NATURAL ARSENIC IN GROUNDWATER: OCCURRENCE, REMEDIATION AND MANAGEMENT - CHAPTER 7 pptx

Natural arsenic in the groundwater of the alluvial aquifers of
Santiago del Estero Province, Argentina
Prosun Bhattacharya, Mattias Claesson & Jens Fagerberg
Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering,
Royal Institute of Technology (KTH), Stockholm, Sweden
Jochen Bundschuh, Angel del R. Storniolo, Raul A. Martin & Juan Martin Thir
Facultad de Ciencias Exactas y Tecnologias, Universidad Nacional de Santiago del Estero (UNSE),
Santiago del Estero, Argentina
Ondra Sracek
Institute of Geological Sciences, Faculty of Science, Masaryk University, Brno,
Czech Republic
ABSTRACT: Natural occurrences of arsenic has been documented in groundwater of the shal-
low aquifers of the Chaco-Pampean Plain, Argentina. The distribution of arsenic and mechanisms
of its mobilization in the shallow alluvial aquifers was investigated around the city of Santiago del
Estero in Northwestern Argentina in order to provide an insight into the complex hydrological and
geochemical conditions that yields high As concentrations in groundwater. Significant spatial
variations of total arsenic (As
tot
) concentrations were observed with an average value of 743 ␮g/L.
Arsenate was a dominant species in most samples. Average concentrations of Al, Mn, and Fe were
360 ␮g/L, 574 ␮g/L, and 459 ␮g/L, respectively. The 7M HNO
3
extraction of sediments and vol-
canic ash-layer indicated As
NO3
concentrations ranging between 2.5–7.1 mg/kg. As
NO3
indicated a
significant positive correlation with Mn
NO3
, Al

NO3
, and Fe
NO3
. Oxalate extractions revealed sig-
nificant fractions of As (As
ox
) in the sediments (0.4–1.4 mg/kg) and a dominance of oxalate
extractable Al- and Mn. Speciation calculations indicate that Al oxide and hydroxides have the
potential to precipitate in the groundwater, suggesting that As adsorption processes may be to
some extent controlled by Al oxides and hydroxides. Mobility of As at local scale seems to depend
on high pH values, related to the dissolution of carbonates driven by cation exchange, and dissol-
ution of silicates. There is a clear relationship of As with F, V, B and Si, suggesting their common
origin in volcanic ash layer. Preliminary conceptual model of arsenic input includes release of As
and Al from dissolution of volcanic ash layer, precipitation of Al oxides and hydroxides followed
by adsorption of As on Al and Fe phases in sediments, and release of As under high pH conditions.
1 INTRODUCTION
Arsenic (As) is a natural inorganic contaminant in drinking water, which is known to have caused
serious environmental health problems globally (Bhattacharya et al. 2002a, Smedley & Kinniburgh
2002). Elevated concentrations of As from geogenic sources are reported in groundwaters in dif-
ferent parts of the world such as Argentina, Bangladesh, China, Nepal, Mexico, Vietnam, and
United States among others (Bhattacharya et al. 2002a, Smedley & Kinniburgh 2002, Bhattacharya
et al. 2004). Natural occurrences of As has been documented in groundwaters in Argentina from
the alluvial aquifers of the Chaco-Pampean Plain, where a population of approximately 1.2 mil-
lion mostly in rural settlements are exposed to As from local drinking water sources. The concen-
trations of As in the groundwater are mostly above the limit of safe drinking water (10 ␮g/L; WHO
57
Natural Arsenic in Groundwater: Occurrence, Remediation and Management –
Bundschuh, Bhattacharya and Chandrasekharam (eds)
© 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X
Copyright © 2005 Taylor & Francis Group plc, London, UK

