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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: loisongley@earthlink.net (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:loisongley@earthlink.nethttp://www.elsevier.com/locate/envpolhttp://www.elsevier.com/locate/envpolmailto:loisongley@earthlink.net

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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.

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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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