The role of geosciences in the assessment of low-temperature geothermal resources (N-Portugal): a review

Abstract

The aim of this paper is to review the results of the assessment of low-temperature geothermal resources (issue temperatures between 41 and 77°C) that occur in the Portuguese mainland. For this purpose, a multidisciplinary approach, including geologic, tectonic, geochemical, geophysical and isotopic (δ 2 H, δ 18 O, δ 13 C, 3 H and 14 C) techniques, was applied in order to update local and/or regional conceptual circulation models. Three case studies of N-Portugal are presented and discussed. This paper describes different low-temperature geothermal waters presenting similar hydrogeological conceptual models but rather different geochemical signatures (e.g., HCO 3 -Na with pH ≈ 8, HCO 3 /Na/ CO 2 -rich with pH ≈ 7 and HCO 3 -Na with pH ≈ 9, type waters). In fact, in the studied low-temperature geothermal systems, local/ regional high altitude sites associated with highly fractured rocks play an important role in conducting the infiltrated meteoric waters towards the discharge zones near the Spas. The discharge zones are mainly related to the intersection of major regional fault lineaments (and conjugate structures), responsible for creating the mineral waters ascent. In some cases, geochemical and isotopic data point out to the existence of anthropogenic contamination of some geothermal spring waters related to the intense use of fertilizers in areas of widespread agricultural practices.

Assessment of Chaves Low-Temperature CO2-Rich Geothermal System (N-Portugal) Using an Interdisciplinary Geosciences Approach

“…In the Chaves region, the ascending low-temperature CO 2 -rich geothermal waters are structurally controlled by the so-called Verin-Régua-Penacova fault zone (VRPFZ - Figure 2) [40,41,46], related to Alpine Orogeny, which trends 70°-80°E and is hydrothermally active along a belt extending 150 km through mainland Portugal (Figure 3). Along this tectonic megalineament lie not only the Chaves low-temperature CO 2 -rich geothermal waters (the only geothermal waters in the study region) but also numerous emanations of cold (17°C) CO 2 -rich mineral waters (e.g., Vilarelho da Raia, Vidago, and Pedras Salgadas), with no signs of a geothermal origin as discussed in detail by [27][28][29][30][31][32][33][34][35][36][37][38], which are used in local spas (see Figures 2 and 3).…”

“…Several studies carried out on the northern part of the Portuguese mainland (e.g., [27][28][29][30][31][32][33][34][35][36][37][38]) have provided a comprehensive characterization of the Chaves low-temperature CO 2 -rich geothermal system. Several hypotheses have been formulated to assess the origin of the low-temperature geothermal waters and the mechanisms of their being upward from the reservoir towards the surface.…”

Global Isotope Hydrogeology―Review

“…Recharge elevations have been estimated with groundwater δ 18 O data in dozens of areas around the globe (Table ; e.g., Gonfiantini et al, ; Carreira et al, ; Marques et al, ; N'da et al, ; Leketa et al, ; Tsuchihara et al, ). Studies are concentrated, unsurprisingly, in areas of high topographic relief, including the northern China, Morocco, Ethiopia, Europe, and mountain ranges spanning the Americas (Figure ).…”

Geofluids Assessment of the Ayub and Shafa Hot Springs in Kopet-Dagh Zone (NE Iran): An Isotopic Geochemistry Approach

“…Also the temperatures were estimated using Aq.QA software by defining Na-K and SiO 2 as geothermometers. The depth of geothermal reservoir of these springs was calculated using (3) [32] and all estimated temperature and depth are listed in Table 2. K + + Na-feldspar = K-feldspar + Na +…”