ly1 . Si Žmg ly1 . Al Žmg ly1 . Fe Žmg ly1 . FeŽII. Žmg ly1 . Mn Žmg ly1 .
Type of stream I Ž6 points.
II Ž4 points.
IIIa Ž7 points.
IIIb Ž4 points.
2.80]3.20 4.07]4.90 579]664 3906]4736 0.27]0.94 0.37]9.21 321.0]440.0 155.0]249.2 10.4]24.5 2.3]7.8 12.8]44.1 47.8]156.4 709.0]1050.0 330.0]868.0 13.8]102.0
4.58]5.67 3.01]3.52 214]432 2257]2906 0.15]0.44 0.67]1.53 423.2]464.0 120.1]169.0 14.8]20.2 4.4]5.7 4.1]9.4 0.2]27.7 360.9]599.0 252.6]535.0 43.3]63.2
3.13]3.45 1.67]2.82 615]695 1082]2616 0.20]0.47 0.29]0.80 178.1]298.0 71.4]108.3 13.2]18.0 2.9]6.0 11.2]14.0 22.9]144.9 6.2]296.5 3.0]239.8 29.9]50.9
5.75]7.15 2.26]2.59 382]426 1551]1900 0.83]1.66 0.15]1.17 396.0]476.8 89.1]139.5 19.4]27.6 5.2]9.8 3.8]6.6 0.1]2.1 0.3]14.6 0.1]11.20 18.6]50.9
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As a consequence of all of these processes, the resulting drainage waters have a high ionic strength; values for electrical conductivity of between 800 and 6500 m S cmy1 were obtained, which are between 15 and 150 times more concentrated than the natural waters in the region, characterised by being extremely dilute. Finally, the ionic strength of the streams is determined by the extent of the influence of sulfide oxidation, as is seen by the extremely strong correlation between electrical conductivity and the concentration of sulfates in the water Ž r s 0.97, P0.001.. 3.2. Types of drainage streams in the mine dump The wide range of values found for almost all parameters measured in assessing water quality clearly indicates the different origins of the streams, with the type of material through which they flow and the time of contact with this material being the factors which determine their composition. Among the samples in this study were found; run-off waters in contact with the surface material and with generally short residence times, permeating waters which reach unknown depths and which then emerge at the surfaces of lower slopes after variable residence times and also pockets of water, trapped during the piling of spoils on the dump which are in contact with the spoils for long periods before emerging at the surface in the form of springs. These streams emerge from different zones after contact with materials of different permeabilities. In relation to physicochemical parameters, groups of streams can be differentiated whose qualities can be related to their origin, the level of influence of oxidation of sulfides and the extent of neutralisation of the acidity generated. Thus the following groups can be distinguished ŽTable 3.: 1. Group I: Streams which are strongly affected by oxidation of sulfides and undergo little neutralisation; 2. Group II: Streams which are strongly affected by oxidation of sulfides and are moderately neutralised; and
3. Group III: Streams moderately affected by oxidation of sulfides and which are: IIIa, not neutralised; IIIb, neutralised.
3.2.1. Streams which are strongly affected by oxidation of sulfides and undergo little neutralisation The streams in this first group, which differ significantly from the others, are strongly acidic, oxidising, have a very high ionic strength and include those from six control points at the oldest area of the mine dump Žarea CS.. The extreme values of pH, Eh and EC measured were, 2.2]3.5, 521]707 mV and 1.0]6.5 mS cmy1 , respectively. Along with these parameters, very high concentrations of sulfates Žgenerally between 4500 and 5000 mg ly1 and occasionally up to 7000 g ly1 . and of dissolved iron Žaverage values of between 700 and 1000 mg ly1 and occasionally up to 1600 mg ly1 . have been registered. In addition, very high Si contents Žup to 54 mg ly1 . and Al Žup to 450 mg ly1 . were found, which reflects the aggressive nature of these streams and their potential to accelerate mineral hydrolysis processes Ža strongly acidolysis process.. The high contents of Ca and Mg, with average concentrations of 320]400 mg ly1 and 150]250 mg ly1 , respectively, were incapable of neutralising the acidity in the system. 3.2.2. Streams which are strongly affected by oxidation of sulfides and are moderately neutralised The second group of streams, which have characteristics markedly different from the previous ones, includes all the water arising from areas E1 and E2 at the point of emergence on the surface Žfour control points.. The most relevant and distinctive characteristics of this group of streams are their acidic to weakly acidic nature Žaverage pH values of between 4.6 and 5.7. and low redox potential values Žaverage Eh between 200 and 400 mV.. The low Eh values, clearly less than in well aerated surface waters suggest that these streams originate in deep, poorly aerated areas. However, these conditions do not inhibit the oxidation of pyrite, as is clear from the high concentration of sulfates found, although they are less than those seen in group I streams Žaverage values between
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2200 and 2900 mg ly1 and maximum concentrations of 4500 mg ly1 .. Despite the pH of these streams not being excessively low, they maintain high levels of dissolved iron Žderived from the oxidation of pyrite. due to the low Eh values. The average concentration of dissolved iron in this group of streams varies between 360 and 600 mg ly1 and the molar ratio of SO42y: Fe is between 2.7 and 3.8, values which are very close to the theoretical value derived from the oxidation of pyrite ŽSO42y:Fe s 2:1.. This indicates that significant processes involving the precipitation of Fe compounds during the flow of these streams from deep areas to the surface do not exist andror there is precipitation of some kind of sulfate which also removes this anion from solution, so that the relative proportion of both are maintained. The concentrations of Si and Al were generally low, with average values usually being less than 10 and 1 mg ly1 , respectively. The metal content of these streams is notably lower than most of those from area CS, although high levels of Mn are maintained in solution Žaverage values of between 43 and 63 mg ly1 .. The high concentrations of Ca were notable, the average values being between 423 and 464 mg ly1 and maximum values of up to 550 mg ly1 , which along with the lesser degree of oxidation of pyrite than in the area CS, helps to explain the partial neutralisation seen in these streams. Regarding the change which the emerging streams undergo at the surface, it was found that gradual oxidation and acidification took place, from the point of emergence to the confluence in the general circulation ditches. Acidification is without a doubt due to the precipitation of Fe compounds which are produced as a consequence of the changes in redox potentials and which cover the bases of the channels excavated in the banks by these streams. In some cases a decrease of almost 1 unit of pH is produced in a distance of only 100 m with a parallel increase of 155 mV in Eh ŽFig. 2.. 3.2.3. Streams moderately affected by oxidation of pyrite The third group of streams includes those from
127
Fig. 2. Changes of Eh and pH of the stream with the distance from the point of its emergence at the surface.
