BIBLIOS

Document type
Conference papers

Document subtype
Extended abstract/Short paper

Title
Angra Hydrothermal Aquifer (Terceira Island, Azores, Portugal): Geological and Morpho-tectonic Constraints.

Participants in the publication
J.C. Nunes (Author)
Instituto Dom Luiz (IDL), Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749–016, Lisboa, Portugal
V. Almeida (Author)
FACULDADE DE CIÊNCIAS DA UNIVERSIDADE DE LISBOA
C. Ferreira (Author)
FACULDADE DE CIÊNCIAS DA UNIVERSIDADE DE LISBOA
M.R. Carvalho (Author)
Dep. Geologia
IDL
H. Sá (Author)
S. Medeiros (Author)

Summary
In the Azores islands (Portugal), the mineral and thermal waters arise in dependence on high enthalpy geothermal systems. The contamination of shallow aquifers by volcanic gases and/or hydrothermal steam arising from geothermal systems in depth results in the occurrence of mineral and thermal waters in perched and basal aquifers. The Angra hydrothermal aquifer is located on the southern slopes of the Basaltic Fissural Zone (BFZ) and the Guilherme Moniz central volcano, close to the Angra do Heroísmo city, the major urban area in Terceira Island.\nA hydrothermal aquifer in the Angra area has been known for centuries, with several thermal springs discharging along the shoreline with 20-25°C temperatures. However, only in the mid-90s of the 20th century a set of boreholes located about 2 km inland intersected this thermal aquifer, with temperatures from about 27°C (Farrouco and Terra Chã wells) to 39°C (Caminho do Posto Santo well), the latter located in the centre of a thermal anomaly that extends to the South until the shoreline.\nAimed for a better understanding of the geological and morpho-tectonic constraints associated with this hydrothermal aquifer, that helped to explain its hydrogeological signature (e.g. a basal fissured aquifer, with a Na-HCO3 facies water, and with a clear deep contribution of volatiles with hydrothermal/volcanic origin) and to support a robust conceptual model for this aquifer, a detailed 1:10,000 geological map of the area was performed. The mapped area extends N-S for about 8x6 km from the central volcanic ridge of the island until the shoreline, along the southern slopes of the BFZ and the Guilherme Moniz and Santa Barbara polygenetic volcanoes.\n\nFollowing the same methodology applied to map the northern branch of the Terceira Island central BFZ, the final map identifies the several volcanic formations associated with the BFZ as well as its detailed volcanostratigraphy, namely in relation to the surrounding Guilherme Moniz and Santa Bárbara geological formations. In addition, the mapping of the tectonic fractures and lineaments existing in the area, the interpretation of drilling geological logs, and some 14C age determinations done (coupled with other dating information available) allowed to obtain a detailed and well constrained picture of the geological and morpho-tectonic features interacting with the Angra hydrothermal aquifer, and useful for on-going and planned future exploration studies of that geothermal resource.\nThe presence of the Santa Bárbara NW-SE graben-like structure (associated with the Terceira Rift regional fracture zone), coupled with local fractures and tectonic lineaments associated with the Guilherme Moniz and the Pico Alto volcanoes, respectively with a N-S to NW-SE and an ENE-WSW trend, seems to play a relevant local control on the rising of volatiles with hydrothermal/volcanic origin, and thus on the high heterogeneity found on many hydraulic parameters of the Angra hydrothermal aquifer.


Event
European Geothermal Congress

Publication Identifiers

Address
Zürich, Switzerland.

Organizers
European Geothermal Energy Council (EGEC)


Keywords
Hydrothermal resources geological mapping morpho-tectonic Terceira Island Azores.