Transcript Document 7429664

Arsenic enrichment of ground water at
two regions of the Chacopampean plain,
northwest Argentina
Ondra Sracek1,2, María Gabriela García3
1 OPV s.r.o., Praha, Czech Republic
2 Pontificia Universidade Católica, Rio de Janeiro, Brazil
3 Universidad Nacional de Córdoba, Córdoba, Argentina
ARSENIC IN
ARGENTINA
● In Argentina, > 1 million of
people are exposed to risk
linked to natural arsenic
● They depend on drinking water
with over 0.05 mg/L arsenic
(limit WHO 0.01 mg/L,
Argentina limit 0.05 mg/L)
● Most affected: parts of the
Chaco-Pampean plains
(+ Andes; minor extent)
● Arsenic is related to
sedimentary aeolian and
fluviatile sediments
● These sediments contain
up to 25 wt % of volcanic ash
Santiago del Estero:
Río Dulce Alluvial Cone
Arsenic history
Semiarid region (precipitation 532 mm/year) with distinct rainy
(summer) a dry (winter) periods
Principal plant is cotton cultivated on fields irrigated by ground
water
Shallow groundwater is used by rural population living in
dispersed settlements
Conditions in shallow aquifers are generally oxidizing
In 1984 first shallow groundwater monitoring (many sites with >
0.4 mg As/L)
First symptoms of chronic endemic regional hydroarsenicism in
1983
Geology and Hydrogeology
General profile of Río Dulce alluvial cone
● thickness of sediments from 150 to 0 m
● alternating layers od gravel, sand, silt and clays
PERFIL GEOLOGICO
CONO ALUVIAL DEL RIO DULCE
PERFIL GEOLOGICO
e
ayán
CONO ALUVIAL DEL RIO DULCE
400
300
PAMPEANO
Srra. de
Guasayán
PLIOCENO
200
Humayampa fault
CUATERNARIO
MIOCENO
PAMPEANO
r
PLIOCENO
100
0
0
Nivel del Mar
5
10
15
20 Km
PLIOCENO
CUATERNARIO
CONSEJO DE INVESTIGACIONES CIENTIFICAS Y TECNOLOGICAS
UNIVERSIDAD NACIONAL DE SANTIAGO DEL ESTERO
MIOCENO
CENTRO DE INVESTIGACIONES HIDROGEOLOGICAS
FACULTAD DE CIENCIAS EXACTASPLIOCENO
Y TECNOLOGIAS
RESULTS
Volcanic ash
Unsaturated zone
Saturated zone
as a distinct layer (max. thickness 1.35 m, mean 0.54 m; in 52% of area)
also dispersed in the sediment (up to 25 wt %)
c
0
2.5
0
d
ilw
ay
0.30
0.50
0.00
ra
50
cm II
0.25
0.65
0.00
0.20
0.00
IV
0.50
Du
0.00
0.96
ve
r
I
0.20
1.35
Ri
ilw
ay
III
N
0.30
0.00
0.00
0.40
0.20
Du
lce
0.00
0.25
V
Ri
ve
r
0.00
0.25
pH
thickness (m)
distance between base volcanic ash layer and groundwater table:
>2
1-2
0-1
(= below
< 0 water table)
5
km
ra
0.34
lce
2.5
km
1.00
0.00
5
pH
7.0 - 7.5
8.0 - 8.5
7.5 - 8.0
8.5 - 9.0
N
RESULTS
Volcanic ash
Unsaturated zone
Saturated zone
as a distinct layer (max. thick. 1.35 m, mean about 0.50 m; in 52% of area
also dispersed in the sediment (25 %)
c
0
2.5
1.00
0.00
VOLCANIC
ASH
0
d
km
ra
ilw
ay
0.34
20-21 ppm Vanadium
0.30
0.50
5
0.00
50
cm II
0.65
1.35
0.20
0.00
Du
0.2-3
lce ppm Molybdenium
0.00
0.96
0.40
I
IV
0.50
0.00
wa
y
0.20
3-6 ppm Arsenic
0.00
Ri
ve
r
ra
il
III
N
0.30
0.00
0.20
0.00
0.25
Du
lce
Ri
ve
r
V
0.00
0.25
pH
thickness (m)
distance between base volcanic ash layer and groundwater table:
>2
1-2
0-1
(= below
< 0 water table)