the general circulation channels in zone E2 and all of those from zone EH. The common characteristics of these streams were, the SO42y concentration, with average values generally being between 1100 and 1900 mg ly1 , and ionic strength, with average values of conductivity being between 1.7 and 2.8 mS cmy1 . These values are substantially lower than those found for the previous two groups, which implies that there is less influence from pyrite oxidation. Within this group, two subgroups can be clearly distinguished by the level of neutralisation. The group IIIa streams are acidic, although less so than those from zone CS, and the group IIIb streams are close to being neutral. Group IIIa principally includes the streams from zone E2. The acidity generated during the oxidation of pyrite is barely neutralised and average pH values of between 3.1 and 3.5 were measured. They are, like those from zone CS, oxidising Žhaving average Eh values of between 600 and 700 mV. and maintain relatively high quantities of iron in solution, although always much less than was found in the previous groups Žaverage values generally being between 20 and 200 mg ly1 with extremes of 4 and 340 mg ly1 .. The molar ratios of SO42y:Fe in solution were always much higher than 2, which indicates that there is a marked precipitation of Fe compounds in the water circulation channels. This fact is reflected in the large accumulation of iron compounds in the base of the channels. As well as the acidic conditions, elevated concentrations of Si Žaverage
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128
values of between 11 and 14 mg ly1 . and Al Žgenerally between 20 and 30 mg ly1 , but with extreme average values of 145 mg ly1 ., as well as of different heavy metals, were found. The final group of neutralised streams ŽIIIb. encompasses those from zone EH, the youngest area of the dump. This group of streams comes from the zones where the surface is largely made up of slates of an alkaline nature. As a consequence, most of the acid generated during the oxidation of pyrite was neutralised Žaverage values of pH between 5.7 and 7.1., mainly by the dissolution from the slates of large quantities of Ca which at these points was present at the
highest concentrations of the whole dump Žaverage values of between 450 and 500 mg ly1 .. The levels of Fe and Al were, consequently very low Žless than 1.5 and 0.1, respectively., as with most of the heavy metals ŽZn, Co, Ni, Cu and Pb.. High concentrations of Mn Žbetween 19 and 51 mg ly1 . were, however, maintained in solution, favoured by the relatively low redox potential values Ž382]426 mV.. 3.3. Ion acti¨ities and mineral stability Table 4 lists the most abundant ions in some ‘type’ streams from the dump, in order of activity.
Table 4 Ionic activities in the different stream types identified from the mine dump and from the River Eume before discharge of mine waste Stream type
Ion activities Žmol ly1 . 10y2
10y3
10y4
10y5
10y6
10y7
I
SO42y , FeSO4
Ca2q, Mg2q, Naq, Cly, Hq, Fe2q, CaSO4 , HSO4 y, MgSO40
Kq, H4 SiO4 , Al3q, Mn2q, AlSO4q, AlŽOH4 .2 , FeŽSO4 .2 , MnSO4
PO43y, AlF2q, FeOH2q, NaSO4 y, ZnSO4
Cu2q, Fe3q, CuSO4 , FeClq, KHSO4 , KSO4-, NO3 -
AlF2 q, ALOH2q, H2 PO4 , FeF2q
II
Ca2q, SO42y , CaSO4 , FeSO4
Mg2q, Naq, Cly, Fe2q, MgSO4 , MnSO4 0
Kq, H4 SiO4 , Mn2q, NaSO4 y
Zn2q, Fy, NaSO4y, ZnSO4
Hq, HSO4 y, AlF 2q, AlF2q, NO3y, FeClq, FeOHq, MgFq
AlF3 , ALOH2q, ALSO4q, NaCl
IIIa
Ca2q, Mg2q, SO4 2y, CaSO4 , MgSO4
Naq, Kq, H4 SiO4 Hq, Al3q, Fe2q, Mn2q, AlSO4q , AlŽSO4 .2y, FeSO4 , HSO4y, MnSO4
Zn2q, NaSO4 y, FeSOq 4 , FeOH2q, AlF2q
Cly, Cu2q, Fe3q, AlOH2q, CuSO4 , KSO4 y, NO3 y, ZnSO4
AlOH2q, FeOH2q
IIIb
Ca2q, Mg2q, Naq, Cly, SO42y, CaSO4 , MgSO4
Kq, H4 SiO4 , NaSO4
F, KSO4y , MnSO4 , NO3y
Mn2q, AlOH2q, AlŽOH.y 4 , FeŽOH.2 , FeŽOH.3 , MgFq
Hq, OHy, Zn2q, H3 SiO4y, NaCl, ZnSO4
Natural
Naq, Cly
Ca2q, Mg2q, Kq, SO42y,
NOy 3
Hq, Fe2q, Mn2q, Fy, AlŽOH.4y, CaSO4 , HPO42y, H2 PO4y, MgSO4
OHy, AlOH2q, H3 SiO4 y, NaSO4
Eume river
H4 SiO4
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129
Fig. 3. Speciation of aluminium in the four different types of streams identified.
For purposes of comparison, a sample from Eume River taken before dumping of the mine effluent was included. This comparison shows, in addition to the effects already commented on, the high
activity of ion pairs with sulfate, which is low in natural waters. The results of the SOLMINEQ chemical speciation show the predominance of complexes of Al bound to SO42y in the most
Fig. 4. Speciation of iron in the four different types of streams identified.
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acidic sulfate-rich waters, Al 3q being the next predominant species ŽFig. 3.. In the neutralized and less sulfide-influenced waters, Fy ions compete favourably with SO42y ions and AL]F complexes are dominant and Al]OH complexes are the next prevailing species. The distribution of Fe species is mainly determined by Eh]pH conditions. In group I streams, several forms of FeŽII. and FeŽIII. occur, mainly FeSO40 and FeSO4q. FeŽIII. forms were virtually absent in the waters II because of the low Eh values, so that Fe 2q and FeSO40 were the dominant species. Hydroxylated monomers of FeŽIII. ŽFe]OH. only occur in the less sulfide-influenced waters and they are important in the neutralized waters ŽFig. 4.. According to the calculations of the saturation indices the theoretical stability of various mineral phases were evaluated ŽTable 5.. The samples from groups I and IIIa were unsaturated with respect to the secondary minerals typical in Galician rivers and soils; kaolinite and gibbsite ŽFernandez Marcos et al., 1979., due to the highly ´ acidic conditions. Oversaturation with respect to aluminium hydroxysulfates, alunite and jurbanite-type mineral, were normally found in these streams, but conditions close to equilibrium were often attained for jurbanite; hence this mineral phase may control the aluminium solubility under these conditions. In addition to jurbanite and alunite, the group II streams were oversaturated with respect to gibbsite, kaolinite and a great number of primary silicates Ždata not shown.. The SI calculated for group IIIb streams indicate that gibbsite, kaolinite and several primary silicates were stable, while jurbanite and alunite were always unsaturated. Most the samples were oversaturated with respect to the Fe-bearing phases traditionally considered for these systems; goethite, FeŽOH. 3am and jarosite. A new mineral has recently been observed that may be the dominant solid phase controlling the activities of Fe and other elements in acid sulfate waters ŽBigham et al., 1996.. This mineral, schwertmannite, is poorly crystalline, has an ideal formula Fe 8 O 8 ŽOH. 6 ŽSO4 .4 and K eq s 18 " 2.4. The SI calculated with these values show that this mineral is near to equilibrium in the samples of group I, II and IIIa streams of this study, while
Table 5 Mineral stability in the different types of streams: equilibrium Žs., oversaturation Žq. and unsaturation Žy. conditions for some iron minerals, deduced from the mineral saturation indexes Mineral phase
Type of stream I
II
IIIa
Q C K G Gm A Jb Gh F Ja Sh Gy
q q y y y q srq q q q s s
q s q q srq q srq q q q srq s
q srq " " yrs q srq q q q srq sry
IIIb q s q q q y y q q q q s
Note. Q, quartz; C, calcedony; K, kaolinite; G, gibbsite; Gm, microcrystalline gibbsite; A, alunite; Jb, jurbanite; Gh, goethite; F, ferrihydrite, FeŽOH. 3am ; Ja, jarosite; Sh, schwertmannite; Gy, gypsum.
