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Applied Geochemistry 20 (2005) 1320–1356

www.elsevier.com/locate/apgeochem

AppliedGeochemistry

Acid mine drainage in the Iberian Pyrite Belt (Odielriver watershed, Huelva, SW Spain):

Geochemistry, mineralogy and environmental implications

Javier Sanchez Espana *, Enrique Lopez Pamo, Esther Santofimia,Osvaldo Aduvire, Jesus Reyes, Daniel Barettino

Direccion de Recursos Minerales y Geoambiente, Instituto Geologico y Minero de Espana (IGME), Rios Rosas, 23, 28003 Madrid, Spain

Received 15 July 2004; accepted 2 January 2005

Editorial handling by R. Fuge

Available on
line 24 May 2005
Abstract

This work reports the physical properties and water chemistry of 64 AMD discharges from 25 different mines in the

IPB draining to the Odiel river watershed, which have been investigated during the hydrologic year 2003–2004. These

AMD solutions vary largely in flow rate and chemical composition both spatially (between the different mine sites, sug-

gesting a strong geologic control on AMD chemistry) and seasonally (due to marked hydrologic variations), and

include cases with very low pH (mostly in the range 1.4–4), and extreme sulphate (up to 44 g/L SO2�4 ) and metal content

(e.g., up to 7.7 g/L Fe, 2.6 g/L Al, or 1.4 g/L Zn). Different hydrogeochemical facies of AMD (namely, Fe(II)/anoxic,

Fe(III)/suboxic, and aluminous/oxic) are recognized in the field, as a response to the continuous oxidation and hydro-

lysis of dissolved Fe. Relevant geochemical aspects of these AMD environments are discussed, including: (i) the redox

chemistry of the Fe(II)/Fe(III) couple, (ii) the reaction rates for bacterially catalyzed oxidation of Fe(II) and hydrolysis

of Fe(III), (iii) the role played by dissolved Fe and Al in the acidity and chemical buffering of the AMD systems, and

(iv) the solubility and trace metal retention capacity of the Fe oxyhydroxysulphate and hydrated sulphate minerals com-

monly associated with AMD. In addition, the mineralogy and chemistry of the Fe precipitates (schwertmannite, jaro-

site, goethite, ferrihydrite), Al phases (e.g., basaluminite) and Mg–Fe–Al efflorescent SO4 salts (e.g., epsomite,

hexahydrite, copiapite, halotrichite, rozenite, coquimbite) present in the AMD-generating mine sites, have also been

studied. The mineralogy of the Fe precipitates is well correlated with the water pH (with jarosite at pH � 2, schwert-

mannite at pH 2–4, basaluminite at pH 4–5, and ferrihydrite at pH > 6). Schwertmannite appears to be the most impor-

tant mineral phase, both in controlling the Fe solubility at pH 2–4, and as sorbent of trace elements (As, Cu, Zn), which

favours natural attenuation. Finally, a basin-scale environmental perspective is given in order to evaluate the impact of

AMD on the water quality, including calculation of metal loadings transported by AMD from the most important mine

districts in the province.

� 2005 Elsevier Ltd. All rights reserved.

0883-2927/$ - see front matter � 2005 Elsevier Ltd. All rights reserv

doi:10.1016/j.apgeochem.2005.01.011

* Corresponding author. Fax: +34 91 349 5834.

E-mail address: [email protected] (J. Sanchez Espana).

1. Introduction and scopes

A long history of metalliferous mining (currently ex-

hausted) has left the Iberian Pyrite Belt (IPB) massive

ed.

mailto:[email protected]

J. Sanchez Espana et al. / Applied Geochemistry 20 (2005) 1320–1356 1321

sulphide province (Huelva, SW Spain) with a legacy of

abandoned mines and attendant spoil tips, including

enormous sulphide-bearing waste rock piles, tailings

and flooded pits. These mine wastes are continuing

sources of environmental contamination, mostly in the

form of acid mine drainage (AMD). These acid mine

waters have caused severe pollution of the Odiel and

Tinto fluvial systems, with transference of large amounts

of acidity and dissolved metals (Fe, Al, Mn, Cu, Zn, Cd,

Pb), As and SO4 (see for example Van Geen et al., 1991,

1997; Nelson and Lamothe, 1993; Elbaz-Poulichet et al.,

1999, 2001; Davis et al., 2000; Cossa et al., 2001; Achter-

berg et al., 2003; Braungardt et al., 2003; for a review on

trace metal anomalies in the Tinto-Odiel estuarine sys-

tem and the Gulf of Cadiz). Acidity and metal concen-

trations of the AMD-affected streams have caused the

loss of most forms of aquatic life, with the exception

of some types of microorganisms adapted to these ex-

treme environments (Lopez-Archilla and Amils, 1999;

Lopez-Archilla et al., 2001; Gonzalez-Toril et al., 2003;

Amaral Zettler et al., 2003).

During the last decade, the Junta de Andalucıa (Re-

gional Government) has made a considerable economic

contribution to various remediation and restoration at-

tempts aimed at reducing the environmental impact of

acid mine waters (e.g., Serrano et al., 1995). These ini-

tiatives have included the geotechnical stabilization and

revegetation of waste piles, construction of rain water

drainage systems and sealing of mine adits, as well as

passive treatments such as anoxic limestone drainage

(ALDs) and anaerobic compost wetlands (Vinas and

Lopez Fernandez, 1994; AYESA, 1996). Unfortu-

nately, these attempts have been highly ineffective

due to chemical and climatic constraints (high acidity

and metal contents, seasonal variability of water

discharge).

A range of papers have been published on the effects

of this mine-related pollution in the Tinto-Odiel estua-

rine system (see above), although unfortunately, very lit-

tle work has been carried out to unravel the

geochemistry of these acid mine waters in origin, their

seasonal chemical variations, their interaction with the

stream waters, or the mineralogical and chemical com-

position of the AMD-related ochreous precipitates.

These questions are important not only from a theoret-

ical viewpoint, but also from a practical aspect, as they

are critical for the design of remediation/attenuation

strategies.

The Geological Survey of Spain (IGME) is investigat-

ing the geochemistry of the AMD systems and their

interaction with fresh water courses in the Odiel river ba-

sin, which receives the great majority of AMD discharge

from the IPB mines. This study includes the following

objectives: (1) physico-chemical characterization of

AMD and its relation to geological, mineralogical and

hydrological factors; (2) seasonal variations of AMD

chemistry and metal loads; (3) mineralogical and chem-

ical transformations taking place in the AMD-generat-

ing mine sites; (4) processes favouring natural

attenuation; and (5) GIS-based, basin-scale, hydrogeo-

chemical mapping according to water quality criteria.

This paper summarises the results obtained from

February-2003 to May-2004, describing the main geo-

chemical features of the AMD systems, the mineralogi-

cal and chemical characteristics of mine drainage

precipitates and efflorescent sulphate salts, and the envi-

ronmental implications.

2. Methodology and analytical techniques

2.1. Sampling

Field work was performed during several seasons

through 2003 (February–March, April, June, July, Sep-

tember–October, November) and 2004 (January, March,

May) and included a comprehensive and detailed explo-

ration of the study area, with identification and sam-

pling of the different acid mine effluents and stream

waters, as well as sediments and precipitates naturally

occuring in the AMD systems, and subsequent chemical

and mineralogical-diffractometric analyses of the col-

lected samples.

Water samples for chemical analyses of major ions

were taken with 60 ml-syringes and Millipore standard

sampling equipment. All samples were filtered in the

field with 0.45 lm cellulose acetate membrane filters,

stored in 125 ml-polyethylene bottles, acidified down

to pH < 2 with concentrated HNO3, and refrigerated

during transport. The polyethylene bottles were washed

with HNO3 for 12 h prior to sampling, and a trace metal

clean procedure was always employed during sample

collection.

The solid samples (ferric crusts, sulphate efflores-

cences) were directly stored in 125 ml-polyethylene bot-

tles, while the ochreous, colloidal precipitates were

taken with 60 ml-syringes and/or by filtering acid water

using a manual suction kit.

2.2. Field measurements

Field parameters such as pH, Eh, temperature (T),

dissolved O2 (DO), and electric conductivity (EC), were

measured in situ with Hanna portable instruments

(probe types HI 9025C, HI9033 and HI9025, respec-

tively) properly calibrated on site against supplied cali-

bration standards (Hanna standard solutions HI 7004

(pH 4.01) and HI 7007 (pH 7.01) for pH; Hanna stan-

dard solutions HI 7021 (240 mV) and 7022 (470 mV)

for Eh). For Eh measurements, the mV reading obtained

with the Pt–Ag/AgCl electrode system was corrected for

temperature and adjusted to a potential relative to that

1322 J. Sanchez Espana et al. / Applied Geochemistry 20 (2005) 1320–1356

of the standard hydrogen electrode (see Method in

Nordstrom, 1977).

Flow rates have been calculated in all cases by con-

ventional methods using digital flow meters (GLOBAL

WATER) in previously defined stream sections.

Quantitative measurement of acidity, alkalinity and

Fe(III)/Fe(II) concentrations, were performed in situ

with Hach titration-based test kits using the digital titra-

tion methods 8201 (bromophenol blue) and 8202 (phe-

nolphthalein (total) acidity; AC-DT model), 8203

(phenolphthalein (total) alkalinity using H2SO4; AL-

DT model), and 8214 (citrate, sodium periodate and

sulfosalicylic acid with TitraVer� standard solution

titration cartridge for Fe(III) and total Fe) reported by

the Hach Instruments company. Titration tests for the

measurement of AMD acidity were performed at inter-

mediate pH values of 3.7 and 5, and total acidity (pH

8.3), with NaOH 1.6 N and continuous measurement

of pH and the amount of base used in every step.

For routine, semiquantitative estimations of Fe(II)

concentration, Merckoquant�-type (MercK) Fe(II)

strips were also used. Similar methods were occasionally

applied for semiquantitative estimations of other ele-

ments such as SO2�4 ; Zn and Mn.

2.3. Laboratory analyses

All samples were analyzed in the IGME laboratories.

Natural water samples were analyzed using diverse spec-

trometric techniques, such as spectrophotometry (ALLI-

ANCE Integral Plus) for SO4, NO3, Cl and PO4, AAS

(flame operation, VARIAN FS-220) for Na, K, Mg,

Ca and Fe, and ICP-MS (LECO Renaissance) for trace

metals. AMD samples were analyzed by AAS for Na, K,

Mg, Ca, Fe, Cu, Mn, Zn and Al, ICP-AES for Be, Ni

and Se, and ICP-MS for Ag, As, Ba, Cd, Co, Cr, Hg,

Mo, Pb, Sb, Th, Tl, U and V. Sulphate was gravimetri-

cally measured as BaSO4, whereas Cl was analyzed titri-

metrically with AgNO3. The accuracy and precision of

the analytical methods were verified against certified ref-

erence waters such as SRM 1643 (trace elements in

water, NIST), APG 4073 (trace metals in waste water,

APG) and others (e.g., internal samples of industrial

mine effluents, control samples of the Water Research

Centre�s AQUACHECK international program) and

close agreement with certified values was achieved for

all metals. 115In was used as internal standard for cali-

bration of the ICP-MS analyses. The detection limits

for trace elements in natural waters were 2.5 lg/L for

Se, 2 lg/L for Al, Be, Cr, Hg and Zn, 1 lg/L for Ni,

and 0.5 lg/L for Ag, As, Ba, Cd, Co, Cu, Mn, Mo,

Sb, Tl, Th, U and V. The detection limits for trace ele-

ments in acid mine waters were 100 lg/L for Se and

Ni, 10 lg/L for Be and Zn, 2 lg/L for Ag, Ba, Co, Cr,

Hg and Pb, 1 lg/L for U, 0.5 lg/L for V, 0.4 lg/L for

As, Cd, Cu and Sb, and 0.2 lg/L for Mo, Tl and Th.

The detection limits for major cations (Na, K, Ca,

Mg, Mn, Fe, Al) was <1 mg/L in all cases.

Solid samples were analyzed using XRF (PHILIPS

1404) for the elements Si, Al, Fe, Ca, Ti, Mn, K, Mg

and P, elemental analyzer (Eltra CS-200) for total S,

ICP-MS (after digestion with HNO3 and H2O2 follow-

ing the method described in USEPA 3050B) for Cd,

Co, Cr, Hg, Mo, Sb, Th, Tl, U, V, Zn, and AAS for

Na, Cu, As and Pb. A number of certified international

reference materials (BCS 175/2, BCS 302/1 and BCS 378,

from the British Chemical Standards; FER-1 and FER-2

from the Canada Centre for Mineral and Energy Tech-

nology) were used to check the accuracy of the analytical

data. The detection limits for trace elements in solids

were 10 ppm for Ni and Se, 2 ppm for Be, Co, Cu,

Hg, Th, V and Zn, 1 ppm for Cr, 0.5 ppm for Mo and

0.2 ppm for Cd, Sb and U.

The solid samples have been mineralogically charac-

terized by powder XRD using a PHILIPS PW 1710 dif-

fractometer with Cu Ka radiation (40 kV, 30 mA), and a

diffracted-beam monochromator. For routine XRD

inspections (for samples of sediment and sulphate salts),

0–60� 2h scans were used with 0.5 s counting time per

step. For more detailed XRD profiles (for the Fe colloi-

dal precipitates), slower counting times of 50 s per step

interval in 10–80� 2h scans, were selected.

2.4. Geochemical modelling

The PHREEQC code (version 2.0; Parkhurst and

Appelo, 1999) was used for geochemical modelling,

including calculation of theoretical Eh (pe) values basedon the Fe(II)/Fe(III) redox couple, activity and chemical

speciation of dissolved species, and saturation index of

analyzed minerals in the parent acid solutions. The ther-

modynamic database of PHREEQC was enlarged with

data from other geochemical codes (MINTEQA2,

WATEQ4F (Ball and Nordstrom, 1991)) for the solubil-

ity of schwertmannite and ferrihydrite.

3. Geological and hydrological framework

3.1. Geology

The Odiel fluvial system (1360 km in length) drains a

vast area of 2300 km2, crossing the Province of Huelva,

from Sierra de Aracena in the North to the Huelva estu-

ary in the South, and receives most of the acid mine

effluents emerging from IPB mines (Fig. 1).

The upper, northernmost part of the basin is under-

lain by plutonic and metamorphic rocks (sericitic schists,

quartzites, granites, gneises, marbles and tuffs) of Upper

Precambrian to Devonian age. The middle and lower

basin is characterized by the Upper Devonian to Middle

Carboniferous volcanic and sedimentary rocks of the

Fig. 1. Location map of the Odiel river watershed and the IPB massive sulphide mines studied in the present work.


IPB (comprising the PQ group, with phyllites and

quartzites, the volcanic-sedimentary complex (VSC),

formed by shales, greywakes, acid to basic volcanic

and volcaniclastic rocks, and the Culm group, made of

a flysch-like sequence of shales and greywakes). Finally,

the southernmost area of the Odiel basin drains Miocene

detrital materials (sands, silts, clays).

The IPB represents the largest concentration of mas-

sive sulphide on Earth, and despite its long history of

extensive mining (dating from the Chalcolitic age, 3 ka


B.C.) still hosts 1700 Mt of unmined massive sulphides,

including 35 Mt Zn, 14.6 Mt Cu, 13 Mt Pb, 46,100 t Ag

and 880 t Au (e.g., Leistel et al., 1998). From the North

of Seville to the South of Lisbon, the IPB comprises

more than 80 mines, including historical deposits with

more than 100 Mt such as Rıo Tinto, Tharsis, La Zarza,

Sotiel and Aznalcollar in the Spanish part, or Aljustrel,

Loussal and Neves-Corvo in the Portuguese zone.

This mining legacy has resulted in a total of 57 aban-

doned waste piles (with a total volume of 107 Hm3) and

10 tailings dumps (42 Hm3) in the province of Huelva

alone (IGME, 1998), which represents one of the

World�s largest accumulations of pyritic mine wastes.

The mineralization is dominated by pyrite (>90% in

volume), with variable amounts of sphalerite, chalcopy-

rite and galena. Other minor minerals are tetrahedrite-

tennantite, arsenopyrite, pyrrhotite, cassiterite,

magnetite and hematite. Pyrite is texturally variable from

fine-grained (colloform, framboidal) to coarse-grained

crystals (Velasco et al., 1998; Sanchez Espana, 2000).

Gangue minerals include abundant silicates (quartz,

chlorite, sericite and feldspars), and minor amounts of

carbonate and barite (Sanchez Espana, 2000).