2001) as well as the local drinking water standard of 50␮g/L. The first symptoms of arsenic
related diseases were detected in 1983 within the counties of La Banda and Robles, and in the fol-
lowing year investigations by the government agencies confirmed elevated arsenic concentrations
(above 1000␮g/L) in groundwater of the shallow aquifers around the provincial capital of
Santiago del Estero in North-western Argentina (Martin 1999). In the recent years, chemistry of
groundwater of the shallow aquifers within selected areas of the alluvial cone of the river Río
Dulce of Santiago del Estero province (Bejarano & Nordberg 2003, Claesson & Fagerberg 2003,
Bundschuh et al. 2004) have been studied in order to understand the mechanisms of arsenic mobil-
ization in these aquifers.
The aim of this study was to investigate the distribution of As in the groundwater of shallow
aquifers located within the alluvial cone of the river Río Dulce around the city of Santiago del
Estero and the associated shallow sediments in order to develop a better understanding of the com-
plex hydrogeological and geochemical conditions that are responsible for the mobilization of As
in groundwater.
2 LOCATION AND GEOLOGICAL SETTING
2.1 The study area
Santiago del Estero is the provincial capital of the province with the same name. It is located on
the dry, western part of the Chaco plain in northwestern Argentina at an altitude of about 200
meters above sea level (Fig. 1). The study area is located to the east of the capital on the alluvial
deposits formed by the Río Dulce (hereinafter referred as the Río Dulce alluvial cone), covering
an area of approximately 2000km
2
. The area is rural, but densely populated in comparison with
surrounding countryside due to its fertile soils and irrigation systems built up by channels from the
Río Dulce. Small agricultural settlements dependent on artificial irrigation are common through-
out the Río Dulce cone.
Hot, rainy summers lasting from November to March and very dry winters from April to October
characterize the climate. The average annual precipitation (1938–90) is 532 mm. Evapotranspiration
is very high, especially in summer period. The area was originally forested but due to intensive
timber harvesting only limited forest areas remain. Vegetation generally consists of low bushes

and ground vegetation. The terrain is very flat with few shallow depressions resulting from his-
torical flow-paths of the river. Strong winds are common and carry a lot of dust during winter
when binding surface water is scarce.
2.2 Geological and hydrogeological characteristics
Río Dulce alluvial cone is limited to the west by the Huyamampa fault. A sequence of 30m
Pleistocene Pampean loess is found to the west of this fault comprising dispersed volcanic ash and
calcareous crusts, which is underlain by lower Pliocene and green Miocene clays with a thickness
of 70 m. Holocene to recent sediments of the Río Dulce cone (Post-Pampean formation) occur to
the east of the fault, and the thickness of the fluvial and aeolian sediments is nearly 150m close to
the fault margin and pinches out about 50 km towards the east. A sequence of alternating layers of
gravel, sand, silt and clay is deposited in discordance with the underlying Pliocene sediments. The
coarser sediments represent the deposits of the palaeo-channels of the river Río Dulce, which form
a multi-layered aquifer system in the region.
Santiago del Estero is located just south of where Río Dulce passes the fault of Huyamampa
(Fig. 2). The fault runs from north to south and the river passes from northwest to southeast. West
of the fault Pliocene and Miocene clays are covered by approximately 30m of Pampean loess
(Bundschuh et al. 2004). Southeast of the fault, the river have deposited alluvial sediments making
up the Río Dulce cone. Depth to ground water table ranges from 1 to 6m. The upper-most aquifer,
the one of major interest in this study, reaches an approximate depth of 15 m (Martin 1999). An
58
Copyright © 2005 Taylor & Francis Group plc, London, UK
important fraction of groundwater recharge to all aquifers, including the shallow one, takes place
in a limited section of coarse material along the Huyamampa fault in the northwestern part of the
study area. Here all aquifers in the alluvial cone are connected to the surface and are recharged by
infiltrating river and surface water. The aquifers are considered to be semi confined with little
interactions among them. Major groundwater flow is horizontal in the separate aquifers.
The upper-most aquifer consists of aeolian and fluvial sediments (Bundschuh et al. 2004) and
has important recharge from the river and surface not only near the Huyamampa fault, but all over
its area. Irrigation channels are non-lined which may lead to significant infiltration losses. Local
groundwater flow patterns yielding long residence times in natural depressions are likely to be of