conditions of oversaturation were always attained for group IIIb. Goethite is always the most stable mineral phase. Fig. 5 also shows the stability and metastability of these Fe-bearing mineral in an Eh]pH diagram for the Fe]S]K]O]H system constructed by Bigham et al. Ž1996.. Finally, it was noted that gypsum was always close to conditions of equilibrium and this may be the mineral phase which controls Ca2q activity. 4. Conclusions The quality of the drainage water is the final result of a combination of complex geochemical interactions in which are involved numerous ions from different sources, principally from the oxidation of sulfides and the accelerated hydrolysis of the accompanying minerals. The composition of these streams is thus determined by the type of material through which they flow and the time of contact with that material. The influence of these factors is reflected in the wide variation of the Eh]pH conditions and the ion concentrations found. Four main groups of streams from different
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spoils have been preselected and slates which are alkaline in nature and have low contents of sulfur, have been placed on the surface. These results demonstrate that the selective management of spoils is the restoration practice which offers the best protection against contamination of surface and subsurface waters and is the method which should be applied in future construction of dump surfaces. The improvement in quality of drainage systems using this practice can significantly reduce the cost of treatment in the purification plant before discharge to the receiving catchment area. Acknowledgements This work forms part of the ‘Project on the Monitoring of the behaviour and Evolution of a Coal Mining Waste Dump in the Process of Reclamation, 1990]1993’, financed by the EC within the ACE Programme. Authors thank the Reclamation and Chemistry Departments of ENDESA for their collaboration in the field and in the laboratory. References
Fig. 5. Location of the various types of drainage streams in the Eh]pH diagram for Fe]S]K]O]H system after Bigham et al. Ž1996.; Jt s K-jarosite, Sh s shwertmannite, Fh s ferrihydrite, Gt s goethite, Py s pyrite. Fields of metastability are shown by dashed lines. Clearing areas demostrate expansion of Jt and Fh fields if lower pKs are selected.
zones were distinguished in which the level of influence of the processes of oxidation of sulfides and the level of neutralisation reached, were the variable features. The poorest quality, extremely acidic streams, originate in the oldest zones of the dump where pyrite rich, carbonaceous sediments are abundant and there is a low capacity for neutralisation of acidity. The streams least affected by the oxidation of sulfides and which have the highest capacity for neutralisation, originate in the most recently constructed areas, where
Alvarez E, Perez ´ A, Calvo R. Aluminium speciation in surface waters and soil solutions in areas of sulphide mineralization in Galicia ŽNW Spain .. Sci Total Environ 1993;133:17]37. ARC ŽAppalachian Region Commission.. Acid Mine Drainage in Appalachia. Washington, D.C.: Appalachian Regional Commission Report, 1969. Brady KS, Bigham WF, Jaynes WF, Logan TJ. Influence of sulfate on Fe-oxide formation: comparisons with stream receiving acid mine drainage. Clays Clay Min 1986;34:266]274. Bigham JM, Schwertmann U, Traina SJ, Winland RL, Wolf M. Schwertmannite, and the chemical modeling of iron in acid sulfate waters. Geochim Cosmochim Acta 1996;12:2111]2121. Calvo de Anta R, Macıas ´ F. Procesos de alteracion ´ inducidos por actividades humanas en materiales con sulfuros de Galicia. II. Composicion ´ de las fases fluıdas ´ y tendencias de neoformacion ´ mineral. In: Blanco J, editor. Alteraciones y Paleoalteraciones. Salamanca, Spain: Graficas Baroba, ´ 1992;I:262]271. Calvo de Anta R, Perez ´ Otero A. Soil affected by acid mine waters in Galicia ŽNW Spain.. Water Air Soil Pollut 1994;73:247]263.
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Caruccio FT, Hossner LR, Geidel G. Pyritic materials: acid drainage, soil acidity, and liming. In: Hossner, editor. Reclamation of Surface-Mined Lands. Boca Raton, Florida: CRC Press, 1989:129]148. Evangelou VP, Zhang YL. A review: Pyrite oxidation mechanisms and acid mine drainage prevention. Crit Rev Environ Sci Technol 1995;25:141]199. Fernandez Marcos ML, Macıas ´ ´ F, Guitian ´ F. A contribution to the study of the stability of the clay minerals from the soil solution composition at different pH values. Clay Min 1979;14:29]37. Gil Bueno A, Val Caballero C, Macıas ´ F, Monterroso C. Influence of waste selection in the dump reclamation at Puentes Mine. In: Rainbow, editor. Reclamation, treatment and utilization of coal mining wastes. Rotterdam: Bolkema, 1990:203]208. Glover HG. Mine water pollution: An overview of problems and control strategies in the United Kingdom. In: Odendaal PE, editor. Mine water pollution. New York: Pergamon Press, 1982:59]70. Herlihy AT, Kaufmann PR, Mitch ME, Brown DD. Regional estimates of acid mine drainage impact on streams in the Mid-Atlantic and Southeastern United States. Water Air Soil Pollut 1990;50:91]107. Johnson CA, Thornton I. Hydrological and chemical factors controlling the concentrations of Fe, Cu, Zn and As in a river system contaminated by acid mine drainage. Water Res 1987;21:359]365. Kempe JO. Review of water pollution problems and control strategies in the South African mining industry. In: Odendaal PE, editor. Mine water pollution. New York: Pergamon Press, 1982:27]58. Kinney EC. Extent of Acid Mine Pollution in the United States Affecting Fish and Wildlife. Washington, DC: US Dept Interior, Fish and Wildlife Circular 191, 1964.
Kharaka YK, Gunter D, Aggarwal A, Perkins EH, Debraal D. SOLMINEQ-88: A computer program for geochemical modeling of water]rock interactions. Mento Park, California, 1989. Lazaroff N. Sulfate requirement for iron oxidation by Thiobacillus ferroxidans. J Bacteriol 1963;85:78]83. Letterman RD, Mitsch WJ. Impact of mine drainage on a mountain stream in Pennsylvania. Environ Pollut 1978;17:53]73. Macıas ´ F. Proyecto Eume: Caracterizacion ´ y seguimiento del medio ´ fisico-quımico, impactos, recursos hıdricos y ´ ´ biologicos de la Cuenca del Eume. Spain: Informe interno, ´ Universidad de Santiago-FEUGA, 1994. Macıas ´ F, Calvo de Anta R. Construccion ´ de infraestructuras lineales en materiales con sulfuros, un ejemplo de impacto ambiental sobre el medio acuatico. II Simposio Nacional de ´ Carreteras y Medio Ambiente. Asociacion de Car´ Tecnica ´ reteras, MOPT, 1993:53]61. Miller SD. Chemistry of a pyritic strip-mine spoil. Ph.D. Dissertation. New Haven: Yale University, 1979:189. Mills AL. Acid mine waste drainage: microbial impact on the recovery of soil and water ecosystems. In: Tate, Klein DA, editors. Soil reclamation processes. New York: Marcel Dekker, 1985:35]80. Nordstrom DK. Aqueous pyrite oxidation and the consequent formation of secondary iron minerals. In: Kitrick JA, Fanning DS, Hossner LR, editors. Acid sulfate weathering. Madison, WI: Soil Science Society of America, 1982:37]56. Short TM, Black JA, Birge WJ. Effects of acid-mine drainage on the chemical and biological character of an alkaline headwater stream. Arch Environ Contam Toxicol 1990;19:241]248. Winland RL, Traina SJ, Bigham JM. Chemical composition of ochreous precipitates from Ohio coal mine drainage. J Environ Qual 1991;20:452]460.
