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| Titel |
Mineral water discharges at the Azores archipelago (Portugal): hydrogeological setting, chemical composition and mapping |
| VerfasserIn |
P. Freire, J. Cruz, R. Coutinho, A. Costa, P. Antunes |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029576
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| Zusammenfassung |
| The geological setting of the Azores archipelago, located in the North Atlantic ocean, about
1500 km form Portugal mainland and made of 9 islands of volcanic origin, enhances the
multiplicity of surface hydrothermal manifestations. Therefore, a field survey made possible
to identify 101 mineral water discharges in the Azores, mainly of CO2-rich cold
waters and thermal waters, spread along São Miguel (75%), Terceira (6%), Graciosa
(7%), Pico (2%), Faial (3%), São Jorge (5%) and Flores ( 2%) islands, as well as
fumarolic grounds. Furnas and Fogo central volcanoes, two of the three composite
active volcanoes that dominates the geology of São Miguel, the largest island of the
archipelago, represent respectively about 41% and 24% of the discharges from the
Azores.
Discharges are mainly from fissured aquifers, made of basaltic or trachitic lava flows.
Instead, discharges from porous aquifers, made of pyroclastic deposits, mainly of pumice
type, are less common, and are more frequent at São Miguel island.
The studied discharges correspond mainly to springs (75), and also to boiling pools (10),
at fumarolic grounds, 14 drilled wells and 2 large-diameter wells. The boiling pools are only
observable at São Miguel island, while drilled wells were made at São Miguel, Terceira and
Graciosa.
Groundwater at Azores occurs in two major aquifers systems: (1) the basal aquifer
system, which corresponds to fresh-water lenses floating on underlying salt water, and (2) in
perched-water bodies. The basal aquifer system is in the coastal area, presenting generally a
very low hydraulic gradient. From the 14 drilled wells only two are in perched-water bodies.
Considering mineral springs, the majority discharge from perched-water bodies (77%),
while all the boiling pools also discharge in altitude, also from perched –water
bodies.
During the field survey an extensive campaign of sample collection was made in all
islands, in order to characterize the chemical composition of these waters, which presents a
large range of water types and mineralization magnitude. Several groups of waters are
defined: (1) Na-HCO3 and Na-HCO3-Cl type waters, to which almost all the thermal and
CO2-rich waters belong, (2) Na-Cl type waters, to which discharges from the basal aquifer
system belong and (3) acid-SO4 type waters, to which some of the boiling waters of São
Miguel island belong. A few samples show intermediate facies between these main water
types.
The pH range between 2.2 and 7.82, discharge temperature between 15ºC and 99.5ºC
(median=35ºC), and conductivity varies between 139 and 43100 μS/cm (median=906
μS/cm).
The main hydrogeochemical processes are the CO2-dominated volatile absorption,
water-rock interaction and mixture with hydrothermal fluids. Sulfate dominated composition
is explained by the influence of steam heating, and the Na-Cl water type result from mixture
with sea salts.
For the purpose of mapping mineral water discharges at the Azores a geochemical atlas
was made using ESRI ArcGis 9.1 software. Data was divided in classes according to quartile
values and spatial analysis was made through thematic mapping, for several features, as
hydrogeological setting, water types and variables as discharge temperature, pH,
conductivity, free CO2 and major elements content. In the present contribution several
examples of the hydrogeological maps are shown. |
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