importance for groundwater chemistry. General direction of flow in the upper aquifer is towards SE
(Fig. 2) and several localized discharge areas occur in topographic lows. However, data on local
groundwater flow patterns and hydraulic conductivity of the upper-most aquifer are very limited.
59
Figure 1. Digital elevation model of the southern part of South America with location of the project
area “Río Dulce alluvial cone” located near the city of Santiago del Estero in North western Argentina (digital
elevation model modified from PIA03388 image; ). Other areas with
groundwater arsenic are also shown.
Copyright © 2005 Taylor & Francis Group plc, London, UK
3MATERIALS AND METHODS
3.1 Groundwater and sediment sampling
Groundwater samples were collected during September–October 2002 from the counties Robles
and La Banda (Fig. 2), both within a distance of not more than 50 kilometers from the city of
Santiago del Estero. Forty well sites were selected where groundwater was mainly abstracted by
hand-pumped tube-wells penetrating the shallow aquifers, that reached a maximum depth of 12 m.
Sampling points are shown in Figure 2.
The well positions at each of the sampling site were determined using Global Positioning
System (GPS). The values of pH, redox potential (Eh), temperature and electrical conductivity of
groundwater were measured in the field. Water samples collected from each well involved: (i) fil-
tered samples for alkalinity and major anion analysis; (ii) filtered, acidified samples for major
cation and trace elements analysis; (iii), sample for DOC analysis, and (iv) filtered through a
Disposable Cartridge
®
for field separation of As(V) and As(III).
Sediment samples were collected from two sites, Balsamo Cuatro Horcones (CH 47) and Nuevo
Libano (NL 30) (Fig. 2) up to a depth of 1.2m using an auger drill in the immediate vicinity of
tube wells to study the sediment-groundwater interactions. Groundwater and the sediment were
analysed following the procedure outlined by Bhattacharya et al. 2001, 2002b.
3.2 Analytical methods
Anions such as Cl

Ϫ
and SO
4
2Ϫ
were analyzed by a Dionex 120 ion chromatograph, and NO
3
Ϫ
and
PO
4
3Ϫ
were analyzed using Tecator AQUATEC 5400 analyzer at wavelengths 540 nm and 690 nm,
respectively. The major and trace metals were analyzed on a Perkin Elmer Elan 6000 ICP-MS.
As(V) was calculated as a difference between total As and As(III) in the samples. Certified standards,
SLRS-4 (National Research Council, Canada) and GRUMO 3A (VKI, Denmark) and synthetic
60
28
23
El Quebrachal
FORRES
FERNANDEZ
Pozo Suni
Mistol
26
24
30
25
Buey Muerto
Las
Lomitas

27
31
Villa Hipólita
Morcillo
Cnia. Bobadal
Colonia
Chingolo
Buey Muerto
32
10
1
El Zanjón
Rubia Moreno
LA BANDA
del ESTERO
SANTIAGO
2
Villa Robles
Maco
La Florida
Los Arias
14
15
Mili
Cara
Pujio
22
Cnia. Jaime
Romano
Sto. Domingo

VILMER
4
12
3
La Bajada
11
8
6
7
20
21
13
16
San José
9
Chilca
La Mirella
BELTRAN
17
18
El Refugio
19
29
Janta
BANDA COUNTY
ROBLES COUNTY
Huyamampa fault
Rí
o Dulce
64˚10´

