Minissale, Angelo

This paper presents and discusses the water and gas geochemistry of a large number of thermal springs occurring along the N-S trending Strymon Valley, from its source, near Sofia (Bulgaria), to the Aegean Sea (Greece). In Bulgaria springs have markedly alkaline pH, relatively low Total Dissolved Solids and prevalent Na-HCO3 to Na-Cl(SO4) in composition while the associated gas phase is mostly N2-dominated. When moving to the Greek sector, the thermal springs, Ca(Mg)-HCO3 to Na-HCO3, become less alkaline and more saline whereas the associated gas phase is CO2-dominated. The abrupt geochemical change in the Greek sector is caused by a variation in the thickness and nature of the sediments filling the Strymon Valley, the latter being characterized by a relevant amount of Neogene marine material. Such changes occur south of an important E-W lineament named Middle Mesta, south of which marble formations extensively crop out and are likely occurring below the sedimentary succession. The presence of these carbonate sequences embedded in the Neogene sediments is explaining the CO2-rich gases associated to the Greek springs. Water isotopes indicate a meteoric origin for the studied waters. From a geothermometric point of view, solute (previous studies) and gas (this work) geothermometers suggest that no high enthalpy systems occur in the Bulgaria and northern sector of Greece with estimated temperatures <120 °C. Consequently, these thermal springs can be regarded as tectonically-derived along the many fault systems that border the Strymon Valley. The convective circuits are thus originated from rainfall in the crystalline massifs that border the valley, i.e. the Serbo-Macedonian to the west and the Rhodope to the east.

A new and unusual eruptive event occurred on 29 February 2016 within the summit crater of the Mount Nyiragongo volcano. Based on field campaigns performed between July 2015 and September 2017, and building on a previously published buoyancy‐driven bidirectional magma flow model explaining the progression of Mount Nyiragongo lava lake level, we provide the first quantitative estimations of volumes of erupted lava outpouring from the new spatter cone. Besides matching field data of the lava lake level covering the period December 2002 to September 2017, numerical solutions of the model reveal that the most probable dike path is one originating from the shallow magma reservoir, and not from the lava lake or branching from its feeding conduit. According to these simulations, the reservoir and erupted lava volumes are respectively estimated to the order of 10 km3 and 20 M m3. Magma overpressure at the level of the shallow reservoir is estimated in the range 12 to 16 MPa, high enough to potentially initiate new erupting events.

In recent years, two research projects specifically conceived by Italian Institutions of Research to promote the implementation of the use of geothermal energy in Southern Italy has allowed the review of most data on chemical and isotopic compositions of natural thermal manifestations in the territory of Italy. Two large databases, one for thermal springs and CO2-rich springs, and a second one for fumarolic condensates and associated gas phase have been produced and are available on line, with data spanning in time from the early 70's to the present. We have used those data, after careful evaluation of the quality and reliability of them, to produce correlation diagrams and isodistribution maps of some relevant geochemical/geothermal parameters, such as: pCO2 in thermal springs, %CO2 and δ13C in CO2 of gas phases, 3He/4He ratio and %He. In this way, we have been able to delimit the areal patterns of thermal anomalies potentially related with geothermal reservoirs. The cross correlation among the many parameters (>40) selected has allowed the overview on the circulation of fluids at shallow crust, in one of the most active tectonic boundary of the Earth between the African and the Eurasian continents. Shallow circulation of hot fluids is particularly active in the Roman Comagmatic Province, the Neapolitan area and Sicily (both at Etna, Aeolian Archipelago and Pantelleria island in the Sicily Channel) where active geothermal systems are already known, whose areal extension is probably much larger than what envisaged at present. The geothermometric evaluation of data has not allowed to clearly identity new areas apart from those already known but, nevertheless, some areas in the inner Apennines, as well as Sicily and Sardinia have shown anomalous 3He/4He values that point to the presence of mantle fluids located inside the crust. Being most of active volcanic islands likely much smaller than the thermal anomaly they are associated with, a futuristic perspective of utilizing geothermal fluids off shore is suggested. Moreover, the database and the proposed maps can be a useful tool both scientific community and stakeholders to perform geothermal favourability maps and to identify potential new areas interesting from a geothermal perspective.

