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| Titel |
Hydrogeology of the Azores volcanic archipelago (Portugal) |
| VerfasserIn |
J. Cruz, R. Coutinho, P. Antunes, P. Freire |
| 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 |
250029380
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| Zusammenfassung |
| The archipelago of the Azores is made of nine islands of volcanic origin located in the North
Atlantic Ocean, with an area of 2333 km2 and approximately 237500 inhabitants, which are
98% dependant from groundwater sources for their water consumption. Therefore,
groundwater is a resource that plays a vital role as drinking water source and as ecosystem
support matrix. Nevertheless, besides the environmental, social and economical value of
groundwater, this resource is subject to an increase pressure and in several islands water
quality deterioration is shown by monitoring data. This pressure is also shown by the 42.7%
increase expected for domestic use until the year 2020 at the Azores, with higher groundwater
abstraction.
The Azores climate can be considered as marine temperate, which is reflected by the low
thermal amplitude and high precipitation. A well-established difference between a dry season
and a colder and wet season occurs, as from October to March about 75% of the annual
precipitation is registered. The average annual precipitation at the Azores is 1930 mm,
exceeding by far the average annual actual evapotranspiration, which is 581 mm. Recharge
rates range from 8.5% to 62.1%, and the highest values are observed at Pico, Terceira, Faial,
São Miguel and Graciosa islands, especially in areas where the terrain is covered by recent
basaltic lava flows and the soil cover is sparse. Groundwater resources estimates point to a
total volume of about 1600x106 m3/yr. Values above the recharge median, equal to
101.3x106 m3/yr. were estimated for the São Miguel, São Jorge, Terceira and Flores
islands.
Despite differences in the islands growth, as a result of successive volcanic
eruptions of various types, groundwater occurrence can be described in function of
two main aquifers systems: (1) the basal aquifer system, which corresponds to
fresh-water lenses floating on underlying salt water, and (2) perched-water bodies, which
are usually drained by springs spread in the volcanic edifices slopes. The basal
aquifer system is in the coastal area, presenting generally a very low hydraulic
gradient.
Hydrogeological surveys that have been made in the Azores archipelago points out
to more than 1000 springs and wells spread all over the archipelago (950 springs
and 83 drilled wells). Spring distribution is heterogeneous, with densities varying
between 0.01 springs/km2 at Pico island and 0.72 springs/km2 estimated at Santa
Maria.
Specific capacity ranges from 1.4x10-2 to 266.7 L/sm, with a median value of 32.3 L/sm.
Transmissivity also present a large range, with values ranging between 1.65x10-5 and
4.03x10-1 m2/s, and a median of 3.66x10-2 m2/s. The heterogeneous distribution shown by
these values expresses the influence of the hydrogeological characteristics of volcanic
terrain, resulting from syngenetic characteristics and secondary processes, like
weathering. The highest values are observed in wells drilled in recent basaltic lava flows,
which generally are thin and fractured, with frequent clincker levels interbedded,
and the lowest data was estimated in the older volcanic formations of Santa Maria
island.
Groundwater on perched-water bodies, excluding the numerous mineral waters that are
spread in several islands of the archipelago, present usually a low mineralization, shown by
the electrical conductivity values (36-725 μS/cm; median=158.0 μS/cm). The average
temperature is equal to 15ºC. Waters have an average temperature of 15ºC and are mainly
slightly acid to slightly alkaline, with a pH range from 4.7 to 8.6, but showing a median value
of 7.2. The main water types are Na-Cl to Na-HCO3 waters, with numerous samples lying in
the intermediate compositional fields that characterize Na-Cl-HCO3 and Na-HCO3-Cl
waters.
The groundwater composition in the basal aquifer system is usually from the Na-Cl
type and presents a higher mineralization, resulting in a median value for electrical
conductivity equal to 1044 μS/cm, expressing the influence of sea salts mixture. |
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