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Geochemical and environmental controls on the genesis of solubleeforescent salts in Coastal Mine Tailings Deposits: A discussion based onreactive transport modeling

S.A. Bea a,, C. Ayora a, J. Carrera a, M.W. Saaltink b, B. Dold c

a Institute of Environmental Assessment and Water Research (IDAEA), CSIC, c/Lluis Sol Sabars, s/n, 08028 Barcelona, Spainb Technical University of Catalonia (UPC) c/Jordi Girona 1-3, 08034, Barcelona, Spainc Institute of Applied Economic Geology University of Concepcin, Victor Lamas 1290, 4070386 Concepcin, Chile

a r t i c l e i n f o a b s t r a c t

Article history:

Received 19 April 2009

Received in revised form 30 November 2009

Accepted 11 December 2009

Available online 24 December 2009

Water-soluble eforescent salts often form on tailings in hyperarid climates. Their high

solubility together with the high risk of human exposure to heavy metalssuch as Cu, Ni,Zn, etc.,

makes this occurrence a serious environmental problem.

Understanding their formation (genesis) is therefore key to designing prevention and

remediation strategies.

A signicant amount of these eforescences has been described on the coastal area of Chaaral

(Chile). There, highly soluble salts such as halite (NaCl) and eriochalcite (CuCl22H2O) form on

4 km2 of marine shore tailings. Natural occurrence of eriochalcite is rare: its formation requires

extreme environmental and geochemical conditions such as high evaporation rate and low

relative air humidity, and continuous Cl and Cu supply from groundwater, etc. Its formation

was examined by means of reactive transport modeling.A scenario is proposed involvingsea water andsubsequently a mixture of seawater/freshwater

in the groundwater composition in the formation of these eforescences. The strong

competition from other halides (i.e. halite and silvite (KCl)) for the Cl may inhibit the

precipitation of eriochalcite. Therefore, the Cl/Na ratio trend N1 is a key parameter in its

formation. Cation-exchange between Na+ and other major ions such as K+, Ca2+, Mg2+ and

Cu2+ in the clay fraction of tailings is proposed to account for realistic Cl/Na ratios.

With regard to preventing the formation of eriochalcite, a capillary barrier on the tailings

surface is proposed as a suitable alternative. Its efciency as a barrier is also tested by means of

reactive transport models.

2009 Elsevier B.V. All rights reserved.

Keywords:

Reactive transport modeling

NaClCuCl2system

Pitzer

Tailings

MultiphaseowUnsaturated zone

Water-soluble eforescences

1. Introduction

Water-soluble eforescent salts often form on tailings in

arid climates. Their formation on the surface increases the

risk of human exposure to heavy metals such as Cu, Ni, and Zn

through wind transport. Signicant amounts of eforescence

salts (mainly halite (NaCl), eriochalcite (CuCl22H2O) and

gypsum (CaSO42H2O)) form on 4 km2 of a porphyry copper

tailings deposit in the coastal area of Chaaral (Chile, Fig. 1A).In 1983, the Chaaral contamination case was classied by

the United Nations Environmental Programme (UNEP) as one

of the most serious cases of contamination in the Pacic area.

Despite the fact that Cl and Cu are widespread elements in

the Earth, eriochalcite (also termed antofagastite, eriocalcite or

erythrocalcite) is a highly soluble salt, commonly obtained in a

laboratory, but rarely occurring under natural conditions.In fact,

eriochalcite has never before been reported as eforescence in

tailings (Dold, 2006). Its natural occurrence is mainly asso-

ciated with fumaroles of volcanoes, e.g. Mt. Vesuvius (Italy)

or associated with copper mineralization, e.g. Mina Quetena,

Journal of Contaminant Hydrology 111 (2010) 6582

Corresponding author. University of British Columbia, 6339 Stores Road,

Vancouver, Canada V6T 1Z4, BC. Tel.: +1 604 827 5607; fax: +1 604 822 9014.

E-mail address:[email protected](S.A. Bea).

0169-7722/$ see front matter 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jconhyd.2009.12.005

Contents lists available at ScienceDirect

Journal of Contaminant Hydrology

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j c o n h yd
mailto:[email protected]://dx.doi.org/10.1016/j.jconhyd.2009.12.005http://www.sciencedirect.com/science/journal/01697722http://www.sciencedirect.com/science/journal/01697722http://dx.doi.org/10.1016/j.jconhyd.2009.12.005mailto:[email protected]

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west of Calama, Chile. In the latter case, eriochalcite is

accompanied by other copper halides such as blandylite

(CuB2

O4

CuCl2

4H2

O) and atacamite (CuCl2

3Cu(OH)2

) in a

leached zone above massive iron suldes(Palache and

Foshag, 1938). Previous works dealing with the environ-

mental problem in Chaaral include those devoted to the

impact on the local and regional marine fauna (Castilla,

1983; Farina, 2001; Lee and Correa, 2005; Lee et al., 2001),

and the study of mobilization of contaminant elements such

as Cu, Zn and Ni, and oxyanions such as As and Mo (Dold,

2006). Environmental and geochemical conditions are parti-

cular for formation of the eriochalcite. A range of water

activity between 0.61 and 0.68 was measured in a solution

saturated in halite and eriochalcite (Filippov et al., 1986). As

a consequence, their natural occurrence is restricted to

extremely arid climates such as the Atacama region in Chile.Moreover, a continuous supply of Cl and Cu is necessary.

Thus, this limits their formation, for instance, to coastal areas

(Cl supply) and to cupric tailings (Cu supply). A priori, these

particular environmental and geochemical conditions are

important for the precipitation of eriochalcite. However, its

rare natural occurrence suggests complex mechanisms in its

formation.

Reactive transport modeling in a vadose zone is relatively

uncommon. This may be attributed to the lack of codes

necessary to deal with multiphase ow associated with

geochemical reactions, and also to the difculty encountered

in obtaining reliable sets of eld or experimental data to

compare with model results. Therefore, the majority of earlier

works to develop reactive transport models for the vadose

zone of mine tailings have assumed time-invariable boundary

conditions and thermohydraulic properties and have ap-

proached it as a single reactor (Bain et al., 2000; Gerke et al.,1998; Gerke et al., 2001; Mayer et al., 2002; Wunderly et al.,

Fig. 1.(A) Location map of Chaaral's tailings. (B) A sketch of geochemical zones in Chaaral tailings. Three geochemical zones are observed: (1) an evaporation

zone mainly formed by eforescences, (2) an oxidation zone characterized by secondary ferric mineralogy (i.e. goethite and K-jarosite), (3) a primary zone

preserving the initial mineralogy of the tailings.

66 S.A. Bea et al. / Journal of Contaminant Hydrology 111 (2010) 6582

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1996). In those studies which have takenuctuating bound-

ary conditions or thermohydraulic properties into account,

a simplied reactive transport problem (usually with pyrite

as the only mineral phase present in the system) has been

considered (Lefebvre et al., 2001a; Lefebvre et al., 2001b; Xu

et al., 2000). In some cases, the developed models have been

used as a tool for predicting the long-termevolution of sulde

wastes. The objective of this paper is to propose the probable

mechanisms in the origin of eriochalcite, so as to design the

best remediation measures. In nding the hypothesis that

best explains the observed eforescences, we will shed light

on the climatic and geochemical conditions that favor preci-

pitation of eriochalcite. To accomplish this, different scenarios

based on the predominance of a particular groundwater and

solute source were simulated. The results of the calculations

were compared with chemical analysis of interstitial water

and sequential extractions of the solid phase from boreholes

in Chaaral (Dold, 2006). The analysis of the multiphaseow,

dissolution/precipitation of the eforescent crust, and sulde

oxidation was carried out by means of reactive transport

modeling. The atmospheric boundary conditions set in the

model account for the hyperarid features of the climate.

