NATURAL ARSENIC IN GROUNDWATER: OCCURRENCE, REMEDIATION AND MANAGEMENT - CHAPTER 15 ppsx

125
Natural Arsenic in Groundwater: Occurrence, Remediation and Management –
Bundschuh, Bhattacharya and Chandrasekharam (eds)
© 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X
Environmental behavior of arsenic in a mining zone:
Zimapán, Mexico
M.A. Armienta, R. Rodríguez, O. Cruz, A. Aguayo, N. Ceniceros
Instituto de Geofísica, UNAM, Circuito exterior C.U., México D.F., Mexico
G. Villaseñor
Instituto de Geología, UNAM, Circuito exterior C.U., México D.F.
L.K. Ongley
Oak Hill High School, Sabatus, USA
H. Mango
Dept. of Natural Sciences, Castleton State College, Castleton, USA
ABSTRACT: The distribution and fate of arsenic in various environmental reservoirs have been
studied in Zimapán, México. Natural arsenic in groundwater is released by oxidation and dissolu-
tion of arsenic-bearing minerals. Mining wastes (tailings and smelter particulates) pollute some of
the shallow wells. Tailings contain high concentration of As. However, As mobility is limited by
oxidation-reduction, sorption, and formation of secondary minerals. Sequential extraction showed
that As is mainly retained in the residual and Fe/Al oxyhydroxides fractions. Arsenic occurs also
in beudantite and K-jarosite. High arsenic concentrations are found in soils and river sediments
near tailings, slag piles and mineralized areas. Due to the semi-arid climate, few crops are grown
in this area. Nevertheless, wild flora has grown on tailing and slag piles, and As-rich soils. Arsenic
is absorbed from these As-rich substrates and translocated to the stem, leaves, flowers and fruits.
The concentrations and fate of arsenic varies depending on the arsenic source.
1 INTRODUCTION
Arsenic concentrations above the drinking water standard have been found in the groundwater of
various parts of México (Armienta 2003). Some of this pollution has occurred in mineralized
areas. Mining has been an important economic activity in México since the XVI Century. Many
towns were founded and developed around mines. As a result of the ore extraction and processing,
wastes were produced and accumulated in these towns. The accumulated residues have the poten-

tial to release metals and metalloids that pollute the environment. Furthermore, high concentration
levels of toxic metals may be present naturally in mining zones (Runnels et al. 1992). Arsenic min-
erals are widespread in México which is the fifth largest As producer in the world.
Although health effects of arsenic-polluted groundwater do not represent a major nation-wide
problem, in locations like Comarca Lagunera it became an endemic problem (Cebrián et al. 1983).
Besides, about 10% of milk samples collected from dairy farms at Comarca Lagunera were
reported to contain arsenic over the permitted level (Rosas et al. 1999). More than 60% of the
drinking water in México comes from groundwater. It is thus important to be able to identify the
sources and understand the fate of arsenic in all the exploited aquifers in México.
Zimapán is a low-income community with nearly 15,000 inhabitants. Groundwater is the only
drinking water source for residents of this region. This mining area has been active for more than
400 years. The population living in the urban part of Zimapán has consumed water containing As
Copyright © 2005 Taylor & Francis Group plc, London, UK
varying from 0.19 mg/L to 0.65 mg/L (average 0.38mg/L) for more than twelve years. In addition,
some residents in the outskirts of the urban area were supplied with water of higher As content for
these years. Health effects related to As intake have been detected in Zimapán inhabitants (Armienta
et al. 1997a).
2 ARSENIC IN GROUNDWATER
Zimapán, located in a semi-arid zone, in the central part of México, is one of many mining dis-
tricts in the country. Silver, Pb and Zn are currently exploited in Zimapán. The ore occurs as skarn
and chimney/manto mineralization. The main minerals occur as massive sulfide ores: pyrite,
pyrrhotite, sphalerite, galena, chalcopyrite, arsenopyrite, tetrahedrite and lead sulfosalts (Villaseñor
et al. 1987). In addition to arsenopyrite, several other arsenic minerals also occur: scorodite
(FeAsO
4
и2H
2
O), lolingite (FeAs
2
), tennantite ((Cu,Fe)

12
As
4
S
13
), adamite (Zn
2
(AsO
4
)(OH), mimetite
(PbS(AsO
4
)3Cl) and hidalgoite (PbAl
3
(AsO
4
)(SO
4
)(OH)
6
).
A complex aquifer system underlies the Zimapán basin. A deep fractured aquifer is developed
in Cretaceous limestones (Soyatal and Tamaulipas formations). A granular shallow aquifer exists
within Quaternary alluvium and Tertiary volcanic rocks in the center of the basin. A volcanic
aquifer is developed in the eastern part of the valley (Armienta et al. 1997b). The more productive
wells (more than 30 L/s) drilled in the fractured limestone aquifer also contain the highest As con-
centrations. Those wells are located in the proximity of intrusive bodies and dikes where As min-
erals are found (Armienta et al. 2001). Groundwater with low As concentration (from non-detectable
up to 0.05 mg/L) is found in the volcanic aquifer, which has low productivity (less than 10L/s).
Arsenopyrite oxidation and scorodite dissolution have produced arsenic contamination in some

