Stefania Venturi

Geochemical characterization of the ground waters from the former Hg-mining area of Abbadia San Salvatore (Mt. Amiata, central Italy): criticalities and perspectives for the reclamation process

This study was aimed to geochemically characterize the groundwater system of the Abbadia San Salvatore (Mt. Amiata, Siena, central Italy) former Hg-mining area, whose activity closed at the end of the seventies, in order to accomplish the reclamation process after that the former ownership of the mining concession (E.N.I. National Agency for Hydrocarbons, AGIP Division) passed the property to the Munici pality of Abbadia San Salvatore in 2008. The study area covers a surface of about 65 ha and since February 2013 old and new piezometers were used to assess the main chemical features along with the concentrations of As, Hg and Sb. Four sampling campaigns were carried out up to January 2014 and a relatively large spatial and temporal geochemical variability was observed. Apparently, the working activities related to the construction of an artificial channel (commenced in March 2013 and terminated at the end of 2013), which crosscuts the whole mining area to drain the surface waters in order to minimize the interaction between the meteoric waters and the by-products deriving by the production of metallic mercury, did not affect the groundwater system. Slag, roasted material and other by-products deriving from the local and surrounding Hg mining activities were indeed used to fill the terrain where most of the mining structures lie. The dominating geochemical facies was Ca(Mg)-SO4 and, subordinately, Ca(Mg)-HCO3, while Na-HCO3 compositions were rarely found. Dissolution of gypsum/anhydrite and carbonates and hydrolysis of sulfide minerals are likely the main geochemical process that produced the observed geochemical compositions. The contents of As and Sb only sporadically exceeded the maximum allowable concentrations intended for human consumption (98/83 EC Directive 1998), i.e. 10 and 5 mg L-1. Conversely, those of Hg were constantly above the EC directive, e.g. 1 mg L-1, with the exception of those waters located up- and downstream the groundwater flow. This indicates that the exotic filling terrains in the mining area, mainly consisting of roasting products, likely play a pivotal role in regulating the concentrations of Hg, which reached values up 853 mg L-1. The construction of permeable reactive barriers, located downstream the water flow, appears to be the most promising solution for the removal of Hg, although, according to the literature, several materials, tested with laboratory experiments, can be used and, as a consequence, it is necessary to individuate specific piezometers with different Hg concentrations where pilot investigations are to be carried out before undertaking any remediation actions.

The biogeochemical vertical structure renders a meromictic volcanic lake a trap for geogenic CO2 (Lake Averno, Italy).

Volcanic lakes are characterized by physicochemical favorable conditions for the development of reservoirs of C-bearing greenhouse gases that can be dispersed to air during occasional rollover events. By combining a microbiological and geochemical approach, we showed that the chemistry of the CO2- and CH4-rich gas reservoir hosted within the meromictic Lake Averno (Campi Flegrei, southern Italy) are related to the microbial niche differentiation along the vertical water column. The simultaneous occurrence of diverse functional groups of microbes operating under different conditions suggests that these habitats harbor complex microbial consortia that impact on the production and consumption of greenhouse gases. In the epilimnion, the activity of aerobic methanotrophic bacteria and photosynthetic biota, together with CO2 dissolution at relatively high pH, enhanced CO2- and CH4 consumption, which also occurred in the hypolimnion. Moreover, results from computations carried out to evaluate the dependence of the lake stability on the CO2/CH4 ratios, suggested that the water density vertical gradient was mainly controlled by salinity and temperature, whereas the effect of dissolved gases was minor, excepting if extremely high increases of CH4 are admitted. Therefore, biological processes, controlling the composition of CO2 and CH4, contributed to stabilize the lake stratification of the lake. Overall, Lake Averno, and supposedly the numerous worldwide distributed volcanic lakes having similar features (namely bio-activity lakes), acts as a sink for the CO2 supplied from the hydrothermal/magmatic system, displaying a significant influence on the local carbon budget.

The Geothermal Resource in the Guanacaste Region (Costa Rica): New Hints From the Geochemistry of Naturally Discharging Fluids

The Guanacaste Geothermal Province (GGP) encompasses the three major volcanoes of northern Costa Rica, namely from NW to SE: Rincon de la Vieja, Miravalles and Tenorio. The dominant occurrence of (i) SO4-rich acidic fluids at Rincon de la Vieja, (ii) Cl-rich mature fluids at Miravalles and (iii) HCO3--rich and low-temperature fluids at Tenorio was previously interpreted as due to a north-to-south general flow of thermal waters and a magmatic gas upwelling mostly centered at Rincon de la Vieja, whereas Miravalles volcano was regarded as the typical geothermal system. The uniformity in chemical and isotopic (R/Ra and 34S) compositions of the neutral Cl-rich waters suggested to state that all the thermal discharges in the GGP are linked at depth to a single, regional geothermal reservoir. In this scenario, the thermal manifestations related to Tenorio volcano were regarded as a distal and diluted fluid outflow. In this study, a new gas geochemical dataset, including both chemical and isotopic (13C-CO2 and R/Ra) parameters of fluid discharges from the three volcanoes, is presented and discussed. Particular attention was devoted to the Tenorio thermal manifestations, since they were poorly studied in the past because this area has been considered of low geothermal potential. The aim is to provide insights into the magmatic-hydrothermal fluid circulation and, to verify the spatial distribution of the heat fluid source feeding the fluid manifestations. Our results only partially confirm the previously depicted model, because the geochemical and isotopic features (e.g. relatively high concentrations of temperature-dependent gases and high R/Ra values) shown by fluids seeping out from the southern sector of Tenorio volcano are more representative of medium-to-high enthalpy volcanic systems than those typically occurring in distal areas. This implies that the geothermal potential in the south of the GGP is higher than previously thought.

The 2012–2016 eruptive cycle at Copahue volcano (Argentina) versus the peripheral gas manifestations: hints from the chemical and isotopic features of fumarolic fluids

This study presents the chemical and isotopic compositions of hydrothermal gases from fumaroles discharging around Copahue volcano (Argentina). Gas samples, including those from two fumaroles at the active summit crater, were collected during 13 surveys carried out by different research teams from 1976 to February 2016. The time-series of H2, CO and light hydrocarbons showed episodic increases related to the main events of the last eruptive cycle that started on 19 July 2012. Concentration peaks were likely caused by enhanced input of hot magmatic fluids affecting the hydrothermal reservoir. These data contrast with the temporal variations shown by Rc/Ra and δ13C-CO2 values in 2012–2014, which indicated an increasing input from a crustal fluid source. In 2015–2016, however, these isotopic parameters showed opposite trends; their composition became closer to that of the two summit fumaroles, which possibly corresponds to that of the deep magmatic-related end-member. The delayed and reduced compositional changes in the peripheral hydrothermal fluid discharge in response to the 2012–2016 eruptive events suggest that geochemical surveys of these emissions are unlikely to provide premonitory signals of volcanic unrest if the volcanic activity remains centered in the main crater. Instead, an instrument which is able to provide measurements of volcanic gases in the air (e.g. MultiGAS) may be used to detect changes at the summit crater. Otherwise, monitoring of seismic activity and ground deformation, as well as the periodic measurement of the chemistry of the water in the Rio Agrio, which is fed by thermal discharge from the summit crater, seem to represent the most reliable means of monitoring at Copahue. However, the relative compositional stability of the hydrothermal reservoir is a great advantage in terms of geothermal resource exploitation and could encourage new investments in the Copahue geothermal project which was abandoned in the 1990s.