Open AccessGeothermal fluids circulation at Caldas do Moledo area, Northern Portugal: geochemical and isotopic signatures
Author(s) -
Marques J. M.,
Marques J. Espinha,
Carreira P. M.,
Graça R. C.,
AiresBarros L.,
Carvalho J. M.,
Chaminé H. I.,
Borges F. S.
Publication year - 2003
Publication title -
geofluids
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.44
H-Index - 56
eISSN - 1468-8123
pISSN - 1468-8115
DOI - 10.1046/j.1468-8123.2003.00059.x
Subject(s) - geothermal gradient , groundwater recharge , geology , groundwater , borehole , geochemistry , hydrogeology , hydrology (agriculture) , spring (device) , hot spring , aquifer , geomorphology , geophysics , mechanical engineering , paleontology , geotechnical engineering , engineering
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 14 C 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 SO 4 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 SO 4 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.