Water chemistry, fluid-rock interaction processes and new estimates of deep temperatures in the Colli Albani volcano, central Italy

In the framework of a multidisciplinary project funded by the Italian “Dipartimento della Protezione Civile”, focused on the Alban Hills volcanic district (Central Italy), a detailed geochemical survey in groundwater was carried out in 2006-2007 in the Alban Hills area (central Italy), sampling a total of 183 water sites (springs and wells), Figure 1. Physical-chemical parameters and, on selected samples, main chemistry, minor and trace element contents, as well as dissolved gases were analyzed. The study had the main goals to: i) characterise the chemical background of the discharging fluids, gathered in a period with low seismicity; ii) define the main gas-water-rock interaction processes presently ongoing and iii) give reliable estimates of the deep aquifer temperatures, only partially performed till now. The bulk of the samples fall in the field of the earth (alkaline)- bicarbonate waters, while some of them show alkaline-bicarbonate and acid-sulphate chemistry. Earth (alkaline)-bicarbonate waters have a relatively fast circulation in the volcanic rocks in a low temperature environment; in some sectors of the volcano they receive a huge gas input from below (mainly CO2). Waters with longer interaction with volcanic rocks and/or clays, in presence of CO2, evolve towards the alkaline-bicarbonate field. Acid-sulphate waters are formed by dissolution of acid and reducing gases (CO2, H2S) into oxygen-rich shallow aquifers. CO2 and N2 are the principal dissolved gases. Nitrogen, in particular, characterises shallow waters (atmospheric component), while carbon dioxide has a prevalent deep origin. H2S, He and H2 show very low contents, while methane was found both in same CO2-rich waters and shallow samples. The main gas-water-rock interaction processes and their extent were assessed by means of activity plots. Waters can be divided into two main groups: i) earth (alkaline)- bicarbonate waters fall prevalently in the field of kaolinite, representing the early stage of the silicate weathering. They show under-saturation with respect to the main rock-forming minerals, and are considered as “immature” ii) alkaline-bicarbonate show a partial equilibrium with the main clay minerals, representing an evolution towards more “mature” terms. Acid-sulphate waters are out of any equilibrium with the host rocks and were not considered. Physical-chemical parameters and observed chemistry fully fit this chemical scenario. Statistical assessment (Factor Analysis) was used to emphasise the main geochemical processes affecting groundwater, accounting for their observed different chemical evolutions. Geothermometric estimations allowed to define the geothermal system beneath the volcano as characterised by a medium-low enthalpy, with temperatures comprised in the range 110-140°C