5
km
0.25
2-20 ppm Uranium
2.5
pH
7.0 - 7.5
8.0 - 8.5
7.5 - 8.0
8.5 - 9.0
N
RESULTS
Electron microprobe
Highly weathered volcanic glass Altered biotite with precipitated barite
Other results:
- partially altered titano-magnetite, biotite and ilmenite
- ferrihydrite in isolated spots rather than in coatings
- gypsum with less hydrated margins present in many samples
Piper diagram
80
3
40
20
Cl
-
+
a
2+
20
SO 24 +
40
b
g
+M
e
2+
Ca
60
60
NO -
80
c
g
d
20
20
80
80
3
OHC
40
80
80
4
Mg
2-
+
+K
40
60
SO
2+
40
40
f
+
Na
60
20
2+
-
80
60
40
20
40
60
80
Ca
20
20
-
Cl + NO3
A clear evolution trend from Ca-HCO3 ground water type
towards Na-HCO3 ground water type with high
concentrations of SO4 and Cl
Correlation:
Depth to groundwater table
Groundwater arsenic
(thickness unsaturated zone)
2.5
5
a
4.0
0
f
km
ra
ilw
ay
0
3.5
3.0
N
2.5
km
Cara
Pujio
ra
il
wa
y
I
III
3.0
2.0
II
pa
leo
-ri
v
er
4.0
IV
5.0
Du
lc
eR
ive
r
1.0
Du
lce
V
Ri
ve
r
total
arsenic
< 0.05
0.3 - 0.5
0.05 - 0.1
> 0.5
mg As L
-1
5.5
0.1 - 0.3
depth to
groundwater
table (m)
5
● No relation between the depth to water table, which also
determines the groundwater recharge time
N
Correlation:
Groundwater flow velocities
Groundwater arsenic
5
b
km
ilw
ay
2.5
5
km
ra
155
ra
0
160
2.5
165
0
f
170
(residence time)
N
ilw
ay
N
I
III
II
IV
Du
lc
V
eR
iv e
r
< 0.05
0.3 - 0.5
0.05 - 0.1
> 0.5
0.1 - 0.3
mg As L
-1
groundwater
level contours
(m s.n.m.)
15
7
15
7
total
arsenic
eR
ive
r
15
5
Du
lc
● moderate correlation between high As and low hydraulic
gradient zones (= highest groundwater residence times)
Milli
Correlation:
Groundwater arsenic
Volcanic ash layer
0
f
2.5
5
c
ilw
ay
ra
il
0.30
0.50
N
5
km
0.34
I
III
2.5
1.00
0.00
km
ra
0
0.00
wa
y
N
0.25
0.30
II
0.65
0.20
0.00
0.00
1.35
IV
0.20
0.00
0.50
Du
lc
eR
ive
r
Du
lce
R
ive
r
V
0.00
0.96
0.00
0.40
0.20
0.00
0.25
0.00
total
arsenic
thickness (m)
< 0.05
0.3 - 0.5
0.25
0.05 - 0.1
> 0.5
distance between base volcanic ash layer and groundwater table:
0.1 - 0.3
mg As L
-1
>2
1-2
0-1
(= below
< 0 water table)
● no correlation between high As zones and presence of volcanic
ash layer and its position regarding water table (above-below)
volcanic ash layer is not the (only) source of groundwater As
presence of other ash lentils or As from dispersed ash
Correlation:
Groundwater arsenic
pH value
0
f
2.5
5
0
d
2.5
km
km
ra
ra
ilw
ay
II
total
arsenic
Ri
ve
r
N
I
II
IV
lce
ilw
ay
III
N
I
III
Du
5
IV
Du
lce
V
< 0.05
0.3 - 0.5
0.05 - 0.1
> 0.5
0.1 - 0.3
mg As L
-1
Ri
V
ve
r
pH
pH
7.0 - 7.5
8.0 - 8.5
7.5 - 8.0
8.5 - 9.0
● clear correlation between As hot spots and areas with high pH
Correlation:
Groundwater arsenic
Electrical conductivity
0
f
2.5
5
0
e
2.5
km
km
ra
ra
ilw
ay
il w
ay
N
III
I
III
II
lce
II
IV
total
arsenic
Du
Ri
ve
r
N
I
IV
Du
5
lce
< 0.05
0.05 - 0.1
0.1 - 0.3
V
Ri
ve
r
V
0.3 - 0.5
> 0.5
mg As L
-1
electrical
conductivity
500-1000
4000-6000
1000-2000
> 6000
2000-4000
uS cm
-1
● high As zones are related to zones of high electrical conductivity