2. Site description

2.1. Ore geology

The Chaaral deposits were otation tailings from the El

SalvadorPotrerillos mining district (Fig. 1A). The Potrerillos

porphyry copper deposit was exploited until 1959. Produc-

tion in El Salvador started that year. Therefore, it can be

assumed that the lower portion of the Chaaral tailings comes

from Potrerillos and that they are overlain by tailings from

El Salvador (Dold, 2006).

The Potrerillos deposit (1.8106 t Cu) is located in the

Atacama Desert, 120 km east of Chaaral. It is characterized

by a granodiorite intrusion affected by three hypogene alter-

ation and mineralization events and an important supergene

alteration. The early potassic and propylitic alterations are

responsible for the K-feldspar, biotite, chlorite, quartz, ankerite,

and anhydrite content. Associated suldes are chalcopyrite,

bornite, pyrite, molybdenite, minor enargite, sphalerite, and

galena (Camus, 2003).

The El Salvador deposit (5.7106 t Cu) is located about

100 km east of Chaaral. Primary mineralization is char-

acterized by granodiorite with quartz veins and a matrix of

K-feldspar, hornblende, biotite, minor oxides, anhydrite and

suldes (Gustafson and Hunt, 1975; Gustafson and Quiroga,

1995).

2.2. Mineralogy of tailings

Tailings grain size varies between silty/clayey to sandy

toward the sea. A sketch of geochemical zones in the Chaaral

tailings is shown in Fig. 1B. The tailings surface presents

abundant greenish-blue and white eforescent salts. XRD

analysis revealed that these were mainly halite (NaCl) and

eriochalcite (CuCl2 2H2O) (Dold, 2006).

The unsaturated zone is characterized by oxidation. Its

depth varies between 0.73 and 1.88 m, with pH ranging from2.5 to 4, underlain by a primary zone of neutral pH. The

oxidation zone is characterized by secondary ferric min-

eralogy with a typical yellowish-brown color (dominated by

K-jarosite) with ochre to orange streaks dotted with goethite.

Gypsum is also present. Clay mineralogy is made up of illite,

kaolinite and a vermiculite-type mixed-layer, the latter re-

sulting from biotite alteration (Dold, 2006; Dold and Fontbote,

2001). Mineralogy from the primary zone consists of quartz,

plagioclase, muscovite, biotite, kaolinite, magnetite and rutile.

In addition, pyrite with minor chalcopyrite and chalcocite are

also observed.

Sequential extractions are widely used for exploration

purposes and for the study of element speciation in tailings. A

seven-step sequence was applied to samples collected at

Chaaral (Dold, 2003). Sequential extractions for Cu, Na, Ca

and Mg, in samples corresponding to the rst meter of

borehole CH1 are shown inTable 1. They showed a high level

of Cu enrichment in the water-soluble fraction in the

evaporation zone at the top of the tailings. This is consistent

with the abundance of eriochalcite described in the previous

section. Thus, eriochalcite content was estimated to be

around 30 mol m3. However, a large fraction of Cu was

associated with clay minerals (probably adsorbed on illite or

as an interlayer ion-exchange cation in the vermiculite-type

mixed-layer), or adsorbed to the surface and/or co-precipi-

tated with Fe(III)-oxides and hydroxides (see Cu(ex) and Cu

(ox) inTable 1, respectively). It is important to note that only

a very small amount of Cu remains in the sulde fraction,

indicating that most of the original chalcopyrite and chalco-

cite has oxidized and dissolved in the last thirty-two years

(see Cu(sph) inTable 1).

Na is also present in the water-soluble fraction (mainly as

halite) at the top (around 700 mol m3, see Na(sol) in

Table 1). However, a signicant portion of the Na is found in

the residual phase (around 440 mol m3, see Na(r) in

Table 1). The sequential extractions indicate a halite/

eriochalcite ratio in eforescences of about 20.

2.3. Hydrological setting

The west coast of South America, from northern Peru to

Central Chile, is extremely arid (Fuenzalida, 1950). Aridity

results from a combination of subsidence generated by a

permanent high-pressure area over the Pacic Ocean and the

Table 1Sequential extractions in mol for Cu, Na, Ca and Mg, in samples

corresponding to the rst meter of borehole CH1 (Dold, 2003). sol = soluble

fraction (Step I), ex = exchangeable fraction (Step II), ox = Fe(III) oxides

and hydroxides fraction (Steps III and IV), sg = organic matter and Cu-

supergene fraction (Step V), sph = primary suldes fraction (Step VI), r =

residual fraction (Step VII) and t = total. A halite/eriochalcite ratio to 20

could be estimated according to Na(sol)/Cu(sol).

Step Cu Na Ca Mg

I (sol) 33.77 684 152.56 59.0

II (ex) 10.18 16.39 20.48 6.45

III and IV (ox) 9 20.57 7.85 33.16

V (sg) 0.4 6.82 4.4 6.45

VI (sph) 0.68 20.59 10.62 47.95

VII (r) 1.01 442.15 63.9 120.73

Total (t) 53.63 1190.6 259.8 273.64

67S.A. Bea et al. / Journal of Contaminant Hydrology 111 (2010) 6582

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atmospheric stability induced by the cold northward-owing

Humboldt Current.

The climate in the coastal area of the Atacama region is

classied as arid with abundant clouds (Fuenzalida, 1950;

Fuenzalida, 1965). On an annual average 28% of the days are

cloudy, 60% are partially cloudy and only 12% display clear

skies. A cloudy sky means less solar radiation on the tailings

surface and reduced evaporation.

Average precipitation of 30 mm per year was recorded for

Chaaral (Larrain et al., 2002). Therefore, the hydrological

system of coastal tailings deposits at Chaaral is mainly

controlled by tides, an inow of brine originating from Salar

de Pedernales, and leakage from freshwater supply which

feeds the village of Chaaral (Dold, 2006; Wisskirchen et al.,

2006). The temperature in this coastal area is characterized

by low variation between summer and winter and between

day and night. (Thompson et al., 2003) provided a temporal

record of temperature and relative humidity in Pan de Azcar

Park, located 20 km north of Chaaral. There, ve meteoro-

logical stations were established at different elevations

(between 210 m and 800 m) and distances from the Pacic

Ocean. Temperature and relative humidity were measured

between June, 1999 and October, 2000 in a station located

close to the Pacic Ocean. The variation in temperatures for

the entire period does not exceed 10 C (approximately 21 C

and 13 C for summer and winter, respectively), and the

Vapor Pressure Decit (VPD) was highest during the summer

(at 0.8 to 1.0 kPa), though not particularly high for desert

ecosystems. Peak temperatures are observed in the months of

January and February. The predominantly WSW winds form

sand dunes, up to 1.5 m high, which migrate over the tailings

surface toward the village of Chaaral. The portion of the

tailings surface not covered by dunes displays abrasion

marks, concordant with the predominant wind direction.

2.4. Hydrogeology

The distribution of major ions (see Na, K, Ca, Mg, Cl and

SO4 in Fig. 2A) suggests that three different groundwater

types were mixed in these tailings deposits (Fig. 2B)

(1) Brine ows through the deepest portion of tailings. It

may have originated at Salar de Pedernales, 150 km

east of Chaaral (Dold, 2006). Therefore, it ows below

the surface along its riverbed in the El Salado valley

and discharges into the ocean.