27˚50´
64˚10´
64˚
5 km
Sampled Well
Paved road
Irrigation canel
County limit
Urban area
groundwater
flow direction
Eastern limit of
alluvial cone
Figure 2. Detailed map of the Río Dulce alluvial cone, the location of the sampled wells and the generalized
patter of groundwater flow within the alluvial cone.
Copyright © 2005 Taylor & Francis Group plc, London, UK
chemical standards prepared in the laboratory, and duplicates were analyzed after every 10
samples during the runs. Trace element concentrations in standards were within 90–110% of their
true values. In case of wider variations, the standards were recalibrated and the preceding batch of
10 samples reanalyzed. Relative percent difference between the original and duplicate samples were
within Ϯ10%. Dissolved organic carbon (DOC) in the water samples were determined on a Shimadzu
5000 TOC analyzer (0.5mg/L detection limit with a precision of Ϯ10% at the detection limit).
4 RESULTS
4.1 Groundwater chemistry
Groundwater pH ranged between 6.4 and 9.3 with an average of 7.6. Field measured redox poten-
tial ranged from Ϫ60 to ϩ348 mV with an average value of ϩ153 mV. Electric conductivity (EC)
ranged between 804 and 9800 µS/cm with an average is 2422␮S/cm. Major ion composition indi-
cated Na
ϩ
(average concentration 427mg/L) and HCO

3
Ϫ
(581 mg/L) as the dominating ions in
groundwaters (Fig. 3). DOC concentrations in groundwater varied between below detection limit
and 18.2 mg/L with an average of 7.6mg/L.
Total arsenic (As
tot
) concentration indicated considerable spatial variations with an average of
743 ␮g/L. Some wells exhibited extremely high values, reaching a maximum of 14,969 ␮g As/L
(Fig. 4b). Speciation of As indicated the dominance of As(V) with average concentration of
617 ␮g/L, while the concentration of As(III) averaged around 125␮g/L. Among the other trace
elements, dissolved Al concentrations were low (average 360␮g/L, median 17␮g/L), while the
concentrations of Mn (average 574␮g/L, median 128␮g/L) and Fe (average 459 ␮g/L, median
140 ␮g/L) were higher. These groundwaters were characterized by high Si concentrations (average
28.1 mg/L). Fluoride concentrations were elevated (average 2.55 mg/L, median 1.26 mg/L), which
also exceeded the WHO limit for safe drinking water (1.5 mg/L; WHO 1993) in 16 out of the 40
sampled wells.
61
80
80
20
HCO
3
2
0
40
60
80
Ca
2+

20
40
60
80
80
Na
+
+
K
+
40
f
20
80
60
4
0
60
-
40
40
SO
4
60
20
d
b
a
40
60

e
c
20
g
Ca + Mg
2+
40
20
2-
SO
4
+ Cl
+ NO
3
20
-
-
80
80
Mg
2+
Cl
-
+ NO
3
-
2-
Figure 3. Representation of major ion chemistry in groundwater samples plotted on a Piper diagram.
Copyright © 2005 Taylor & Francis Group plc, London, UK
4.2 Sediment chemistry

Geochemical investigations have revealed considerable enrichment of arsenic in shallow aquifer
sediments. Extraction of the sediments samples collected at two sites (NL30 and CH47, Fig. 2) by
7M HNO
3
revealed As
NO3
concentrations in the range between 2.5–7.1 mg/kg which is higher than
the average As concentrations in soils and sediments (Taylor & McLennan 1985). The volcanic
ash-layer also had appreciable As
NO3
content (3.0 mg/kg). A significant positive correlation (Fig. 4)
was observed between As
NO3
and the concentrations of Mn
NO3
, Al
NO3
, and Fe
NO3
(R ϭ 0.76–0.79).
Sequential leaching of sediment samples was performed using deionized water (DIW), bicar-
bonate (HCO
3
), acetate and oxalate media, which extracted As at varying concentrations (Fig. 5).
Oxalate extractions reveal significant fractions of extractable As (As
ox
) in all the samples ranging
between 0.4–1.4 mg/kg (Fig. 5). Comparison of Fe, Al and Mn concentrations extracted by oxalate
clearly indicated dominance of Al- and Mn-oxides and -hydroxides as compared to Fe-oxides and
hydroxides (Fig. 6). Much higher concentrations at site CH47 seem to be related to the preferen-

tial dissolution of volcanic ash layer located at this site at about 1 m depth.
4.3 Speciation calculations
Results of saturation indices (SI) calculations for selected minerals calculated with program
MINTEQA2 are listed in Table 1. They show that crystalline forms of Al oxide and hydroxides
such as gibbsite are stable in groundwater, implying that As adsorption processes may be to some
62
0
2
4
6
8
0 10 20 30 40 50 60
As
NO
3
(mg/kg)
As
NO
3
(mg/kg)
Fe
NO
3
and Al
NO
3
(mg/kg)
a
0
2

