Use of geochemical tools to study groundwater
salinization in volcanic islands: a case study in
the Porto Santo (Portugal) and Santiago (Cape
Verde) islands
M. Teresa Condesso de Melo, J. Silva, A. Lobo Pina, A. Mota Gomes, F. Almeida,
Moura, M. A. Marques Silva
R.
20th SWIM
1. Study areas
Archipelago:
R. A. Madeira
Island:
Porto Santo
Area:
Porto Santo
42,2 km2
Population:
4 474 inhabitants
Maximum altitude:
517 m
Archipelago:
Cape Verde
Island:
Santiago
Area:
991 km2
Population:
Santiago
236 627 inhabitants
Maximum altitude:
1 394 m
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2. Challenges & objectives
►
volcanic islands with scarce freshwater
resources;
►
population rely on groundwater (and
seawater desalination plants) to guarantee
public water supply;
Porto Santo
►
groundwater is very important for crop
irrigation and increasing salinities are
obliging the agricultures to abandon the
fields;
►
high salinity waters are having health
impacts on the populations (specially due
to high F and As)
Santiago
to identify areas and origins of
high salinity
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3. Climate
Semi-arid regions
influence of the Sahara
desert
Annual rainfall:
< 400 mm/year
Porto Santo
unreliable and erratic
rainfall leading in some
years to prolonged droughts
Potential
Evapotranspiration:
> 1300 mm/year
3-4 x rainfall
Winds:
NE winds are humid but
bring no rainfall
Dry winds from Africa
SW monsoon winds bring
rainfall between July-Ago
Santiago
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160
140
mm/month
120
100
80
Porto Santo
60
Potential
Evapotranspiration
40
20
Rainfall
0
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
Surface water run-off just after short
heavy events that may produce flash
floods and strong landscape erosion.
Limited groundwater recharge.
Santiago
Need to irrigate.
20th SWIM
Average annual rainfall
2
Altitude = 1.80 Rainfall - 188,59; r = 0,78
800
Curralinho
Serra Malagueta
600
Porto Santo
400
S. Jorge dos Orgãos
200
Praia
Chão Bom
0
0
100
200
300
400
500
600
Rainfall (mm/year)
1750
Strong altitude
effect on rainfall
distribution in
Santiago.
1500
Precipitação total (mm/ ano)
Altitude (m above m.s.l.)
1000
1250
1000
750
500
Santiago
250
0
1960
1970
1980
1990
2000
2010
X Data
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4. Geological characteristics that condition groundwater flow
and hydrochemistry
►
Volcanism in the studied regions occurred intermittently since at least
Palaeogene (in Santiago) and Lower Miocene (Porto Santo) in an
environment characterized by dramatic eustatic and isostatic sea-level
fluctuations and active tectonics.
►
Periods of prolonged volcanic activity under submarine or subaerial
conditions alternated with phases of volcanic quiescence dominated by
intensive erosion and deposition of sedimentary formations.
►
The islands are mainly composed of basaltic-trachytic-rhyolitic lava
flows and some alkaline rocks, with a subordinate amount of pyroclastic
and sedimentary rocks.
Santiago, Cape Verde
Santiago, Cape Verde
Porto Santo, Portugal
Porto Santo, Portugal
20th SWIM
5. Hydrogeological setting
Island: Porto Santo
low permeability,
weathered volcanic
formations
volcanic
formations
Carbonate eolianites
Beach sands
Eruptive rocks
Weathered volcanic rocks
with clay
Sedimentary calcarenite
formation, eolinites
20th SWIM
5. Hydrogeological setting
Island: Porto Santo
low permeability,
weathered volcanic
volcanic
formations
formations
no creaks,
higher
infiltration
volcanic
formations
Carbonate eolianites
Beach sands
Eruptive rocks
spring fed
creaks
Weathered volcanic rocks
with clay
Principal aquifer formation
(calcarenites)
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5. Hydrogeological setting
Island: Porto Santo
Calcarenite (eolinite) formation
►
it can present thicknesses ~40-50 m in
the central part of the island
►
heterogeneous, interstratified levels of
palaeosoils and shells
►
degree of consolidation increases with
depth
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5. Hydrogeological setting
Island: Santiago
Pillow lavas
Sedimentary formations, alluvial deposits
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5. Hydrogeological setting
Island: Santiago
recent unit
intermediate unit
principal aquifer
bottom unit bedrock
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1700000
5. Hydrogeological setting
LEGEND:
1690000
200
Atlantic
1670000
Ocean
I l h a de
1660000
Santiago
Atlantic
1650000
UTM (m)
Piezometric heads
below mean sea level
Piezometers
1680000
Risk of salt water
intrusion
Piezometric heads (m above m.s.l)
(data INGRH, Praia, C. Verde)
Ocean
200000
210000
220000
230000
240000
UTM (m)
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6. Groundwater sampling
►
sampled springs, water mines,
boreholes and wells in both islands;
►
analysed for major, minor and trace
elements;
►
stable isotopes;
►
tritium.
