A contribution to groundwater flow models in hydrothermal systems from North of Portugal

Geothermal resources are considered a sustainable and environmentally friendly alternative to produce energy. The north of Portugal seems to be a suitable area for the exploitation of geothermal resources due to its geothermal gradient and the occurrence of several thermal areas. A study based on 18O and 2H water isotope and geochemical data of shallow and deep geothermal aquifers was performed in selected seven hydrothermal systems located in the Cávado and Ave River watersheds, NW Portugal. The main goal of the study is the proposal of conceptual groundwater flow models, including information on recharge areas, groundwater flow paths, water-rock interaction, and mixing processes between water from different sources. The studied hydrothermal systems are ascribed to groundwater circulation in fractured granitic host rocks. Calcium is dominant cation in the thermal water of Caldelas, while sodium is dominant in the other studied thermal waters. Hydrogen carbonate is the dominant anion in the thermal water of Caldelas, Moimenta, Taipas and Vizela, while Cavadinho is represented by Cl-type water. The hydrothermal systems of Gestal and Eirogo are characterized by a mixed water type between chloride and hydrogen carbonate. The intensity of water-rock interaction suggests rather short residence times in the reservoir for the geothermal waters of Caldelas and Taipas compared to the other thermal waters. The 18O and 2H water isotope data indicate a dominant meteoric origin of the thermal waters. Considering the isotope altitude effect, the preferential thermal water recharge areas are located at altitudes between 315 and 585 m asl, which is coincident with the fault networks and could be a flow water circulation paths. Several geothermometers were applied, considering the geothermal gradient of the region, to estimate the reservoir depth and temperature. Estimated reservoir temperatures range from 40 °C for the Caldelas thermal area to 130 °C for the Vizela thermal area, indicating that correspond to low-temperature geothermal systems. The estimated temperatures suggest depths of the geothermal reservoirs between 0.8 and 3.3 km. Indications for mixing processes between upwelling geothermal waters and shallow aquifers were found at Vizela and Taipas area

Quasiparticle losses at the surface of superconducting tunnel junction detectors

Superconducting tunnel junctions (STJs) are promising as high energy resolution x-ray detectors. However, the theoretical limit of the energy resolution of STJs has not yet been reached for several reasons. In many cases quasiparticle losses limit the energy resolution. We have investigated STJs with different multilayer structures by means of low temperature scanning electron microscopy. By measuring the quasiparticle lifetime of Nb junctions with and without Ta passivation at the surface, we have identified quasiparticle losses at the surface of nonpassivated junctions as the dominant loss process. The temperature dependence of the quasiparticle lifetime gives information about the loss mechanism. The influence of quasiparticle traps on the effective quasiparticle lifetime is discussed. © 1999 American Institute of Physics

Risikoorientierte Bewilligung von Erdwärmesonden

Die Zahl an Erdwärmesonden nimmt in ganz Europa stetig zu. Wie mit anderen Technologien der Energiegewinnung sind auch mit Erdwärmesonden Risiken verbunden. Deshalb bedarf es Leitlinien als Grundlage für die Bewilligungsverfahren, die den Bau und Betrieb einer solchen Anlage regeln. Um dabei den zu erwartenden Nutzen gegen mögliche Risiken abwägen zu können, sind Kenntnisse über die hydrogeologischen Verhältnisse am Standort und die dort stattfindenden Prozesse erforderlich. Am Beispiel des Erdwärmenutzungskonzepts des Kantons Basel-Landschaft wird gezeigt, wie die Bewilligung von Erdwärmesonden differenziert geregelt werden kann, und wie die geologischen Merkmale einer Region in ein Erdwärmenutzungskonzept integriert werden können. Dabei werden insbesondere Karstgebiete, Gebiete mit der Gefahr des Gebirgsquellens und der Subrosion, Gewässerschutzbereiche, und Gebiete mit Grundwasserstockwerkbau oder gespannten Grundwasserverhältnissen berücksichtigt. Der Artikel will damit einen Beitrag zur derzeit stattfindenden gesellschaftlichen Diskussion über den Nutzen und die Risiken der untiefen Geothermie leisten, indem er Risiken durch den Bau und Betrieb von Erdwärmesonden beschreibt, und Möglichkeiten aufzeigt, diese Risiken in der Bewilligungspraxis zu berücksichtigen. The number of borehole heat exchangers (BHEs) is continuously rising in Europe. Just like other energy producing technologies, there are risks associated with BHE systems. Therefore, guidelines are necessary for regulating the construction and operation of such installations. In order to be able to trade-off the expected benefits against possible risks, knowledge about the local hydrogeological conditions, and the processes taking place, is required. Using the geothermal energy use concept developed for Canton Basel-Landschaft as an example, it is shown how the approval of BHE systems can be regulated in a differentiated way, and how the geological characteristics of a region can be integrated into a geothermal energy use concept. In particular, karst areas, areas with the risk of ground swelling and subrosion, water protection areas, and areas with multiple or confined aquifers are considered. The article aims at making a contribution to the present societal discussion about the benefits and risks of shallow geothermal energy systems by describing the risks associated with BHEs, and by presenting options to account for these risks in approval practice. Geothermie in der Nordwestschwei