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

Abstract

This paper reviews the results of a multi- and interdisciplinary approach, including geological, geomorphological, tectonic, geochemical, isotopic, and geophysical studies, on the assessment of a Chaves low-temperature (77°C) CO2-rich geothermal system, occurring in the northern part of the Portuguese mainland. This low-temperature geothermal system is ascribed to an important NNE-trending fault, and the geomorphology is dominated by the “Chaves Depression,” a graben whose axis is oriented NNE-SSW. The study region is situated in the tectonic unit of the Middle Galicia/Trás-os-Montes subzone of the Central Iberian Zone of the Hesperic Massif comprising mainly Variscan granites and Paleozoic metasediments. Chaves low-temperature CO2-rich geothermal waters belong to the Na-HCO3-CO2-rich-type waters, with pH≈7. Total dissolved solids range between 1600 and 1850 mg/L. Free CO2 is of about 500 mg/L. The results of SiO2 and K2/Mg geothermometers give estimations of reservoir temperature around 120°C. δ18O and δ2H values of Chaves low-temperature CO2-rich geothermal waters indicate a meteoric origin for these waters. No significant 18O-shift was observed, consistent with the results from the chemical geothermometry. δ13CCO2 values vary between −7.2 and −5.1‰ vs. V-PDB, and CO2/3He ratios range from 1×108 to 1×109, indicating a deep (upper mantle) source for the CO2. 3He/4He ratios are of about 0.9 (R/Ra). The Chaves low-temperature CO2-rich geothermal waters present similar 87Sr/86Sr ratios (between 0.728035 and 0.716713) to those of the plagioclases from granitic rocks (between 0.72087 and 0.71261) suggesting that water mineralization is strongly ascribed to Na-plagioclase hydrolysis. Geophysical methods (e.g., resistivity and AMT soundings) detected conductive zones concentrated in the central part of the Chaves graben as a result of temperature combined with the salinity of the Chaves low-temperature CO2-rich geothermal waters in fractured and permeable rock formations. This paper demonstrates the added value of an integrated and multi- and interdisciplinary approach for a given geothermal site characterization, which could be useful for other case studies linking the assessment of low-temperature CO2-rich geothermal waters and cold CO2-rich mineral waters emerging in a same region.

Geochemical and Isotope Characterisation of Thermo-Mineral Springs of Corsica Island: From Geological Complexity to Groundwater Singularity

“…Temperature and pH measured in 2014 are consistent with respective measures from 1980 [34]. High temperature and high pH are typical of a deep magmatic rocks circulation [13,48,49]. This 6 months' survey highlights strong mineralisation stability and is characteristic of the involvement of deep groundwater processes controlling the water quality.…”

“…FONT and ACQ, both bubbling springs, show the highest HCO 3 content (up to 35 meq/L). Their high mineralisation is due to the intensive water-rock interactions boosted by the aggressiveness of low pH induced by the high CO 2 solubility at low temperatures [48]. However, ACQ shows a seasonal variability of the dissolved content, explained by the surface waters contribution.…”

Lithium in Portuguese Bottled Natural Mineral Waters—Potential for Health Benefits?

“…If bottled as such it must be labelled as “naturally carbonated natural mineral water” ( Table 1 : NC waters). For example, in the case of Vidago and Pedras Salgadas waters, following [ 44 ] and references therein, their δ 13 C CO2 values vary between −7.2 and −5.1‰ vs. V-PDB and CO 2 / 3 He ratios range from 1 × 10 8 to 1 × 10 9 , indicating a deep (upper mantle) source for the CO 2 . It may also be possible to capture the natural source of CO 2 and re-inject it into the water prior to bottling or added it artificially, being, in the second case, described as “artificially carbonated natural mineral water” ( Table 1 : AC waters).…”

“…These waters are mainly exploited from Hercynian granitic rocks in the north of Portugal, from within the Minho and Trás-os-Montes regions, in the Geotectonic unit Galiza-Trás-os-Montes Zone ( Figure 1 ). The catchment areas of these Li-rich waters is well correlated with regional fault systems, such as the “Penacova-Régua-Verin Fault”(Campilho, Vidago and Pedras Salgadas), the “Vilariça Fault” (Frize and Bem-Saúde) and the “River Minho Fault” (Melgaço) ( Figure 1 ), since they normally provide the best conditions for the rising of fluids from deep crustal zones [ 43 , 44 , 45 , 53 , 54 ]. With exception of Campilho, all these Li-rich natural mineral waters are naturally carbonated, with free CO 2 contents above 250 mg/L, identified on the bottle label as a “gasocarbonic” water.…”

Laboratory Investigation and Modelling of the Thermal-Mechanical Properties of Soil in Shallow Mineralized Groundwater Area

“…According to the United Nations Educational, Scientific and Cultural Organization (UNESCO), over 1 billion hectares of saline soils or salt-affected soils are widely distributed in nearly 100 countries and regions, especially in arid and semiarid areas [1]; many of these areas are facing severe challenges of substantial air pollution and energy shortage caused by consuming fossil fuels for heating and cooling of the buildings [2]. Therefore, the renewable geothermal energy becomes a promising alternative to achieve thermal comfort due to its low cost, high availability, stability, and minimal environment impact and can be utilized by various geothermal subsurface earth structures such as geothermal energy piles (GEP), ground source heat pumps (GSHP), earth-air tunnel heat exchangers (EATHE), borehole thermal energy storage (BTES), geological carbon dioxide sequestration and storage, and geothermal heated pavement [3][4][5][6][7]. The basic principle of these projects is simple: a cold fluid (CO 2 or water) is injected into the buried pipe, absorbs heat from hotter surrounding soil, and then is pumped out [8].…”