(2) A lens of freshwater overlies the brine. These fresh-

waters may have originated from leaks in Chaaral's

freshwater supply (Wisskirchen et al., 2006).

(3) Sea water, less dense than the brine described above.

In Chaaral, the ocean level varies approximately 1 mduring the tidal cycle.

2.5. Aqueous chemistry

Three piezometer nests (CH1, CH2 and CH3) were

installed to a maximum depth of 9 m, along a prole in the

southern part of the tailings deposits, directly in the west of

Fig. 2.Aqueous chemistry distribution and hydrogeology. (A) Vertical proles of pH and major ions (K, Ca, Mg, Cl and SO 4) corresponding to borehole CH1 (its

location is shown inFig. 1A). (B) Hydrogeology: three different water types are identied in the hydrogeologic system: (1) a brine in the deepest part of thetailings, (2) a lens of freshwater, and (3) sea water.


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the Chaaral village (seeFig. 1A). The 20 pore-water samples

were analyzed for major chemical components (Na, K, Mg, Ca,

Cl, NO2, NO3, and SO4), and trace metals (Fe, Al, Cd, Cu, Mn,

Zn, Pb, V, Cr, Co, Ni, As, Se, Sr, Mo, Ba, Ti, and U) by (Dold,

2006).

Vertical proles for pH and major chemical components

corresponding to borehole CH1 are shown in Fig. 2A. Pore-

water pH ranged between2.6 and 4 in the oxidation zone,and

increased sharply to neutral values below the oxidation front.

Furthermore, redox potential (Eh) reached 600 mV in the

oxidation zone and dropped to 200 mV in the reducing

environment of the saturated zone. Majorions such as Ca, Mg,

Na, Cl and SO4increase their concentrations in the oxidation

zone of these tailings, whereas K decreases its concentration

in the same zone.

3. Conceptual and numerical model

A conceptual model for the thermohydraulic and geo-

chemical processes involved in the formation of eriochalcite

is shown inFig. 3.

3.1. Thermohydraulic model

Several atmospheric processes are considered in the

model: rain, evaporation, radiation and heat exchange

between tailings/atmosphere. Water ux (jw, k g r s1)

through the tailings surface results from

jw = P+ E+ jwg +jsr 1

wherePis rainfall, Eevaporation,jgw

ux of water vapor and

jsr of the surface runoff. The evaporation rate in Eq. (1) is

given by an aerodynamic diffusion relation according to

E= fva;za;z0eae

t 2

wheree

a

and e

t

are vapor pressures in the atmosphere andpore tailings, respectively, fis a constant of proportionality,

which takes into account the wind speed (va, m s1), the

roughness length (z0, m), and the screen height at which

va and ea were measured (za, m). Vapor pressure in the

atmosphere (ea) is computed from its relative humidity

(Hr) and temperature (Ta) through the psychometric law

(Edlefson and Anderson, 1943). However, the vapor pres-

sure in pore tailings (et) depends on its temperature (Tt),

salinity (through water activity), and the capillary proper-

ties of tailings.

On the other hand, the energy ux (Je, J m2 s1) consists

of sensible heat (Hs), latent heat (Hc) and radiation (Rn)

uxes. As with evaporation, Hs is a function of air tempera-ture, wind speed and the specic heat of the gas. Hc is equal to

the evaporation rate times the latent energy of vapor. Rn is

calculated according to direct solar short-wave radiation,

long-wave atmospheric radiation, the albedo of the tailings

surface, etc. The calculation of the direct solar short-wave

radiation takes into account seasonal variations, local latitude

and the cloud index (this index represents completely clear to

completely cloudy conditions). The daily variation inRnis not

considered in the model for the purposes of simplicity.

Fig. 3.Flux, transport, geochemical and energy balance conceptual model for Chaaral tailings.


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The low relative humidity during the summer causes eva-

poration, making water ow upwards. Solute concentrations

in the upowing water are dramatically increased, with sat-

uration resulting in some salts forming an eforescent crust.

As evaporation progresses, pore-water content is reduced,

preventing liquidux from rising to the surface. Therefore,the

evaporation front is displaced slightly downwards, and efo-

rescences continue to grow within the pore volume. This

causes the tailings top to become increasingly cemented by

salts. The associated reduction in permeability favors an in-

crease in downward vapor diffusion. Vapor condenses at depth

and concentrations decrease.

The model is conceptualized as a 1D clayey tailings ac-

cording to grain size described for borehole CH1 by (Dold,

2006). Physical parameters and constitutive laws used in the

simulations are shown in Table 2. The 1D column is open

at the top and bottom. Atmospheric boundary conditions are

applied at the top, whereas prescribed gas and liquid pres-

sures (approximately 0.1 MPa) are applied at the bottom in

order to simulate the groundwater level.

As for the atmospheric parameters, maximum and minimum

mean values for temperature (Ta) and relative humidity (Hr)

are calculated over a period of thirty-two years (19752007)

from data reported by (Thompson et al., 2003). The relative

humidity varies between 0.63 and 0.74 and the temperature

between 21 C and 13 C from summer to winter, respec-

tively. Thus, sinusoidal functions between these values are

simulated. On the other hand, the mean value for temper-

ature (17 C) is set at the bottom of the model.

As laid out above, the wind speed and a clear/cloudy sky

affect the evaporation rate on the tailings surface. Thus, a

constant wind speed of 4 m s1 and a partially cloudy sky are

considered in the model.

3.2. Geochemical model

In parallel to the reduction of liquid saturation, oxygen

(O2(g)) diffuses downwards, activating oxidation reactions.

Oxidation of pyrite (FeS2) and chalcopyrite (CuFeS2) pro-

duces sulfate, metals and acidity. The initial pathway consists

of oxidation of the disulde to sulfate by O2(aq). The second

pathway for sulde oxidation is by reaction with Fe(III). Both

pathways are fast. They yield a low pH and Fe(II), which may

become oxidized by O2(aq) to Fe(III). This oxidation is known to

beveryslow atlow pH(Singer and Stumm, 1970) except in the

presence of microorganisms, which can increase the rate of Fe

(III) production by up to six orders of magnitude.

The decrease in pH produced by pyrite and chalcopyrite

oxidation increases the dissolution rate of accompanying

silicates, which become the main source of alkalinity in these

environments.

The evolution of tailings impoundments is also controlled

by the precipitation of secondary phases into the pores

of waste materials and/or over their surface. Thus, the SO4,

Fe(III) and K generated from dissolution are consumed

by gypsum (CaSO42H2O), K-jarosite (KFe(SO4)2(OH)6) and

goethite (FeO(OH)).

Furthermore, cation-exchange could occur on the clay

fraction of these tailings (e.g. on the vermiculite-type mixed-

layer and illite). Major cations such as Ca2+, Mg2+, Cu2+, Na+

and K+ could be exchanged onto the clay fraction, thus mod-

ifying the major cation relationships.

