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Arsenic in the soils of Zimapan, Mexico
Lois K. Ongley a,*,1, Leslie Sherman b, Aurora Armienta c, Amy Concilio d,Carrie Ferguson Salinas e,2
a Oak Hill High School, P.O. Box 400, Sabattus, ME 04280, USAb Department of Chemistry, Washington College, 300 Washington Avenue, Chestertown, MD 21620, USA
c Instituto de Geofsica, UNAM, Mexico D.F. 04510 , Mexicod Department of Earth, Ecological, and Environmental Sciences, University of Toledo, Toledo, OH 43606, USA
e Department of Agronomy and Environmental Management, Louisiana State University, Baton Rouge, LA 70803, USA
Received 15 December 2004; accepted 1 May 2006
Much of the arsenic is relatively immobile but presents long-term source of arsenic.
Abstract
Arsenic concentrations of 73 soil samples collected in the semi-arid Zimapan Valley range from 4 to 14 700 mg As kg1. Soil arsenic con-
centrations decrease with distance from mines and tailings and slag heaps and exceed 400 mg kg1 only within 500 m of these arsenic sources.
Soil arsenic concentrations correlate positively with Cu, Pb, and Zn concentrations, suggesting a strong association with ore minerals known to
exist in the region. Some As was associated with Fe and Mn oxyhydroxides, this association is less for contaminated than for uncontaminated
samples. Very little As was found in the mobile water-soluble or exchangeable fractions. The soils are not arsenic contaminated at depths greater
than 100 cm below the surface. Although much of the arsenic in the soils is associated with relatively immobile solid phases, this represents
a long-term source of arsenic to the environment.
2006 Elsevier Ltd. All rights reserved.
Keywords: Arsenic; Mining; Trace metals; Pollution; Smelter; Sequential extraction
1. Introduction
1.1. Arsenic in mining regions
Elevated soil arsenic concentrations due to mining and
smelting have been reported around the world (see for
example, Davis et al., 1996; Filippi et al., 2004; Ghoshet al., 2004; La Force et al., 2000; Lombi et al., 2000; Matera
et al., 2003; Moore et al., 1988; Nriagu, 1994; Razo et al.,
2004; Stuben et al., 2001). Soil arsenic concentrations in these
contaminated locations range from 500 to 17 000 mg kg1. In
most cases, arsenic concentrations decrease with increasing
distance from tailings piles and impoundments and from
active or retired smelters (Garca et al., 2001; Magalh~aes
et al., 2001; Nriagu, 1994; Razo et al., 2004). Arsenic concen-
trations in uncontaminated soils average 5e6 mg kg1 with
a range of 0.1e
40 mg kg1
(NAS, 1977) although high natu-rally occurring arsenic concentrations have been reported
(Hansen et al., 2001; Lalor et al., 1999).
Arsenic is known to adsorb to iron and manganese oxyhydr-
oxides, clays, carbonates and organic matter (Cheng et al.,
1999; Dixit and Hering, 2003; Goldberg, 2002; Goldberg and
Glaubig, 1988; Lin and Puls, 2000; Manning et al., 1998;
Romero et al., 2004; Thanabalasingam and Pickering, 1986).
In soils contaminated by mining activities, arsenic has been
found to be primarily associated with amorphous iron oxyhydr-
oxides in soils (Ahumada et al., 2004; Filippi et al., 2004;
* Corresponding author. Tel.: +1 207 948 3131.
E-mail address: [email protected] (L.K. Ongley).1 Associate Professor of Chemistry, Unity College, Unity, ME 04988 USA;
Formerly at Bates College, Lewiston, ME, USA.2 Formerly at Department of Geology, Centenary College, Shreveport, LA,
USA.
0269-7491/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.envpol.2006.05.014
Environmental Pollution 145 (2007) 793e799www.elsevier.com/locate/envpol
mailto:[email protected]://www.elsevier.com/locate/envpolhttp://www.elsevier.com/locate/envpolmailto:[email protected] -
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Ghosh et al., 2004; Lombi et al., 2000; Matera et al., 2003; Van
Herreweghe et al., 2003). Arsenic can also form secondary min-
erals, such as scorodite and sulfidic minerals, or can co-precip-
itate with other minerals (Fendorf et al., 2004; Filippi et al.,
2004; Moore et al., 1988; Reynolds et al., 1999).