of the deep wells (up to 180 m depth) drilled in limestones. A correlation may be observed
between As and sulfate in those wells (Fig. 1). Other processes (mainly FeS dissolution and oxi-
dation of As-rich pyrite) may release As and sulfate to those wells. FeAsS oxidation is reflected
also in the relatively lower Eh, higher temperature (Armienta et al. 2001) and lower HCO
3
/SO
4
2Ϫ
ratios recorded in polluted wells.
Tailing piles oxidation has polluted nearby shallow wells. These wells had higher sulfate con-
tents than deep contaminated wells. A mixing line among As-free water within the shallow aquifer
and tailings leachate reflects the interaction between tailings and shallow groundwater (Fig. 2).
Sulfate ions result from oxidation of various sulfide minerals within the tailings, mainly pyrite,
pyrrothite, arsenopyrite, chalcopyrite, and sphalerite (Romero et al. 2004a). The correlation between
arsenic and sulfate shows the influence of the oxidation of the tailings.
126
R
2
= 0.444
0
20
40
60
80
100
120
140
160
0 0.2 0.4 0.6 0.8 1 1.2
As (mg/L)

SO
4
(mg/L)
Figure 1. Arsenic vs sulfate concentrations in deep wells in limestone aquifer.
Copyright © 2005 Taylor & Francis Group plc, London, UK
Lower As concentrations were measured in shallow wells within the Soyatal limestone, in a
small zone near mineralized dikes away from the influence of the tailings (Fig. 3). A good correl-
ation was observed between As and SO
4
2Ϫ
in these wells further indicating that As results from
oxidation of arsenic-bearing sulfides.
Arsenic is mobilized by different mechanisms in the shallow granular aquifer and in the deep
fractured limestone aquifer. Arsenic in the deep aquifer is transported through fractures, with a
higher water-rock ratio. Arsenic in shallow wells flows as a function of sediment permeability in
the granular aquifer, resulting in a lower water-rock ratio. Released As may be retained on calcite,
iron oxyhydroxides and clays in the aquifer matrix (Romero et al. 2004b).
3 ARSENIC IN TAILINGS
Old and some recent tailing piles are found along the Zimapán town margins. The main mineral-
ogy includes quartz, calcite, gypsum, arsenopyrite, and jarosite. Sulfates in water leachates ranged
127
R
2
= 0.9026
0
200
400
600
800
1000

1200
1400
1600
1800
2000
0 0.2 0.4 0.6 0.8 1
As (mg/L)
SO
4
(mg/L)
Figure 2. Arsenic vs sulfate in shallow wells. Polluted wells are located next to tailing piles. Open circle
corresponds to a tailings’ leachate composition. High sulfate concentrations reflect tailings’ oxidation.
R
2
= 0.7642
0
20
40
60
80
100
120
140
160
180
200
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
As (mg/L)
SO
4

(mg/L)
Figure 3. Arsenic vs sulfate concentration in shallow wells away from tailings. Arsenic contamination
results from natural mineralization.
Copyright © 2005 Taylor & Francis Group plc, London, UK
from 512 mg/kg in unoxidized tailings to 2750mg/kg in oxidized tailings. The influence of calcite
on the physico-chemical characteristics of these wastes is reflected in the near neutral pH values
of most samples (from 6.4 to 7.6). A low pH (3.3) was measured in one oxidized pile whose min-
eralogy indicates calcite consumption and formation of secondary minerals such as gypsum and
jarosite.
High As contents have been measured in all tailing piles (up to 22,000mg/kg) in spite of their
different oxidation degress (Armienta & Rodríguez 1996). Nevertheless, As concentration in shal-
low wells next to tailings reached only 0.44 mg/L. These relatively low As concentrations in
groundwater may be explained by As retention processes occurring within the tailings. Mendez &
Armienta (2003) determined, through sequential extraction, that arsenic is mostly associated with
the low mobility Fe and Al oxyhydroxides, and other residual fractions in four tailing piles in
Zimapán. Arsenic associated with the most mobile soluble and exchangeable fractions was low in
most tailings (from 3 to 9%). However, this percentage reached 34% in one of the oxidized piles
that at the same time had a lower proportion in the Fe and Al fraction . This indicates that only part
of the arsenic may be available to be mobilized. The formation of arsenic-bearing secondary min-
erals, mainly beudantite and jarosite, and As sorption onto iron oxyhydroxides are also controlling
As dispersion (Romero et al. 2004a). These interactions have limited arsenic dispersion prevent-
ing higher concentrations than those observed in shallow wells next to tailings. Tailing piles occur
on top of alluvial material of high permeability; this has allowed leachate percolation. Differences
between tailings leachate concentration measured by Ongley et al. (2001) and groundwater As in
nearby wells indicates that water-rock interactions are playing an important role in As retention.
4 ARSENIC IN SOILS AND SEDIMENTS
Arsenic enrichment in soils next to tailing piles, former smelters and mineralized zones has
occurred at Zimapán. Superficial soils near tailings contain up to 2580 mg/kg of total As and
7.3 mg/kg of soluble As. Total and soluble As kept high values in 30 cm depth samples (up to
1070 mg/kg of total As, and 8.4mg/kg of soluble As) (Armienta & Rodríguez 1995). Arsenic con-

