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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:sbea@eos.ubc.ca(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:sbea@eos.ubc.cahttp://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:sbea@eos.ubc.ca8/9/2019 Bea et al. (2010)
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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
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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.
68 S.A. Bea et al. / Journal of Contaminant Hydrology 111 (2010) 6582
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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.
69S.A. Bea et al. / Journal of Contaminant Hydrology 111 (2010) 6582
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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).
70 S.A. Bea et al. / Journal of Contaminant Hydrology 111 (2010) 6582
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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.
71S.A. Bea et al. / Journal of Contaminant Hydrology 111 (2010) 6582
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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.
References
Acero, P., Ayora, C., Carrera, J., 2007. Coupled thermal, hydraulic andgeochemical evolution of pyritic tailings in unsaturated column experi-ments. Geochimica et Cosmochimica Acta 71, 5325 5338.
Appelo, C., D. Postma, 1993. Geochemistry, Groundwater and Pollution,Balkema, A.A. Ed.
Bain, J., Blowes, D.W., Robertson, W., Frind, E., 2000. Modelling of suldeoxidation with reactive transport at a mine drainage site. Journal ofContaminant Hydrology 41 (12) 23.47.
Bea, S.A., Carrera, J., Batlle, F., Ayora, C., Saaltink, M., 2009. CHEPROO: aFortran 90 object-oriented module to solve chemical processes in EarthScience models. Computers & Geosciences 35 (6), 10981112.
Bea, S.A., J. Carrera, C. Ayora, . Pitzer algortithm: Efcient implementation ofPitzer equations in geochemical and reactive transport models, Compu-ters & Geosciences (in press).
Camus, F., 2003. Geologa de los sistemas porfricos en los Andes de Chile.Santiago de Chile, Servicio Nacional de Geologa y Minera.
Castilla, J.C., 1983. Environmental-impact in sandy beaches of copper minetailings at Chaaral. Chile, Marine Pollution Bulletin 14, 459 464.
Chou, L., Wollast, R., 1985. Steady-state kinetics and dissolution mechanismsof albite. American Journal of Science 285, 963 993.
Dold, B., 2003. Speciation of the most soluble phases in a sequentialextraction procedure adapted for geochemical studies of copper suldemine waste. Journal of Geochemical Exploration 80, 55 68.
Dold, B., 2006. Element ows associated with marine shore mine tailingsdeposits. Environmental Science & Technology 40, 752758.
Dold, B., Fontbote, L., 2001. Element cycling and secondary mineralogy inporphyry copper tailings as a function of climate, primary mineralogy,and mineral processing. Journal of Geochemical Exploration 74, 3 55.
Edlefson, N., Anderson, A., 1943. Thermodynamics of soil mix- water-wettedporous media. Water Resources Research 35, 635649.
El-Bayaa,A.A., Badawy, N.A., AbdAlKhalik, E.,2009. Effectof ionic strength onthe adsorption of copper and chromium ions by vermiculite pure claymineral. Journal of Hazardous Materials 170, 12041209.
Farina, 2001. Temporal variation in the diversity. Marine Pollution Bulletin.Filippov, V.K., Charykov, N.A., Fedorov, Y.A., 1986. NaCl NiCl2(CuCl2)H2O
systems at 25 C. Zhurnal Neorganicheskoi Khimii 31, 18611866.Foscolos, A.E., 1968. Cation-exchange equilibrium constants of aluminum-
saturated montmorillonite and vermiculite clays. Soil Science Society ofAmerica Journal 32, 350354.
Fuenzalida, P., 1950. Geografa econmica de Chile I. CORFO 188 254.Fuenzalida, P., 1965. Biogeografa. Geografa econmica de Chile, CORFO.Gerke, H., Molson, J., Frind, E., 1998. Modelling the effect of chemical
heterogeneity on acidication and solute leaching in overburden minespoils. Journal of Hydrology 209 (14), 166185.
Gerke, H., Molson, J., Frind, E., 2001. Modelling the impact of physical and
chemical heterogeneity on solute leaching in pyritic overburden minespoils. Ecological Engineering 17 (23), 91101.
Gustafson, L.B., Hunt, J.P., 1975. Porphyry copper-deposit at El Salvador,Chile. Economic Geology 70, 857912.
Gustafson, L.B., Quiroga, J., 1995. Patterns of mineralization and alterationbelow the porphyry copper orebody at El Salvador. Chile, EconomicGeology and the Bulletin of the Society of Economic Geologists 90, 2 16.
Harvie, C.E., Moller, N., Weare, J.H., 1984. The prediction of mineralsolubilities in natural waters: the NaKMgCaHClSO4OHHCO3CO3CO2H2O system to high ionic strengths at 25 C. Geochimica etCosmochimica Acta 48, 723751.
Haung, H.H., 1989. Estimation of Pitzer ion interaction parameters forelectrolytes involved in complex-formation using a chemical-equilibriummodel. Journal of Solution Chemistry 18, 10691084.
Kalinowski, B.E., Schweda, P., 1996. Kinetics of muscovite, phlogopite, andbiotite dissolution and alteration at pH 14, room temperature.Geochimica et Cosmochimica Acta 60 (3), 367 385.
