Genesis of Low-Temperature Sulfurous Mineral Waters (NORTHERN Portugal): A Geochemical and Isotopic Approach

Abstract

Chemical and isotopic (18O, D and 3H) analysis of thermomineral waters and dilute spring waters sampled in the surroundings of the low-temperature geothermal areas of Carlao, Sao Lourenco and Moledo (Northern Portugal) were used to determine the source of the waters and to investigate the existence of mixing of a deep thermal component with recent meteoric waters. ɬ18O and ɬD values indicate the meteoric origin of the geothermal waters. The lighter composition of Carlao and S. Lourenco thermal waters could result from the influence of altitude effect. Mixing between deep mineralised waters and dilute superficial waters of meteoric origin might explain the higher 3H activity found in Moledo spring waters. Geothermometric interpretation was used to estimate circulation depth of the groundwater flow systems indicating that maximum depth reached by the waters systems was between 2.0 km and 3.1 km. The results obtained provide extremely important guidelines for the next investigations that form the basis for future drilling strategies and development plans.