The geochemistry of natural thermal fluids discharging in the Mondragone Plain has been investigated. Thermal spring emergences are located along the Tyrrhenian coast in two different areas: near Padule-S. Rocco (41°7.5′N 13°53.4′E) at the foot of Mt. Petrino, and near Levagnole (41°8.5′N 13°51.3′E) at the foot of Mt. Pizzuto. The water isotopic composition of both thermal discharges is lighter than the one of local shallow groundwater (δ18O ≅ −6.3‰ SMOW vs. ≅ −5.9‰; δD ≅ −40‰ SMOW vs. ≅ −36‰, respectively) as a consequence of inland higher altitude of recharge by rainfall, suggesting that thermal water undergoes a deep and long flow-path before emerging along the coast. The chemical composition of the highest temperature samples of two areas points that fluids in the hydrothermal reservoir(s) interact with similar lithologies, since they are both hosted in the lower sedimentary carbonate formations of the Campanian–Latial Apennine succession. However, the two spring systems are different in terms of temperature and salinity (Levagnole: ≅50 °C and 8.9 g/L vs. Padule: ≅32 °C and 7.4 g/L, respectively). The higher salinity of Levagnole springs is due to a longer interaction with evaporite material embedded in Miocene sedimentary formations and to the eventual mixing, during rising, with fresh seawater close to the seashore. The chemical and isotopic composition of the free gases associated with the springs, again suggests a different source of the two hydrothermal systems. Comparing the 3He/4He measured ratios with other gas emissions located NE and SE of Mt. Massico-Roccamonfina alignment, it is evident that the Levagnole thermal springs are related to the northern Latial mantle wedge where the 3He/4He is about 0.5 R/Ra, whereas the Padule-S. Rocco springs, although being only 3.5 km south of Levagnole, are related to the Campanian mantle wedge where R/Ra is always ≥2.0. Such a difference in 3He/4He ratio in a very short distance, clearly, suggests a different source of the Padule-S.Rocco gas phase rising to the surface through the deep transpressive regional fault(s) system related to the NE–SW Ortona–Roccamonfina tectonic alignment.

In this paper thematic maps produced on the basis of the principles of linear geostatistics are described in order to characterise the superficial and ground waters of the municipal territory of Arezzo. The aim is to assess the correlation with lithology, use of soil, drainage density and anthropogenic pressure and identify sensible areas to be periodically monitored in time. This study has used an original geochemical data-base consisting of about 500 water samples. A statistical approach has been used for the identification of anomalous values and homogenous populations. This step was followed by a detailed variographic study in order to analyse the spatial behaviour of the variables. Subsequently, an estimation procedure based on the application of ordinary kriging algorithm and sequential Gaussian simulation methods was applied to obtain the maps.

Since 2000 a joint project between the Department of Earth Science of Florence and the Unit of Environment and Health of the Municipality of Arezzo has allowed to geochemically characterise the superficial and ground waters of the municipal territory of Arezzo in order to establish the water quality and to investigate the main natural and anthropic processes responsible of their composition. The available geochemical data-base consists of more than 500 samples sites (90% of which are private wells and 7 and 3% are springs and running waters, respectively) on which physical parameters (temperature and electrical conductivity) and major, minor and trace dissolved species (pH, Ca, Mg, Na, K, NH4, HCO3, SO4, NO3, NO2, Cl, Br, F, heavy metals) have been performed by using the same sampling procedure and analytical methodology in order to have a consistent set of data. Fifteen selected sites have been analysed twice per year to evidence possible seasonal effects. No significant differences have been recorded. The Arezzo Basin, formed since Upper Pliocene, is a structural depression limited to the North and to the East by the Pratomagno and Chianti belts, respectively, and to the South and to the East by two tectonic lineaments (Val d’Arbia-Val Marecchia transversal and Chitignano normal faults). Along these tectonic discontinuities CO2-rich manifestations either seep out or exploited by private companies. Hydrogeologically, three main aquifers are recognised: i) a relatively deep aquifer hosted in Tertiary sandstone formations; ii) an intermediate aquifer hosted in Quaternary fluvio-lacustrine sediments and iii) a shallow aquifer in recent alluvional sediments. The content in Total Dissolved Solids (TDS) allows to classify the Arezzo waters in: oligomineral (69%), medium-mineral (30%) and mineral (1%) and they can be regarded as Ca(Mg)-HCO3 (87 %), Na(K)-HCO3 (7%), Ca(Mg)-SO4 (5%) and Na(K)-Cl (1%). It is noteworthy to point of that the Na(K)-HCO3 waters are aligned along the above mentioned tectonic systems. The quality of Arezzo waters has been referred to the Italian legislation that is addressed to the definition of the Maximum Admissible Concentration (MAC, DPR 236/88, Dlgs 31/01) and the Reference Value (RV, DPR 236/88) in terms of waters for the human consumption. Waters from the northern area of Arezzo overcome MAC for chlorides, sulphates and sodium; if we consider nitrogen species (NH4, NO2, NO3) the values overcome CMA for those waters collected into the city, its peripheral areas and in the south-western suburbs. Thematic maps has been produced, on the basis of the principles of linear Geostatistics, in order to analyse the spatial behaviour of the analysed variables. The aim was to find correlations with lithology, use of the soils, drainage density, pressure of antrophic activities and so on, and to identify sensible areas to monitor in their time evolution. The investigation has been developed starting from a detailed variographic analysis by means of the geochemical behaviour of each variable has been analysed in the different directions of the space while the estimation procedure to obtain the maps has been based on the application of sequential Gaussian simulation procedures.