Evaporation trends for water samples collected in Chaaral

are simulated using CHEPROO code (Bea et al., 2009). The ionic

strength of water samples rangedfromaround 0.5to 15 M, thus

requiring the Pitzer ion interaction approach (Bea et al., in

press). The evaporation trend for a sample located in the oxi-

dation zone, 0.3 m below the tailings surface(sample CH1-1),is

shown inFig. 4A. Here, evaporation progress is dened as the

ratio between the initial and the remaining mass of water

after the evaporation step.Halite precipitates whenthe evap-

oration progress is approximately equal to 6. This sample

presents a Cl/Na ratio higher than 1. Therefore, precipitation

causes Na concentrations to decrease and Cl to increase but

with a lower slope (geochemical divide). Copper speciation

changes during evaporation. Cu2+ is the predominant

species at the onset of evaporation whereas CuCl+ is pre-

dominant after halite precipitation (Fig. 4A1). Eriochalcite

precipitates after evaporation progress equal to 150, when

the ionic strength of the solution is 8 M and water activity

0.7. At this point, the total Cu in solution decreases and Cl

concentration continues to increase.

The previous evaporation trend implies that the key

parameter for eriochalcite to form is the Cl/Na ratio of the

starting solution. For a Cl/Na ratio less than 1, Cl decreaseswhen halite precipitates but Na does not decrease, and the

Table 2

Parameters and the most important constitutive laws used in the simula-

tions.Slis the pore saturation (m3 m3),Pl,Pgand e are the liquid, gas and

vapor pressures, respectively (MPa), W is the molecular weight of water

(kg wmol1),Ris the universal gasconstant, Tthe temperaturein Kelvin, aw is

the water activity in the pore-water, and lis the liquid density (kg m3).

Parameter or constitut ive law Tailings

Porosity, 0.4

Intrinsic permeability (m2) 1014

Retention curve a S1 = 1 + PgPl

P0

1

1n

n

P0(MPa) 0.002

n 0.19

Liquid relative permeability, krl = krlCSl

1

Constant,C 1

Residual saturation,Srl 0.01

Maximum saturation,Sls 0.99

Thermal conductivity, = S1sat +

1Slsat

sat = 1sol

liq

dry = 1sol

gas

solb (W m1 K1) 2

gas(W m1 K1) 0.026liq(W m

1 K1) 0.6

Solid density,sol(kg m3) 1700

Solid specic heat (J kg1 K1) 789

Saturated vapor pressure, es(MPa) eS = awAexp B

T

A(MPa) 136,075

B(K) 5239.7

Psychrometric law c e= esexp PlPg

RT1W

Evaporation by aerodynamic diffusion (E) E= k2 va

lnzaz0

h i2tvav Wind speed,va, (m s

1) 4

Stability factor, 1

Screen height,za(m) 2

Roughness length,z0(m) 0.01

Karman's constant,k 0.4

a Van Genuchten model (Van Genuchten, 1980).b

Computed from mean values of mineral phases in tailings.c Edlefson and Anderson (1943).


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solution never reaches eriochalcite saturation. This is the case

of the sample CH14, for instance (Fig. 4B, located 0.3 m

below the water-table). The Cl/Na ratio of the pore-water

samples from the three boreholes is plotted in Fig. 5. Besides

sea water, only three samples display a Cl/Na ratio higher

than 1. Two of them are located in the upper part of the

oxidation zone, below the evaporation front. The third is

located in the brine, in the deepest part of the tailings. The

higher Cl/Na ratiocould be due to:(1) the exchange of Na+ by

Ca2+, Mg2+, Cu2+ and K+ onto the clay fraction, (2) the

inuence of the marine aerosol on the tailings surface, and

(3) the precipitation of Na secondary phases (e.g. mirabilite,

Na2SO410H2O). In this work, the above-mentioned process-

es are evaluated in the proposed alternative scenarios.

In this study, the numerical nite element model CODE-

BRIGHT (Olivella et al., 1994) and the object-oriented module

CHEPROO (Bea et al., 2009) are applied to simulateow and

reactive transport through tailings. The rst solves the

multiphase ow processes in the unsaturated zone while

the second solves the geochemical processes. Both codes are

coupled by direct substitution of the chemical equations into

the transport equations (Saaltink et al., 1998).

Fig. 4.Evaporation trends of samples CH1-1 (Cl/Na ratio higher than 1), and CH14 (Cl/Na ratio less than 1). (A1 and B1) Evolution of the concentration for Cl, Na

andcupric species (i.e. Cu2+, CuCl+ and CuCl2). (A2and B2)Evolution of water activity andionic strength.(A3 andB3) Evolution of thesaturation indices forhalite

(NaCl) and eriochalcite (CuCl22H2O). Evaporation is dened as the ratio between initial and the remaining mass of water after the evaporation step.


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The domain for reactive transport modeling is 1 m of the

unsaturated zone. The 1D nite element grid consists of 100

vertical elements.

Modeled aqueous species are Cu2+, CuCl+ and CuCl2(aq),

Ca2+, Mg2+, MgOH+, Al3+, Na+, K+, Fe2+, Fe3+, SO42, HSO4

,

Cl, SiO2(aq), H2O and OH. Virial coefcients for most of the

ions are taken from (Harvie et al., 1984). Halite and

eriochalcite solubility in the NaClCuCl2

system were mea-

sured by (Filippov et al., 1986). (Haung, 1989) modeled these

experimental results and provided the best t for the constant

equilibrium of the two minerals and Pitzer virial coefcients

between dissolved ions. The same author included copper

chloride complexes for a better t of experimental solubility.

Modeled exchange complexes are CaX2, MgX2, CuX2, NaX and

KX. Due to the absence of experimental exchange coefcients

in Chaaral clays (mainly vermiculite and illite), we have

tentatively used the selectivity coefcients given by (Appelo

and Postma, 1993), and performed a sensitivity analysis of the

inuence of these data on the nal results.

The main reactions considered in the geochemical model

are listed in Table 3. Homogeneous reactions are assumed to be

in equilibrium except for Fe(II) oxidation, which is modeled

using a modied kinetic expression provided by (Singer and

Stumm, 1970). Once the oxidation rates of suldes have been

dened in the model, the evolution of iron in the pore-water

could only be made to t by calibrating simultaneously the

scaling factor for rates of Fe(II)-oxidation and the precipitation

of goethite and K-jarosite.A factorequal to 107 times the abiotic

Fe(II)-oxidation rate is applied, which indicates that Fe(II)-

oxidation in the pore-water is biocatalyzed.

Fig. 5. Cl/Na ratio for water samples collected in boreholes CH1, CH2 and

CH3. Besides sea water, only three samples display a Cl/Na ratio higher than

1. Two of them are located in the upper part of the oxidation zone, below the

evaporation front. The third is located in the brine, in the deepest part of the

tailings.

Table 3

Reactions considered in the model. Equilibrium constants are taken of EQ3

data base (Wolery and Daveler, 1992) at 25 C.