The chemical composition of the sulphide ores is

highly variable among the different deposits and ore fa-

cies (massive, banded, stockwork). Whole-rock compo-

sitions of sulphide ores normally show ranges of

18–42% S, 11–57% Fe2O3, 0–20% SiO2, 0–5% Al2O3,

0–5% MgO, 0–1.5% MnO, 0–7% CaO, with element

contents of 400–305,000 ppm Zn, 300–84,000 ppm Cu,

75–230,000 ppm Pb, 56–17,600 ppm As, 23–4500 ppm

Sb, 7–800 ppm Cd and 8–612 ppm Co, among others

(e.g., Sanchez Espana, 2000).

In short, the mineralogical and textural characteris-

tics of the IPB ores (namely, (1) dominantly pyritic,

fine-grained, usually brecciated and fractured, highly

reactive sulphide grains and (2) a lack of carbonates to

neutralize acidity), favour pyrite oxidation/dissolution

and the subsequent formation of AMD.

3.2. Hydrology

The hydrological characteristics of the province of

Huelva are typical of a semi-arid, Mediterranean-type

climate. The average water flow at Gibraleon (near

the Odiel river mouth) has been estimated to be around

320 Hm3/a (10 m3/s) for the period 1969–1997 (Confed-

eracion Hidrografica del Guadiana, unpublished data).

However, this is just an oversimplified calculation that

masks a complex reality defined by marked variations

in water discharge and subsequent water flow rates

from summer (June–September) to winter, as well as

between different hydrologic years (Fig. 2). Rainfall

discharge (up to 10–70 mm/day in winter, approaching

0 mm/day in summer; Fig. 2), evapotranspiration

(<1 mm/day in winter, and up to 10 mm/day during

the summer), and consequently, acidity and metal loads

transported by AMD (see Section 5.3.1, Table 10), are

also very randomly distributed through the year, mak-

ing the predictions and calculations neccessary for the

design of any remediation/attenuation technique very

difficult.

4. Results

4.1. Chemical characterization of stream waters

With the aim of obtaining a chemical reference back-

ground for stream water quality in the area, 12 samples

of natural stream waters collected from several locations

were chemically analyzed (Table 1). These samples in-

cluded water courses close to or upstream of the mine

sites, but always unaffected by AMD discharge based

on EC, Eh and pH field data. Six of these samples come

from streams draining unmineralized rocks of the miner-

alization-hosting lithostratigraphic unit (VSC), and the

rest come from streams draining undifferentiated sedi-

mentary rocks of the IPB (comprising shales, phyllites,

quartzites and greywakes from the PQ and Culm

groups) and Sierra de Aracena (including sericitic

schists, quartzites, granites, gneises, marbles and tuffs).

Additionally, water alkalinity has been measured by

titration-based tests performed in situ in 31 different

stream water samples to cover the whole basin (Table 2).

The chemical composition of these waters shows

lower SO4 and Cl concentrations (6–32 mg/L SO2�4 ,

12–26 mg/L Cl�) with respect to bicarbonate content

(69 mg/L HCO�3 on average). There is no clear chemical

distinction between waters draining VSC materials and

those draining undifferentiated rocks (e.g., SO2�4 vs.

pH; Fig. 3).

In all cases, natural stream waters are characterized

by low redox potential (Eh = 250–470 mV), circumneu-

tral pH, low conductivity values (EC = 98–578 lS/cm),

and very low metal concentrations. The correlations be-

tween trace metals are poor and do not show any signif-

icant trend. The most significant contents are those of Fe

(26–445 lg/L), Al (23–253 lg/L), Mn (6–53 lg/L) and

Ba (5–61 lg/L). The Cu and Zn contents are in the

ranges of 1–7 and 1–12 lg/L, respectively. The concen-

trations of other trace elements such as Ag, Be, Cd,

Co, Cr, Hg, Ni, Mo, Sb, Se, Th, Tl and U were always

close to or below the detection limit of the analytical

technique (some of these elements have been conse-

quently omitted in Table 1). Such element concentra-

tions are always below drinking-water standards

(50 lg/L for As, Cu, Pb, Mn and Cr; 5 lg/L for Cd,

3 mg/L for Zn, 0.3 mg/L for Fe; EU Council Directive

75/440/EEC of 16 June 1975 concerning the quality re-

quired of surface water intended for the abstraction of

drinking water in the Member States).

Daily Water Flow (m3/s)

Fre

quen

cy

Yearly Flow RateOdiel outlet 1969-1997

0

200

400

600

800

1000

1200

1400

1600

1800

2000

1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997

Years

Flo

w r

ate

(Hm

/yea

r)3

A

B

C

0

10

20

30

40

50

60

70

80

90

DecNovOctSepAugJulJunMayAprMarFebJan

Months

Rai

nfal

l dis

char

ge (

mm

/day

)

Diary rainfall discharge

Odiel waterheads 2003

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

0

0.2

0.4

0.6

Fig. 2. Hydrological characteristics of the Odiel river watershed, including: (A) annual flow rate, (B) daily water flow, and (C) rainfall

discharge. Annual and daily flow rate data (A and B) correspond to the period 1969–1997 for the Odiel river mouth in Gibraleon

(unpublished data from Confederacion Hidrografica del Guadiana), whereas (C) shows the distribution of rainfall discharge in the

uppermost basin during 2003.


Consequently, it can be stated that, from a geochem-

ical viewpoint, the IPB stream waters of unmined areas

do not show any significant chemical signature, with

water quality standards akin to water courses draining

siliciclastic rocks.

Another important chemical feature of the stream

waters is their relatively low alkalinity values (Table

2). Alkalinity of these waters is basically determined

by the concentration of HCO�3 ions, being therefore very

variable between different lithological areas. Thus, water

alkalinity is in the range 120–240 mg/L CaCO3 eq. in the

upper basin (which drains Precambrian to Devonian

metamorphic and plutonic rocks of the Sierra de Arac-

ena), and between 10 and 115 mg/L CaCO3 eq. in the

middle and lower basin, characterized by volcanic and

sedimentary rocks of the IPB (PQ, VSC and Culm

units).

In general, these data indicate a limited acid-neutraliz-

ing capacity for most streamwaters in the area, which has

drastic environmental consequences (the potential acidity

of the acidmine effluents is about 2–3 orders ofmagnitude

higher than alkalinity of stream waters – see below –, and

therefore, they cause a rapid pH decrease of the natural

water courses, even when AMD volumes are low in rela-

tion to stream water volumes).

4.2. Location and description of acid mine effluents

By May-2004, 64 AMD discharges from 25 different

mines of the IPB have been studied. AMD-generating

Table

1

Field

parametersandchem

icalcompositionofnaturalstream

waters

oftheOdielriver

watershed

Thecorrespondingsample

number,site

description,drained

area(km

2),are

provided

inallcases.Allsampleswerecollectedin

April2003except(*),whichwascollectedin

March2004.Abbreviations:Q,

waterflow

rate;DO,dissolved

O2;EC,electric

conductivity;n.a.,notanalysed.


mine sites include, in decreasing order of importance,

waste rock piles (50%), mine adits (30%), tailings

impoundments (10%), mine holes (7%), and mine pit

lakes (3%). These mine sites include some of the largest

and most important deposits in the IPB (Rıo Tinto,

Tharsis, La Zarza, San Telmo, Sotiel-Almagrera) and

many others of medium size (Lomero-Poyatos, San

Miguel, Cueva de la Mora, Aguas Tenidas, Concepcion,

San Platon, Poderosa, Tinto-Sta. Rosa), which repre-

sent, as a whole, the total of acid emissions received

by the Odiel fluvial system.

A site description with field parameters of the studied

AMD discharges is given in Table 3. The chemical com-

position of these acid effluents is reported in Table 4,

Fe(II)/Fe(III) and acidity data are given in Table 5,

and some significant AMD effluents from various mine

sites of the area are shown in Fig. 4. All analyses have

been performed in the laboratory except for acidity

and Fe(II)/Fe(III) (calculated in situ by titration-based

tests). Most AMD discharges have been sampled and

analyzed several times through the period 2003–2004,

in order to study their seasonal variations in chemistry

and flow rate.

Both water volume and chemical composition of

AMD are very variable seasonally and among different

deposits (Table 3). The most important AMD dis-

charges come from the largest deposits (e.g., Rıo Tinto,

Tharsis, La Zarza and San Telmo), whereas acid mine

waters coming from the smaller deposits (e.g., El Carpio,

La Torerera, San Platon, Esperanza, El Soldado,

Angostura) always show very low water volumes. Many

acid leachates from waste rock piles (e.g., Monte Ro-

mero, Confesionarios) are ephemeral, being active only

after rainfall episodes.

4.3. Chemical composition of AMD

The data reported in Tables 4 and 5 show also that

the acid emissions are far from being chemically uni-

form. In fact, they can be differentiated in terms of sea-

sonal continuity (permanent, seasonal or ephemeral

drainage), water volume (flow rates are in the range

0.1–220 L/s), acidity (200–30,000 mg/L CaCO3 eq.), re-

dox conditions (Eh = 400–800 mV; pe = 6.7–13.5), elec-

tric conductivity (EC = 1000–24,000 lS/cm), dissolved

O2 content (from totally anoxic to O2-saturated), dis-

solved Fe(II) to Fe(III) ratio (Fe(II)/Fet = 0.1–1), and

colour (green, yellow to reddish, white, blue). Based

on this field evidence, 3 main hydrogeochemical facies

for AMD can be recognized in the area, namely: (1) fer-

rous/anoxic (pH 1.4–4.0, Eh = 400–640 mV, DO � 0–

20% sat., Fe(II)/Fet � 0.5–1, green-coloured), (2) ferric/

suboxic (pH 2.0–3.5, Eh = 640–800 mV, DO = 50–

100% sat., Fe(II)/Fet � 0.1–0.5, yellow to reddish-

coloured), and (3) aluminous/oxic (pH 4.0–5.7,

Eh 6 500 mV, DO � 100% sat., Fe(III) � 0 mg/L,

Table 2

Alkalinity values (mg/L CaCO3 eq.) for stream waters in the Odiel river basin

Sample

No.

River/creek Lithostratigraphic

unit

Sampling

date

pH EC Alkalinitya

959-3 Aserrador Miocene, PQ, VSC May-04 7.4 557 146

981-5 Ayo. San Bartolome Miocene, PQ, VSC May-04 7.3 578 157

937-11 Ayo. Sepultura Culm May-04 7.4 168 58

959-3 Aserrador Culm Nov-03 7.0 460 66

937-18 Bco. Ovejeros Culm May-04 7.4 266 69

937-14 Tallisca Culm May-04 7.2 388 100

959-8 Chapinero Culm May-04 7.3 270 104

937-5 Escalada Culm, Pulo de Lobo, VSC May-04 7.1 131 21

937-12 Fresnera Culm, Pulo de Lobo, VSC May-04 6.5 123 11

960-11 Palanco VSC May-04 6.9 322 94

960-6 Carrasco VSC May-04 7.8 331 114

938-52 Bco. Hocino VSC, Culm May-04 7.8 220 88

937-9 Tamujoso VSC, Culm May-04 7.4 370 68

959-9 Cascabelero VSC, Culm May-04 7.8 302 83

938-26 Rivera Seca VSC, Granites, PQ May-04 7.4 135 39

960-12 Rivera del Villar VSC, PQ May-04 7.8 353 107

959-19 Vega del Almendro VSC, PQ May-04 8.4 433 113

938-45 Olivargas PQ Nov-03 7.7 162 34

938-5 Olivargas (before reservoir) PQ, Culm Jan-04 9.0 165 65

937-12 Fresnera Pulo de Lobo, PQ May-04 6.5 123 11

937-5 Rivera Pelada Pulo de Lobo, PQ, VSC Nov-03 6.8 166 18

938-55 Valdehornos Pulo de Lobo, Culm May-04 7.4 98 20

938-54 Bco. del Perro Pulo de Lobo, Culm May-04 8.8 161 65

917-1 Acebuche Pulo de Lobo, Sierra de Aracena Maassif May-04 8.4 211 80

938-28 Escalada (before San Miguel) Sierra de Aracena Maassif May-04 8.3 275 121

917-3 Casares Sierra de Aracena Maassif May-04 8 235 135

917-2 Almonaster Sierra de Aracena Maassif May-04 8.1 324 137

938-8 Santa Eulalia Sierra de Aracena Maassif May-04 8.4 300 143

938-1 Odiel Sierra de Aracena Maassif May-04 8.7 335 148

917-4 Linares Sierra de Aracena Maassif May-04 8.5 383 197

918-1 Jabuguillo Sierra de Aracena Maassif May-04 8.1 472 236

The corresponding sample number, site description, lithostratigraphic units drained, date of sampling, and some other relevant field

data (pH and EC), are provided in all cases. Data have been organized from top (Miocene, southernmost basin) to bottom (Upper

Precambrian to Devonian, Northernmost basin) in increasing age of the rocks drained by the corresponding rivers and creeks. See text

for explanation.a In mg/L CaCO3 eq.

0

5

10

15

20

25

30

35

5 6 7 8pH

SO4

(mg/

L)

VSC rocks

Undifferentiated rocks

Fig. 3. SO2�4 vs. pH diagram for natural stream waters in the

Odiel river basin. Waters draining different types of rocks

(siliceous volcanic and sedimentary rocks of the volcanic-

sedimentary complex (VSC), and undifferentiated rocks from

other lithostratigraphic units of the IPB such as the Culm, PQ

or Pulo do Lobo Units) are distinguished.


white-coloured). This distinction is only valid at the

source points (mine adits, waste pile leachates), as the

oxidation and hydrolysis of Fe at pH � 2.5–3 in these

waters is very fast after their contact with atmospheric

O2 (see Section 5.1.2), and provokes the transition from

Fe(II)/anoxic to Fe(III)/suboxic and, when mixed with

stream waters, aluminous/O2-saturated facies, in a very

short distance. Type 2 (ferric/suboxic) AMD is, by far,

the most common and volumetrically important among

the acid emissions in the IPB, with ferrous and alumi-

nous effluents being scarce and always having very low

water volumes (Table 3).