1.2. Arsenic in Zimapan, Mexico
In Zimapan, Pb, Zn, and Ag minerals have been extensively
mined since about 1576 (Daz, 1995). The ore at Zimapan
sometimes contains an appreciable amount of arsenic up to
16 wt% (Ongley et al., 2001). The dominant arsenic-bearing
mineral is arsenopyrite, however, some ore bodies include ten-
nantite, lead sulfosalts, and lollingite; the most common arse-
nic-bearing secondary mineral is scorodite (Armienta et al.,
2001).
Large tailings piles (up to 1.9 wt% As) dominate the land-
scape along the Toliman River (Fig. 1). Fresh tailings are a dry
gray powder fine enough to form aeolian ripples on the flankof the tailings. Weathered tailings are red-orange in color
and often display white and yellow crystallization on the sur-
face. While there are no smelters currently in operation in
Zimapan, tall smokestacks and slag piles still mark a few of
the former smelter sites.
In Zimapan, elevated groundwater arsenic concentrations
exceed the WHO drinking water standard more than 10-fold
(Armienta et al., 1997a; Ongley et al., 2001). Arsenic-related
health impacts have been found in the region, including hypo-
pigmentation, hyperpigmentation, and hyperkerathosis
(Armienta et al., 1997b).
1.3. Scope of this study
The focus of this study was to determine the distribution of
arsenic in the surface soils of Zimapan. We hypothesize that
soil arsenic concentrations will be highest in the immediate vi-
cinity of mines, tailings and former smelter sites. Soil arsenic
concentrations should decrease with depth reflecting the his-
toric record as well as indicating minor movement of arsenic
down the profiles. We also hypothesize that arsenic should
be found primarily associated with metal oxyhydroxides or
with refractory minerals such as arsenopyrite.
2. Materials and methods
2.1. Site description
Zimapan is located in the state of Hidalgo, Mexico, about 150 km north of
Mexico City at an elevation of about 1770 m. PbeZneAg ore is found as mas-
sive sulfide skarn deposits in the limestones to the north and west of town and
in occasional hydrothermal veins in the extreme eastern portion of the valley.
The soils of the region are predominately regosols and lithosols developed
from fresh alluvium and rock, respectively (INEGI, 2005). Soil thickness
Fig. 1. Soil arsenic concentration in the Zimapan Valley, Mexico (Group A, open circles; Group B, open triangles; Group C, closed circles; Group D, closed
triangles). The locations of sequential extraction (SE) and profile (P) samples are indicated. Mines are indicated by an asterisk, tailings by gray polygons.
794 L.K. Ongley et al. / Environmental Pollution 145 (2007) 793e799
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ranges from
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less than 3500 m from tailings, mines and smelters. These
soils appear to have received some particulate deposition
from smelters and tailings.
Soil arsenic concentrations exceeding 400 mg kg1 were
concentrated in a radius of about 500 m around mines, tailingsheaps, and former smelters. The very contaminated soils were
taken in the vicinity of the slag heaps or from an arroyo
(Arroyo Santa Maria) that drains a process wastewater reser-
voir from the San Miguel Nuevo tailings, where the aqueous
arsenic concentration is about 0.4 mg l1; the fine gray wind-
blown tailings particles are evident around the arroyo. Indeed
one of the samples was originally taken because of its similar ap-
pearance to the nearby tailings, 400 m away; it had an arsenic
concentration of 30 000 mg kg1. (This sample was not in-
cluded in the correlations.) The developing soil on top of one
slag heap was extremely arsenic rich (14 700 mg kg1).
3.2. Arsenic correlation to other metals
Rank correlation coefficients were calculated for As with
Ca, Cu, Fe, Mn, Pb, and Zn (Table 2). Arsenic was signifi-
cantly correlated with Cu, Pb, and Zn (p< 0.01) and Ca
(p< 0.05). Upon examination of a plot of As vs. Ca, the cor-
relation seems fortuitous and due simply to the fact that at high
Ca concentrations, the As concentrations are constant
(Fig. 2a). This is distinctly different from the graphs of As
with Cu, Pb and Zn (Fig. 2b). There is no significant correla-
tion of As with Fe.
The relationships of As with Cu, Zn, and Pb, as well the
significant correlation between these metals suggest thatmuch of the arsenic in the soils is from primary minerals of
the local ores and tailings, as was found in a neighboring
Mexican mining area (Razo et al., 2004). The semi-arid cli-
mate likely minimized the weathering of these minerals.
