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 (δ2H, δ18O, δ13C, 3H and 14C) 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., HCO3-Na with pH ≈ 8, HCO3/Na/CO2-rich with pH ≈ 7 and HCO3-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.