-
[show abstract]
[hide abstract]
ABSTRACT: Geochemical and isotopic studies have been undertaken to assess the origin of CO2-rich waters issuing in the northern part of Portugal. These solutions are hot (76°C) to cold (17°C) Na–HCO3 mineral waters. The δ2H and δ18O signatures of the mineral waters reflect the influence of altitude on meteoric recharge. The lack of an 18O-shift indicates there has been no high temperature water–rock interaction at depth, corroborating the results of several chemical geothermometers (reservoir temperature of about 120°C). The low 14C activity (up to 9.9 pmC) measured in some of the cold CO2-rich mineral waters (total dissolved inorganic carbon) is incompatible with the presence of 3H (from 1.7 to 4.1 TU) in those waters, which indicates relatively short subsurface circulation times. The δ13C values of CO2 gas and dissolved inorganic carbon range between −6‰ and −1‰ versus Vienna-Peedee Belemnite, indicating that the total carbon in the recharge waters is being diluted by larger quantities of CO2 (14C-free) introduced from deep-seated (upper mantle) sources, masking the 14C-dating values. The differences in the 87Sr/86Sr ratios of the studied thermal and mineral waters seem to be caused by water–rock interaction with different granitic rocks. Chlorine isotope signatures (−0.4‰ < δ37Cl < +0.4‰ versus standard mean ocean chloride) indicate that Cl in these waters could be derived from mixing of a small amount of igneous Cl from leaching of granitic rocks.
Geofluids 10/2006; 6(4):273 - 287. · 1.50 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A hydrogeological conceptual model of the Caldas do Moledo geothermal site is proposed that shows mixing between geothermal waters and local shallow groundwaters. Stable isotope values of Caldas do Moledo geothermal waters indicate recharge areas located at relatively high altitudes (850–1250 m a.s.l.). The NW–SE Vigo–Régua shear zone plays an important role in fluid recharge and circulation towards the NNE–SSW Régua–Verin fault system, forming a path for ascent of geothermal fluids. The apparent 14C age of geothermal fluids (15.66 ± 2.86 ka BP) was estimated in the total dissolved inorganic carbon (TDIC). Geothermometer calculations indicate that, assuming a conductive temperature gradient of 32°C per kilometer for northern Portugal, the maximum depth of circulation is roughly 1.8 ± 0.4 km. The K, Ca and SO4 concentrations found in some Caldas do Moledo geothermal spring waters show mixing between deep geothermal and shallow groundwater systems. Local shallow groundwaters showing the highest SO4 concentrations were found at low elevation areas, originating from fertilisers and pesticides applied to the Port wine vineyards in the Douro River valley. Geothermal waters from boreholes AC1 and AC2 do not show evidences of direct pollution from the spreading of such agrochemicals.
Geofluids 07/2003; 3(3):189 - 201. · 1.50 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Chaves thermal waters (76C) are the most important external manifestations of low-temperature geothermal systems occurring in Portuguese mainland. They are related to crystalline granitic rocks. This paper describes the use of the environmental isotopic composition (oxygen-18, deuterium, and tritium) of hot and cold waters of the Chaves area as an important hydrogeological tool to solve specific problems arising in the appraisal of the geothermal resources of the area (e.g., origin and age of waters, recharge area, and underground flow paths).18O and D analyses seem to confirm the meteoric origin of Chaves thermal waters. The local altitude dependence of meteoric waters was determined by18O and D analyses of superficial and shallow groundwaters of the Chaves plain and its bordering mountains. Chaves thermal waters seem to be related to meteoric waters, infiltrated on the highest topography areas (Bolideira granitic outcrop NE Chaves), that percolate at great depth and emerge in a discharge area at lower altitude (Chaves plain). Chaves thermal waters showing little oxygen-18 shift and low tritium concentrations could be considered external manifestations of an old geothermal system in which the isotopic water-rock interaction is adjusted to equilibrium.
Environmental Geology 05/1995; 25(4):232-238. · 1.13 Impact Factor
-
Cadernos do Laboratorio Xeolóxico de Laxe: Revista de xeoloxía galega e do hercínico peninsular, ISSN 0213-4497, Nº 28, 2003, pags. 147-172.