The chemical composition of these waters include ex-

treme concentrations of dissolved SO4 (up to 44 g/L)

and metals (up to 7.7 g/L Fe, 2.6 g/L Al, 2.9 g/L Mg,

1.4 g/L Zn, 435 mg/L Cu and 440 mg/L Mn; Tables 4

and 5). Trace elements are also significantly enriched

Table 3

Relation of the AMD discharges recognized and studied in the IPB (Odiel river basin), with description of the mine sites, AMD type, geographic location (in UTM coordinates), and field measurements (including water flow (Q), pH, dissolved

O2 (DO), Eh, electric conductivity (EC), and temperature (T))

Mine AMD type AMD No. Sampling date Site position Field measurements

Year Month Day X-utm Y-utm Altitudea Q

(L/s)

pH DO

(mg/L)

DO

(%)

Eh

(mV)

EC

(ls/cm)

T

(�C)

Aguas Tenidas Mine hole 938-109 2003 6 19 160425 4188518 305 0 2.5 1.6 17 704 – 25.5

2003 2 28 160425 4188518 305 2 3.1 0.0 2 604 1950 20.0

Angostura Mine adit 938-110 2003 6 19 172771 4186494 276 0.1 2.7 8.7 95 655 2240 18.9

2003 3 1 172771 4186494 276 0.2 2.7 7.4 77 678 1945 17.0

Angostura Mine hole 938-11 OB 2003 6 19 172771 4186494 276 0 2.2 2.2 20 636 3750 20.5

2003 3 1 172771 4186494 276 0 2.2 2.2 17 653 3150 15.0

Angostura ALD outflow 938-111 2003 6 19 172976 4186597 324 0.1 3.2 1.3 17 521 2140 24.3

2003 3 1 172976 4186597 324 0.2 3.0 0.0 0 563 2000 15.6

Campanario Waste-rock pile leachate 960-105 2003 7 30 162318 4162098 166 0 3.4 1.7 20 495 3720 21.2

Campanario Mine hole 960-109 2004 3 12 162182 4162073 167 1 2.6 0.8 9 589 3970 19.2

Castillo Buitron ALD inflow 960-108 2003 9 25 167621 4171913 139 0.7 2.5 0.2 2 616 1010 19.5

Concepcion Waste-rock pile leachate 938-105 2004 3 12 176582 4187684 224 2.8 2.3 6.1 66 703 4560 16.4

2003 9 26 176582 4187684 224 2.4 2.5 4.1 45 733 3050 22.0

Confesionarios Waste-rock pile leachate 937-101 2003 2 27 157912 4190185 344 2 2.0 0.0 0 733 5700 20.0

Confesionarios Waste-rock pile leachate 937-113 2004 3 18 157875 4189355 311 15 2.6 8.9 100 633 12,500 21.0

Confesionarios Waste-rock pile leachate 937-114 2004 3 18 157592 4189631 314 7 2.0 6.2 68 648 18,230 20.0

Cueva de la Mora Mine adit 938-107 2004 3 10 165893 4187971 254 3.5 3.3 1.3 14 565 4190 20.7

2003 9 27 165893 4187971 254 1.6 3.2 4.6 52 548 3790 20.7

Descamisada Mine adit/ALD outflow 960-106 2004 3 12 161488 4162711 145 0.2 3.9 0.3 4 438 2740 19.2

2003 7 30 161488 4162711 145 3 4.2 1.1 14 360 2050 22.0

Descamisada Mine hole 960-107 2003 7 30 161557 4162741 151 1 2.4 1.7 21 687 3080 24.0

El Carpio Mine adit 937-110 2004 3 10 149233 4192241 289 1.56 2.8 6.3 75 663 2980 24.8

2003 9 27 149233 4192241 289 0 2.1 1.5 19 643 4700 27.0

2003 7 29 149233 4192241 289 0 2.6 3.4 47 641 4660 32.0

El Soldado Mine adit 938-119 2003 4 1 178259 4185728 339 2 2.5 6.5 73 661 1800 20.9

Esperanza Mine adit 938-122 2004 3 12 175738 4185620 248 2.2 2.7 0.4 4 599 4470 20.3

2003 6 19 175738 4185620 248 2.3 2.8 0.0 0 551 3580 21.6

Gloria Mine adit 960-103 2003 6 23 165708 4172769 130 0.1 2.8 4.5 53 673 1970 24.0

La Poderosa Waste-rock pile leachate 938-103 2003 9 23 177782 4183954 258 0.2 1.9 2.9 32 659 5040 21.1

La Poderosa Mine adit 938-113 2003 3 6 177896 4184244 298 0 3.2 3.5 38 633 5500 20.0

La Poderosa Mine adit 938-114 2004 3 18 177028 4184430 241 4.5 1.7 0.0 0 618 13,000 21.0

2003 9 23 177028 4184430 241 0.3 1.6 0.1 1 632 9540 21.3

2003 6 18 177028 4184430 241 1.6 1.4 0.6 7 595 10,080 20.4

2003 4 1 177028 4184430 241 3.5 1.6 1.7 19 614 16,000 24.0

2004 3 6 177028 4184430 241 3.7 1.7 0.3 3 599 15,200 22.0

La Poderosa Waste-rock pile leachate 938-118 2003 4 1 177861 4184226 289 0.05 5.4 8.2 87 501 2200 20.9

La Torerera Waste-rock pile leachate 959-100 2003 3 3 156170 4168339 85 1 3.1 9.5 96 683 1200 16.0

La Zarza-Perrunal Mine adit 937-108 2004 3 10 159407 4180411 229 2.0 2.9 2.1 27 486 7450 29.0

2003 9 28 159407 4180411 229 0.5 3.2 1.4 18 513 7230 27.9

2003 6 21 159407 4180411 229 0.6 2.9 0.7 10 491 7750 31.1

La Zarza-Perrunal Mine adit 937-109 2004 3 10 159961 4180148 198 0.9 3.6 4.4 47 610 2470 18.5

2003 9 28 159961 4180148 198 0.3 3.4 2.2 25 618 1990 20.8

2003 6 21 159961 4180148 198 0.5 3.5 2.7 32 615 2820 21.2

Lomero-Poyatos Mine adit 937-102 2004 3 13 154380 4191826 348 0.5 2.8 8.3 89 648 4510 17.8

2003 9 27 154380 4191826 348 0.4 3.2 4.3 53 591 4770 24.3

2003 6 21 154380 4191826 348 5.0 2.7 3.2 38 612 4880 22.7

2003 2 28 154380 4191826 348 8 3.0 3.6 45 571 6000 25.5

Monte Romero Waste-rock pile leachate 938-101 2003 2 28 166209 4188237 264 – 2.8 8.2 82 693 1150 15.6

Rio Tinto Ore dump leachate 938-116 2003 4 1 181556 4183015 306 0.1 2.7 7.4 75 621 7300 15.7

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Rio Tinto Ore dump leachate 938-117 2003 4 1 181215 4182846 339 0.05 2.6 8.7 92 689 6100 16.7

Rio Tinto (embalse del Cobre) Water reservoir outflow 938-48 2004 3 18 183632 4182736 370 48 4.0 9.8 99 633 2200 15.0

2003 4 5 183632 4182736 370 220 4.6 9.5 102 478 1240 19.1

Rio Tinto (Corta Atalaya) Mine adit 938-102 2004 3 16 180507 4180177 221 1 2.3 5.0 57 667 10,050 21.0

2003 9 23 180507 4180177 221 <0.5 2.2 7.7 91 799 12,370 13.4

2003 7 29 180507 4180177 221 <0.5 2.4 5.0 60 762 9760 22.2

Rio Tinto (Corta Atalaya) Waste-rock pile leachate T-1 2004 3 16 181427 4179918 270 28 2.6 0.5 6 570 17,200 24.7

2003 7 29 181427 4179918 270 10 3.0 0.9 11 537 17,120 25.9

2003 6 17 181427 4179918 270 15 2.8 0.8 10 544 15,330 24.8

2003 3 5 181427 4179918 270 90 2.7 0.0 0 614 16,000 24.0

Rio Tinto (Corta Atalaya) Waste-rock pile leachate 938-112 2003 6 17 181429 4179951 328 1 2.6 0.4 4 622 19,400 30.8

2003 4 1 181429 4179951 328 2 2.5 1.1 12 655 10,500 16.5

Rio Tinto (Corta Atalaya) Waste-rock pile leachate 938-124 2004 3 16 180135 4180482 – 21 2.8 9.3 97 823 7700 17.0

2004 1 15 180135 4180482 – 25 2.5 – – 848 6850 16.8

2003 11 13 180135 4180482 – 20 – – 60 – 18,000 16.2


2004 1 15 182091 4180510 331 20 2.5 – 7 636 14,300 21.0

2003 11 13 182091 4180510 331 15 2.7 4.2 48 620 14,200 21.0


2004 1 15 180607 4179004 290 17 2.3 0.0 0 636 11,850 17.0

2003 11 13 180607 4179004 290 5 1.7 4.4 46 656 8300 17.6

Rio Tinto (Corta Atalaya) Waste-rock pile leachate 938-127 2004 3 17 181473 4181168 277 <0.5 2.6 9.1 95 748 9400 16.0

Rio Tinto (Corta Atalaya) Ore dump leachate 938-128 2004 3 17 181527 4181164 298 1 4.2 0.0 0 396 7000 20.0

Rio Tinto (Corta Atalaya) Waste-rock pile leachate 938-129 2004 3 17 181360 4181323 – 13 3.8 8.5 93 477 5850 19.0

San Miguel Waste-rock pile leachate 938-108 2003 9 24 169325 4185855 167 0.02 1.5 3.1 41 646 13,200 30.3

2003 2 27 169325 4185855 167 4 1.8 8.5 85 641 8500 19.1

San Miguel Mine hole 938-121 2003 9 24 169336 4186144 174 0.2 2.2 0.2 3 552 5970 19.8

2003 6 19 169336 4186144 174 1.5 2.3 0.7 6 564 5710 19.3

2003 4 3 169336 4186144 174 10 2.3 0.0 0 613 6000 20.0

San Platon ALD outflow 938-106 2004 3 12 176602 4186051 177 2.0 2.5 0.6 5 599 6030 19.0

2003 9 26 176602 4186051 177 0.2 2.7 2.1 25 558 5750 17.4

San Platon Mine adit 938-123 2004 3 12 176678 4186540 218 0.1 3.0 4.7 47 699 1690 14.6

2003 10 2 176678 4186540 218 0.1 3.1 8.8 100 786 1770 18.6

San Telmo Waste-rock pile leachate 937-104 2004 3 10 150048 4191059 273 2.0 2.5 0.4 4 636 13,810 21.6

2003 9 27 150048 4191059 273 0.4 2.5 0.4 5 647 10,480 24.2

2003 7 29 150048 4191059 273 – 2.6 2.1 26 625 11,950 24.4

2003 6 21 150048 4191059 273 1.5 2.3 0.4 5 619 13,270 24.0


2003 9 27 150116 4191087 280 1.3 2.5 0.6 5 608 7660 21.4

2003 7 29 150116 4191087 280 – 2.4 0.9 11 605 9080 20.6

2003 6 21 150116 4191087 280 3.0 2.2 0.1 1 604 9850 21.1

San Telmo Pit lake outflow 937-106 2004 3 10 150492 4191108 263 40 2.9 9.9 101 750 4050 15.2

2003 9 27 150492 4191108 263 0 2.8 8.3 100 782 4550 23.8

2003 7 29 150492 4191108 263 3 2.7 6.7 92 775 4430 30.4

2003 6 21 150492 4191108 263 5 2.5 7.1 92 779 4280 26.8

San Telmo Waste-rock pile leachates 937-107 2004 3 10 150703 4191494 278 38 3.2 8.2 93 629 3480 20.1

2003 9 27 150703 4191494 278 9 3.3 7.1 88 621 3250 24.5

2003 7 29 150703 4191494 278 – 3.2 7.6 99 615 3270 27.3

2003 6 21 150703 4191494 278 10 3.0 7.2 90 630 3150 26.7


2003 9 27 149942 4191151 282 0.7 2.8 6.7 80 725 12,240 21.1

2003 7 29 149942 4191151 282 – 2.7 5.7 70 719 12,230 22.5

Sotiel-Almagrera Ore dump leachate 960-102 2003 4 8 160171 4173540 315 0 1.8 6.7 79 713 5650 23.5

Sotiel-Almagrera Ore dump leachate 960-110 2004 5 10 160171 4173540 315 1.5 2.7 3.5 40 546 6470 18.0

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(continued on next page)

Table 3 (continued)

Mine AMD type AMD No. Sampling date Site position Field measuremen

Year Month Day X-utm Y-utm Altitudea Q

(L/s)

pH DO

(mg/L)

DO

(%)

Eh

(mV)

EC

(ls/cm)

T

(�C)

Sotiel-Almagrera Ore dumps outflow 960-5 2003 11 13 162584 4172286 83 80 2.8 9.9 101 642 2200 14.2

Sotiel-Almagrera Ore dumps outflow 960-5 2004 5 10 162584 4172286 83 130 7.9 9.0 101 325 3200 21.0

Tharsis Waste-rock pile leachate 959-101 2003 3 4 138400 4168773 250 0.5 3.1 9.3 95 697 3800 20.7

Tharsis Pit lake 959-103 2003 6 20 136192 4168648 252 0.0 2.2 – – 790 3950 26.8

Tharsis Waste-rock pile leachate 959-104 2004 3 11 138752 4168503 203 0.3 2.9 8.3 86 616 4940 15.1

2003 9 29 138752 4168503 203 0.1 2.5 7.4 81 723 6030 22.0

Tharsis Waste-rock pile leachate 959-105 2004 3 11 138332 4167778 216 – 2.3 0.2 2 636 16,670 16.7

2003 9 29 138332 4167778 216 0.4 2.3 0.7 9 658 23,900 26.0

Tharsis Mine adit 959-106 2004 3 11 136336 4167549 242 3.3 2.0 1.5 15 615 6010 17.4

2003 9 29 136336 4167549 242 0.4 2.6 0.4 3 609 4030 20.7

Tharsis Waste-rock pile leachate DB-1 2003 4 7 135355 4169059 236 1 2.4 8.2 87 800 2870 17.4

Tharsis Waste-rock pile leachate DB-2.1 2003 9 29 135347 4168661 249 0.5 2.5 0.3 4 628 8440 20.2

2003 7 31 135347 4168661 249 – 2.5 1.9 22 434 8300 20.6

2003 6 20 135347 4168661 249 1.5 2.3 0.0 0 639 6710 18.6

2003 4 7 135347 4168661 249 4 2.2 0.0 0 663 5560 18.6

Tharsis Waste-rock pile leachate 959-107 2004 5 13 138231 4169650 226 15 2.7 3.0 40 609 15,330 28.7

Tharsis Waste-rock pile leachate 959-108 2004 5 13 138431 4169298 234 0.5 2.2 4.9 63 614 22,700 22.7

Tharsis Waste-rock pile leachate 959-110 2004 5 13 138495 4168461 243 – 2.9 6.3 72 728 3700 19.3

Tinto-Sta Rosa Mine adit 960-100 2003 6 23 164566 4172338 120 1.8 3.3 6.7 71 560 3500 21.0

2003 3 3 164566 4172338 120 0.6 3.4 7.1 78 522 3630 21.5

Tinto-Sta Rosa Mine adit 960-101 2003 6 23 164591 4172420 116 0.5 3.3 0.3 4 604 1750 19.2

2004 3 11 164591 4172420 116 – 3.3 0.0 0 599 1830 12.4

Tinto-Sta Rosa Waste-rock pile leachate 960-102 2003 6 23 164575 4172420 110 1.0 5.7 8.8 90 428 539 19.8

2004 3 11 164575 4172420 110 0.6 5.5 9.4 96 447 685 14.7

Trincheron Mine adit/ALD outflow 960-104 2004 3 12 161590 4162444 162 0.3 3.2 1.5 15 692 1142 14.7

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ts

Table 4

Chemical composition of AMD discharges in the IPB (Odiel river watershed)

Abbreviations: b.l., below detection limit; n.a., not analyzed.


Table 5

Fe2+ and Fe3+ concentrations, and acidity (measured at pH values of 3.7, 5.0 and 8.3) of the AMD discharges in the IPB (Odiel river watershed)

Mine site Sample

No.

Sampling

date

Field measurements Iron content (mg/L) Fe2+/Fet (%) Acidity (mg/L CaCO3 eq.)