Porto Santo
Santiago
Santiago, Cape Verde
Porto Santo, Portugal
20th SWIM
CE (mS/cm)
1.9 to 2.5
2.5 to 5
5 to 6
6 to 7
7 to 8
3652000
UTM (m)
3657000
UTM (m)
3662000
Distribution of electrical
conductivity:
368500
373500
378500
UTM (m)
1645000 1650000 1655000 1660000 1665000 1670000 1675000 1680000 1685000 1690000 1695000 1700000
5. Hydrogeochemistry
CE (microS/cm)
300 to 400
400 to 500
500 to 750
750 to 1000
1000 to 1500
1500 to 2000
2000 to 6000
200000
205000
210000
215000
220000
225000
230000
235000
240000
245000
UTM (m)
20th SWIM
5. Hydrogeochemistry Santiago island
LEGENDA:
Distribution of chemical facies:
Facies hidrogeoquímicas
1690000
Geochemical processes:
- cation exchange with sea
water along the coast (Na, Cl)
due to saline intrusion
Na-Cl
1680000
Na-HCO3
1670000
UTM (m)
Oceano
Atlântico
I l h a de
Santiago
1660000
- molar relation Ca/Mg < 1,
implying that dissolution of
magnesian minerals are
controlling calcite dissolution
Mg-HCO3
Oceano
Atlântico
1650000
- dissolution of carbonates
minerals (HCO3) like
magnesite (Mg) and calcite
(Ca).
Mg-Cl
200000
210000
220000
230000
240000
UTM (m)
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0 to 50
50 to 100
100 to 200
200 to 300
300 to 500
500 to 2000
2000 to 5000
slight evaporation effect
mixing with sea water
0.0
-2.0
-4.0
-6.0
-8.0
0
200
400
600
800
Cl (mg/ L)
1000
1200
800
Altitude (m)
Cl (mg/L)
Delta O-18 (per mil)
5. Hydrogeochemistry Santiago island
600
400
200
0
0
2000 4000 6000 8000 10000 12000 14000
CE (microS/cm)
lower EC are identified in higher
altitudes, with shorter residence times
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5. Hydrogeochemistry Porto Santo island
80
80
Mg
O4
Cl
+S
60
40
40
K
20
20
H HH
HA
H
LA
L H
H
H
H
H
H
HH
H
Mg
SO4
80
=>
+K
<=
HC
O
Na
60
Água Dessalinizada
K Dessalinized
water
L Zarco Shopping e Hotel Porto Santo
Resort
(Maio 2008)
H Colombo's
Coastal
waters
Colombo's
Resort
(Dezembro 2007)
A
Porto
Santo
H Interior waters
3
80
Na > Mg > Ca
+
Ca
<=
=>
Legenda:
60
60
Cl > HCO3 > SO4
40
40
K
20
Na+K
HCO3
80
H
LLH
H
A
H
A
H
H
20
40
60
80
Ca
20
HH
H
K
H
H
A
L
H
H
H
L
H
H H
H
H
60
H
40
20
Cl
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5. Hydrogeochemistry Porto Santo island
0.14
80
se
0.08
at e
aw
-1
r
0.10
Na (mmol L )
-1
Br (mmol L )
0.12
io
rat
0.06
0.04
r
ate
w
a
60
io
rat
LEGEND;
Samples collected inland
(> 20 m above mean sea level)
se
with high salinities
40
(CE>1.6 mS/cm) due to water-rock
interaction processes.
20
0.02
0.00
Samples collected along coastline
0
0
20
40
60
80
0
20
-1
40
60
80
-1
Cl (mmol L )
Cl (mmol L )
10
water-rock interaction and salt-water
aw
ate
-1
Na (mmol L )
r ra
t io
intrusion processes.
se
-1
Mg (mmol L )
6
4
with high salinities (CE>4.8 mS/cm )
due to the cumulative effect of
80
8
(< 20 m above mean sea level)
2
0
60
40
20
0
0
1
2
3
4
-1
Ca (mmol L )
5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
-1
Si (mmol L )
20th SWIM
6. Conclusions
• Groundwater samples collected in Porto Santo are all
very mineralized (EC>1.6 mS/cm) and with
characteristic pH values above 7.5.
• Groundwater along the coastline and near the saline
intrusion interface is characterized by higher electrical
conductivity (>4.78 mS/cm), lower Na/Cl ratios and
slightly higher Ca/Mg ratios compared to the
groundwater sampled in areas further away from
coastline.
Porto Santo
• It suggests that Na in these samples is being partially
removed from the groundwater and replaced by Ca,
which is confirmed by the higher Ca/Na ratios.
• The samples collected in a short distance from the
coastline present Br/Cl very close to seawater mixing
line (0.0016) but samples collected inland show
enrichment in Si, which may be attributed to longer
rock-water interactions.
Santiago
20th SWIM
6. Conclusions
• Groundwaters in Santiago display a great variability
in their chemical and physical features. They range
from dilute waters (EC=362 mS/cm) in higher altitudes to
groundwaters that are generally more saline (CE~1 000–
12 000 mS/cm).
Porto Santo
• Groundwater samples collected along the coast have
often Cl concentrations exceeding 500 mg/l as a result
of saline intrusion.
• Stable isotopic data shows a slight enrichment in
shallow high salinity groundwater samples which may
indicate a contribution of evaporation for increasing
salinities.
• Thus the geochemical results obtained show that
different processes – evaporation, water-rock
interaction and sea water intrusion are contributing for
groundwater salinization and limiting its use for public
supply.
Santiago
20th SWIM