4
6
8
200 400 600 800 1000
Mn
NO
3
(mg/kg)
b
Figure 4. Bivariate plots showing the correlation of As
NO3
with: a) Fe
NO3
(ᮀ – dashed line; R ϭ 0.79) and
Al (᭡ – solid line; R ϭ 0.79), and b) Mn
NO3
(R ϭ 0.76) in the shallow aquifer sediments.
0.0
0.5
1.0
1.5
2.0
0.0 1.0 2.0 3.0
Leached As (mg/kg)
a
0.0
1.0
2.0
3.0
4.0

0.0 1.0 2.0
Leached As (mg/kg)
b
DIW
Depth (m)
Depth (m)
HCO
3
Acetate
Oxalat
e
3.0
Figure 5. Sequential leaching of As from the sediment samples from: left: Balsamo Cuatro Horcones
(CH 47); right: Nuevo Libano (NL 30). Sediments were sequentially leached by de-ionized water (DIW),
bicarbonate (HCO
3
), acetate and oxalate.
Copyright © 2005 Taylor & Francis Group plc, London, UK
extent controlled by Al mineral phases. This is consistent with an important role of Al mineral
phases in oxalate extractable fraction. However, also ferric minerals like goethite are stable and
may be important As adsorbents.
5 DISCUSSION AND CONCLUSIONS
Potential primary source of both As and Al is volcanic ash layer, which comprises very soluble vol-
canic glass. Principal Component Analysis (Bhattacharya et al. 2004, submitted) shows the relation
of As with F, V, B and Si; most likely due to their common origin in volcanic ash, indicating its
importance as a source of As in shallow groundwater. This is exhibited by a fairly high correlation
(Fig. 7a–d) observed between the concentrations of As
tot
with F
Ϫ

(R
2
ϭ 0.43; p Ͻ 0.0001),
V (R
2
ϭ 0.67, p Ͻ 0.0001); B (R
2
ϭ 0.43; p Ͻ 0.001) and Si (R
2
ϭ 0.43; p Ͻ 0.001) in the
analyzed groundwater samples.
Redox conditions in the study area are oxidizing or moderately reducing. This is consistent with
predominating arsenate. Thus, reductive dissolution of ferric minerals observed, for example, in
Bangladesh (Smedley & Kinniburgh 2002, Ahmed et al. 2004) can be ruled out as a principal
mechanism of arsenic input. In contrast, high pH values seem to promote desorption of As
63
0.0
0.5
1.0
1.5
2.0
0 100
200 300 400 500 600
a
Oxalate leached Fe, Al, Mn (mg/kg)
Depth (m)
0.0
1.0
2.0
3.0

4.0
0 100
200 300 400 500
b
Depth (m)
Fe
Al
Mn
Oxalate leached Fe, Al, Mn (mg/kg)
Figure 6. Trends in the distribution of oxalate extractable Fe, Al and Mn with depth of the sediment samples
from: left: Balsamo Cuatro Horcones (CH 47); right: Nuevo Libano (NL 30).
Table 1. Results of speciation modeling using the MINTEQA2 program (Alison et al. 1991) and the SI val-
ues for selected Al, Fe, and Mn phases for ground water samples from five wells.
NL30 SC8 23 24 16
(As
tot
ϭ 8083 (As
tot
ϭ 1574 (As
tot
ϭ 669 (As
tot
ϭ 36 (As
tot
ϭ 23
Mineral ␮g/L) ␮g/L) ␮g/L) ␮g/L) ␮g/L)
SI SI SI SI SI
Al(OH)
3
(a) Ϫ1.98 Ϫ2.68 Ϫ2.50 Ϫ3.04 Ϫ1.35