(predominantly zones with high Na+ - HCO3- ground water type)
● these zones also correspond to zones of high pH
3.0
b
km
Cara
Pujio
0
2.5
5
km
ra
ra
ilw
ay
il w
ay
155
3.0
5
160
2.5
165
0
3.5
170
4.0
a
N
N
pa
2.0
leo
-ri
v
er
4.0
Summary
5.0
1.0
depth to
groundwater
table (m)
c
0
2.5
15
7
5
2.5
ra
ilw
ay
0.30
0.00
il w
ay
III
N
N
0.25
0.30
0.00
1.35
0.20
0.00
IV
0.50
Du
lc
eR
ive
r
I
II
0.20
0.00
Du
0.00
0.96
0.00
0.40
lce
0.00
0.25
0.20
V
Ri
ve
r
0.00
0.25
pH
thickness (m)
distance between base volcanic ash layer and groundwater table:
>2
1-2
pH
(= below
0-1
< 0 water table)
0
e
2.5
5
7.0 - 7.5
8.0 - 8.5
7.5 - 8.0
8.5 - 9.0
0
f
2.5
km
ra
ilw
ay
III
il w
ay
N
I
II
IV
eR
IV
Du
V
ive
r
electrical
conductivity
I
III
II
Du
lc
5
km
ra
not related to zones:
● where volcanic ash layer is
present
● where volcanic ash layer is
below or above the water
table
5
km
0.65
● of high residence time
● of high pH
● high EC, Na-HCO3 ground
water type
Milli
0
d
ra
0.34
0.50
related to zones:
Ri
ve
r
km
1.00
0.00
lce
groundwater
level contours
(m s.n.m.)
5.5
Zones with high arsenic
concentrations in
groundwater are
ive
r
15
7
Du
eR
15
5
Du
lc
500-1000
4000-6000
1000-2000
> 6000
2000-4000
uS cm
-1
total
arsenic
lce
Ri
ve
r
V
< 0.05
0.05 - 0.1
0.1 - 0.3
0.3 - 0.5
> 0.5
mg As L
-1
N
CORRELATION DIAGRAMS
seems to be
related to zones
of cation
exchange (Ca,
Mg for Na)
(Na-HCO3–waters)
R = 0.18
1
0.1
0.01
1
10
2+
1
0.1
0.01
0.001
0.1
100
0.1
0.01
0.001
8
pH
10
100
9
10
As (mg L-1 )
As (mg L-1 )
1
0.1
0.001
100
1000
-1
10000
-1
HCO3 (mg L )
R = 0.26
0.1
0.01
1000
1000
-
1
0.001
100
R = 0.33
1
Ca (mg L )
R = 0.41
7
10
0.01
2+
-1
Mg (mg L )
10
R = 0.36
10000
-1
electrical conductivity (uS cm )
10
As (mg L-1 )
0.001
0.1
10
As (mg L-1 )
As (mg L-1 )
10
and by
positive correlation
of As with
● pH, HCO3 , EC
negative correlation
of As with
● Ca, Mg
characterized
by high
● pH,
● Na
● EC
As (mg L-1 )
high arsenic
concentrations
R = 0.44
1
0.1
0.01
0.001
10
100
10000
1000
+
-1
Na (mg L )
CORRELATION OF As WITH MINOR AND TRACE ELEMENTS
gw-arsenic has a
good correlation
found in high
concentrations
in volcanic ash
21
20
6
3
V, Mo, U, and F
ppm
ppm
ppm
ppm
V
U
As
Mo
10
10
10
R = 0.83
1
0.1
0.01
R = 0.45
As (mg L-1 )
As (mg L-1 )
R = 0.9
As (mg L-1 )
volcanic ash is
assumed to be
primary source of As
in the shallow
groundwater
1
0.1
0.01
0.01
0.1
1
10
0.01
-1
0.1
10
-1
V (mg L )
Mo (mg L )
10
1
0.1
0.01
0.001
0.01
0.1
-1
U (mg L )
1
SUMMARY OF RESULTS FROM SANTIAGO DEL
ESTERO
1. Areas with high and low groundwater arsenic concentrations
could be delimited
2. Areas of high groundwater As concentrations are related to areas
● with slow groundwater flow (long residence times)
● high electrical conductivity and Na-HCO3 type of GW
● high pH
– probably caused by cation exchange and dissolution of silicates
3. Probable primary source of groundwater As seems to be volcanic
ash
● present as a distinct layer
● and dispersed in the sediment.
