Subsidies on Geomorphological and Geological-Structural Elements for the Knowledge of Deep Aquifer Systems of Hot Sulphurous Groundwaters in the Meda Region (Portugal)

Abstract

"The Meda region is a territory in the interior of Portugal, made up of essentially rocky terrain of the Hercynian granite type and
the ante-Ordovician schist type. The local population lives mainly from agriculture; only a small percentage lives from
services, commerce, and other residual activities. The supply of drinking water to the population is provided from surface
water, rainwater, stored in the Ranhados Dam, on the Torto River. The most common groundwater is essentially used for
agricultural activities and is abstracted from surface aquifer systems, mainly in granite massifs, up to around 100m deep. The
waters of these systems have hydrogeochemical characteristics typical of short underground circuits, shallow, with an acidic
pH of around 6, total mineralization generally less than 300 mg/L, with the dominant anion being bicarbonate and the dominant
cation being sodium and/or calcium. However, there are some occasional occurrences of groundwater, which although very
rare, are very important due to their specific characteristics These rare groundwaters are sulphureous, with the presence of
reduced Sulphur species, are alkaline due to their pH generally being higher than 8, have total mineralization between 300
mg/L and 600 mg/L, belong to the sodium bicarbonate facies, have significant concentrations of silica, with SiO2 between 25.0
mg/L and 63.5 mg/L, have fluoride values higher than 10 mg/L, and also have the presence of carbon dioxide (total CO2);
present several trace elements with relevance to Boron (B), Rubidium (Rb), Strontium (Sr), Cesium (Cs), and Tungsten (W).
There is also the particularity that the latter waters are warm, resurfacing to the surface at temperatures higher than normal in
the region ( 15oC), with the highest temperature at the Well AC1A of the Longroiva Medical Spa, with 47.4oC.
Geothermometer studies indicate reservoir temperatures of around 78oC for Águas do Graben, 84oC for Areola and 115oC for
Longroiva, which are the three main natural discharge sites for these deep aquifer systems. The potential use of these special
waters is for thermalism activities and geothermal exploitation, as is already the case at the Longroiva Medical Spa. Therefore,
in order to promote the exploration of those waters and new applications, it is necessary to know as accurately as possible the
geohydraulic model of the circuit of those waters, and the geomorphological and geological-structural aspects are absolutely
central to this process. So, this article, after a brief introduction, presents the main geomorphological and geological-structural
characteristics of the region and explains the various details that led to the compartmentalization of the region into three
potential exploitation poles: Longroiva Medical Spa, Areola Medical Spa, and Águas do Graben. Finally, some conclusions and
final notes are presented on proposals for new studies to support detailed knowledge of the geohydraulic models of each Pole
and other potential locations to explore water from the deep aquifer system."