Reaction

Homogeneous

HSO4

H+ + SO42

MgOH+ + H+H2O+Mg2+

OH+H+H2O

Fe2 +0.25O2(aq)+H+Fe3+ +0.5H2O

CuCl+Cu2+ + Cl a

CuCl2(aq)Cu2+ +2Cl a

Cation-exchange

CuX2 +2Na+2NaX+Cu

2+ b

MgX2 +2Na+2NaX+Mg2+ b

CaX2 +2Na+2NaX+Ca2+ b

KX+Na+NaX+K+ b

Dissolution/precipitation

Pyrite+3.5 O2(aq)+H2O2 SO42 + Fe+2 +2H+

Pyrite+14Fe3++ 8H2O2SO42 +15Fe+2 +16H+

Chalcopyrite+4O2(aq)2 SO42+17Fe2+ +Cu2+16H+

Chalcopyrite+16Fe 3+ + 8H2O2SO42+17Fe2+Cu2+ +16H+

Albite+4.2H+2.95 SiO2(aq)+ 1.05 Al3+ +0.95Na+ +0.05Ca2+

+2.1 H2O

Anorthite+8H+2 Al3+ +2SIO2(aq)+Ca2+ + 4H2O

EriochalciteCu2+ +2Cl+ 2H2O a

HaliteNa+ + Cl a

SylviteK+ +Cl

Biotite+10H+3Al3+ +3SiO2(aq)Fe2+ + K+ +2Mg2+ +6H2OMuscovite+10H+3Al3+ +3SiO2(aq)+K

+ + 6H2O

Geothite+3H+Fe3+ + 2H2O

QuartzSiO2(aq)

GypsumCa2+SO42+2H2O

MirabiliteSO42+2Na+ +10H2O

Kaolinite+6H+2Al3++2SiO2(aq)+5H2O

Pickeringite2Al3+ + Mg2+ +4SO42+22H2O

K-jarosite+6H+K+ +2 SO42+3 Fe3+ + 6H2O

(K,Na)-jarosite+6H+0.5Na+ +0.5 K+ +2 SO42+3 Fe3++ 6H2O

HexahydriteMg2+ +Fe2+ +SO 42+ 6H2O

Halotrichite2Al3+ + Fe2+ +4SO42+22H2O

Gas dissolution/exsolution

O2(g)O2(aq)

a

Equilibrium constants taken fromHaung (1989).b Selectivity coefcients taken fromAppelo and Postma (1993).

Table 4

Summary of kinetic expressions used in the model.

Reaction process Rate expression (mol m2 s1) Ref.

Pyrite oxidation

(oxygen path)

R =108.19aH+0.11aO2(aq)

0.5 (1) a

Pyrite oxidation

(Fe(III) path)

R =106.7aFe3+0.93 aFe2+

0.4 (1) a

Chalcopyrite oxidation

(oxygen path)

R =1010.58aH+0.15 (1) b

Chalcopyrite oxidation

(Fe(III) path)

R =106.75aFe3+0.43 (1) c

Fe(II) oxidation R =103aO2(aq) (1) d

Albite dissolution R =109.17aH+0.5 (1) e

Anorthite dissolution R =109.17aH+0.5 (1) f

Muscovite dissolution R =101223aH+0.38 (1) g

Quartz dissolution R =1013.38 (1) h

Kaolinite precipitation R =(1010.77aH+0.5 + 10166aH+

0.3)(1) i

Goethite precipitation R =10

11(1) jK-jarosite precipitation R =104(1) j

(K,Na)-jarosite

precipitation

R =104(1) j

Biotite dissolution R =(108.5aH+0.57 + 1014.1aH+

0.29)(1) k

a Williamson and Rimstidt (1994).b Acero et al. (2007).c Rimstidt et al. (1994).d Modied after Singer and Stumm (1970), calibrated to reproduce

observed iron evolution.e Chou and Wollast (1985).f Equal to albite.g Kalinowski and Schweda (1996).h Tester et al. (1994).i Nagy et al. (1991).j

Calibrated to reproduce observed iron evolution.k Malmstrom and Banwart (1997).


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Mineral dissolution reactions are modeled using kinetic laws.

The kinetic rates presented inTable 4are based on the general

expression presented byLasaga et al. (1994). Most secondary

phasesareconsideredto precipitatein equilibrium.Theseinclude

halite, eriochalcite, sylvite, gypsum, mirabilite, halotrichite,

hexahydrite and pickeringite. K-jarosite, goethite and kaolinite

are considered to precipitate kinetically (seeTable 4).

Dissolved oxygen is modeled in equilibrium with the

partial pressure of oxygen gas in pores (pO2 = 0.21 atm).

A mixture of suldes (mainly pyrite and chalcopyrite) and

aluminosilicates (mainly quartz, muscovite, biotite, and pla-

gioclase) is considered as initial mineralogy. Their initial min-

eral fractions are presented inTable 5. The volumetric fraction

occupied by each mineral is calculated from its weight fraction

by considering its density. Weight fractions are estimated from

sequential extractions and mineralogical descriptions reported

by (Dold, 2006). A total reactive surface corresponding to silty

grain size tailings is considered (104 m2 m3). It is proportion-

ally distributed in accordance with the volumetric fraction of

each mineral phase. A vermiculite content of 3 wt.% is assumed

in the model. Thus, the total exchange capacity considered in

the calculations is computed from this clay content and the

exchange capacity of Jeffersite vermiculite (Foscolos, 1968).

The chemical composition of the initial and boundary

solutions is shown inTable 6.

3.3. Simulated scenarios

Six scenarios (shown in Fig. 6) are simulated to account

for the different genetic hypotheses about the formation of

eforescences. The aim is to obtain a Cl/Na ratio higher than 1

in the pore-water, while using the observed types of water.

Sea water is imposed at the bottom in Scenarios I and II

(SWCE and SW, with/without the cation-exchange, respec-

tively), thus implying a sea water wedge reaching the inner

portion of the tailings, which may well have been the case

during early sedimentation. Mixtures (mixing proportion

= 0.5) of sea water/freshwater and brine/freshwater are

considered for Scenarios III and IV (SFWCE and BFWCE),

respectively. Cation-exchange is also considered in both

scenarios.

There are situations in which the Cl/Na ratio is less than 1

in the original groundwater composition and can be reversed

through cation-exchange, or by adding marine aerosol. Both

cases are tested in Scenarios V and VI (FWCE and FWMASL),

respectively. With regard to the latter, a slow, constant ow

of sea water (20 mm year1, its chemical composition is

described in Table 6) is imposed at the upper 0.1 m of the

model.

4. Modeling results and discussion

All simulations are run for thirty-two years, beginning

after discharges from the El Salvador were stopped in 1975

(period 19752007).

Scenario I (SWCE) is chosen as the base case. Therefore,

we thoroughly describe its features and results in the fol-

lowing sections. Later, the remaining scenarios are analyzed

and compared.

Table 5

Initial conditions. Summary of initial volumetric fraction and reactive surface

used in the reactive transport model.

Mineral phase vol.% Initial surface area (m2 m3)

Pyrite 0.39 130

Chalcopyrite 0.32 108

Anorthite 18 5852

Albite 6.8 2277

Quartz 24 8231

Goethite 0.18 61

Muscovite 1.7 576

Biotite 2.5 845

Kaolinite 5.8 1920

Gypsum 0 0

Sylvite 0 0

Halite 0 0

Eriochalcite 0 0

Mirabilite 0 0

Pickeringite 0 0

Hexahydrite 0 0

(K,Na)-jarosite 0 0

K-jarosite 0 0

Halotrichite 0 0

Total 60 20,000

Table 6

Chemical composition of initial, marine aerosol and boundary waters (mol kg w1).

Initial solution Scenarios I, II and marine

aerosol (sea water)

Scenario III

(0.5 sea-water+0.5 freshwater)

Scenario IV

(0.5 brine+0.5 freshwater)

Scenarios V and VI

(freshwater)

pH 7.48 7.48 7.41 7.45 7.03

Fe 6.35106 4.8106 6.8105 7.5104

Cu 104 103 2.1105 8.0106 9.4106

Zn 7.64108 104 9.0105 1.6106

Ca 104 9.8103 1.06102 2.23102 1.1102

Mg 107 5.2102 2.9101 8.7101 6.2103

Na 107 5.0101 3.5101 1.4102 2101

K 107 1.07102 7.21103 4.8106 3.7103

Cl 5.75 101 5.75101 3.69101 8.7101 1.6101

SO4 2.9102 2.9102 2.73102 2.27102 2.5102

O2 2.5104

Si

Al

Ionic strength 0.347 0.726 0.5 1.01 0.27


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Fig. 6. Simulatedscenarios according to thecomposition ofthe water imposed at thebottom. Legend: S = seawater;F = freshwater; B = brine; W = water; MASL =

marine aerosol; CE = cation-exchange.