Drainage waters affected by pyrite oxidation in a coal mine in Galicia Ž NW Spain. : Composition and mineral stability C. MonterrosoU , F. Macıas ´ Department of Edafologıa Agrıcola, Facultad de Biologıa, ´ y Quımica ´ ´ ´ Uni¨ersidad de Santiago, 15706 Santiago de Compostela, Spain Received 1 December 1997; accepted 9 February 1998
Abstract The quality of the drainage waters from the As Puentes lignite mine dump ŽGalicia, Spain. was evaluated along with the geochemical processes which determine its composition. Analysis of water from different areas of the dump was carried out at monthly intervals over a period of 2 years. During this period, pH values ranged between 2.1 and 8.0 and redox potential values between 150 and 750 mV, although most of the water samples were strongly acidic ŽpH 2.5]3.5. and oxidising ŽEh 600]750 mV.; conditions which are usually found in mine drainage systems where pyrite is present. In general, the water samples were characterised by the presence of elevated concentrations of Fe, SO42y and Hq, liberated from the oxidation of pyrite, and of Si, Al, Ca and Mg derived from the accelerated mineral hydrolysis occurring under these conditions. At the same time, very high concentrations of elements, in particular Mn, Zn, Ni and Co, which were liberated from both processes, were recorded. The best water quality was found in the most recently constructed areas of the dump, comprising pre-selected material low in sulfides. Q 1998 Elsevier Science B.V. Keywords: Acid mine drainage; Land reclamation; Mineral stability; Pyrite oxidation
1. Introduction Acid drainage represents one of the largest environmental problems associated with surface mining, due to oxidation of the sulfides found in many spoils. The mine waters affected by this
U
Corresponding author. Tel.: q34 81 563100, ext. 13290; fax: q34 81 596904; e-mail: [email protected]
process may show great variability in their physicochemical characteristics, but are usually highly acidic and have elevated concentrations of SO42y, Fe and some heavy metals ŽMills, 1985; Caruccio et al., 1989; Winland et al., 1991; Calvo de Anta and Macıas, ´ 1992; Evangelou and Zhang, 1995; Bigham et al., 1996.. The damage produced in aquatic ecosystems by acidic mine effluents has been widely studied all over the world since the beginning of the 1960s ŽKinney, 1964; ARC, 1969;
0048-9697r98r$19.00 Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00149-1
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Letterman and Mitsch, 1978; Kempe, 1982; Glover, 1982; Johnson and Thornton, 1987; Herlihy et al., 1990; Short et al., 1990, amongst others.. In Galicia ŽNW Spain., the impacts made on the river Brandelos, tributary of the Ulla, the Xestosa and the Eume ŽAlvarez et al., 1993; Macıas ´ and Calvo de Anta, 1993; Calvo de Anta and Perez ´ Otero, 1994; Macıas, ´ 1994., have been exhaustively studied. Although a large number of studies exist, the oxidation of sulfides is a complex subject and its effects vary enormously between different sites and under different conditions. The river Eume receives the effluents generated at the As Puentes lignite mine ŽGalicia, NW Spain.. The generation of acid drainage becomes problematic because of two main factors } the high rainfall and the existence of a large surface area where streams intercept and are exposed, both of which favour the oxidation of sulfides. Today, the impact on the receiving catchment area is avoided by physicochemical treatment of the water generated during excavation of the mine. However, the production and circulation of acidic streams in the dump area creates a problem for land reclamation as it impedes the establishment of vegetation and even causes the disappearance of already well established vegetation ŽGil Bueno et al., 1990.. To minimize these effects, a reclamation programme has been developed whose objectives are to reduce the surfaces affected by the water flows, slow down the kinetics of the oxidation of sulfides and increase the pH of the rhizosphere. The programme includes selective management of spoils, use of soil correctors and drainage work and the results are evaluated by following the quality of the circulating water and studying the soil solutions. In the present study, analysis of the composition of the drainage water of the As Puentes lignite mine dump was made, the geochemical processes which give rise to its character were identified and the effects of the selective spoil management on its characteristics were evaluated. 2. Materials and methods The climate of As Pontes is characterised by high rainfall Žaverage annual rainfall: 1684 mm.
and moderate temperatures Žaverage annual temperature: 11.68C.. The lignite mine dump is designed to store 800 M m3 of spoils and has a forecasted final height of 220 m and a final surface area of 1300 ha, of which 370 ha are presently undergoing restoration. Most of the spoils are slates and Tertiary sediments containing different amounts of carbonaceous material, with pyrite often being present Žbiogenic pyrite in the carbonaceous sediments and geologic pyrite in the slates .. To control the run-off, the banks have been subdivided into partial banks and berms and water is conducted and evacuated through a system of channels and ditches operating by gravity. The contact between spoils of different permeabilities allows the frequent outflow of subsuperficial water from the banks which are interconnected with the general circulation. The soil restoration work includes using correctors, such as lime or ashes from the lignite combustion, inorganic and organic fertilisers and on some occasions, the spreading of a layer of topsoil. The different kinds of spoils dumped and the procedures used have given rise to a wide variety of physicochemical conditions at the dump surface. To ensure that all types of water emerging and circulating were represented, a total of 21 water quality control points were selected from the different restoration areas. These were termed, in chronological order: CS, E1, E2, EH and EF. The presence of sulfide rich materials is common in the oldest areas ŽCS, E1., whereas the more recent areas have been constructed from material selected for having low or zero pyrite content. Sampling was carried out monthly over a period of 2 years and a total of approx. 450 water samples were obtained. Measurements of pH, Eh and electrical conductivity were made in the field and duplicate solution samples were collected and filtered through 0.45-m m Nucleopore membranes. One of the subsamples was acidified with ultrapure HNO3 for measurement of dissolved metals. Total concentrations of Ca, Mg, Na, K, Fe, Al, Si, Mn, Zn, Ni and Co were measured by ICP-AES with a Thermo Jarrel Ash Mod. Atom Scan 25 equipment. The emission intensity, detection limits and potential spectral interferences were taken in
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consideration for selecting a wavelength for each element; the instrument operating parameters were improved for each element and background correction was applied ŽTable 1.. Sample rate was 1 ml miny1 , nebulizer pressure was 30 psi, Ar flow was 15 l miny1 for the plasma and 1 l miny1 as auxiliary, Ar optics flush was on or below 200 nm. The ICP was calibrated by using two-point standardization and multielement standards were prepared in 2% Žvrv. HNO3 from 1000 m g mly1 stock standard ŽMerk KGaA, Germany.. Trace concentrations of Cu, Cd and Pb were determined by means of differential pulse anodic stripping voltammetry ŽDPASV., using a Metrohm 646VA Stand with three electrodes: a hanging mercury drop electrode, an AgClrAg reference electrode and a Pt counter electrode. To avoid the problems derived from the matrix of the samples an automatized standard-addition method was used, by making two or three successive additions. Sulfate and Cly were measured by ion chromatography ŽDionex 2000., Fy, NOy and 3 Ž NHq 4 , with ion selective electrodes Orion 94-09, Orion 93-07 and Orion 95-10, respectively., PO43y, by visible spectrophotometry with molybdate reagent, as well as Fe 2q using o-phenanthroline reagent. The concentration of Fe 3q was calculated from the difference between total dissolved Fe determined by ICP-AES and Fe 2q determinations. At least 20% of samples were analyzed in duplicate and for each batch of samples analyzed two spiked samples were also analyzed. The CV for replicate analyses was F 5%. Ion activities were calculated with the SOLMINEQ88 geochemical computer programme ŽKharaka et al., 1989.. The stabilities of various mineral phases were evaluated by using Saturation Indices ŽSI., calculated by comparing measured ion activities with those expected from the solubility product of a given mineral phase. The measured activities, expressed as an ion activity product ŽIAP. were divided by the equilibrium constant of each mineral phase ŽK eq .: SI s log ŽIAPrK eq .. Positive, negative and zero SI values indicate, respectively, that the system is oversaturated, unsaturated or at equilibrium with respect to a particular mineral phase.