3.3. Arsenic speciation in surficial samples
Most As in the soils of the Zimapan area is relatively immo-
bile, bound to iron and manganese oxyhydroxides, organic mat-
ter, and carbonates or found in the residual mineral fraction, i.e.
in refractory minerals. In each sample, the soluble and
exchangeable fractions have low to undetectable arsenic
concentrations (
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west of town, had slightly elevated arsenic concentrations
(59 mg kg1) at the surface.The Via Real profile, which had sur-
face arsenic concentration of 640 mg kg1, was only 100 m
from tailings. The Banamex profile, located in a construction
site, also hadelevated arsenic in the layer 20 cm from the surface
(880 mg kg1). The nearest old smelter was less than 100 m
away and so the contamination is likely due to smelter particu-
late deposition. The surface layer was not sampled for the pro-
file, as the material appeared to be construction fill.
In each profile, arsenic concentrations decrease with depth
within the first 50 cm, most dramatically for profiles within
arsenic-influenced zones (Fig. 3). Below this depth, the soilswere uncontaminated or only slightly contaminated. The pro-
file with surface contamination of arsenic suggests that some
movement of arsenic may have occurred down the profiles
due to leaching perhaps from garden irrigation. This urban
recharge has high arsenic concentrations, up to at least
0.3 mg L1, as well as the capability to dissolve some soil
arsenic and carry it slightly deeper (Ongley et al., 1999).
3.5. Arsenic speciation in soil profiles
In the contaminated Banamex soil profile, as with the con-
taminated surface soils, most of the As was associated with the
residual fraction. The percentage of As associated with the re-
sidual fraction decreased with depth in the profile, while the
As percentage in the Fe and Mn oxyhydroxide fraction in-
creased. These two Fe-rich fractions may explain the lack of
correlation of As with total Fe and with Mn. The high level
of residual arsenic in the top layer most likely reflects the orig-
inal smelter dry deposition deposits, some of which has
leached down the profile to adsorb to or co-precipitate withsecondary Fe and Mn oxyhydroxides. In the oxidizing condi-
tions of the soil, these oxyhydroxides are stable. In the event of
anaerobic conditions, this As phase would likely be mobilized
by reduction to As(III). Since the total As concentration at
120 cm in the soil is only slightly contaminated and is in min-
eral form or bound to minerals, it is unlikely to leach to the
groundwater.
1
10
100
1000
10000
100000
0 5 10 15 20 25 30
Arsenic(mg/k
g)
Fe Ca
1
10
100
1000
10000
100000
1 10 100 1000 10000 100000
Metal (mg/kg)
Arsenic(mg/k
g)
Cu Pb Zn
Calcium or Iron (wt )
(a) (b)
Fig. 2. (a) Arsenic plotted as a function of Fe and Ca concentrations for the soil samples. (b) Arsenic in soils plotted as a function of the metals: Cu, Pb and Zn.
Table 3
Arsenic sequential extraction results
Sample Water-soluble (%)
(range)
Exchangeable (%)
(range)
Carbonate (%)
(range)
Fe/Mn bound (%)
(range)
Organic (%)
(range)
Residual
(%)
Total As
(mg/kg)
SurfaceTenguedho nd nd nd 102 (3.3e200) nd 2 6
San Pedro nd nd 16 (nde31) 40 (34e60) 20 (20e24) 24 10
La Perla III nd nd 3.5 (3.4e3.6) 100 (45e140) 73 (13e91) 76 11
Ojo de Agua
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4. Conclusions
The spatial distribution of arsenic in the soils of the Zimapan
Valley is highly influenced by the mining and ore processing
activities of the past four centuries. The most contaminated
area is concentrated along the southern boundary of town
near the tailings along the river. Soil contamination generallydecreases with distance from a known arsenic source. In gen-
eral, soil arsenic concentration decreases with depth in the
profile.
There was no significant correlation between As and Fe in
the soils. Arsenic is more abundant in residual fractions in areas
of anthropogenic influence, while in areas not likely contami-
nated by smelter fumes or tailings material, a greater arsenic
fraction was found to occupy non-residual fractions, such as
the Fe/Mn bound fraction. In the arid environment of the
Zimapan Valley, arsenic in both of these fractions is stable,
and so in the short term, this As is not very mobile. However,
the soils may represent a long-term source of arsenic to theenvironment with weathering.
Acknowledgements
This material is based on work supported by the National
Science Foundation under Grant Nos. 9619810 and 9424249.
Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the authors and do
not necessarily reflect the views of the National Science Foun-
dation, Bates College, or the Univerisdad Nacional Autonoma
de Mexico.
Peter Beeson developed the Geographic Information Sys-
tem database. Samples and data were collected by more than
45 members of Team REU Zimapan over the course of the
program.
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