Q

(L/s)

pH Eh (mV) EC

(lS/cm)

DO

(mg/L)

DO

(%)

T (�C) Fe2+ Fe3+ Fet pH 3.7 pH 5.0 pH 8.3

Campanario 960-109 Mar-04 1 2.6 593 3970 0.8 9 19.2 755 85 840 90 1120 1410 1970

Concepcion 938-105 Mar-04 2.8 2.3 707 4560 6.1 66 16.4 80 517 597 13 1450 2520 3060

Confesionarios 937-16 Feb-04 15 2.5 672 4500 7.0 70 14.0 60 1455 1515 4 3330 – 6,540

Confesionarios 937-113 Mar-04 7 2.0 645 18,250 6.2 68 20.0 n.d. n.d. 7730 14,800 24,100 29,450

Cueva de la Mora 938-107 Mar-04 3.5 3.3 572 4190 1.3 14 20.7 575 45 620 93 600 1090 2090

Cueva de la Mora 938-51 Feb-04 15 3.2 679 1850 9.3 91 12.0 50 n.d. n.d. – 830 1150 1770

Descamisada 960-106 Mar-04 0.2 3.6 442 2740 0.3 4 19.2 750 22 772 97 940 1310 1780

El Carpio 937-110 Mar-04 1.6 2.8 667 2980 6.3 75 24.8 175 252 427 41 880 1290 1620

Esperanza 938-122 Mar-04 2.2 2.7 603 4470 0.4 4 20.3 830 135 965 86 1330 2090 2660

La Poderosa 938-114 Mar-04 4.5 1.7 622 13,000 0.0 0 21.0 2100 560 2660 79 5500 7480 8100

La Zarza 937-108 Mar-04 0.8 2.9 490 7450 2.1 27 29.0 2200 90 2290 96 3290 4660 6320

La Zarza 937-109 Mar-04 0.8 3.6 614 2470 4.4 47 18.5 27 28 55 49 110 390 790

Rio Tinto

(Corta Atalaya)

938-102 Mar-04 1 2.3 671 10,050 5.0 57 21.0 1300 2760 4060 32 5700 8690 11,300

Rio Tinto

(Corta Atalaya)

938-112 Mar-04 2 2.6 574 17,200 0.5 6 24.7 1949 121 2070 94 5100 11,470 15,150

Rio Tinto

(Corta Atalaya)

938-124 Mar-04 21 2.8 827 7700 9.3 97 17.0 5 334 339 1 1160 4010 5660

Rio Tinto

(Corta Atalaya)

938-125 Mar-04 6 2.5 626 13,500 0.0 0 21.0 850 775 1625 52 3800 10,400 13,690

Rio Tinto

(Corta Atalaya)

938-126 Mar-04 10 2.4 647 10,200 0.0 0 17.5 588 462 1050 56 2450 6210 8700

Rio Tinto

(Corta Atalaya)

938-127 Mar-04 1 4.2* 400 7000 0.0 0 20.0 1270 0 1270 100 1550 1900 2750

Rio Tinto

(Corta Atalaya)

938-128 Mar-04 <0.5 2.6 752 9400 9.1 95 16.0 0 0 0 0 – – –

Rio Tinto

(Corta Atalaya)

938-129 Mar-04 13 3.8* 481 5850 8.5 93 19.0 910 0 910 100 1120 1480 2100

Rio Tinto

(Corta Atalaya)

938-48 Mar-04 48 4.0 637 2200 9.8 99 15.0 0 0 0 0 0 100 200

Rio Tinto

(Corta Atalaya)

B-2 Mar-04 15 2.6 742 8900 9.2 100 19.0 0 730 730 0 – – –

Rio Tinto

(Corta Atalaya)

T-1 Mar-04 28 2.6 608 17,100 5.9 68 22.0 1400 585 1985 71 8130 12,200 16,730

San Platon 938-106 Mar-04 2 2.5 603 6030 0.6 5 19.0 1535 245 1780 86 2630 3790 4840

San Platon 938-123 Mar-04 0.5 3.0 703 1690 4.7 47 14.6 10 28 38 26 44 120 231

San Telmo 937-104 Mar-04 2 2.5 640 13,810 0.4 4 21.6 1185 1040 2225 53 4370 9390 12,750

San Telmo 937-105 Mar-04 2.7 2.4 611 10,650 0.5 4 21.2 2070 520 2590 80 4360 7740 10,400

San Telmo 937-106 Mar-04 40 2.9 754 4050 9.9 101 15.2 10 182 192 5 550 1160 1860

San Telmo 937-107 Mar-04 38 3.2 633 3480 8.2 93 20.1 100 46 146 68 150 630 1060

Sotiel-Almagrera 960-110 May-04 1.5 2.7 346 6470 3.5 40 18.0 1085 90 1175 92 1450 2130 3750

Tharsis 959-104 Mar-04 0.3 2.9 620 4940 8.3 86 15.1 400 185 585 68 1090 2350 3520

Tharsis 959-105 Mar-04 5 2.3 640 16,670 0.2 2 16.7 2050 2620 4670 44 9010 16,950 23,160

Tharsis 959-106 Mar-04 3.3 2.0 612 6010 8.1 17.4 985 230 1215 81 2120 3110 3670

Tharsis 959-107 May-04 15 2.7 607 15,330 3.0 40.0 28.7 1540 830 2370 65 4000 8030 10,910

Tharsis 959-108 May-04 0.5 2.2 613 22,700 4.9 63.0 22.7 3140 1270 4410 71 7490 11,100 14,500

Tharsis 959-110 May-04 – 2.9 728 3700 6.3 71.6 19.3 187 10 197 95 – – –

Tharsis 959-16 Feb-04 0.15 2.7 480 11,500 9.3 92.8 14.2 860 872 1732 50 3900 8600 11,350

Tharsis 959-17 Jan-04 11 2.9 774 3500 10.6 96 9.0 3 n.d. n.d. – 430 – 1730

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Tinto-Sta

Rosa

960-101

Mar-04

–3.3

603

1830

0.0

012.4

133

39

172

77

140

260

470

Trincheron

960-104

Mar-04

0.3

3.2

696

1142

1.5

15

14.7

35

65

100

35

51

167

262

AverageIP

BAMD

Mar-04

92.7

624

7925

4.6

48

18.7

801

476

1494

57

2885

5237

6803

Additionalfielddata

atthemomentofsampling(includingwaterflow

(Q),pH,dissolved

O2(D

O),Eh,electric

conductivity(EC),andtemperature

(T))are

alsoprovided.


in these acid waters (maximum values of 17 mg/L As,

8 mg/L Cd, 48 mg/L Co, 17 mg/L Ni, and 725 lg/LPb). Also, U and Th, with peak values around 1100

and 400 lg/L, respectively (sample T-1 from Corta Ata-

laya waste pile, Rıo Tinto, Table 4) are unusually con-

centrated. On the other hand, other trace elements

such as Ag, Be, Hg, Mo, Sb or Se were always close to

or below the detection limits (these elements are not in-

cluded in Table 4).

In general, these AMD compositions indicate con-

centrations of total dissolved solids (sulphate and met-

als) which are comprised within the ranges normally

reported for VMS-type deposits (Fig. 5) (see Plumlee

et al., 1999, for a comprehensive revision).

The higher SO4 and metal concentrations are usually

found in the acid leachates from the huge waste piles of

Corta Atalaya (Rıo Tinto) and Filon Norte (Tharsis),

and indicate an important oxidation and subsequent dis-

solution of pyrite and the accompaning sulphides (chal-

copyrite, sphalerite, galena, arsenopyrite).

An excellent correlation (r = 0.97 for n = 72;

p < 0.01) has been found between electric conductivity

(EC) of these waters and (1) total dissolved solids (Fig.

6A) and (2) dissolved SO4 (Fig. 6B), which confirms con-

ductivity as a good indicator of the degree of contamina-

tion of the stream waters by AMD, especially when

coupled with pH/Eh measurements.

4.4. Mineralogy and chemistry of iron precipitates

Probably, the most widespread feature of the study

area is the ubiquitous presence of yellow to reddish-

brown sediments in the banks of the rivers affected by

AMD. These so-called ‘‘ochre precipitates’’ consist of

Fe-phases precipitated from the Fe dissolved in AMD

and coming from pyrite oxidation in the mine sites.

However, and in contrast to other well crystallized prod-

ucts of pyrite oxidation such as hematite or goethite, the

fine-grain size and low crystallinity of these chemical

sediments make their mineralogical characterization dif-

ficult. To date, these mine drainage minerals (MDM;

Murad et al., 1994) have been well characterized in

many mine districts all over the world by a combination

of different techniques (XRD, electron microscopy, dif-

ferential dissolution with ammonium oxalate, Fe/Smolar

ratio) and have been proved to consist of extremely small

particle size (from <10 nm to 5–10 lm in diameter),

poorly crystallized oxyhydroxides and oxyhydroxysul-

phates of fibrous to spherical morphology such as fer-

rihydrite and schwertmannite, in addition to jarosite

and goethite (Carlson and Schwertmann, 1981; Bigham

et al., 1990, 1994, 1996; Murad et al., 1994; Nordstrom

and Alpers, 1999a; Yu et al., 1999; Kawano and Tomita,

2001; Williams et al., 2002; Dold, 2003; Fukushi et al.,

2003; Regenspurg et al., 2004; Majzlan et al., 2004).

Fig. 4. Photographs of several mine sites in the IPB showing the typical field aspect of some relevant acid mine effluents. (A) The

Tintillo acid river (front), which is formed by different acid leachates (pH � 2.7, Fet � 2 g/L) emanating from the base of large waste-

rock piles in the surroundings of the Corta Atalaya open pit (Rio Tinto mine). The laminated, yellowish brown to orange formations

deposited on the stream bed are composed of a mixture of hydrous Fe oxide precipitates (mainly schwertmannite, jarosite and goethite)

with plant debris and filamentous algae. (B) AMD discharge (pH � 3, Fet � 300 mg/L) in the Lomero-Poyatos mine adit (ochre

deposits include jarosite and schwertmannite). (C) AMD discharge emanating from the Perrunal mine adit (pH � 3.1, Fet � 2.6 g/L).

(D) Acid leachate (pH � 2.5, Fet � 500 mg/L) from the waste-rock piles situated around Filon Centro open pit (Tharsis mine). The

AMD stream is profusely collonized by green filamentous algae, and surrounded by efflorescent sulphate salts (halotrichite,

pickeringite, hexahydrite). (E) AMD discharge (pH � 2.3, Fet � 4 g/L) and associated efflorescent sulphates in a mine adit close to

Corta Atalaya (Rio Tinto). (F) Partial view of the Santa Barbara pit lake (San Telmo mine) showing the water inflow (formed by acid

leachates emanating from several waste-rock piles, and their related efflorescent salts). (G) Acid sulphate waters (pH � 2.5) in the Filon

Norte open pit.


Table 6

Mineralogical and chemical composition of sediments found in different AMD discharges and streams receiving AMD in the Odiel river basin

Mine/river site Sample No. Sampling

date

Mineralogy (XRD) Major oxides (%) Trace metals (ppm)

Major Minor SiO2 Al2O3 Fe2O3 CaO TiO2 MnO K2O MgO Na2O P2O5 LOI Total As Cr Cu Pb V Zn

Bco. Las Vinas P-937-19 Apr-03 Al-comp Gyp 72.44 7.30 9.65 0.10 0.51 0.00 2.08 0.10 1.07 0.02 6.81 100.08 66 10 25 11 29 54

Confesionarios P-937-101 Feb-03 Qtz, Jar Alb, Ms, Fe-ox 48.57 5.52 24.70 0.10 0.23 0.00 1.58 0.10 1.00 0.01 18.34 100.15 373 1 31 34 16 <2

Confesionarios

(Las Vinas)

P-937-19 Feb-03 Qtz K-feld, Ms, Jar 72.44 7.30 9.65 0.10 0.51 0.00 2.08 0.10 1.07 0.02 6.81 100.08 66 10 25 11 29 54

Corta Atalaya

(rıo Tintillo)

P-938-32 Feb-03 Schw Jar, Qtz 4.76 1.73 61.45 0.10 0.20 0.04 0.34 0.10 0.11 0.13 31.16 100.12 1134 9 657 316 42 107

Corta Atalaya

(rıo Tintillo)

P-938-34 Feb-03 Schw Jar, Qtz 5.53 3.38 52.62 0.11 0.50 0.06 0.37 0.35 0.90 0.14 36.05 100.01 734 6 462 374 26 312

Corta Atalaya

(rıo Tintillo)

P-T-1 Apr-03 Goet, Jar Qtz 5.30 1.74 60.31 0.15 0.32 0.06 0.26 0.25 0.14 0.21 31.04 99.76 860 11 1107 406 42 442

Corta Atalaya

(rıo Tintillo)

P-T-2 Apr-03 Schw – 1.78 1.35 58.40 0.11 0.41 0.06 0.10 0.21 <0.1 0.20 37.34 99.96 709 14 421 297 38 383

Corta Atalaya

(rıo Tintillo)

P-T-3 Apr-03 Schw Qtz 3.83 2.47 55.33 0.17 0.24 0.08 0.28 0.52 <0.1 0.22 37.00 100.14 1114 10 393 222 42 418

Corta Atalaya

(rıo Tintillo)

P-T-4 Apr-03 Schw Qtz 3.40 1.82 58.36 0.11 0.20 0.06 0.26 0.15 <0.1 0.14 35.81 100.30 818 12 435 236 48 286

Corta Atalaya

(rıo Tintillo)

P-T-6 Apr-03 Qtz, Ms, Jar – 2.82 1.50 63.95 0.10 0.11 0.05 0.18 <0.10 <0.1 0.15 31.14 99.99 794 13 814 107 75 212

Corta Atalaya

(rıo Tintillo)

P-T-6-II Apr-03 Goet Schw, Qtz 4.45 1.96 60.35 0.07 0.10 0.04 0.47 <0.10 0.34 0.11 32.38 100.25 1369 7 930 386 35 122

Dehesa Boyal P-DB-8 Apr-03 Schw, Qtz – 33.38 24.91 4.92 0.83 0.75 0.08 1.00 0.85 1.04 0.18 32.07 100.00 37 2249 149 40 402

La Torerera P-959-11 Feb-03 Qtz Mic, Alb,

Jar, Cl

45.31 14.17 18.34 0.10 0.53 0.04 2.43 0.41 0.55 0.08 18.07 100.03 2969 24 337 1810 31 159

La Zarza P-937-8 Feb-03 Schw Jar 1.07 0.73 60.68 0.18 0.03 0.05 0.15 0.10 0.08 0.15 36.83 100.04 742 4 91 9 83 71

Lomero-Poyatos P-937-102 Feb-03 Jar, Schw Qtz, Ms,

Cl, Gyp

4.86 2.39 47.40 1.22 0.15 0.05 0.26 1.99 0.33 0.14 41.25 100.04 2221 25 236 1080 88 447

Mina el Soldado P-938-119 Apr-03 Goet Qtz, Schw 1.35 1.12 59.16 0.05 0.02 0.02 0.11 <0.10 <0.1 0.06 38.22 100.10 1147 <2 282 14 9 5

Rıo Odiel P-938-43 Apr-03 Qtz, Alb, Ms, Ka – 23.65 19.15 25.86 0.91 0.33 0.09 0.67 0.90 0.47 0.40 27.69 100.12 135 21 4127 78 56 18,766

Rıo Odiel

(Est. 54)

P-960-2 Apr-03 Schw Qtz 61.23 12.27 12.06 1.28 1.16 0.08 2.05 1.04 1.74 0.09 7.01 100.00 95 30 248 61 62 108

Rivera de

la Panera

P-937-1 Apr-03 Qtz, Fe-ox Alb 23.92 10.05 30.45 0.38 0.38 0.09 1.60 1.12 0.39 0.11 31.31 99.81 226 12 508 38 19 944

Rivera

Olivargas

P-938-7 Apr-03 Qtz, Jar Ms 67.94 8.50 9.98 0.10 0.37 0.01 1.34 0.10 2.64 0.01 9.08 100.07 172 1 111 677 7 8

Sotiel P-960-8 Feb-03 Qtz Ms, Schw 52.75 20.50 10.45 0.18 0.59 0.02 1.57 0.47 0.31 0.06 13.11 100.00 278 33 90 180 50 173

Tharsis P-959-2 Feb-03 Schw Qtz, Ms, Cl 26.25 8.46 35.26 0.16 0.37 0.12 1.24 0.85 0.53 0.36 26.41 99.99 4729 42 537 221 179 887

Tharsis

(Dehesa Boyal)

P-DB-1 Apr-03 Jar Goet, Qtz 37.28 13.55 28.56 0.07 0.61 0.03 2.30 0.12 0.20 0.10 16.79 99.62 717 20 299 1015 24 142

Tharsis

(Dehesa Boyal)

P-DB-3 Apr-03 Qtz, Schw Ms, K-feld,

Jar

8.80 4.75 52.32 0.13 0.12 0.06 0.41 0.29 0.11 0.19 33.12 100.30 351 19 537 127 26 182

Tharsis

(Dehesa Boyal)

P-DB-5 Apr-03 Qtz, Schw Ms 20.07 10.52 34.34 0.35 0.26 0.14 1.07 0.95 0.49 0.14 32.70 101.02 164 36 803 65 38 392

Tharsis

(Dehesa Boyal)

P-DB-7 Apr-03 Qtz, Ms Ms, Schw 15.75 9.12 40.45 0.19 0.23 0.07 0.88 0.17 0.49 0.14 32.70 100.18 68 42 546 101 14 73

Tharsis

(Aguas Agrias)

P-959-7 Feb-03 Schw Jar, Goet 2.18 1.21 60.21 0.10 0.06 0.04 0.45 0.10 0.14 0.20 35.37 100.05 3189 7 299 426 96 223

Tintillo P-T-7 Apr-03 Qtz, Schw Goet, Ms 40.09 7.53 28.38 0.42 0.43 0.07 1.09 0.70 0.39 0.13 20.61 99.84 884 14 226 129 30 60

Tintillo P-T-8 Apr-03 Schw, Qtz – 21.45 5.61 43.75 0.36 0.45 0.05 0.81 0.25 0.39 0.22 26.45 99.78 1043 12 697 799 30 341

Tinto-Sta Rosa P-960-9 Feb-03 Qtz, Ms, Cl – 31.20 30.18 3.62 0.36 0.20 0.06 0.82 0.49 0.23 0.03 32.80 100.00 168 7 16,956 243 8 637

Abbreviations: Qtz, quartz; K-feld, potassium feldspar; Alb, albite; Ms, muscovite; Cl, chlorite; Gyp, gypsum; Al-comp, amorphous aluminium compounds; Fe-ox, undifferentiated iron oxides; Jar, jarosite; Goet, goethite; Schw,

schwertmannite; Ka, kaolinite.