Fig. 7.Scenario I (SWCE): Vertical distribution of (A) temperature (C), (B) salinity, (C) water activity, and (D) pore saturation, at the last winter/summer.


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4.1. Thermohydraulic evolution in Scenario I (SWCE)

Spatial distribution of temperature, salinity, water activity

and pore saturation are shown inFig. 7. The time evolution of

temperature, evaporation rate and energy balance are plotted

inFig. 8.

As expected, these parameters uctuate seasonally. The

tailings surface temperature declines in the winter as solar

radiation and air temperature do the same (Figs. 7A and 8).

This temperature: (1) is always higher than the atmosphere

temperature, and (2) increases during the summer (approx-

imately 3 C) and over time (Fig. 8A). Both observations are

explained by the energy balance at the upper boundary

(Fig. 8B). Net radiation is at its maximum and most of the

inowing radiation is used in evaporating water. However,

the mean evaporation rate decreases over time as a

consequence of the increase in salinity of the pore-water.

Thus, the latent heat ux (invested in evaporation) declines

over years. To maintain the heat balance, the sensible heat has

to be increased (Fig. 8B), which requires increasing the

surface/air temperature differential (Fig. 8A). The evapora-

tion is produced during the summer, but in the winter thereis

slight condensation (Fig. 8B).

During the summer as a consequence of the high level of

evaporation (Fig. 7B), salinity increases in the uppermost

portion of tailings. However, during the winter, salinity

decreases slightly or undergoes a slight increase at the top

of tailings.

An opposite evolution is predicted for water activity

(Fig. 7C). It decreases in the uppermost portion of tailings due

to an increase in salinity during the summer. However, it

increases at the top of tailings during the winter. It decreases

slightly between 0.1 m and 0.5 m below the surface during

the same season.

The water saturation prole (Fig. 7D) shows that the

water saturation reached a steady-state prole. The satura-

tion declined from the initial saturated state to about Sl =0.68

at the top andSl =0.99 at the base.

4.2. Evolution of pore-water chemistry in Scenario I (SWCE)

Vertical proles for Cl, K, Ca, SO4, Cu, Na and pH in the last

winter/summer are shown in Fig. 9A to G. In general,

concentrations of ions increase in the oxidation zone,especially at the top. As expected, seasonal uctuations are

predicted for them. Concentrations of solutes decrease during

Fig. 8. Scenario I (SWCE): (A)the time evolutionof temperature in theatmosphere andtailings. (B)The time evolutionof evaporation rate andenergy uxesin thetailings.


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Fig. 9.Scenario I (SWCE): Vertical proles of the main species at the last winter/summer. (A) Cl, (B) K, vertical distribution corresponding to Scenario II (SW) isalso shown, (C) Ca, vertical distribution corresponding to Scenario II (SW) is also shown, (D) pH, (E) SO 4, (F) Cu, and (G) Na.


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the winter in the upper part, but increase slightly below the

evaporation front (between 0.1 m and 0.5 m, e.g. see Cl in

Fig. 9A). Similar behavior is also predicted for Na, K, Cu, Ca,Mg, Al, Zn, Fe(total) and SiO2(aq). However, SO4 shows

different behavior. It decreases its concentration in the upper

part of the model during the summer (seeFig. 9E).

pH is found to decrease in tailings as a consequence ofsulde oxidation (Fig. 9D). It increases sharply to neutral

Fig. 10.Temporal evolution of eforescences obtained from probable scenarios: (A) Scenario I (SWCE), (B) Scenario III (SFWCE), and (C) Scenario IV (BFWCE).

Vertical distribution of mineral volumetric contents in the last summer: (D) Pyrite and chalcopyrite, (E) geothite and K-jarosite, and (F) gyspum and kaolinite.


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values below the oxidation front (located 0.8 m below the

surface). pH increases slightly in the upper part of the model

during the winter, and the oxidation front rises 0.1 m in the

same season. The concentration of K decreases within the

oxidation zone due to precipitation of K-jarosite and its

exchange onto the clay fraction (Fig. 9B). Ca concentration

increases mainly due to the exchange with Na and limited

plagioclase dissolution (Fig. 9C).

In general, concentration and pH proles are consistent

with eld observations. The total concentration of Cu

predicted after thirty two years is one order of magnitude

higher than eld observations (Fig. 9F). Other processes (not

considered in this model) could probably explain Cu

depletion in the pore-water. For instance, the adsorption of

Cu onto secondary Fe-oxides/hydroxides (i.e. onto goethite

and K-jarosite, see Cu(ox) inTable 1). Although the trend ts

the experimental one, the predicted concentrations of Cl are

clearly higher than measured. A possible explanation for the

calculated high Cu and Cl concentrations could also be an

insufciently accurate solubility value for eriochalcite in such

complex solutions.

4.3. Precipitation of secondary phases and suldes oxidation in

Scenario I (SWCE)

Eforescences precipitate in the upper part of the model

(Fig. 10A). However, seasonal variation in them is predicted.

This process of formation starts in the summer and partial

dissolution (total in the case of eriochalcite) is produced

during the winter. The total dissolution of eriochalcite was

veried in Chaaral tailings during the winter of 2008.

Eriochalcite was only identied in eforescence samples

collected during the summer.

Halite increased over time, whereas eriochalcite almost

reached a constant value (between 15 and 25 mol m3).

Thus, the halite/eriochalcite ratio observed during the

summer increased over time from 10 to 70 after thirty-two

years.

Goethite and K-jarosite form at the expense of Fe(II)

oxidation (Fig. 10E). Their precipitated amount is consistent

with that observed in Chaaral (0.07 wt.% K equivalent for K-

jarosite). In agreement with the eld observations, gypsum

(CaSO42H2O) and kaolinite (Al2Si2O5(OH)4) form through-

out the oxidation zone (Fig. 10F). Kaolinite forms as a product

of the dissolution of aluminosilicates (i.e. muscovite, biotite,

and plasgioclase) in an acidic environment. Gypsum is mainly

predicted in the upper part of the model, and its amount

(150 mol m3) is consistent with Ca measured in the soluble

fraction of soil samples (see Ca(sol) inTable 1).

Pyrite and chalcopyrite oxidation is predicted in the

model. However, a proportion of chalcopyrite remained

after thirty-two years (Fig. 10E) despite the fact that it was

practically exhausted in the analysis of the tailings (see Cu

(sph) inTable 1). Due to the uncertainty in its initial value, it

is probable that chalcopyrite was overestimated in the initial

condition (1.2 pyrite and 0.8 chalcopyrite equivalent). Based

on sequential extractions and assuming that suldes were

preserved by the oxidation processes below the water-table, a

lower initial proportion of pyrite and chalcopyrite equivalent

could be considered (0.65 pyrite and 0.15 chalcopyrite). Chal-copyrite is exhausted in a simulation taking the latter wt.%

as initial conditions after thirty-two years. After thirty-two

years, all silicate proles remain essentially unchanged from

their initial proles. However, the sum of all mineral volume

changes induces a net mineral volume gain, which results in a

maximum porosity decrease of about 0.3% mainly in the upper

part with virtually no change in other parts (not represented).