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Table 1 Some operating parameters for each element analyzed by ICP-AES
Ca Mg Na K Fe Al Si Mn Zn Ni Co
Wavelength Žnm.
Sensitivity Ž m g ly1 .
RF power ŽW.
Observation height Žmm.
317.933 279.553 589.590 766.490 259.940 167.081 251.612 257.610 213.856 231.604 228.616
1 1 10 100 2 2 10 2 2 5 3
1350 1350 950 950 1150 1750 1150 1350 1350 1350 1350
15 15 10 20 12 12 12 12 10 10 10
3. Results and discussion 3.1. Water quality
The results obtained from the analysis of the water showed high variability in Eh]pH and ionic concentrations which illustrates the great diversity of geochemical conditions in the drainage systems ŽTable 2.. However, they also have some characteristics which differ markedly from those of natural waters. These characteristics are determined by the process of sulfide oxidation, which affects, to a greater or lesser extent, all of the waters analysed, as is clear from the high concentration of SO42y Žbetween 500 and 7500 mg ly1 .. The high concentrations of sulfates is important in these type of waters for two reasons; firstly, because of the metabolic requirement of this anion for the oxidation of Fe 2q by chemolithotrophic, acidophilic bacteria, such as T. ferroxidans ŽLazaroff, 1963. and secondly, due to the influence that sulfate appears to have on the thermodynamics and kinetics of the formation of various minerals Žjarosite, schwertmannite, ferrihydrite, goethite,.... from mine drainage systems ŽNordstrom, 1982; Brady et al., 1986; Bigham et al., 1996.. As well as liberation of SO42y, the oxidation of
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Table 2 Composition of streams from the mine dump: average, maximum and minimum values of different parameters Variable
Average S.D.
pH 4.03 1.40 CE ŽmS cmy1 . 3.15 1.25 Eh ŽmV. 547 147 SO42y Žmg ly1 . 2602 1458 NO3y Žmg ly1 . 1.12 3.77 Cly Žmg ly1 . 22.89 12.14 Fy Žmg ly1 . 1.00 1.21 PO43y Žmg ly1 . 0.04 0.07 NH4q Žmg ly1 . 1.38 2.30 Ca Žmg ly1 . 352.6 114.1 Mg Žmg ly1 . 140.0 70.6 Na Žmg ly1 . 18.2 6.6 K Žmg ly1 . 5.7 2.2 Si Žmg ly1 . 13.2 10.4 Al Žmg ly1 . 43.00 62.01 Fe Žmg ly1 . 395.50 432.37 FeŽII. Žmg ly1 . 327.08 375.75 Mn Žmg ly1 . 49.62 30.47 Ni Žmg ly1 . 2.38 1.42 Co Žmg ly1 . 2.09 1.24 Zn Žmg ly1 . 3.18 2.95 Cu Žmg ly1 . 0.33 0.55 Pb Ž m g ly1 . 2.8 5.2 Cd Ž m g ly1 . 16.3 22.1
Minimum Maximum 2.23 0.82 143 424 0.03 6.25 - 0.01 - 0.01 0.04 50.7 14.6 4.6 0.7 1.6 - 0.05 - 0.01 - 0.01 1.30 - 0.01 - 0.01 - 0.01 - 0.01 - 0.5 - 0.5
7.99 6.51 754 7404 43.99 96.40 5.26 0.643 12.48 602.5 370.0 53.2 19.0 54.0 450.00 1592.00 1463.00 148.70 8.60 6.60 17.00 5.610 92.3 141.0
sulfides gives rise to large quantities of iron and Hq. The concentrations of dissolved Fe in the water samples under study varied greatly; from 0.1 to 1600 mg ly1 , most of which Žbetween 70 and 100%. was in the reduced form. However, if we take into account the ratio of SO42y:Fe theoretically obtained from the oxidation of pyrite Ž2:1., the concentration of dissolved Fe in most of the water samples is lower than that expected, indicating that part of the dissolved Fe has precipitated through oxidationrhydrolysis reactions ŽFig. 1a.. High concentrations of dissolved Fe 3q were found only in the most acidic streams ŽpH3.. The relationship found between the concentration of SO42y and the pH of the water samples from the mine dump is far from being linear ŽFig. 1b., since the processes of oxidation and of neutralisation of the acid liberated occur simultaneously. Thus, one group of water samples, which despite being highly affected by the oxidation of sulfides Žand having a concentration of SO42y of
Fig. 1. Relationship between the concentration of SO42y and Ža. dissolved iron, Žb. pH in the streams under study. Line in Ža. indicates the theoretical molar ratio SO42y:Fe derived from the oxidation of pyrite.
between 1500 and 3000 mg ly1 ., has effectively neutralised the acid liberated, principally by the dissolution of large quantities of Ca and Mg, leading to pH values of between 5 and 8. However, most of the water samples are very acidic with pH values lying within a very narrow range, between 2.5 and 3.5, independent of SO42y concentration. This suggests that most of the dump streams are buffered to this range of pH. Miller Ž1979. suggests that the mine dumps containing pyrite may be in equilibrium with jarosite and ferrihydrite, forming a buffering system with a pH value of around 3.19 Ž"0.17.. Most of the streams in this study would thus fall into the buffering range estimated by this author.
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The redox potential values also varied widely ŽEh s 150]750 mV., although the most common range was between 600 and 750 mV. There was a slight relationship between pH and Eh Ž r s y0.84, P- 0.001.. The Eh]pH conditions most commonly found in the dump waters were, therefore, acidic and oxidising ŽTable 2.. The association between high acidity and high redox potential values is mainly due to the presence of significant quantities of dissolved Fe 3q, which acts as a more effective oxidizing agent than O 2 , in the most acidic environments. The high acidity generated during the oxidising process gives rise to accelerated hydrolysis of the other minerals in the spoil materials, causing large quantities of the constituent elements to be made soluble. Concentrations of Ca and Mg were especially high Žthe average values being 352 and 140 mg ly1 , respectively. as were those of Si Ž13.2 mg ly1 . and Al Ž43.0 mg ly1 .. The strong correlation between Si and Al Ž r s 0.71, P- 0.001. reflects the origin of Al, which is principally from the alteration of silicates. Using Si as a tracer of mineral hydrolysis, a strong correlation with Fy Ž r s 0.85 and 0.75, P- 0.001, respecand NHq 4 tively. was found. These are elements which can reach concentrations much higher than those found in natural waters Žup to 5.3 and 12.5 mg
125
ly1 , respectively.. Minerals rich in Fy are scarce in slates and are very resistant, which gives some idea of the extraordinary aggressiveness of some of these stream waters to be able to occasionally produce such high concentrations of Fy. NHq 4 may originate from the retention of slate in silicate meshes and from organic binding in carbonaceous sediments. Žusually - 1 mg ly1 . Concentrations of NOy 3 3y Ž and PO4 usually - 100 m g ly1 ., in general lower than those in natural waters, were found. The low concentrations of PO43y may be related to processes of immobilisation by adsorption of Fe hydroxides precipitated in the water circulation channels andror the precipitation of Fe insoluble phosphates. The high acidity of most of the Žusually streams favours the presence of NHq 4 0.5]2 mg ly1 . and makes the appearance of NOy 3 more unlikely. The processes of oxidation of sulfides and of mineral hydrolysis have given rise to the mobilisation of heavy metals, causing abnormally high concentrations of these in the streams. This was especially true for Mn, for which values of up to 140 mg ly1 were seen. Mn, Zn, Ni and Co were strongly correlated with each other and with the concentration of SO42y Žcoefficients of correlation between 0.63 and 0.87, P- 0.001..