J.Sanchez

Espanaet

al./Applied

Geochem

istry20(2005)1320–1356

1335

Table 7

Mineralogical and chemical composition of mine drainage minerals (MDM) found in different mine sites of the IPB (Odiel river basin)

Abbreviations: Jar, jarosite; Goet, goethite; Schw, schwertmannite; Ferr-2L, ‘‘two line’’-ferrihydrite; Bas, basaluminite; Qtz, quartz; Ms, muscovite; Cl, chlorite; n.a., not analyzed; n.d., not detected. The provided field data (pH and Eh in mV)

1336

J.Sanchez

Espanaet

al./Applied

Geochem

istry20(2005)1320–1356


In the IPB, these Fe(III) phases had not been stud-

ied in detail, being traditionally referred to under

ambiguous terms such as ‘‘ochreous precipitates’’,

‘‘amorphous iron oxides’’, ‘‘ferric hydroxides’’, or

‘‘poorly crystalline oxyhydroxides’’. Although the pres-

ence of schwertmannite has been recently suggested in

the Rıo Tinto sediments (e.g., Hudson-Edwards et al.,

1999), it has not been unequivocally recognized or

characterized.

In this work, the authors present preliminary miner-

alogical and chemical characterization of the MDM usu-

ally found in the IPB (Tables 6 and 7; Figs. 7 and 8).

These samples were collected from different deposits

and included: (1) recent, ochreous, colloidal precipitates

directly taken from the AMD water column, (2) film-like

thin Fe(III) layers floating on the water surface in low

flow or stagnant AMD sites, (3) aged, hard Fe(III)

crusts, and (4) laminated Fe(III) formations formed by

the covering of streambed cobble by filamentous algae

and hydrous Fe oxyhydroxides. Also, two samples of

‘‘synthetic’’ precipitates formed during titration of

AMD solutions with NaOH 2 M to pH � 4, have also

been analyzed for comparison with the naturally occur-

ring samples (Fig. 7).

The older and more consistent samples (Fe(III)

crusts, laminated formations) detailed in Table 6 show

1

10

100

1,000

10,000

100,000

1,000,000

-3.0 -1.0 1.0

SO4

(mg/

L)

0

1

10

100

1,000

10,000

100,000

1,000,000

10 100 1,000

SO4 (

Fe+

Cu+

Zn

(mg/

L)

IPB

A

B

Iron Mountain

Fig. 5. (A) SO4 vs. pH diagram and (B) (Fe + Cu + Zn) vs. SO4 diagra

comparison purposes, the chemical data available for AMD waters fr

see references therein) have been also plotted.

always trace to major amounts of detritic silicates such

as quartz, feldspars or clay minerals which are cemented

by Fe oxyhydroxides, and always have high contents of

SiO2 and Al2O3.

The recent ochre precipitates show recognizable

bands in the XRD profiles (Fig. 7) and consist mainly

of schwertmannite (Fe8O8(SO4)(OH)6), either as a

monomineralic phase, or combined with jarosite (KFe3(SO4)2(OH)6) and/or goethite (FeOOH).

Schwertmannite is poorly crystallized in comparison

with jarosite and goethite. However, it is possible to rec-

ognize 8 bands at 1.46, 1.51, 1.66, 1.95, 2.28, 2.55, 3.39

and 4.86 A in the XRD patterns (Fig. 7), in agreement

with other schwertmannite samples described in the lit-

erature (e.g., Bigham et al., 1996; Williams et al., 2002;

Dold, 2003). These results, coupled with pe–pH data

(pe = 12–15, pH 2.0–3.5) of parent acid solutions (as-

sumed to be in equilibrium with analyzed minerals),

and chemical composition of these precipitates (averag-

ing 61% Fe2O3, 37% LOI, and (Fe/S)molar � 5.3), are

conclusive evidence for schwertmannite identification

(Table 7; Fig. 8).

Additionally, 2 line-ferrihydrite (Fe5HO8 Æ 4H2O)

and an Al-bearing amorphous phase, have been

identified in a sample collected downstream of Con-

cepcion AMD discharge on the Odiel river (pH � 7;

3.0 5.0 7.0 9.0

pH

IPB

10,000 100,000 1,000,000

mg/L)

Iron Mountain

m, for AMD discharges in the IPB (Odiel river watershed). For

om different types of deposits (compiled in Plumlee et al., 1999,

102 103 104 105

EC (µS/cm)

SO4

(mg/

L)

R2 =0.95

EC (µS/cm)

TD

S (m

g/L

)

R2 =0.93

A

B

102

103

104

105

103 104 105103

104

105

Fig. 6. Binary plots showing the correlation between electrical

conductivity (EC) field measurements and (A) total dissolved

solids (TDS) and (B) SO2�4 , for the studied AMD discharges in

the IPB (Odiel river watershed).


(Fe/S)molar = 12.1). The artificially obtained phases after

titration of AMD samples from Rıo Tinto (sample 938-

102) and Tharsis (sample 959-16) are composed of a

mixture of schwertmannite and an XRD-amorphous,

white-coloured, Al-phase that could be basaluminite

(Al4(SO4)(OH)10 Æ 5H2O).

XRD results are in perfect agreement with pe–pHdata and chemical analyses of these oxyhydroxysul-

phates. Most samples plot within their respective theo-

retical stability fields in a pe–pH diagram (Fig. 8) and

show (Fe/S)molar ratios similar to those reported else-

where (Bigham et al., 1990, 1994, 1996).

The average chemical composition of monomineralic

schwertmannite includes minor amounts of SiO2 (0.4–

3.8 wt%) and Al2O3 (0.7–3.0 wt%), which could suggest

either (1) some silicate contamination of samples, and/

or (2) some degree of sorption/coprecipitation of Si

and Al onto the schwertmannite colloidal particles. In

fact, the two mixed schwertmannite-basaluminite sam-

ples formed by titration of acid solutions (samples 959-

16 and 938-102; Table 7), show also SiO2 contents of

0.4–1.5%, which strengthens the second possibility, and

agrees with the results of Carlson and Schwertmann

(1981), who found Si contents of 2.5–4.5% in AMD pre-

cipitates (in this case, consisting of ferrihydrite) from

Finland.

Average compositions of the analyzed Fe minerals

also include significant trace element contents

(23–12,770 ppm As, 80–4800 ppm Cu, 5–6230 ppm Zn,

2–1080 ppm Pb, 4–274 ppm Co, 4–452 ppm Ni,

1–31 ppm Cd; Table 7). This suggests variable sorption

of dissolved trace metals onto schwertmannite, ferrihy-

drite, jarosite and goethite mineral surfaces. Other trace

metals (Ag, Be, Hg, Mo, U, Th) were always below the

detection limit.

4.5. Mineralogy and chemistry of efflorescent sulphate

salts

In addition to the hydrous Fe oxide phases, a number

of sulphate efflorescences from different mines (e.g.,

Corta Atalaya-Rıo Tinto, Tharsis, La Zarza, San

Telmo, Lomero-Poyatos, San Miguel) were collected

during the dry season (June and September; Fig. 4D–

F). These minerals are formed by evaporation of the

acid mine waters, being ubiquitous around waste piles,

taillings and river banks during this season (evaporation

rates of about 5–10 mm/day were calculated during field

experiments conducted in September 2003). These salts

have also been mineralogically identified by XRD and

chemically analyzed for major and trace element con-

tents (Table 8).

These soluble metal sulphate salts are usually found

as botryiodal (colliform-like) efflorescences of variable

colour (white, greenish blue, pale green, yellow, or-

ange, dark reddish), being normally zoned from the in-

ner to the outer zones of the AMD sources and

streams, and suggesting a paragenetic sequence of sul-

phates with distinct solubilities and degrees of dehy-

dration. They are rarely found as monomineralic

phases, and most commonly consist in mixtures of

Mg–Fe–Al hydrated sulphates such as epsomite, hexa-

hydrite, pickeringite, melanterite, szomolnokite, roze-

nite, halotrichite, copiapite or rhomboclase. Most of

these phases have been indentified in other mining dis-

tricts (e.g., Iron Mountain; Alpers et al., 1994; Nord-

strom, 1999; Nordstrom and Alpers, 1999b) and

have also been recently recognized and analyzed by

Buckby et al. (2003) in the adjacent Tinto river.

Among these sulphates, the Fe(II)-rich salts (melante-

rite, szomolnokite, rozenite, halotrichite) are dominant

at the discharge points of the more acidic and Fe(II)-

rich acid waters, whereas the Fe(III)-rich (copiapite,

coquimbite) and Mg-rich (epsomite, hexahydrite) min-

erals are typical of the banks of most streams affected

by AMD. Gypsum is also frequent as acicular crystals

in mine adits and waste piles. Stalactites composed of

gypsum ± copiapite ± jarosite are commonly observed

at the AMD discharge points.

Fig. 7. Selected XRD profiles for the mine drainage minerals (MDM) studied in the IPB mine sites. These diffractometric patterns

include, 4 examples of schwertmannite found in several AMD discharges at pH of 2.5–3.5 (left), two cases of schwertmannite-bearing

precipitates obtained during titration of AMD solutions at pH about 4.0 (center), one sample of jarosite–schwertmannite from the

Tinto river (pH 2.1; upper right), and a ferrihydrite sample taken from the Odiel river and occurring at neutral pH (lower right). As an

example, in sample 937-8, the d-spacing (A) of the identified reflections are indicated. See Table 7 for a detailed sampling site

description and chemical composition of these Fe precipitates.


Chemical analyses of mixtures of these sulphates re-

veal metal contents (e.g., average values of 2800 ppm

Cu and 9000 ppm Zn, with local Zn values of several

percent; Table 8) which are significantly higher than

those corresponding to analyzed schwertmannite, fer-

rihydrite, jarosite and goethite (810 ppm Cu and

750 ppm Zn on average; Table 7). On the other hand,

As contents of the sulphates are very low in comparison

with those of schwertmannite (195 and 1700 ppm,

respectively), whereas Co, Ni and Cd show average val-

ues of 240, 155 and 30 ppm, respectively. Other trace ele-

ments (Ag, Be, Hg, Sb and Th) were always below their

respective detection limits.

The environmental significance of sulphate minerals

precipitated during AMD evaporation has been pro-

fusely documented (e.g., Alpers et al., 1994; Nord-

strom and Alpers, 1999b). The large accumulation of

these salts in the mine sites plays an important role

in the transient storage of metals and acidity, which

can be easily redissolved and incorporated in the water

courses during rainfall episodes.

5. Discussion

5.1. Geochemistry of the acid mine drainage

5.1.1. Eh and Fe(II)/Fe(III) redox chemistry

The field Eh data are well correlated with the Fe(II)/

Fetotal ratio, which means that the redox potential of the

acid mine waters is basically governed by the oxidation of

Fe(II) (Fig. 9). The observed variation of Eh with Fe(II)/

Fetotal allows the quantitative estimation of the Fe(II) to

Fe(III) ratio of the acid mine waters from Eh measure-

ments. An approximate Eh value of � 640 mV differenti-

ates the dominantly Fe(II) (>50% Fe(II)) from the

dominantly Fe(III) (<50% Fe(II)) acid drainage (Fig. 9).

Additionally, theoretical Eh values for the AMD

solutions (calculated with PHREEQC from Fe(II)

and Fe(III) concentrations) are highly correlated

(r = 0.88 for n = 34) with the Eh values measured on

site for all Fe molal concentrations (10�7–10�1 mol;

Fig. 10). These data reinforce the validity of Eh as

indicator of the Fe oxidation state in AMD.

Fig. 8. pe–pH diagram for the studied hydrous Fe oxides

(oxyhydroxysulphates). The pe (Eh (mV)/59.2) and pH data

correspond to the ambient conditions measured in the parent

AMD solutions from which the samples were taken, at the

moment of sampling. Field boundaries for stability of the

different mineral phases are taken from Bigham et al. (1996).

The shaded areas represent the expansion of the K-jarosite and

ferrihydrite stability fields if lower solubility products are

selected (Bigham et al., 1996). Abbreviations: Jt, K-jarosite; Sh,

schwertmannite; Fh, ferrihydrite; Gt, goethite; Py, pyrite; Diss.

Spec., dissolved species.


5.1.2. The bacterial oxidation of Fe(II)

The oxidation of Fe(II) is very fast, and therefore, the

AMD waters evolve geochemically downstream in a

short distance (often on a m-scale) as they become equil-

ibrated with atmospheric O2. The hydrolysis of Fe(III)

also occurs rapidly, and precipitation of hydrous Fe oxi-

des takes place at a few meters from the discharge

points.

From detailed field work at selected mine sites (e.g.,

La Zarza-Perrunal and Tharsis mine portals, unpub-

lished data) it has been calculated that the field rate

for Fe(II) oxidation is between 1 and 20 mmol L�1 h�1

(depending on the concentration of bacterial cells, as

well as on temperature). These rates are comparable to

other reaction rates reported for bacterially catalyzed

Fe oxidation in AMD environments (Nordstrom,

1985; Williamson et al., 1992; Kirby and Elder Brady,

1998; Kirby et al., 1999), and about 106 times faster than

published laboratory abiotic rates (Stumm and Lee,

1961; Singer and Stumm, 1970).

These data suggest that bacterial activity is playing a

critical role in the geochemical evolution of the acid

mine waters, catalyzing the oxidation of Fe(II). This

idea is confirmed by several field observations like the

biologically induced, laminated Fe(III) formations,

which are present in most of the studied mine sites. In

fact, several microbiological studies (Waddell, 1978;

Ahonen and Tuovinen, 1989) have confirmed that the

ambient conditions commonly found in these mine-

drainage environments (pH � 3, T � 20–30 �C) are opti-mum for Acidithiobacillus ferrooxidans (which have been

observed at pH 1.3–4.5 and T = 4–37 �C; Waddell,

1978). A number of chemolithotrophic microorganisms,

including Fe- and S-oxidizing bacteria (e.g., Acidithioba-

cillus ferrooxidans, Acidithiobacillus thiooxidans, or

Leptospirillum ferrooxidans) have been described in the

acidic Odiel and Tinto rivers (Lopez-Archilla et al.,

2001; Gonzalez-Toril et al., 2003). Additional studies

have addressed the importance of these bacteria not only

in the oxidation of Fe(II), but also in the nucleation and

growth of hydrous Fe oxide particles (e.g., schwertman-

nite) on their cell surfaces (e.g., Kawano and Tomita,

2001).

5.1.3. Acidity and the role of Fe(III) and Al(III) as

buffering species of AMD systems

The high acidity values reported in Table 5 are fa-

voured by the scarcity of carbonates to neutralize the

acidity released during pyrite oxidation. Although some

aluminosilicates (muscovite, albite, K-feldspar) can also

neutralize acidity, these minerals are much less efficient

as acidity-neutralizers than carbonate dissolution

(Stumm and Morgan, 1981). Furthermore, whereas the

dissolution rate of carbonates (10�5–10�6 mol m�2 s�1)

is faster than that of pyrite (10�8–10�10 mol m�2 s�1),

aluminosilicates show significantly slower rates of disso-

lution (10�11–10�13 mol m�2 s�1; Banwart and Malm-

strom, 2001). Consequently, without the presence of

abundant carbonate, the existing aluminosilicates can-

not neutralise the acidity released during the abundant

pyrite oxidation. On the contrary, aluminosilicate disso-

lution releases important amounts of dissolved Al (in

addition to Fe(II) in the case of chlorite) which can also

be hydrolized and provide additional acidity.

The maximum contents of Ca (529 mg/L), Na

(206 mg/L), K (36 mg/L), Mg (2894 mg/L), and Cl

(580 mg/L) found in the analyzed AMD (Table 4),

suggest an effective dissolution of the abundant alumi-

nosilicates (especially chlorite, in addition to sericite,

K-feldspar and albite) and rare carbonates (calcite,

dolomite, ankerite) at the mine sites.

To achieve a more accurate interpretation of the

acidity potential of the acid mine drainage, detailed

titration tests have been performed on selected AMD

samples (n = 40). The acidity data are reported in Table

5, and some examples of titration curves are shown in

Fig. 12.

The data given in Table 5 show two relevant features:

(1) total acidity is very variable over the range 200–

30,000 mg/L CaCO3 eq. among the different mine efflu-

ents and (2) acidity is excellently correlated with the

Fe and Al contents (R2 = 0.91–0.95; Fig. 11).