Larrain, H., Velasquez, F., Cereceda, P., Espejo, R., Pinto, R., Osses, P.,Schemenauer, R.S., 2002. Fog measurements at the site Falda Verde
81S.A. Bea et al. / Journal of Contaminant Hydrology 111 (2010) 6582
8/9/2019 Bea et al. (2010)
18/18
north of Chaaral compared with other fog stations of Chile. Atmo-spheric Research 64, 273284.
Lasaga, A.C., Soler, J.M., Ganor, J., Burch, T.E., Nagy, K.L., 1994. Chemicalweathering rate laws and global geochemical cycles. Geochimica etCosmochimica Acta 58, 23612386.
Lee, Correa, 2005. Effects of copper mine tailings. Marine EnvironmentalResearch.
Lee, Correa, Castilla, 2001. An assesment. Marine Pollution Bulletin.Lefebvre, R., Hockley, D., Smolensky, J., Gelinas, P., 2001a. Multiphase transfer
processes in waste rock piles producing acid mine drainage I: conceptual
model and system characterization. Journal of Contaminant Hydrology52 (14), 137164.
Lefebvre, R., Hockley, D., Smolensky, J., Lamontagne, A., 2001b. Multiphasetransfer processes in waste rock piles producing acid mine drainage 2.Applications of numerical simulation. Journal of Contaminant Hydrology52 (14), 165186.
Malmstrom, M., Banwart, S., 1997. Biotite dissolution at 25 C: the pHdependence of dissolution rate and stoichiometry. Geochimica etCosmochimica Acta 61, 27792799.
Mayer, K.U., Frind, E.O., Blowes, D.W., 2002. Multicomponent reactivetransport modeling in variably saturated porous media using ageneralized formulation for kinetically controlled reactions. WaterResources Research 38, 1174.
Melchior, S., Berger, O., Vielhaber, G., Miehlich, G., 1993. Comparison ofeffectiveness of different liner systems for top cover. Proceeding of 4th
International Landll Symposium, S. Margherita Di Pula, Cagliari, Itlia,pp. 225234.
Nagy, K.L., Blum, A.E., Lasaga, A.C., 1991. Dissolution and precipitationkinetics of kaolinite at 80 C and pH 3 the dependence on solutionsaturation state. American Journal of Science 291, 649686.
Olivella, S., Carrera, J., Gens, A., Alonso, E.E., 1994. Nonisothermal multiphaseow of brine and gas through saline media. Transport in Porous Media15, 271293.
Palache, C., Foshag, W., 1938. Antofagastite and bandylite, two new copperminerals from Chile. American Mineralogist 23 (2), 8590.
Rimstidt, J.D., Chermak, J.A., Gagen, P.M., 1994. Rates of reaction of galena,sphalerite, chalcopyrite, and arsenopyrite with Fe(III) in acidic solutions.Environmental Geochemistry of Sulde Oxidation 550, 213.
Saaltink, M.W., Ayora, C., Carrera, J., 1998. A mathematical formulation forreactive transport that eliminates mineral concentrations. WaterResources Research 34, 16491656.
Singer, P.C., Stumm, W., 1970. Acidic mine drainage. Rate-determining step.Science 167, 11211123.
Tester, J.W., Worley, W.G., Robinson, B.A., Grigsby, C.O., Feerer, J.L., 1994.Correlating quartz dissolution kinetics in pure water from 25 C to625 C. Geochimica et Cosmochimica Acta 58, 24072420.
Thompson, M.V., Palma, B., Knowles, J.T., Holbrook, N.M., 2003. Multi-annualclimate in Parque Nacional Pan de Azcar, Atacama Desert, Chile. Revista
Chilena de Historia Natural 76, 235254.Van Genuchten, M.T., 1980. A closed-form equation for predicting the
hydraulic conductivity of unsaturated soils. Soil Science Society ofAmerica Journal 44 (5), 892898.
Williamson, M.A., Rimstidt, J.D., 1994. The kinetics and electrochemical rate-determining step of aqueous pyrite oxidation. Geochimica et Cosmochi-mica Acta 58, 54435454.
Wisskirchen, C., Dold, B., Spangenberg, J.E., 2006. Hydrogeochemical andstable isotope study of the watershed of the El Salado valley and itswaters inltrating into marine shore tailings deposit at Chaaral(northern Chile). Congreso Geolgico Chileno Vol. 2. Symposio Hidro-geologa, pp. 671674.
Wolery, T., Daveler, S., 1992. EQ6, a computer program for reaction pathmodeling of aqueous geochemical system: theoretical manual, user'sguide, and related documentation (version7.0). UCLR-MA-110662 PT IV,Lawrence Livermore Natl. Lab. Livermore, California.
Wunderly, M.D., Blowes, D.W., Frind, E.O., Ptacek, C.J., 1996. Sulde mineral
oxidation and subsequent reactive transport of oxidation products inmine tailings impoundments: a numerical model. Water ResourcesResearch 32 (10), 31733187.
Xu, T.F., White, S.P., Pruess, K., Brimhall, G.H., 2000. Modeling of pyriteoxidation in saturated and unsaturated subsurface ow systems.Transport in Porous Media 39 (1), 2556.
82 S.A. Bea et al. / Journal of Contaminant Hydrology 111 (2010) 6582
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