Table 3 Range of average values recorded over a period of 2 years for the parameters analysed in water samples from the four different types of streams identified Variable
pH CE ŽmS cmy1 . Eh ŽmV. SO42y Žmg ly1 . NO3y Žmg ly1 . NH4q Žmg ly1 . Ca Žmg ly1 . Mg Žmg ly1 . Na Žmg ly1 . K Žmg ly1 . Si Žmg ly1 . Al Žmg ly1 . Fe Žmg ly1 . FeŽII. Žmg ly1 . Mn Žmg ly1 .
Type of stream I Ž6 points.
II Ž4 points.
IIIa Ž7 points.
IIIb Ž4 points.
2.80]3.20 4.07]4.90 579]664 3906]4736 0.27]0.94 0.37]9.21 321.0]440.0 155.0]249.2 10.4]24.5 2.3]7.8 12.8]44.1 47.8]156.4 709.0]1050.0 330.0]868.0 13.8]102.0
4.58]5.67 3.01]3.52 214]432 2257]2906 0.15]0.44 0.67]1.53 423.2]464.0 120.1]169.0 14.8]20.2 4.4]5.7 4.1]9.4 0.2]27.7 360.9]599.0 252.6]535.0 43.3]63.2
3.13]3.45 1.67]2.82 615]695 1082]2616 0.20]0.47 0.29]0.80 178.1]298.0 71.4]108.3 13.2]18.0 2.9]6.0 11.2]14.0 22.9]144.9 6.2]296.5 3.0]239.8 29.9]50.9
5.75]7.15 2.26]2.59 382]426 1551]1900 0.83]1.66 0.15]1.17 396.0]476.8 89.1]139.5 19.4]27.6 5.2]9.8 3.8]6.6 0.1]2.1 0.3]14.6 0.1]11.20 18.6]50.9
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As a consequence of all of these processes, the resulting drainage waters have a high ionic strength; values for electrical conductivity of between 800 and 6500 m S cmy1 were obtained, which are between 15 and 150 times more concentrated than the natural waters in the region, characterised by being extremely dilute. Finally, the ionic strength of the streams is determined by the extent of the influence of sulfide oxidation, as is seen by the extremely strong correlation between electrical conductivity and the concentration of sulfates in the water Ž r s 0.97, P0.001.. 3.2. Types of drainage streams in the mine dump The wide range of values found for almost all parameters measured in assessing water quality clearly indicates the different origins of the streams, with the type of material through which they flow and the time of contact with this material being the factors which determine their composition. Among the samples in this study were found; run-off waters in contact with the surface material and with generally short residence times, permeating waters which reach unknown depths and which then emerge at the surfaces of lower slopes after variable residence times and also pockets of water, trapped during the piling of spoils on the dump which are in contact with the spoils for long periods before emerging at the surface in the form of springs. These streams emerge from different zones after contact with materials of different permeabilities. In relation to physicochemical parameters, groups of streams can be differentiated whose qualities can be related to their origin, the level of influence of oxidation of sulfides and the extent of neutralisation of the acidity generated. Thus the following groups can be distinguished ŽTable 3.: 1. Group I: Streams which are strongly affected by oxidation of sulfides and undergo little neutralisation; 2. Group II: Streams which are strongly affected by oxidation of sulfides and are moderately neutralised; and
3. Group III: Streams moderately affected by oxidation of sulfides and which are: IIIa, not neutralised; IIIb, neutralised.
3.2.1. Streams which are strongly affected by oxidation of sulfides and undergo little neutralisation The streams in this first group, which differ significantly from the others, are strongly acidic, oxidising, have a very high ionic strength and include those from six control points at the oldest area of the mine dump Žarea CS.. The extreme values of pH, Eh and EC measured were, 2.2]3.5, 521]707 mV and 1.0]6.5 mS cmy1 , respectively. Along with these parameters, very high concentrations of sulfates Žgenerally between 4500 and 5000 mg ly1 and occasionally up to 7000 g ly1 . and of dissolved iron Žaverage values of between 700 and 1000 mg ly1 and occasionally up to 1600 mg ly1 . have been registered. In addition, very high Si contents Žup to 54 mg ly1 . and Al Žup to 450 mg ly1 . were found, which reflects the aggressive nature of these streams and their potential to accelerate mineral hydrolysis processes Ža strongly acidolysis process.. The high contents of Ca and Mg, with average concentrations of 320]400 mg ly1 and 150]250 mg ly1 , respectively, were incapable of neutralising the acidity in the system. 3.2.2. Streams which are strongly affected by oxidation of sulfides and are moderately neutralised The second group of streams, which have characteristics markedly different from the previous ones, includes all the water arising from areas E1 and E2 at the point of emergence on the surface Žfour control points.. The most relevant and distinctive characteristics of this group of streams are their acidic to weakly acidic nature Žaverage pH values of between 4.6 and 5.7. and low redox potential values Žaverage Eh between 200 and 400 mV.. The low Eh values, clearly less than in well aerated surface waters suggest that these streams originate in deep, poorly aerated areas. However, these conditions do not inhibit the oxidation of pyrite, as is clear from the high concentration of sulfates found, although they are less than those seen in group I streams Žaverage values between
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2200 and 2900 mg ly1 and maximum concentrations of 4500 mg ly1 .. Despite the pH of these streams not being excessively low, they maintain high levels of dissolved iron Žderived from the oxidation of pyrite. due to the low Eh values. The average concentration of dissolved iron in this group of streams varies between 360 and 600 mg ly1 and the molar ratio of SO42y: Fe is between 2.7 and 3.8, values which are very close to the theoretical value derived from the oxidation of pyrite ŽSO42y:Fe s 2:1.. This indicates that significant processes involving the precipitation of Fe compounds during the flow of these streams from deep areas to the surface do not exist andror there is precipitation of some kind of sulfate which also removes this anion from solution, so that the relative proportion of both are maintained. The concentrations of Si and Al were generally low, with average values usually being less than 10 and 1 mg ly1 , respectively. The metal content of these streams is notably lower than most of those from area CS, although high levels of Mn are maintained in solution Žaverage values of between 43 and 63 mg ly1 .. The high concentrations of Ca were notable, the average values being between 423 and 464 mg ly1 and maximum values of up to 550 mg ly1 , which along with the lesser degree of oxidation of pyrite than in the area CS, helps to explain the partial neutralisation seen in these streams. Regarding the change which the emerging streams undergo at the surface, it was found that gradual oxidation and acidification took place, from the point of emergence to the confluence in the general circulation ditches. Acidification is without a doubt due to the precipitation of Fe compounds which are produced as a consequence of the changes in redox potentials and which cover the bases of the channels excavated in the banks by these streams. In some cases a decrease of almost 1 unit of pH is produced in a distance of only 100 m with a parallel increase of 155 mV in Eh ŽFig. 2.. 3.2.3. Streams moderately affected by oxidation of pyrite The third group of streams includes those from
127
Fig. 2. Changes of Eh and pH of the stream with the distance from the point of its emergence at the surface.