Table 8

Mineralogical and chemical composition of efflorescent sulphate salts found in the mine sites and water courses comprised in the Odiel river basin

Mine/river

site

Sample

No.

Sampling

date

Mineralogy (XRD) Major oxides (%) Trace metals (ppm)

Major Minor SiO2 A12O3 Fe2O3 CaO TiO2 MnO K2O MgO Na2O LOI To As Cd Co Cu Ni Pb U V Zn

Angostura 938-110 Jun-03 Pick, Hal – 2.26 9.77 2.87 0.08 0.03 0.13 0.10 6.92 0.27 77.68 100 <10 5 152 4351 55 n.a. 3 1 921

La Zarza 937-109 Jun-03 Sta Gyp 2.36 1.79 0.43 6.50 0.02 1.48 0.10 15.53 0.53 64.37 93. <10 2 274 333 452 22 1 1 2683

Lomero-

Poyatos

937-103 Jun-03 Eps Hex, Hal,

Kal

4.55 7.09 5.23 0.26 0.08 0.41 0.10 9.98 1.36 71.05 100 22 13 21 993 18 33 2 14 2674

Lomero-

Poyatos

937-103b Jun-03 Sta Pent, Gyp,

Anh

2.29 4.19 6.28 0.68 0.04 0.39 0.10 12.82 0.40 72.91 100 88 9 28 609 22 6 1 5 4212

Lomero-

Poyatos

937-103c Jun-03 Hex Pick, Hal,

Kal

0.94 4.43 6.35 0.18 0.03 0.36 0.10 14.77 1.36 71.68 100 <10 7 31 504 18 19 1 1 4916

Oraque

river

E-959-1 Jul-03 Sta, Roz Pick 0.70 7.48 0.71 0.45 001 0.94 0.10 11.58 3.20 65.44 90. <10 80 398 2460 211 5 9 2 30,300

Poderosa 938-1 14-A Sep-03 Eps Alun, Bass 0.85 5.42 21.96 0.32 0.03 0.06 0.16 1.19 0.26 69.73 99. 260 36 n.a. 7173 n.a. 6 n.a. n.a. 6268

Poderosa 938-1 14-B Sep-03 Rom Szo 0.69 3.76 19.45 4.83 0.02 0.05 0.15 1.22 0.19 62.72 93. 194 27 n.a. 7766 n.a. 10 n.a. n.a. 5898

Rio Tinto RT-1 Jun-03 Cop,

Mg-Cop

Hal 0.10 6.05 17.16 0.13 0.01 0.41 0.10 4.77 0.16 71.30 100 417 38 219 7449 60 1 1 3 5810

San Miguel 938-108 Jun-03 Cop,

Mg-Cop

Coq 3.82 4.57 19.77 0.13 0.04 0.23 0.08 1.93 0.09 69.32 99. 344 4 86 1506 37 17 1 5 494

San Miguel 938-121-B Sep-03 Szo Gyp, Rom,

Eps

2.41 5.57 20.10 0.70 0.04 0.47 0.15 4.34 0.20 66.03 97. 28 6 n.a. 443 n.a. 5 n.a. n.a. 1406

San Miguel 938-121-P Sep-03 Pick Szo, Hal 0.69 3.93 18.06 0.25 0.02 0.39 <0.10 3.73 0.08 72.85 99. 2 3 n.a. 502 n.a. 2 n.a. n.a. 1222

San Telmo 937-105a Jun-03 Szo Hal, Pick 0.42 6.51 17.97 0.19 0.01 0.55 0.58 6.30 0.27 67.13 99. 1480 26 54 114 46 19 3 10 3497

San Telmo 937-105b Jun-03 Cop Rom 0.48 2.35 26.26 0.13 0.02 0.07 0.11 0.37 0.07 70.18 100 625 19 53 6580 27 11 2 2 3174

San Telmo 937-105c Jun-03 Pick Hal, Kie 4.60 6.94 2.71 0.25 0.05 1.58 0.10 11.84 0.43 71.57 100 <10 41 57 9592 100 41 3 34 12,676

San Telmo 937-2 Jun-03 Cop,

Mg-Cop

Kie, Hal 2.92 5.84 13.25 0.11 0.06 0.74 0.14 7.36 0.07 69.59 100 102 21 106 2995 107 n.a. 3 7 6883

San Telmo

(pit lake)

937-106 Jun-03 Pick Roz, Sta,

Hal

2.29 6.79 3.14 0.08 009 1.09 0.10 8.66 0.07 77.66 99. <10 26 125 3018 80 n.a. 6 1 8592

San Telmo

(waste pile)

937-104 Jun-03 Gyp Jar, Qtz 8.39 1.45 2.18 29.35 0.04 0.03 0.19 1.37 0.14 23.54 66. 15 3 8 190 7 29 <0.2 3 440

Tharsis 959-104 Sep-03 Pick, Hal Sta <0.10 8.81 0.11 0.06 0.03 4.62 <0.10 9.22 0.34 76.77 100

Tharsis

(Aguas Agrias)

959-7 Jun-03 Pick, Hal – 2.44 9.82 1.04 0.25 0.03 1.10 0.10 9.84 0.61 74.87 100 <10 43 266 4255 159 n.a. 6 1 8400

Tharsis

(Aguas Agrias)

E-959-7A Jul-03 Sta, Hex Hal, Pick 0.35 5.24 6.95 0.15 0.02 0.62 0.10 9.69 0.07 76.89 100 534 73 845 2012 550 14 7 36 33,400

Tharsis

(Aguas Agrias)

E-959-7B Jul-03 Sta, Cop,

Pick

Roz 0.10 5.50 6.57 0.10 0.01 0.62 0.10 10.10 0.15 76.99 100 172 87 832 2107 540 1 8 10 32,200

Tharsis

(Aguas Agrias)

E-959-7C Jul-03 Sta, Cop,

Pick

Kal 0.87 7.17 5.79 0.13 0.01 0.68 0.10 9.25 0.30 75.80 100 176 94 829 2579 482 79 8 11 35,800

Tharsis

(Dehesa Boyal)

DB-2.1 Jun-03 Pick, Hal – 0.98 10.10 5.34 0.18 0.02 0.94 0.10 5.26 0.07 77.18 100 <10 15 214 4285 139 n.a. 1 2 3893

Tharsis

(Dehesa Boyal)

P-DB-6 Apr-03 Hex, Gyp Jar 16.16 5.60 39.76 0.30 0.23 0.10 0.45 1.71 0.39 37.85 102 117 1 n.a. 604 n.a. 71 <1 26 219

Tintillo river 938-112 Sep-03 Gyp Cop 1.40 2.97 18.50 4.58 0.19 0.31 0.12 2.95 0.08 48.76 80. 72 6 n.a. 486 n.a. 4 n.a. n.a. 6184

Tintillo river 938-112a Jun-03 Pick Hal, Kie 1.20 8.43 4.42 0.12 0.02 1.11 0.10 9.77 0.07 74.93 100 <10 36 178 3110 115 10 6 0 8389

Tintillo river 938-112b Jun-03 Sta Hal, Roz,

Pick

1.22 6.37 7.31 0.11 0.02 0.96 0.10 8.31 0.07 75.70 100 <10 34 158 2625 101 12 5 1 7929

Tintillo river 938-112c Jun-03 Sta Hal, Pick 1.08 5.76 8.82 0.11 0.03 0.88 0.10 8.48 0.07 74.79 100 29 28 189 2738 108 7 4 1 12,873

Tintillo river E-T-9 Jul-03 Sta Pick 0.20 8.27 3.55 0.18 0.01 0.83 0.10 8.95 0.27 77.75 100 20 113 600 4001 310 n.a. 1 1 4 27,600

Tintillo river T-1 Sep-03 Hex, Pick Hal O.10 7.32 5.45 0.40 0.01 0.99 <0.10 8.72 <0.065 77.05 99. 72 6 n.a. 486 n.a. 4 n.a. n.a. 6184

Tintillo river T-1-A Sep-03 Sta, Pick Roz, Hal 0.74 8.35 4.68 0.23 0.03 0.97 <0.10 8.35 0.09 76.50 99. 2 32 n.a. 1990 n.a. <2 n.a. n.a. 6823

Tintillo river T-1-B Sep-03 Pick, Sta Hal 0.38 9.08 4.20 0.19 0.02 1.03 <0.10 7.08 0.08 77.93 99. <2 36 n.a. 2189 n.a. <2 n.a. n.a. 6163

Line missing

J.Sanchez

Espanaet

al./Applied

Geochem

istry20(2005)1320–1356

1341

tal

.10

10

.10

.10

.20

61

97

08

.19

99

24

23

93

.03

.07

.06

89

67

.15

.10

.07

.23

.09

.16

.55

01

.17

.17

.10

.11

93

10

54

Tintilloriver

T-1-C

Sep-03

Pick,Hal

Sta

<0.10

3.75

4.65

0.67

0.02

0.64

<0.10

10.57

<0.065

79.69

100.00

<2

18

n.a.

1476

n.a.

<2

n.a.

n.a.

7487

(continued

onnextpage)

Table

8(continued)

Mine/river

site

Sample

No.

Sampling

date

Mineralogy(X

RD)

Majoroxides

(%)

Trace

metals(ppm)

Major

Minor

SiO

2A12O

3Fe 2O

3CaO

TiO

2MnO

K2O

MgO

Na2O

LOI

Total

As

Cd

Co

Cu

Ni

Pb

UV

Zn

Tintilloriver

T-5

Sep-03

Sta,Pick,

Roz

Hal

0.15

9.90

0.33

0.07

0.03

1.55

<0.10

8.88

0.19

78.92

99.67

13

30

n.a.

1442

n.a.

<2

n.a.

n.a.

7241

Tintilloriver

T-7

Jun-03

Hal,Pick

Jar,Ferr

0.91

8.42

5.21

0.56

0.03

0.98

0.10

8.14

0.08

75.68

100.10

<10

35

186

3940

96

74

17312

Tintilloriver

T-7

Sep-03

Pick,Sta

–0.21

9.65

0.38

0.08

003

1.59

<0.10

8.56

0.31

79.20

99.48

15

39

n.a.

2612

n.a.

<2

n.a.

n.a.

7869

Tintilloriver

T-9

Jun-03

Pick,Hal

–1.38

7.75

4.31

0.22

0.02

0.95

0.10

7.99

0.19

77.19

100.10

51

25

41

3765

21

14

16513

Tintilloriver

T-9

Sep-03

Pick,Sta

Hal

0.17

9.16

3.16

0.07

003

0.94

<0.10

8.04

0.28

78.13

99.52

20

35

n.a.

3222

n.a.

<2

n.a.

n.a.

6509


The titration curves are clearly marked by two sharp

slope breaks at pH � 2.7 and around 4.0–4.5, which

indicate strong resistance of the acid solutions to pH in-

crease during titration (Fig. 12), being the result of the

hydrolysis of dissolved Fe(III) and Al(III) ionic species.

Additionally, Fe(II) hydrolysis causes slope breaks at

pH � 7.0–8.0 in the Fe(II)-rich samples that were not

oxidized with H2O2 before titration (e.g., Fig. 12A, C

and F). Although some hydrolysis of dissolved Fe(II)

at circumneutral conditions occasionally occurs in the

field (e.g., La Poderosa and San Miguel mines), this pro-

cess rarely takes place under natural conditions at the

basin-scale.

These figures reflect that Fe(III) and Al(III) hydroly-

sis concurrently can represent around 70% on average of

the total acidity potential of the aqueous acid solutions.

In other words, Fe and Al dissolved in AMD act as

chemical buffers, stabilizing the AMD systems at pH

values around about 2.7 and 4.5, respectively. This buf-

fering is evident in the Odiel river mouth, which shows

an average pH of between 2.5 and 3.5 (with a well-de-

fined median value of 2.7 in the summer months) over

the period 1990–2001 (Confederacion Hidrografica del

Guadiana, unpublished data).

5.1.4. Seasonal variation of the AMD chemistry

As is clearly shown in Tables 3 and 4, AMD compo-

sitions are not chemically constant throughout the

hydrologic year 2003–2004. On the contrary, they show

marked chemical variations between the winter and

summer seasons. Flow rate is strongly lowered and pH

tends to decrease during the dry period, but changes in

SO4 and metal concentrations do not show a clear trend.

Further, the variability in metal contents is not propor-

tional among the different mine effluents.

This suggests that factors, other than simple dilution

by fresh groundwater input after rainfall episodes,

determine the variations in element concentrations. Spe-

cifically, several geological, hydrogeological and miner-

alogical aspects are involved in these chemical changes.

For example, the dissolution of efflorescent sulphates

previously formed during the dry period in partly

flooded galleries, could provoke local increases of the

SO4 and metal contents, as described in other mine dis-

tricts like Iron Mountain, California (Alpers et al., 1994)

or St. Kevin Gulch, Colorado (Kimball, 1999). Also,

mineralogical differences between the mineralization

being oxidized (pyritic to base metal-rich complex ore)

and host rocks (chloritic to sericitic volcaniclastic rocks,

silicified vitric tuffs, black shales, etc.) could be responsi-

ble for the differential dissolution of some elements.

5.1.5. Relation between AMD chemistry and deposit

geology/mineralogy

Fig. 5 shows that the IPB acid mine waters are

more acid and metal-enriched than many of those

0

10

20

30

40

50

60

70

80

90

100

400 500 600 700 800 900

Eh (mV)

Fe2+

/Fe t

otal

(%)

Ferrous AMD

Ferric AMD

IncreasingO

xidising

conditions

Eh~

640

mV

(r=-0.92, 5%-95% range)

>95

% F

e2+

=>

Eh~

570

mV

>95

% F

e3+

=>

Eh~

720

mV

Fig. 9. Diagram showing the relation between the Fe(II) to total Fe ratio (Fe2+/Fetotal) and the redox potential (Eh) of the analyzed

AMD. An approximate reference value of about 640 mV allows differentiation of the Fe(III) (>50% Fe3+) from the Fe(II) (>50% Fe2+)

acid mine waters in the IPB.


reported in the literature. Only the extreme cases re-

ported for Iron Mountain (with negative pH values

and Fe, Al, Zn and Cu contents of tens of grams

per litre; Nordstrom and Alpers, 1999a) show clearly

more extreme conditions than the IPB acid drainage.

Chemical composition of the studied AMD always

shows high contents of Zn and Cu (and to a lesser de-

gree, As, Pb, Cd, Co and Ni), which is a common

feature of mine waters draining massive sulphide

deposits (Ball and Nordstrom, 1989; Goldfarb et al.,

1996; Plumlee et al., 1999; Nordstrom and Alpers,

1999a).

However, the most remarkable observation that can

be made on the AMD chemistry is its dependence on

the deposit geology and mineralogy, so that a strong

geologic control on AMD composition is suggested.

Thus, the increasing contents of Zn, Cu, Mn, As, Cd,

Co, Ni or Pb with increasing SO4 content (Fig. 13)

indicate a progressive dissolution of pyritic and poly-

metallic (‘‘complex’’) ores (containing abundant sphal-

erite, chalcopyrite, galena, arsenopyrite and

tetrahedrite-tenantite) at the mine sites. The observed

Al, Mg, Na, K and Ca concentrations are indicative

of dissolution of the accompanying gangue minerals

(mainly chlorite, sericite and feldspars). Also, a good

correlation of r = 0.96 (p < 0.05; n = 9) has been ob-

tained between the Zn/Cu ratio of acid mine waters

and that of the drained mineralization (only taking into

account the cases of mine adits with AMD discharges

for which both chemical data for water and metal

grades for mineralization are available). In fact, the

average Zn/Cu ratio of AMD in the IPB is 2.7, similar

to the average ratio of the IPB ore mineralization (Zn/

Cu = 2.3; see Leistel et al., 1998).

Another interesting feature of the AMD chemical

composition is the high U and Th contents. The good

correlations found between U and elements such as:

(1) Al (r = 0.92), Mg (r = 0.90), Mn (r = 0.91) and (2)

Zn (r = 0.90), Cd (r = 0.82) together with SO4

(r = 0.89), suggest that U could be derived from the dis-

solution of: (1) some aluminosilicates such as Mg-chlo-

rite and/or K-feldspars (present in the acid volcanic

and vocanoclastic lithologies such as rhyolite and dacite)

and/or (2) other phases related with sphalerite-rich com-

plex ores. Uranium concentrations ranging from 1 to 9

and from 0.3 to 2.3 ppm have been measured in rhyolites

and polymetallic to complex ores from the IPB, respec-

tively (Marcoux, 1998). The Th contents of the IPB vol-

canic and sedimentary rocks (2–25 ppm on average;

Sanchez Espana, 2000) suggest that these lithologies

are the most probable sources of this element.