centrations up to 4200 mg/kg were measured in soils near former smelters or smelter wastes.
Soluble As increases with depth in some sites from 5 mg/kg on the surface to 14.3 mg/kg at 50 cm
depth. Settling of As-rich smelter particulates, wind dispersion of solid tailings, and leachate run-
off from tailings has contaminated soils around smelters and tailings. In addition, natural subsur-
face As mineralization has resulted on As concentration as high as 768 mg/kg in mineralized
zones. Soils more than 4000 m from tailings and slags generally had less than 40mg/kg As
(Ongley et al. 2003). Percolation of water through polluted soils has also increased As water con-
tent in shallow wells near former smelters.
Sediments of the Tolimán river are also contaminated with arsenic. This non-perennial river is
the only surficial water body in the Zimapán basin. It flows next to tailings and reaches the mine
zone. The highest concentrations (up to 6575mg/kg) were measured in front of the tailings.
Arsenic content also increases in the mine area, reaching 5148 mg/kg (García et al. 2001). Arsenic
is mainly associated with Fe and Mn oxyhydroxides and residual fractions in sediments. Arsenopyrite
was identified in As-rich sediments influenced by tailings and in the mineralized zone (García
1997). Higher As contents corresponded to lower pH values. Oxidation of arsenopyrite and other
sulfide minerals may decrease the pH and increase the arsenic concentration. Released As may
then be partly retained onto Fe and Mn oxyhydroxides.
5 ARSENIC IN PLANTS
Arsenic was analyzed in samples from mesquite (Prosopis laevigata), huizache (Acacia
farnesiana), and pepper tree (Schinus molle), all of which are naturally growing in mining
wastes (Fig. 4). The highest concentration was measured in a mesquite root (1400 mg/kg) located
128
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on a non-oxidized tailing pile. Much lower concentration was found in leaves of the same tree
(66 mg/kg). These concentrations are much higher than those found in mesquite growing on agar
solution with 5 ppm As
2
O
3
(Aldrich et al. 2002). A similar As content was measured in the root of

the fern Pteris vittata. This fern has been found to be extremely efficient in extracting arsenic from
soils. Fern specimens growing on a contaminated soil with chromate copper arsenate contained
1442–7526 ppm, and much higher concentrations (up to 21290 ppm) were accumulated by this fern
in plants when grown over artificially contaminated soils containing 500 ppm As (Ma et al. 2001).
A huizache tree growing on a slag pile also had high As content (102 mg/kg in leaves, and 119
in stems). Arsenic concentration in a pepper tree on a slag pile was 99mg/kg in the stem, 115mg/kg
in the leaves and 62mg/kg in the flowers. Lower As concentrations were measured in pods (up to
30 mg/kg). These plants are not edible; however, mesquite and huizache leaflets and pods are con-
sumed by goats, which are the most important livestock in the area. In addition, cultivated areas
cover a small proportion of the total Zimapán region (about 3%). Plants may therefore indirectly
be another source of As for Zimapán inhabitants.
6 CONCLUSIONS
Arsenic is widely distributed in Zimapán. Mining wastes and natural arsenic-bearing mineraliza-
tion release As to water, soils and plants. Geochemical processes create differences among these
sources. The highest As concentrations in groundwater are produced by natural processes, primar-
ily the oxidation and dissolution of As-bearing minerals. This process takes place mainly near
mineralized zones. Arsenic from this source may be transported long distances in the deep frac-
tured aquifer. Mining wastes have polluted shallow wells near the residues. Once in the shallow
aquifer, As transport may be retarded by water-rock interactions. Complex processes occur within
tailings that release and retain arsenic. Although total As concentrations up to 22,000 mg/kg have
been measured in tailings, only a low proportion (less than 10% for most tailing piles) is easily
available to the environment. Soils and sediments are also enriched in As due to the presence of
tailings, smelter particulates and general arsenic mineralization. Water infiltration through these
substances pollute some shallow wells. Wild plants growing on mining residues absorb arsenic
which is then mainly concentrated in roots. However, As is translocated to stems, leaves, flowers and
pods and may enter the food chain by being consumed by goats.
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Figure 4. A mesquite tree growing on a tailings pile.
Copyright © 2005 Taylor & Francis Group plc, London, UK
Understanding of As behavior in mining areas requires comprehensive studies to identify the

sources of As and their contribution to As contamination. This may allow for the development of
remediation alternatives that at the same time allow economic and social development of commu-
nities relying on mining, such as Zimapán.
ACKNOWLEDGEMENTS
The authors thank the financial support given by grant 017 of the National Council of
Science and Technology, CONACyT, -SEMARNAT Mexico, by UNAM, and by the National Science
Foundation (USA).
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