the general circulation channels in zone E2 and all of those from zone EH. The common characteristics of these streams were, the SO42y concentration, with average values generally being between 1100 and 1900 mg ly1 , and ionic strength, with average values of conductivity being between 1.7 and 2.8 mS cmy1 . These values are substantially lower than those found for the previous two groups, which implies that there is less influence from pyrite oxidation. Within this group, two subgroups can be clearly distinguished by the level of neutralisation. The group IIIa streams are acidic, although less so than those from zone CS, and the group IIIb streams are close to being neutral. Group IIIa principally includes the streams from zone E2. The acidity generated during the oxidation of pyrite is barely neutralised and average pH values of between 3.1 and 3.5 were measured. They are, like those from zone CS, oxidising Žhaving average Eh values of between 600 and 700 mV. and maintain relatively high quantities of iron in solution, although always much less than was found in the previous groups Žaverage values generally being between 20 and 200 mg ly1 with extremes of 4 and 340 mg ly1 .. The molar ratios of SO42y:Fe in solution were always much higher than 2, which indicates that there is a marked precipitation of Fe compounds in the water circulation channels. This fact is reflected in the large accumulation of iron compounds in the base of the channels. As well as the acidic conditions, elevated concentrations of Si Žaverage
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128
values of between 11 and 14 mg ly1 . and Al Žgenerally between 20 and 30 mg ly1 , but with extreme average values of 145 mg ly1 ., as well as of different heavy metals, were found. The final group of neutralised streams ŽIIIb. encompasses those from zone EH, the youngest area of the dump. This group of streams comes from the zones where the surface is largely made up of slates of an alkaline nature. As a consequence, most of the acid generated during the oxidation of pyrite was neutralised Žaverage values of pH between 5.7 and 7.1., mainly by the dissolution from the slates of large quantities of Ca which at these points was present at the
highest concentrations of the whole dump Žaverage values of between 450 and 500 mg ly1 .. The levels of Fe and Al were, consequently very low Žless than 1.5 and 0.1, respectively., as with most of the heavy metals ŽZn, Co, Ni, Cu and Pb.. High concentrations of Mn Žbetween 19 and 51 mg ly1 . were, however, maintained in solution, favoured by the relatively low redox potential values Ž382]426 mV.. 3.3. Ion acti¨ities and mineral stability Table 4 lists the most abundant ions in some ‘type’ streams from the dump, in order of activity.
Table 4 Ionic activities in the different stream types identified from the mine dump and from the River Eume before discharge of mine waste Stream type
Ion activities Žmol ly1 . 10y2
10y3
10y4
10y5
10y6
10y7
I
SO42y , FeSO4
Ca2q, Mg2q, Naq, Cly, Hq, Fe2q, CaSO4 , HSO4 y, MgSO40
Kq, H4 SiO4 , Al3q, Mn2q, AlSO4q, AlŽOH4 .2 , FeŽSO4 .2 , MnSO4
PO43y, AlF2q, FeOH2q, NaSO4 y, ZnSO4
Cu2q, Fe3q, CuSO4 , FeClq, KHSO4 , KSO4-, NO3 -
AlF2 q, ALOH2q, H2 PO4 , FeF2q
II
Ca2q, SO42y , CaSO4 , FeSO4
Mg2q, Naq, Cly, Fe2q, MgSO4 , MnSO4 0
Kq, H4 SiO4 , Mn2q, NaSO4 y
Zn2q, Fy, NaSO4y, ZnSO4
Hq, HSO4 y, AlF 2q, AlF2q, NO3y, FeClq, FeOHq, MgFq
AlF3 , ALOH2q, ALSO4q, NaCl
IIIa
Ca2q, Mg2q, SO4 2y, CaSO4 , MgSO4
Naq, Kq, H4 SiO4 Hq, Al3q, Fe2q, Mn2q, AlSO4q , AlŽSO4 .2y, FeSO4 , HSO4y, MnSO4
Zn2q, NaSO4 y, FeSOq 4 , FeOH2q, AlF2q
Cly, Cu2q, Fe3q, AlOH2q, CuSO4 , KSO4 y, NO3 y, ZnSO4
AlOH2q, FeOH2q
IIIb
Ca2q, Mg2q, Naq, Cly, SO42y, CaSO4 , MgSO4
Kq, H4 SiO4 , NaSO4
F, KSO4y , MnSO4 , NO3y
Mn2q, AlOH2q, AlŽOH.y 4 , FeŽOH.2 , FeŽOH.3 , MgFq
Hq, OHy, Zn2q, H3 SiO4y, NaCl, ZnSO4
Natural
Naq, Cly
Ca2q, Mg2q, Kq, SO42y,
NOy 3
Hq, Fe2q, Mn2q, Fy, AlŽOH.4y, CaSO4 , HPO42y, H2 PO4y, MgSO4
OHy, AlOH2q, H3 SiO4 y, NaSO4
Eume river
H4 SiO4
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129
Fig. 3. Speciation of aluminium in the four different types of streams identified.
For purposes of comparison, a sample from Eume River taken before dumping of the mine effluent was included. This comparison shows, in addition to the effects already commented on, the high
activity of ion pairs with sulfate, which is low in natural waters. The results of the SOLMINEQ chemical speciation show the predominance of complexes of Al bound to SO42y in the most
Fig. 4. Speciation of iron in the four different types of streams identified.
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acidic sulfate-rich waters, Al 3q being the next predominant species ŽFig. 3.. In the neutralized and less sulfide-influenced waters, Fy ions compete favourably with SO42y ions and AL]F complexes are dominant and Al]OH complexes are the next prevailing species. The distribution of Fe species is mainly determined by Eh]pH conditions. In group I streams, several forms of FeŽII. and FeŽIII. occur, mainly FeSO40 and FeSO4q. FeŽIII. forms were virtually absent in the waters II because of the low Eh values, so that Fe 2q and FeSO40 were the dominant species. Hydroxylated monomers of FeŽIII. ŽFe]OH. only occur in the less sulfide-influenced waters and they are important in the neutralized waters ŽFig. 4.. According to the calculations of the saturation indices the theoretical stability of various mineral phases were evaluated ŽTable 5.. The samples from groups I and IIIa were unsaturated with respect to the secondary minerals typical in Galician rivers and soils; kaolinite and gibbsite ŽFernandez Marcos et al., 1979., due to the highly ´ acidic conditions. Oversaturation with respect to aluminium hydroxysulfates, alunite and jurbanite-type mineral, were normally found in these streams, but conditions close to equilibrium were often attained for jurbanite; hence this mineral phase may control the aluminium solubility under these conditions. In addition to jurbanite and alunite, the group II streams were oversaturated with respect to gibbsite, kaolinite and a great number of primary silicates Ždata not shown.. The SI calculated for group IIIb streams indicate that gibbsite, kaolinite and several primary silicates were stable, while jurbanite and alunite were always unsaturated. Most the samples were oversaturated with respect to the Fe-bearing phases traditionally considered for these systems; goethite, FeŽOH. 3am and jarosite. A new mineral has recently been observed that may be the dominant solid phase controlling the activities of Fe and other elements in acid sulfate waters ŽBigham et al., 1996.. This mineral, schwertmannite, is poorly crystalline, has an ideal formula Fe 8 O 8 ŽOH. 6 ŽSO4 .4 and K eq s 18 " 2.4. The SI calculated with these values show that this mineral is near to equilibrium in the samples of group I, II and IIIa streams of this study, while
Table 5 Mineral stability in the different types of streams: equilibrium Žs., oversaturation Žq. and unsaturation Žy. conditions for some iron minerals, deduced from the mineral saturation indexes Mineral phase
Type of stream I
II
IIIa
Q C K G Gm A Jb Gh F Ja Sh Gy
q q y y y q srq q q q s s
q s q q srq q srq q q q srq s
q srq " " yrs q srq q q q srq sry
IIIb q s q q q y y q q q q s
Note. Q, quartz; C, calcedony; K, kaolinite; G, gibbsite; Gm, microcrystalline gibbsite; A, alunite; Jb, jurbanite; Gh, goethite; F, ferrihydrite, FeŽOH. 3am ; Ja, jarosite; Sh, schwertmannite; Gy, gypsum.