5.1.6. Metal speciation

Geochemical calculations performed with the PHRE-

EQC code reveal different dissolved metal species in the

acid solutions (Table 9). The geochemical predictions

indicate that the dissolved metals in the acid mine waters

are likely present in the form of either individual free

ions (Cu2+, Zn2+, Mn2+) or as a number of different

sulphate ionic species ðe:g:; FeSOþ4 ; FeðSO4Þ�2 ;

FeHSO2þ4 Þ. Additionally, other ionic species in the form

of hydroxide (e.g., Fe(OH)2+) and/or chloride (e.g.,

CdCl+) complexes, could be locally present, although

they would be quantitatively very minor.

500

550

600

650

700

750

800

850

300 400 500 600 700 800 900

Ehmeasured (mV)

Eh c

alcu

late

d(m

V)

-0.4

-0.2

0

0.2

0.4

0.6

-8 -6 -4 -2 0

Log Total Dissolved Iron, molality

Eh m

easu

red

-Eh

calc

ulat

ed(V

olts

)R2=0.82

A

B

Fe2+ <5% FetFe2+ >95% Fet

Fig. 10. Correlation between measured and calculated electronic potential (Eh in mV). The measured Eh values were obtained in the

field with a Pt electrode, whereas the theoretical Eh was computed with PHREEQC v. 2.0, by introducing measured Fe(II) and Fe(III)

concentrations (modified from Nordstrom, 2000).


A significant difference is predicted by PHREEQC

between divalent and trivalent cations. Whereas triva-

lent cations (Fe3+, Al3+) would be essentially present

as Al- and Fe-sulphate ionic species ðAlSOþ4 ;

AlðSO4Þ�2 ; FeSOþ4 ; FeðSO4Þ�2 Þ, bivalent cations would

be present either as individual free ions (Fe2+, Cu2+)

or sulphate species (e.g., FeSO4, CuSO4).

5.2. Mineralogy of the acid mine drainage solids

5.2.1. Solubility of the AMD-related mineral phases

Considering the measured values of Eh, pH, T, dis-

solved O2, and Fe, Al, SO4, Na and K concentrations,

along with available solubility products of Fe- and Al-

bearing phases (schwertmannite, jarosite, ferrihydrite,

goethite, hematite, basaluminite, jurbanite, gibbsite)

naturally occuring in AMD systems, it has been possible

to calculate the saturation indices (SI) of these minerals

in the AMD solutions (e.g., Table 9). All thermody-

namic constants (logK, DH) have been taken from the

geochemical databases accompanying the program

PHREEQC (Parkhurst and Appelo, 1999), and from

Bigham et al. (1996).

Under the geochemical conditions normally found in

the AMD systems (pH 2–3.5, Eh = 640–800 mV,

Fet = 100–7000 mg/L, Al = 25–2600 mg/L, SO4 = 2–

24 g/L; Tables 3 and 4), the poorly crystalline Fe-phases

(schwertmannite, jarosite), as well as the more crystal-

line (goethite, hematite) are always supersaturated

(SI > 0) in the acid solutions, therefore tending to pre-

cipitate (Table 9). On the other hand, ferrihydrite and

the amorphous to poorly crystalline Al-phases (bas-

aluminite, jurbanite, alunite, gibbsite) are mostly under-

saturated (SI < 0). This is well exemplified in Fig. 14,

where the SI values of schwertmannite and basaluminite

are plotted as a function of pH (considering the initial

conditions of a representative sample such as 938-112;

see Tables 3, 4 and 9).

10

100

1,000

10,000

10 100 1,000 10,000

Fe3+ (mg/L)

Aci

dity

(pH

=3.7

)

R2=0.91

A

10

100

1,000

10,000

10 100 1,000 10,000

Al3+ (mg/L)

Aci

dity

(pH

3.7

-5.0

)

R2=0.95

B

Fig. 11. Binary diagrams showing the correlation between (A) the Fe(III) dissolved in the AMD samples with acidity measured at a

reference pH value of 3.7 and (B) the Al dissolved in the AMD samples with acidity measured at the pH range of 3.7–5.0. Acidity is

given in mg/L CaCO3 eq. and has been calculated by titration with NaOH 1.6 N.


To illustrate the solubility behaviour of these Fe- and

Al-phases, the SI calculations performed on two AMD

systems from Corta Atalaya (Rıo Tinto mine) are shown

in Fig. 15. These plots show the evolution of the solubil-

ity of several Fe- and Al-phases in km-scale profiles

downstream from the discharge points, and include

two possible scenarios of pH evolution, namely: (A) con-

stant pH around 2.7 and (B) pH drop of an aluminous

AMD solution (pH � 4.5) mixing with more acid, ferric

AMD (pH � 2.7). The observed difference between Fe

and Al solubility leads to spatial and temporal separa-

tion of precipitating phases of hydrolyzed Fe and Al.

In all cases, the solubility calculations performed,

as well as kinetic considerations, clearly suggest that

schwertmannite and, to a lesser extent, jarosite are

the dominant phases controlling Fe(III) solubility in

the AMD solutions, whereas Al solubility would be

mainly controlled by amorphous to poorly crystalline

Al-phases like basaluminite. Although goethite has

also been found in variable proportions in several

MDM samples (Table 7), and it has been proved to

precipitate coetaneously with schwertmannite, ferrihy-

drite and jarosite in AMD systems (e.g., Bigham

et al., 1996), this mineral does not appear to play a

relevant role in controlling the Fe(III) concentration

of acid solutions. Goethite is more commonly found

in older Fe(III) deposits (e.g., Fe(III) crusts) at the

mine sites than in the fresh, recently precipitated

Sample 938-51

0200400600800

100012001400160018002000

2 3 4 5 6 7 8 9

pH

Aci

dity

(m

g/L

CaC

O3)

Sample 937-16

0

1000

2000

3000

4000

5000

6000

7000

2 3 4 5 6 7 8 9pH

Aci

dity

(m

g/L

CaC

O3)

35%

26%

Fe3+ hydrolysis

Al3+ hydrolysis

26%

15%

Fe3+ hydrolysis

Al3+ hydrolysis

Sample 938-4

0100020003000400050006000700080009000

2 3 4 5 6 7 8 9

pH

Aci

dity

(m

g/L

CaC

O3)

52%

Fe3+ hydrolysis

Al3+ hydrolysis

11%

Fe2+ hydrolysis

Sample 959-16

0

2000

4000

6000

8000

10000

12000

2 3 4 5 6 7 8 9pH

Aci

dity

(m

g/L

CaC

O3)

A

B

C

D

Fe3+ hydrolysis

Al3+ hydrolysis35%

22%

17%

0

1000

2000

3000

4000

5000

6000

2 3 4 5 6 7 8pH

E

0

500

1000

1500

2000

2500

3000

2 3 4 5 6 7 8 9 10pH

Aci

dity

(m

g/L

CaC

O3) FSample 937-108 Sample 938-127

Aci

dity

(m

g/L

CaC

O3)

Fe2+

Fe2+

Fe3+

Fe3+H

2 O2

addition

H2 O

2

addition

Fig. 12. Several examples of titration curves obtained after titration with NaOH of the acid mine water samples. The respective Fe and

Al contents of these samples are given in Table 4. In cases A, B, D and E, the relative importance of the hydrolysis of Fe and Al in the

total acidity is indicated. In C and F, the effect caused in the titration curves by the addition of H2O2 (over 5 min) to completely oxidize

Fe(II), is also illustrated.


colloidal particulates. This suggests that goethite

should be best considered as a product of mineralogi-

cal transformation of metastable Fe(III) phases (schw-

ertmannite, ferrihydrite, jarosite) than a mineral

directly precipitated from the AMD solutions. In fact,

both goethite and hematite show much slower growth

kinetics than schwertmannite, ferrihydrite and jarosite

(Nordstrom and Alpers, 1999a). Therefore, although

the geochemical calculations predict the precipitation

of goethite and hematite (Table 9; Fig. 15), they seem

to require more stable conditions and a longer time to

form.

Although the Al-phases have not been identified by

XRD, basaluminite is the mineral that best explains

the Al behaviour of acid solutions, and is the authors�preferred option to account for the control of Al concen-

tration at the critical pH range of 4–5. Alunogen be-

comes stable only at negative pH values, and gibbsite

is usually unstable at pH < 5.5 (depending on sulphate

activity), being rare in acid-sulphate environments

(Nordstrom, 1982; Nordstrom et al., 1984; Nordstrom

and Alpers, 1999a). Jurbanite is the most stable mineral

and thermodynamically favoured to precipitate at pH

below 4, although it rarely appears as an AMD precip-

itate. On the other hand, basaluminite is the more wide-

spread mineral phase known to precipitate from AMD

solutions at pH values of 5 or above (Nordstrom

et al., 1984; Nordstrom and Alpers, 1999a).

In summary, and in agreement with previous works

in other mine districts (e.g., Bigham et al., 1996; Lee

et al., 2002), a good correlation has been found between

pH and mineralogy of the MDM at the watershed

scale, with ferrihydrite precipitating at pH values

above 6, amorphous Al-oxyhydroxysulphate (probably

Table 9

Theoretical metal speciation in a ‘‘typical’’ AMD solution from

the IPB (exemplified by sample 938-112, see Tables 3–5 for a

detailed chemical composition), and calculations of the satura-

tion indices (SI) of selected Fe and Al mineral phases with

respect to this AMD fluid

Element Species Molality mol%

Al3+ 6.72E � 02

AlðSO4Þ�2 4.04E � 02 60

AlSOþ4 2.09E � 02 31

Al3+ 5.86E � 03 9

Fe3+ 2.23E � 03

FeSOþ4 1.49E � 03 67

FeðSO4Þ�2 5.35E � 04 24

Fe3+ 1.08E � 04 5

Fe(OH)2+ 7.23E � 05 3

FeHSO2þ4 1.61E � 05 1

Fe2+ 3.60E � 02

Fe2+ 2.11E � 02 59

FeSO4 1.46E � 02 40

FeHSOþ4 2.23E � 04 1

Mg2+ 9.92E � 02

MgSO4 4.93E � 02 50

Mg2+ 5.00E � 02 50

Mn2+ 4.93E � 03

Mn2+ 2.90E � 03 59

MnSO4 1.99E � 03 40

MnCl+ 3.79E � 05 1

Ca2+ 7.33E � 03

CaSO4 3.27E � 03 45

Ca2+ 4.02E � 03 55

Cu2+ 2.61E � 03

CuSO4 1.18E � 03 45

Cu2+ 1.43E � 03 55

Cd2+ 2.02E � 05

CdSO4 7.25E � 06 36

Cd2+ 5.86E � 06 29

CdðSO4Þ2�2 5.16E � 06 26

CdCl+ 1.86E � 06 9

Zn2+ 6.94E � 03

Zn2+ 3.04E � 03 44

ZnSO4 2.53E � 03 36

ZnðSO4Þ2�2 1.34E � 03 19

Saturation indices

Phase Formula logK SI

Dissolving minerals

Al(OH)3(a) Al(OH)3 11.06 �5.90

Alunite KAl3(SO4)2(OH)6 �1.34 �0.61

Basaluminite Al4(OH)10SO4 22.70 �9.41

Gibbsite Al(OH)3 8.34 �3.17

Jurbanite AlOHSO4 �3.23 �0.76

Fe(OH)3(a) Fe(OH)3 4.89 �1.68

Ferrihydrite Fe5HO8 Æ 4H2O 5.00 �1.79

Gypsum CaSO4 Æ 2H2O �4.58 �0.16

(continued on next page)

0

500

1,000

1,500

2,000

2,500

3,000

0 10 20 30 40 50SO4

= (g/L)

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

0 10 20 30 40 50SO4

= (g/L)

A

B

Mn (mg/L; r=0.83)

Cd (r=0.76)

As (r=0.20)

Cu (r=0.70)

Zn (r=0.87)

Al (r=0.90)

Al

Zn

Cu

As

Mn

Cd

mg/

Lµg

/L

Fig. 13. Binary plots with regresion lines and correlation

(Pearson-Product moment) values of SO2�4 with different

dissolved metals: (A) Al, Zn, Cu and (B) As, Mn, Cd.


basaluminite) at pH > 4, and a mineralogical assemblage

of schwertmannite–jarosite–goethite found in AMD

with pH between 2 and 4.

5.2.2. Iron precipitates and efflorescent sulphates as trace

metal sorbents

Iron colloidal precipitates such as schwertmannite or

ferrihydrite have been found to act as sorbents for trace

metal removal in AMD systems, with a high pH-depen-

dence of metal sorption (e.g., Smith, 1999). Sorption of

the less soluble elements like As or Pb on these mineral

phases can occur at pH around 4.5–5, whereas 100%

sorption of other cations like Cu, Zn or Cd only occurs

in circumneutral conditions (e.g., Smith, 1999; Lee et al.,

2002).

The analyzed mineral samples collected from AMD

sites showing a schwertmannite–jarosite–goethite min-

eral paragenesis always have As, Cu, Pb and Zn concen-

trations in the range of tens to a few thousand ppm

(Table 7). These values are higher than those observed

for the samples dominated by quartz and aluminosili-

cates (chlorite, feldspar, mica; Table 6), and suggest that

the high specific surface area of the Fe precipitates

would have favoured metal sorption.

Titration experiments conducted in AMD samples

from the area (unpublished data) have shown virtually

total As removal from solution during schwertmannite

Table 9 (continued)

Phase Formula logK SI

Precipitating minerals

Hematite Fe2O3 �3.70 10.12

Goethite FeOOH 0.64 2.56

K-Jarosite KFe3(SO4)2(OH)6 �14.49 6.66

Na-Jarosite NaFe3(SO4)2(OH)6 �10.84 5.16

Schwertmannite Fe8O8(SO4)(OH)6 18.00 11.30

Relative abundances of the different ionic species in molal

percentage. The logK values used for the SI calculation are

indicated for each mineral phase.


precipitation at pH 3.5. This is in agreement with the

high As contents found in the naturally occurring schw-

ertmannite (1700 ppm As on average; Table 7). Addi-

tional mass-balance calculations performed in the

Odiel river (unpublished data) suggest a considerable

retention of Zn and a practically complete removal of

Pb and Cu at pH � 8. This trace metal retention would

have resulted from adsorption by the abundant Fe and

Al colloidal phases produced after the mixing of the

Odiel river with several AMD discharges.

These results are in agreement with available data of

metal sorption on hydrous Fe(III) oxide in AMD sys-

tems (e.g., Webster et al., 1998; Smith, 1999; Lee et al.,

2002; Fukushi et al., 2003), and indicate that trace metal

retention by sorption onto Fe and Al colloidal precipi-

tates can be an effective mechanism for natural attenua-

tion of dissolved metal loads.

With regard to the chemistry of the salts, chemical

analyses of SO4 efflorescences show that these phases

can contain very high Zn and Cu concentrations (Table

8). These metals are sorbed and/or co-precipitated dur-

Fig. 14. Variation of the solubility of schwertmannite and basalumin

Ksp), where IAP is the ion activity product of the dissolved mineral co

been calculated with PHREEQC v. 2.0 (Parkhurst and Appelo, 1999)

5). The shaded areas correspond to the approximate pH ranges for th

ing the evaporative processes taking place in extremely

concentrated AMD pools. However, these sulphates

are only temporal sinks and can release high quantities

of acidity, SO4 and metals again to the water courses

during rainfall, causing a rapid pH decrease and a high

increase in dissolved solids, as described in other mine

districts (e.g., Alpers et al., 1994; Kimball, 1999; Nord-

strom, 1999). This process was observed during a field

visit performed on 1st October 2003, when the first rain-

fall discharge after the dry season dissolved in a few

hours around 50–90% (varying among the mine sites)

of the salts formed during the summer. This salt dissolu-

tion provoked significant pH decreases and conductivity

increases in the studied streams.

5.3. Environmental implications

5.3.1. Effects on water quality and calculation of metal

loads

It has been estimated that 29% (390 of 1360 km) of

the total length of streams and water courses in the Odiel

river watershed are affected to a variable degree by

AMD pollution (Fig. 16). This mine-related contamina-

tion, which affects about 85% of the Odiel river main

course (104 of 122 km), is evident in the Odiel river

mouth, where pH values of 3.0 ± 0.5, and average (med-

ian) metallic contents of 16 mg/L Fe, 12 mg/L Mn, 7 mg/

L Cu, 20 mg/L Zn and 66 lg/L Cd, are usually measured

(data from the period 1990–2001; COCA network; Con-

federacion Hidrografica del Guadiana, unpublished

report).