4.4. Analysis of scenarios

The precipitation of halite is predicted in all scenarios in

the upper part of the model. However, precipitation of

eriochalcite is only reached in scenarios I (SWCE), III (SFWCE)

and IV (BFWCE) (Fig. 11). The time evolution of halite and

eriochalcite for these scenarios is displayed in Fig. 10A to C.

Notice that cation-exchange is considered in all three scenarios.

In Scenario II (SW), precipitation of eriochalcite is in-

hibited in several ways. First, the saturation rate of eriochal-

cite is reduced as a consequence of Cl consumed by halite

precipitation (i.e. geochemical divide, Fig. 4B2). Second,

water activity decreased quickly in the upper part of the

model due to high concentrations of other major ions such asMg and K, further preventing the saturation of eriochalcite. As

shown in the previous section, the evaporation rate decreases

for low values of water activity. Due to the high concentra-

tions of K and Mg, the precipitation of other KMg salts such

as silvite (KCl) and hexahydrite (MgFe(SO4)26H2O) is

predicted.

No secondary KMg minerals precipitated in scenarios

with cation-exchange. In these cases, as a result of the ex-

change between Mg2+ and K+ by Ca2+, hexahydrite and

silvite did not reach saturation.

Other eforescences are associated with halite in the

scenario affected by marine aerosol (Scenario VI (FWMASL),

mirabilite and silvite). None of these associated efores-cences have been found in Chaaral.

Eriochalcite is also inhibited in Scenario V (FWCE) because

its Cl/Na ratio continuously decreases in the pore-water. This

is enhanced by the Cl/Na ratio of its boundary water (b1) and

cation-exchange processes.

After thirty-two years, scenarios III (SFWCE) and IV

(BFWCE) predicted larger amounts of halite (Fig. 10B and C,

approximately 1400 and 2200 mol m3, respectively). How-

ever, a similar precipitated amount is predicted for eriochalcite

Fig. 11. Evolution of the saturation index of eriochalcite (CuCl22H2O) as a

function of water activity from different scenarios. Eriochalcite precipitationis only reached in Scenarios I (SWCE), III (SFWCE) and IV (BFWCE).


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in all thesescenarios (between20 and 25 mol m3). Therefore,

in both scenarios III and IV, the halite/eriochalcite ratio is much

larger than observed.

The time evolution of evaporation/condensation rates

obtained from different scenarios are shown inFig. 12. All of

them present a similar trend (i.e. the evaporation is produced

during the summer, while there is slight condensation in the

winter). However, some differences are predicted in Scenar-

ios I (SWCE) and II (SW) in spite of the fact that sea water is

imposed in both scenarios. The evaporation rate trend is

mainly controlled by the water activity and eforescence

evolution at the evaporation front. As seen above, different

eforescences are predicted for both scenarios (i.e. halite

eriochalcite and halitesilvitehexahydrite in Scenarios I

(SWCE) and II (SW), respectively). The evolution of these

eforescences is different and consequently, so is the evolu-

tion of evaporation rates.

The scenario affected by marine aerosol (Scenario VI,

FWMASL) predicts a higher evaporation rate. The dissolution/

precipitation of eforescences (i.e. halitesilvitemirabilite)

controls the evolution of evaporation rates.

4.5. Sensitivity analysis in Scenario I (SWCE)

In order to gain further insight into the hydrogeochemical

behavior of the Chaaral tailings, and to address parameter

uncertainty, several additional simulation scenarios are run

using Scenario I (SWCE) as the base case. Therefore, the

following parameters are tested: (1) the grain size of tailings,

(2) the radiation on the tailings surface, (3) the wt.% to the

clay fraction, (4) the order of selectivity (i.e. through

selectivity coefcients) for cation-exchange, and (5) the

inuence of the tidal cycle.

The results are compared with data on: (1) ef

orescenceratios (i.e. halite and eriochalcite), (2) the pore-water

chemistry, (3) the precipitated amount of secondary phases

described in Chaaral (i.e. gypsum, K-jarosite, goethite,

kaolinite), (4) the formation of eforescences not previously

described in Chaaral (e.g. silvite, hexahydrite, mirabilite),

and (5) the dissolution of suldes (i.e. pyrite and chalcopy-

rite), and aluminosilicates (i.e. biotite and muscovite).

With regard to sensitivity to grain size (i.e. varying the

retention curve), sandy tailings are modeled. As set out above,

sandy tailings are characteristic toward the sea. The base case

is repeated here using the same geometry and mineralogy,

but with a modied water retention curve for tailings

(P0 =0.002 (MPa) and =0.6 for Van Genuchten parameters

inTable 2). The modication had the effect of reducing the

water retention capacity of tailings. As a consequence of

evaporation, the liquid saturation decreases signicantly in

the upper part, and the evaporation front is located

approximately ve centimeters below the surface. As a

consequence of this, the evaporation rate also decreases

during the summer. Major ions such as Mg, Na and K are

strongly exchanged onto the clay fraction and their concen-

trations decrease in the pore-water. Cl concentration

increases signicantly at the top, so eriochalcite saturation

is immediately achieved. An amount similar to the base case

is predicted for this eforescence. However, halite saturation

occurs one year later as a consequence of the retardation in

the Na front from groundwater. The halite/eriochalcite ratio

reaches 1 during summers, a much lower ratio than observed.

Only a minor amount of other secondary phases is predicted

(i.e. gypsum, goethite, K-jarosite) due to the lower availabil-

ity of K and SO4 from the groundwater. A clear sky means

greater solar radiation on the tailings surface and increased

evaporation. It affects mainly the amount of halite and

eriochalcite (the precipitate almost doubled) but not their

relative proportion (halite/eriochalcite). It also increases,

however, the K concentration in the pore-water to much

higher values than observed. Due to the higher evaporation

rate in relation to the precipitation rate of K-jarosite, the K

concentration increases in the pore-water.

Low and high clay contents are evaluated (0.3 wt.% and

30 wt.%, respectively). Each one is implemented in the model

using the CEC (Cation-Exchange Capacity) parameter. A

similar geochemical evolution is obtained for low CEC as in

Scenario II (SW) (scenario without cation-exchange). How-

ever, approximately 100 mol m3 of eriochalcite is predicted

for high CEC. In this case, Na+ is strongly exchanged onto the

clay fraction, increasing the Cl/Na ratio in the pore-water.

Thus, saturation of halite is reached two years later. The

precipitated halite/eriochalcite ratio is approximately 1,

which is much lower than observed.

Each type of clay has its own order of cation-exchange

preference (i.e. order of selectivity). The order of selectivity

used in the base case was typical for illite (K+NCa2+N(Cu2+,

Mg2+)NNa+). However, a different order of selectivity was

suggested by other clays (Foscolos, 1968). This varies

according to selectivity coefcients. Sensitivity analysis was

carried out on the NaCu selectivity coefcient in order to

obtain two orders of selectivity: 1) Cu2+NK+NCa2+NMg2+N

Na+ (high afnity for Cu2+), and 2) K+NCa2+NMg2+NNa+N

Cu2+ (low afnity for Cu2+). The concentrations of Na+ and

Cu2+ were modied in the initial solution in order to obtain

the same exchanged equivalent fractions (i.e. for NaX and

CuX2) as the base case simulation. When Cu2+ is strongly

exchanged onto the clay its availability in solution is depleted,

and a maximum of 15 mol m3 of eriochalcite is predicted to

form. When the afnity of Cu2+ in the exchange complex is

low the opposite behavior is obtained, and precipitation of

32 mol m3 of eriochalcite is predicted. Therefore, although

there are differences, the resulting halite/eriochalcite ratiosvary within the same order of magnitude with the selectivityFig. 12.The time evolution of evaporation rates on the tailings surface fromdifferent scenarios (period 19761980).