Gibbsite 0.74 0.03 0.27 Ϫ0.33 1.37
Fluorite 1.09 Ϫ0.33 0.81 Ϫ1.46 n.a.
Fe(OH)
3
(a) 0.31 2.08 2.18 1.87 Ϫ1.58
Goethite 6.18 7.95 8.0 7.73 4.29
MnOOH Ϫ8.21 Ϫ4.19 Ϫ1.38 Ϫ2.94 Ϫ9.36
Rhodochrosite Ϫ0.32 Ϫ1.14 Ϫ0.21 0.28 Ϫ1.19
Siderite 0.57 Ϫ2.53 Ϫ4.18 Ϫ2.54 Ϫ0.81
SiO
2
(a) Ϫ0.57 Ϫ0.67 Ϫ0.47 Ϫ0.60 Ϫ0.60
Vivianite Ϫ1.00 Ϫ10.37 Ϫ21.04 Ϫ13.65 n.a.
n.a. – not available; (a) – amorphous phase.
Copyright © 2005 Taylor & Francis Group plc, London, UK
adsorbed on to the amorphous oxides of Al, Mn and Fe. In iso-curves plots, maximum concentra-
tions of As and high values of pH generally coincide (Fig. 8). High pH seems to be related to the
dissolution of carbonates induced by cation exchange. This is consistent with a negative correl-
ation between Ca and Na observed in earlier studies (Bundschuh et al. 2004). Another factor con-
tributing to high pH values is probably dissolution of silicates in volcanic glass. Smedley et al.
(2001) also postulated high pH as a factor contributing to high As concentrations in La Pampa
region located southeast of the study area.
Tentative mechanism of arsenic mobilization can be summarized as follows: (a) dissolution of
volcanic ash layer with resulting release of As and Al; (b) precipitation of Al oxide and hydroxides
and adsorption of As on Al and Fe mineral phases; and (c) release of As under high pH conditions.
However, the importance of volcanic ash as the source of As still remains unproved and further
investigation of the interaction between the ash layer and aqueous chemistry should be prioritized
in further studies.
64
y = 0.0048x + 0.658

R
2
= 0.4284
0,0
1,0
2,0
3,0
4,0
5,0
6,0
0 100 200 300 400 500 600 700 800
F
Ϫ
(mg/L)
a
y = 0.7583x + 2.1426
R
2
= 0.674
0
100
200
300
400
500
600
700
800
0 100 200 300 400 500 600 700 80
0

V (µg/L)
b
As
tot
(µg/L)
y = 0.0035x + 0.7499
R
2
= 0.484
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
8,0
0 100 200 300 400 500 600 700 800
B (mg/L)
c
As
tot
(µg/L)
y = 3.6975 ln(x) + 13.19
6
R
2
= 0.4246
0

10
20
30
40
50
0 100 200 300 400 500 600 700 80
0
Si (mg/L)
d
Figure 7. Bivariate plots of total arsenic concentration (As
tot
) with: (
a) F
Ϫ
, (
b) V, (c) B and (d) Si in shallow
groundwaters. Note: the data sets with exceedingly high As
tot
concentrations are excluded from the plots to
visualize the trends correlation.
Figure 8. (a) Distribution of As
tot
(␮g/L) and (b) pH in groundwater. Kriging is used to interpolate
isocurves. Note irregular concentration scales.
Copyright © 2005 Taylor & Francis Group plc, London, UK
ACKNOWLEDGEMENTS
The authors would like to acknowledge the Swedish International Development Agency (Sida-
SAREC) for supporting the research on arsenic-rich groundwater in the Santiago del Ester province
of Argentina at the Royal Institute of Technology during 2001–2003. MC and JF acknowledge the
financial support provided by the Swedish International Development Agency (Sida) in the form

of Minor Field Study grants during 2002. We appreciate the constructive criticisms by an anonym-
ous reviewer which has helped to improve the manuscript considerably.
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