conditions of oversaturation were always attained for group IIIb. Goethite is always the most stable mineral phase. Fig. 5 also shows the stability and metastability of these Fe-bearing mineral in an Eh]pH diagram for the Fe]S]K]O]H system constructed by Bigham et al. Ž1996.. Finally, it was noted that gypsum was always close to conditions of equilibrium and this may be the mineral phase which controls Ca2q activity. 4. Conclusions The quality of the drainage water is the final result of a combination of complex geochemical interactions in which are involved numerous ions from different sources, principally from the oxidation of sulfides and the accelerated hydrolysis of the accompanying minerals. The composition of these streams is thus determined by the type of material through which they flow and the time of contact with that material. The influence of these factors is reflected in the wide variation of the Eh]pH conditions and the ion concentrations found. Four main groups of streams from different
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spoils have been preselected and slates which are alkaline in nature and have low contents of sulfur, have been placed on the surface. These results demonstrate that the selective management of spoils is the restoration practice which offers the best protection against contamination of surface and subsurface waters and is the method which should be applied in future construction of dump surfaces. The improvement in quality of drainage systems using this practice can significantly reduce the cost of treatment in the purification plant before discharge to the receiving catchment area. Acknowledgements This work forms part of the ‘Project on the Monitoring of the behaviour and Evolution of a Coal Mining Waste Dump in the Process of Reclamation, 1990]1993’, financed by the EC within the ACE Programme. Authors thank the Reclamation and Chemistry Departments of ENDESA for their collaboration in the field and in the laboratory. References
Fig. 5. Location of the various types of drainage streams in the Eh]pH diagram for Fe]S]K]O]H system after Bigham et al. Ž1996.; Jt s K-jarosite, Sh s shwertmannite, Fh s ferrihydrite, Gt s goethite, Py s pyrite. Fields of metastability are shown by dashed lines. Clearing areas demostrate expansion of Jt and Fh fields if lower pKs are selected.
zones were distinguished in which the level of influence of the processes of oxidation of sulfides and the level of neutralisation reached, were the variable features. The poorest quality, extremely acidic streams, originate in the oldest zones of the dump where pyrite rich, carbonaceous sediments are abundant and there is a low capacity for neutralisation of acidity. The streams least affected by the oxidation of sulfides and which have the highest capacity for neutralisation, originate in the most recently constructed areas, where
Alvarez E, Perez ´ A, Calvo R. Aluminium speciation in surface waters and soil solutions in areas of sulphide mineralization in Galicia ŽNW Spain .. Sci Total Environ 1993;133:17]37. ARC ŽAppalachian Region Commission.. Acid Mine Drainage in Appalachia. Washington, D.C.: Appalachian Regional Commission Report, 1969. Brady KS, Bigham WF, Jaynes WF, Logan TJ. Influence of sulfate on Fe-oxide formation: comparisons with stream receiving acid mine drainage. Clays Clay Min 1986;34:266]274. Bigham JM, Schwertmann U, Traina SJ, Winland RL, Wolf M. Schwertmannite, and the chemical modeling of iron in acid sulfate waters. Geochim Cosmochim Acta 1996;12:2111]2121. Calvo de Anta R, Macıas ´ F. Procesos de alteracion ´ inducidos por actividades humanas en materiales con sulfuros de Galicia. II. Composicion ´ de las fases fluıdas ´ y tendencias de neoformacion ´ mineral. In: Blanco J, editor. Alteraciones y Paleoalteraciones. Salamanca, Spain: Graficas Baroba, ´ 1992;I:262]271. Calvo de Anta R, Perez ´ Otero A. Soil affected by acid mine waters in Galicia ŽNW Spain.. Water Air Soil Pollut 1994;73:247]263.
132
C. Monterroso, F. Macıas ´ r The Science of the Total En¨ironment 216 (1998) 121]132
Caruccio FT, Hossner LR, Geidel G. Pyritic materials: acid drainage, soil acidity, and liming. In: Hossner, editor. Reclamation of Surface-Mined Lands. Boca Raton, Florida: CRC Press, 1989:129]148. Evangelou VP, Zhang YL. A review: Pyrite oxidation mechanisms and acid mine drainage prevention. Crit Rev Environ Sci Technol 1995;25:141]199. Fernandez Marcos ML, Macıas ´ ´ F, Guitian ´ F. A contribution to the study of the stability of the clay minerals from the soil solution composition at different pH values. Clay Min 1979;14:29]37. Gil Bueno A, Val Caballero C, Macıas ´ F, Monterroso C. Influence of waste selection in the dump reclamation at Puentes Mine. In: Rainbow, editor. Reclamation, treatment and utilization of coal mining wastes. Rotterdam: Bolkema, 1990:203]208. Glover HG. Mine water pollution: An overview of problems and control strategies in the United Kingdom. In: Odendaal PE, editor. Mine water pollution. New York: Pergamon Press, 1982:59]70. Herlihy AT, Kaufmann PR, Mitch ME, Brown DD. Regional estimates of acid mine drainage impact on streams in the Mid-Atlantic and Southeastern United States. Water Air Soil Pollut 1990;50:91]107. Johnson CA, Thornton I. Hydrological and chemical factors controlling the concentrations of Fe, Cu, Zn and As in a river system contaminated by acid mine drainage. Water Res 1987;21:359]365. Kempe JO. Review of water pollution problems and control strategies in the South African mining industry. In: Odendaal PE, editor. Mine water pollution. New York: Pergamon Press, 1982:27]58. Kinney EC. Extent of Acid Mine Pollution in the United States Affecting Fish and Wildlife. Washington, DC: US Dept Interior, Fish and Wildlife Circular 191, 1964.
Kharaka YK, Gunter D, Aggarwal A, Perkins EH, Debraal D. SOLMINEQ-88: A computer program for geochemical modeling of water]rock interactions. Mento Park, California, 1989. Lazaroff N. Sulfate requirement for iron oxidation by Thiobacillus ferroxidans. J Bacteriol 1963;85:78]83. Letterman RD, Mitsch WJ. Impact of mine drainage on a mountain stream in Pennsylvania. Environ Pollut 1978;17:53]73. Macıas ´ F. Proyecto Eume: Caracterizacion ´ y seguimiento del medio ´ fisico-quımico, impactos, recursos hıdricos y ´ ´ biologicos de la Cuenca del Eume. Spain: Informe interno, ´ Universidad de Santiago-FEUGA, 1994. Macıas ´ F, Calvo de Anta R. Construccion ´ de infraestructuras lineales en materiales con sulfuros, un ejemplo de impacto ambiental sobre el medio acuatico. II Simposio Nacional de ´ Carreteras y Medio Ambiente. Asociacion de Car´ Tecnica ´ reteras, MOPT, 1993:53]61. Miller SD. Chemistry of a pyritic strip-mine spoil. Ph.D. Dissertation. New Haven: Yale University, 1979:189. Mills AL. Acid mine waste drainage: microbial impact on the recovery of soil and water ecosystems. In: Tate, Klein DA, editors. Soil reclamation processes. New York: Marcel Dekker, 1985:35]80. Nordstrom DK. Aqueous pyrite oxidation and the consequent formation of secondary iron minerals. In: Kitrick JA, Fanning DS, Hossner LR, editors. Acid sulfate weathering. Madison, WI: Soil Science Society of America, 1982:37]56. Short TM, Black JA, Birge WJ. Effects of acid-mine drainage on the chemical and biological character of an alkaline headwater stream. Arch Environ Contam Toxicol 1990;19:241]248. Winland RL, Traina SJ, Bigham JM. Chemical composition of ochreous precipitates from Ohio coal mine drainage. J Environ Qual 1991;20:452]460.