Based on measured chemical compositions and water

flow of the AMD discharges, SO4 and metal loads have

been calculated for the studied mining districts in the

ite with pH. The saturation index (SI), defined as SI = log(IAP/

nstituents, and Ksp is the solubility product for the mineral, has

, considering the initial conditions of sample 938-112 (Tables 3–

e typical ferric and aluminous AMD. See text for explanation.

-20

-15

-10

-5

0

5

10

0 2 4 6 8 10 12

Distance (km)

SI

Gib

Alun

Basal

Jur

B

mixing

-20

-15

-10

-5

0

5

10

15

20

0 2 4 6 8 10 12

Distance (km)

SI

Schw

Ferr

K-JarNa-Jar

Goet

Hem

Basal

Gib

AAluminium phases

Iron phases

pH=4.5 pH=2.7

pH=2.7

Fig. 15. Variation of the solubility (given by the saturation index, SI) of several Fe and Al mineral phases in AMD with distance. Both

profiles are plotted in a downstream direction from the respective discharge points of AMD. (A) Acid leachate from a waste-rock pile

in Corta-Atalaya; (B) outflow from embalse del Cobre (Rio Tinto mines). In (A), the SI profiles of the Fe phases are plotted as solid

lines, whereas the aluminous phases are plotted as dashed lines. The dotted line of SI = 0 separates the fields of precipitation (SI > 0)

and dissolution (SI < 0). In (B), the abrupt change in the SI is due to mixing of the AMD with a more acid (pH 2.7) AMD stream.

Calculations performed with PHREEQC v. 2.0 (Parkhurst and Appelo, 1999) from chemical analysis of sucessive sampling points at

each AMD discharge. See text for explanation. Abbreviations: Schw, schwertmannite; Jar, jarosite; Ferr, ferrihydrite; Goet, goethite;

Hem, hematite; Basal, basaluminite; Gib, gibssite; Alun, alunite; Jur, jurbanite.


IPB for the period 2003–2004 (Table 10). These estima-

tions have been performed considering two different sce-

narios (winter and summer), as yearly average values for

chemical composition and flow rate are highly unrealis-

tic, as discussed previously.

As can be deduced from Table 10, Tharsis and Rıo

Tinto (Corta Atalaya) are, by far, the mine sites that

most profusely contribute to AMD volume, and SO4

and metal loads transferred to the Odiel fluvial system,

followed by San Telmo and Sotiel-Almagrera. These 4

mine districts can represent around from 70% (winter)

to 90% (summer) of the total SO4 and metal loads re-

ceived by the Odiel river watershed from AMD

discharge.

Also, from simple mass-balance calculations, it can

be deduced that around 99% of the Fe and As, and from

60% to 80% of other metals (Al, Mn, Cu, Zn, Cd, Pb)

and SO4 transported by the acid mine effluents as dis-

solved load, would have been retained within the Odiel

basin (Table 10). Thus, the great majority of metal loads

released from the mine sites and transported by the acid

mine waters to the streams and tributaries of the Odiel

river, should essentially remain in the water courses in

the form of solid phases like schwertmannite, jarosite,

ferrihydrite, basaluminite or efflorescent sulphates.

However, most of these minerals are unstable and can

be either transported as colloids downstream during

high flow conditions or, on the other hand, they can

be transformed to more stable minerals (e.g., goethite)

and form cemented and more permanent chemical

sediments.

These results suggest that only 1% of the total Fe and

As dissolved load, and around 20–40% of the total Al,

Mn, Cu, Zn, Cd, Pb and SO4 loads, would have been

transferred from the Odiel river basin to the Huelva

estuary. However, given the water volumes implied in

Fig. 16. Metal-affected map of the Odiel river basin. The total length of streams affected by AMD pollution, and the respective

dissolved metal loadings reported in Table 10 for the different sub-basins are indicated. Daily metal loadings represent the situation

observed in winter 2003. See text for explanation.


this fluvial system, this still represents impressive metal

fluxes. Thus, during the winter months of 2003, an aver-

age of around 4 t of Al, 9 t of Mg, 0.9 t of Mn, 1.4 t of

Zn, 0.6 t of Cu, 0.2 t of Fe, 9 kg of Cd, 6 kg of Pb and

0.5 kg of As, would have been daily transferred as dis-

solved load to the estuarine system. If added to the sum-

mer loads, these data imply a total yearly load of about

3760 t of dissolved metals for 2003–2004.

Table 10

Calculation of sulphate and metal loads transferred by AMD discharge from the mine sites of the IPB to the stream waters of the Odiel river basin

Mine district Season Q (L/s) Sulphate and metal loads

SO4

(Tn/day)

Fe

(kg/day)

Al

(kg/day)

Mg

(kg/day)

Mn

(kg/day)

Cu

(kg/day)

Zn

(kg/day)

As

(g/day)

Cd

(g/day)

Pb

(g/day)

Upper Odiel 184 9759 10,731 14,711 1845 2022 3213 7723 23,425 4274

Angostura Winter 0.4 0.2 4 2 3 0.1 0.7 0.2 0.1 1.6 1

Summer 0.2 0.1 2 1 2 0.1 0.4 0.1 0.1 1 0.6

Concepcion Winter 12 1.1 160 57 18 1.3 5 3 16 21 10

Summer 4 0.8 93 48 21 3 3 48 10 10 7

Corta Atalaya (Rio Tinto) Winter 350 172 6760 10,200 14,370 1820 1872 3100 6020 23,000 4200

Summer 50 61.2 3240 3167 1776 825 570 1110 280 3730 40

El Soldado Winter 2.5 0.2 33 7 7 0.2 1.1 2 7 9 19

Summer* 0.9 0.1 14 3 3 0.1 0.5 0.9 3 4 8

Esperanza Winter 2 0.7 159 29 19 0.6 7 5 3 9 12

Summer 0.6 0.2 64 10 14 0.3 2 1 20 4 2

La Poderosa Winter 8 3.6 960 138 36 0.7 104 52 600 330 2

Summer 2 1.1 295 45 16 1 28 17 630 70 3

San Miguel Winter 11 5.2 1420 255 243 21 20 12 870 0.7 27

Summer 0.8 0.1 42 6 3 0.8 0.2 6 13 1 0.7

San Platon Winter 2 1.0 263 43 15 1 12 39 207 54 3

Summer 0.2 0.1 35 4 2 0.2 0.5 4 19 7 0.2

Lower-middle Odiel 41 1821 305 479 98 55 147 2556 69 12

Campanario Winter 1 0.2 67 5 8 2 1 5 976 4 2

Summer – – – – – – – – – – –

Castillo-Buitron Winter* 2 0.5 190 26 10 2 10 14 n.a. n.a. n.a.

Summer 0.7 0.2 79 11 4 1 4 6 n.a. n.a. n.a.

Descamisada Winter 4 0.6 267 21 33 31 3 9 600 22 6

Summer 0.2 0.02 6 0.5 1 0.7 0.1 0.3 21 0.5 0.2

Gloria Winter* 0.3 0.02 5 1 1 0.2 2 0.1 2 0.2 0.1

Summer 0.1 0.01 2 0.4 0.4 0.05 0.7 0.03 0.7 0.07 0.03

La Torerera Winter 1 0.05 2 1 3 0.8 0.2 0.6 13 1 3

Summer – – – – – – – – – – –

Sotiel-Almagrera Winter 125 38 1040 230 350 43 32 90 n.a. n.a. n.a.

Summer 20 3 43 10 14 2 1 4 n.a. n.a. n.a.

Tincheron Winter 2 0.3 20 4 30 5 0.5 0.5 15 1 1

Summer 0.3 0.05 4 0.7 6 1 0.1 0.1 3 0.2 0.2

Tinto-Sta Rosa Winter 5 1 230 17 44 14 7 27 950 41 1

Summer 3 0.5 45 5 25 8 4 15 398 17 0.1

Olivargas 3 670 105 88 15 13 156 397 349 4001

Aguas Tenidas Winter 3 0.2 52 12 16 2 4 16 73 60 43

Summer – – – – – – – – – – –

Cueva de la Mora Winter 3.5 1.1 208 38 47 5 4 132 n.d. 260 3950

Summer 1.6 0.4 71 17 25 3 0.4 17 n.d. n.d. n.d.

La Zarza Winter 2 1.4 410 55 25 8 5 8 324 29 8

Summer 0.5 0.1 114 13 19 4 1 2.5 103 7 1

Monte Romero Winter – – – – – – – – – – –

Line missing

J.Sanchez

Espanaet

al./Applied

Geochem

istry20(2005)1320–1356

1351

Summer – – – – – – – – – – –

(continued on next page)Table 10 (continued)

Mine district Season Q (L/s) Sulphate and metal loads

SO4

(Tn/day)

Fe

(kg/day)

Al

(kg/day)

Mg

(kg/day)

Mn

(kg/day)

Cu

(kg/day)

Zn

(kg/day)

As

(g/day)

Cd

(g/day)

Pb

(g/day)

Oraque 64 10,842 2802 5582 563 395 1310 38,063 3334 950

Confesionarios Winter 15 5.2 1950 233 242 14 2 6 700 240 10

Summer* 3 1.3 507 61 63 4 0.5 2 180 62 3

El Carpio Winter 1.6 0.3 41 10 14 1 5 3 69 12 2

Summer – – – – – – – – – – –

El Perrunal Winter 1 0.16 1.1 5.3 14 4 0.3 0.7 4 1 1

Summer 0.3 0.23 1 1.5 6 1 0.1 0.2 0.5 0.4 0.5

Lomero-Poyatos Winter 6 3.0 360 52 323 7 4 12 200 20 400

Summer 2.5 0.9 115 17 109 2 1 3 50 8 140

San Telmo Winter 363 12.1 1966 385 820 120 76 263 650 1000 200

Summer 3 1.0 32 31 118 11 5 22 3 40 13

Tharsis Winter 50 433 6523 2117 4169 417 307 1024 36,440 2061 337

Summer 10 13 1960 631 1225 125 89 306 11,032 610 93

Meca 12 1973 275 540 63 50 97 5550 1779 6463

Tharsis Winter 25 12.01 1973 275 540 63 50 97 5550 1779 6463

Summer 2 1 392 127 235 21 15 53 1700 122 19

Total AMD 2003–2004 Winter 671 303 25,064 14,218 21,399 2584 2535 4922 54289 28,956 15,700

Summer 106 85 7157 4210 3687 1014 726 1618 14,466 4694 331

Odiel outlet at Gibraleon (pH 4.2) Winter 2700a 96 224 4043 8986 900 642 1350 450 9435 5616

2003–2004 (pH 3.3) Summer 450a 24 77 1080 2565 350 156 430 156 2020 –

Dissolved metal loads retained in Basin (%) 2003–2004 Winter 68 99 72 58 65 75 73 99 67 64

Summer 72 99 74 30 65 79 73 99 57 –

Two different scenarios (winter and summer seasons) are considered. These calculations have been performed considering the chemical data provided in Tables 3 and 4 from all the mine sites studied in the present work, and are subdivided into

different sub-basins (Upper and lower-middle Odiel, Olivargas, Oraque, Meca). In order to evaluate the rate of basin-scale retention of sulphate and metals dissolved in AMD, the total amount of metal and sulphate loadings provided by

1352

J.Sanchez

Espanaet

al./Applied

Geochem

istry20(2005)1320–1356


5.3.2. Duration and future evolution of the acid mine

drainage

Some of the studied acid emissions have been ema-

nating from underground mines abandoned since the

early 20th century (e.g., El Soldado and Campanario

in 1917, La Poderosa in 1924, Esperanza and Tinto-

Sta Rosa in 1931, Aguas Tenidas and San Platon in

1934) and late 19th century (e.g., Descamisada in

1876, Confesionarios in 1888; Pinedo Vara, 1963). These

remain very acid and have high SO4 and metal loads.

The antiquity of the AMD systems has led to significant

landscape transformations such as: (1) cemented layers

and laminated, algal mat Fe(III) formations of consider-

able thickness (10–50 cm) which record hydrologic epi-

sodes (wet/dry seasons) and (2) elevated terrace

deposits formed by hydrous Fe oxides in the confluence

between AMD and natural streams.

At present, it is difficult to predict whether or not the

AMD chemistry is undergoing some kind of long-term

temporal evolution, which could result in an improve-

ment of stream water quality at the basin-scale. Unfor-

tunately, there is no historical record of water quality

data for the cited mine discharges with which to com-

pare the current chemical signatures, and therefore, it

is not possible to decide if a significant decrease in the

acidity and/or metal contents is taking place at a regio-

nal scale. In this sense, several studies have reported lit-

tle chemical change of acid effluents emanating for

hundreds of years, whereas others state that poor drain-

age quality may last only 20–40 a (Demchak et al.,

2004).

In any case, further research on the studied AMD

discharges has to be performed in the forthcoming years

in order to obtain a realistic model of prediction for

water quality evolution at the basin scale, which could

serve as a basis for future reclamation and/or restoration

plans in the Odiel river basin.

6. Conclusions

The acid mine waters produced by the oxidation of

pyritic mine wastes and massive mineralizations in the

mines of the IPB show marked seasonal variations in

flow rate and chemical composition, in response to cli-

matic, hydrogeological, geological, and mineralogical

factors. This variability in the AMD hydrogeochem-

istry allows recognition of very different conditions of

pH, Eh, DO, oxidation rate of Fe(II) and metal con-

tent among the studied acid effluents. This fact has

strong implications as it introduces additional difficul-

ties into the design of corrective measures at the mine

sites.

Iron is several orders of magnitude more abundant

than any other redox species (such as As(III)/As(V) or

O2/H2O) in the acid mine waters. Consequently, the re-

dox chemistry of the AMD systems is basically defined

by the oxidation of Fe(II), which seems to be strongly

catalyzed by acidophilic, Fe-oxidizing bacteria and is

perfectly correlated with the electronic potential (Eh)

of the acid solutions.

The acidity of AMD is not only given by the concen-

tration of H+, but also by the content of Fe(III) and Al,

whose hydrolysis (around 2.7 and 4.5, respectively) re-

leases important amounts of additional acidity and pro-

vokes the buffering of the AMD systems.

As a result of the rapid oxidation and subsequent

hydrolysis of dissolved Fe, a number of meta-stable

and poorly crystallized Fe mineral phases (mostly schw-

ertmannite, with additional jarosite and/or goethite) are

formed. The mineralogy of these mineral phases is

strongly dependent on the water pH, so that it reflects

the geochemical evolution of these acid sulphate waters.

Thus, schwertmannite (± jarosite ± goethite) precipi-

tates in the vicinity of the discharge points at pH 2–4;

when most of the Fe has been precipitated, amorphous

to poorly crystalline Al-oxyhydroxysulphates form at

pH 4.5; finally, ferrihydrite is usually favoured to precip-

itate at circumneutral pH in the stream courses. Among

the solid phases forming in the AMD systems, schwert-

mannite plays a major role: (1) in the control of Fe sol-

ubility and (2) in the retention of trace elements (As, Pb,

Cu).

The evaporation of AMD causes the precipitation of

hydrated sulphate salts which are temporal reservoirs of

acidity, SO4 and metals.

From mass-balance calculations, it has been shown

that the majority of the SO4 and metal loads released

from the mine sites by the acid effluents would substan-

tially remain in solid form within the Odiel basin. How-

ever, the remaining dissolved loadings that reach the

Huelva estuary still imply impressive metal amounts of

several thousand tonnes (specially the most soluble met-

als such as Mn, Zn, Cd and Cu) which are being yearly

transferred to the estuarine system, and finally, to the

Atlantic Ocean.

Acknowledgements

We sincerely thank Juan Antonio Martın Rubı,

from the IGME laboratory, for his attention dedicated

during the XRD analyses. Ron Fuge, K. Hudson-Ed-

wards and an anonimous reviewer are thanked for their

helpful comments on an earlier version of this manu-

script. This project has been finantially supported with

funds from the CICYT Project No. REN2003-09590-C

O4-04, and from the Junta de Andalucıa (Consejerıa de

Empleo y Desarrollo Tecnologico). The authors dedi-

cate this work to the loving memory of the 192 people

killed by the terrorist bombings in Madrid on 11-

March 2004.


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