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coefcients for Cu2+. In all cases, exchanged copper decreases

with ionic strength, primarily attributed to formation of metal

chlorides (mainly CuCl+, CuCl2(aq)), as was veried exper-

imentally with vermiculite (El-Bayaa et al., 2009).

Regarding sensitivity to the tidal cycle, a tidal oscillation of

0.5 m is imposed at the lower boundary. In this case, a higher

hydrodynamic dispersion is induced by tides and saturation

of halite and eriochalcite is reached much later than for the

base case. However, once saturation is reached, the tidal cycle

did not signicantly affect the evolution and mineral propor-

tion of the eforescences in the upper part of the system.

4.6. Remediation alternative

In general, remediation strategies on tailings are con-

cerned with the capacity of the systems to minimize water

and oxygen percolation towards the bulk of tailings. How-

ever, our remediationstrategy focuses mainly on reducing the

evaporation rate on the tailings surface and then preventing

eforescence precipitation. Therefore, a capillary barrier is

proposed as a suitable alternative (Melchior et al., 1993). This

would consist of two layers overlying tailings (seeFig. 13A):

(1) a coarse sand layer (i.e. drainage layer), and (2) a geo-

membrane. The latter has two main purposes: (a) to isolate

the capillary barrier from the acid mine drainage system, and

(b) to prevent oxygen percolation toward the tailings. The

former could reduce the evaporation rate as a consequence of

its low water content and thermal conductivity. On the other

hand, this cover design might provide an additional organic

soil layer for the development of vegetation (in the present

conditions, vegetation does not grow on tailings due to the

acidic conditions).

The proposed cover design is evaluated through reactive

transport modeling. Thus, the base case mesh was extended

in order to simulate 0.15 m drainage layer and a thick geo-

membrane. Their main physical parameters were (see Table 2

for reference): intrinsic permeabilities, 1014 and 1020 (m2),

respectively; retention curve parameter P0, 0.002 and 0.0001

(MPa), respectively; retention curve parameter n, 0.6 and

0.19 (), respectively; and thermal conductivity sol, 0.16

(W m1 K1) for both layers.

Regarding the results, evaporation is dramatically re-

duced thus preventing eforescence formation (see the tem-

poral evolution of the saturation index for eforescences in

Fig. 13B).

5. Concluding remarks

Although eforescence formation on tailings is common in

hyperarid climates, natural occurrence of eriochalcite is rare.

According to the modeling exercise described above, this is

due to the fact that several environmental and geochemical

constraints are required for its precipitation. A high level of

evaporation and low relative humidity (Hr) have to be

reached. These parameters control water activity evolution

in the pore-water chemistry. Therefore, eforescences form

mainly during the summer when the relative air humidity is

low and solar radiation is high, whereas their dissolution is

produced during the winter when relative humidity increases

and light condensation is produced on the tailings surface.

The modeled result was veried during a visit to Chaaral

tailings during the winter of 2008. Eriochalcite could only be

collected as eforescence during summer.

With regard to geochemical conditions, the Cl/Na ratio is a

key parameter in the formation of eriochalcite. The strong

competition of other cations for Cl may form halite and

sylvite, and inhibit the precipitation of eriochalcite. Precipi-

tation of halite creates a geochemical divide, allowing either

Cl orNa+ concentrations to increase, depending on whether

the Cl or Na+ concentration is higher in the pore-water.

Fig. 13. Remediation alternative: capillary barrier on tailings. (A) Cover design (Melchior et al., 1993). (B) Temporal evolution of the saturation index foreforescences on the tailings surface considering and not considering the capillary barrier.


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Eriochalcite forms only in those samples with a Cl/Na ratio

higher than 1. Three mechanisms are proposed to obtain a

higher Cl/Na ratio: (1) the exchange of Na+ by Ca2+, Mg2+,

Cu2+ and K+ onto a vermiculite-type mixed-layer, (2) the

inuence of the marine aerosol on the tailings surface, and

(3) the precipitation of secondary Na phases. According to the

mineralogy and solute concentration proles of the different

modeling scenarios, cation-exchange is the most likely mech-

anism to obtain a higher Cl/Na ratio in the Chaaral tailings.

In fact, it is also supported by analytical evidence: (1) a sig-

nicant fraction of Cu, Na, and Ca were exchanged at least in

the oxidation zone of these tailings (e.g. see Cu(ex), Na(ex)

and Ca(ex) inTable 1). In the case of Cu, it may be released

from this vermiculite-type mixed-layer mineral from samples

in the low pH oxidation zone (Dold, 2003), (2) water samples

collected in the oxidation zone (below the evaporation front,

corresponding to boreholes CH1, CH2 and CH3) present a

Cl/Na ratioN1.In thiscase, noNamineral phases precipitate in

this zone, and only cation-exchange processes could explain

these ion ratios. With regard to the probable scenario for the

formation of these eforescences, a scenario with two stages is

proposed: a rst stage involving sea water (Scenario I (SWCE),

or a mixture of sea water/freshwater (Scenario III (SFWCE)) in

the groundwater composition, and a second stage with the

intrusion of freshwater. The rst stage is based on the amount

of eriochalcite formed, whereas the freshwater intrusion is

required to explain the observed halite/eriochalcite ratio and

the vertical concentration distribution in therst 0.5 m above

the water-table.

Sensitivity analysis of different physical and geochemical

parameters is evaluated in the model. The result is particu-

larly sensitive to the clay content, because Na exchanged and,

then the Cl/Na ratio increases with the clay content. As a

result of this, a higher amount of eriochalcite is predicted and

a halite/eriochalcite ratio lower than observed. The model is

less sensitive to the cation-exchange selectivity coefcients.

Thus, although higher afnity for Cu2+ in the exchange

complex decreases the amount of eriochalcite formed, the

variations in the halite/eriochalcite ratio are kept close to the

base case value.

Moreover, the model is also sensitive to grain size. Thus, for

more sandy tailings, theliquid content in the uppermost part of

the model is markedly reduced. Then, eriochalcite precipitates

immediately and a lower evaporation rate is predicted.

Therefore, less capillary transport and precipitation of second-

ary mineral phases (i.e. gypsum, goethite and K-jarosite) are

predicted in the oxidation zone. However, for consistence

between grain size and clay content, the latter should be

reduced, and eriochalcite formation decreases accordingly.

On the other hand, the model is not particularly sensitive to

the tidal cycle. In this case, major hydrodynamic dispersion in

the oxidation zone is predicted but it did not signicantly affect

the evolution of eforescences in the upper part of the model.

With regard to preventing the formation of eriochalcite, a

capillary barrier on the tailings surface is proposed as a suitable

alternative (previous excavation of precipitated efores-

cences). Its efciency as a barrier is also tested through reactive

transport (approximately 0.15 m in thickness was considered

in the simulation). Precipitation of eriochalcite is inhibited

because evaporation rate on the tailings surface is dramaticallyreduced.

Acknowledgments

This work was partially funded by a point action CSIC/

CONYCIT (20082009), the project CTM2007-66724-C02/

TECNO of the Spanish Government, and Swiss National Science

Foundation projects No. 200020-117792/1 and 200021-

105507/1. Thanks are also to anonymous reviewer for the

valuable comments and suggestions that have signicantly

improved thenal version of the paper.

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