- This is indicated by
- high concentrations of As, V, U, and Mo in volcanic ash and
the positive correlation of As with V, U, and Mo in
groundwater
Tucumán
Location of study area
Sali River
Río Salí Hidrogeological basin
POPULATION
Concentrated in small settlements
Most of population is located along the Salí River
Water supply is by deep and shallow groundwater
CLIMATE
Subtropical with distinct dry season (winter)
Mean precipitation 600 (east) to 1000 (west) mm
ACTIVITY
Cultivation of sugar cane and soybeans on irrigated
fields
Suggar mills and citric industries near Salí River
Geology and hydrogeology
W-E section of the
Hydrogeological basin
showing the main
Aquifer units and
lithology
Hydrogeochemistry
Large differences in chemical composition
between unconfined and confined
aquifers
Shallow unconfined aquifer:
loessoid sediments
Na-HCO3 type of ground water
pH: 7.1-8.7
EC: 250 - >3,000 mS cm-1
Dissolved Oxygen: 0.2 – 8.1 mg L-1
Elements exceeding standard requirements (WHO):
As (up to > 700 μg/l), V, F, Fe, Mn, NO3-
Spatial variation of As in the shallow
unconfined aquifer
Sources of As in loess
Potential primary source
Secondary source
grains of glass
Microprobe images
coatings of
ferric oxyhydroxides
Deep confined and semiconfined aquifers
General Hydrogeochemistry
Hydrochemical
zones
Semiconfined aquifer
Confined aquifer
pH: 7.0-8.4
EC: 619 - 2182 mS cm-1
Dissolved Oxygen: 0.5 – 7.8 mg L-1
Elements exceeding standard
requirements (WHO): As (up to 70
mg L-1), Fe and Mn (occasionally)
Hydrogeology
Potentiometric surface of
the deep confined aquifer
Spatial variation of As in confined and
semiconfined aquifers
Hydrogeochemical profiles in the semiconfined aquifer
close to the Rio Salí
Salí River
0
20
+
K
2+
40
Mg
Cl
Transition Zone
2+
60
Depth (mbs)
SO4
-
2-
Ca
HCO3
80
-
+
Na
100
120
140
160
180
a)
200
0
100
200
300
-1
Concentration (mg L )
840
Increased HCO3concentration in the
transition zone,
caused by the
degradation of DOC
Hydrogeochemical profiles in the semiconfined aquifer
High organic load in the river,
temporary reducing conditions
during dry period
Salí River
0
-1
COD (mg O2 L )
20
Transition Zone
40
Depth (mbs)
60
80
pH
100
120
140
Salí River
160
0
-1
DO (mg L )
180
Transition Zone
20
c)
As
40
200
5
10
15
20
25
30
35
40
45
Fe
60
Depth (mbs)
0
Mn
80
100
120
140
160
increased concentrations of
As, Fe and Mn in the transition
zone
180
b)
200
0
100
200
300
400
-1
Concentration (mg L )
500
Summary of results from Tucuman





Primary source of dissolved As in the shallow groundwater
seems to be the dissolution of volcanic glass spreads in the
loess matrix
Secondary source seems to be associated with desorption
from Fe oxy-hydroxide coats and/or reductive dissolution
High As concentrations in the unconfined aquifer are
generally associated with high pH values
In the semiconfined aquifer, increasing As concentrations in
the transition zone are associated with increasing load of
organic matter in the Sal River and the occurrence of
reductive conditions in surface waters
In deep confined water (more than 40 m deep) As
concentrations are generally lower than 50 mg L-1
Comparison of both regions






In spite of different climatic conditions and scales of
investigation there seems to be similarity between both
regions
Arsenic concentrations are high in shallow aquifer comprised
of loessoid sediments, they are linked to high pH, Na-HCO3
type of ground water
Primary source of arsenic seems to be highly weathered
volcanic glass in sediments, coatings of Fe(III) minerals on
silicate grains are discontinuous because supply of iron was
limited due to acidic character of original rocks
Competition with other oxyanion forming species like V, B,
Mo, and PO4 may further limit adsorption
Deep aquifers (data only from Tucuman) have much lower
dissolved arsenic concentrations
Ground water arsenic concentrations may be locally elevated
close to surface water bodies affected by discharged organic
contamination;
Acknowledgements
We thank Jochen Bundschuh, who provided many slides
for the Santiago del Estero section.