Environmental pre-exploita ton monitoring of Torre Alfna geothermal system (Central Italy) Carapezza ML. 1 , Gatuso A. 1 , Pagliuca N.M. 1 , Ranaldi M. 2 , Sortno F. 3 , Tarchini L. 2 , Zanolin F. 4 , Badiali L. 4 , Console R. 6 , Frepoli A 5 ., Lisi A. 1 , Marchet A. 5 , Mele G. 1 1 INGV, Roma 1, Rome, Italy; 2 University of Rome Tre, Dept. Sciences, Rome, Italy; 3 INGV, Palermo, Palermo, Italy.; 4 INGV, Rome, Rome, Italy.; 5 INGV, Rome, CNT, Italy; 6 CGIAM, Potenza, Italy. An interestng project of geothermal pilot plant, with no-gas emission in atmosphere, has been submited for approval in the medium-enthalpy geothermal feld of Torre Alfna. This prompted us to develop a geochemical and geophysical monitoring of the area with the aim of establishing a background informaton to reco- gnize anomalous gas emission, induced seismicity and subsidence, possibly related to the feld exploitaton. The exploraton conducted by Enel in the years ‘70 - '80, including the drilling of 9 deep wells, has shown the existence of a medium -enthalpy geothermal feld in the Torre Alfna zone, in central Italy. The area has been afected by a very complex geological evoluton during the Neogene. It was afected by the Quaternary volcanism of the Tyrrhenian margin which, reached its climax between 0.6 and 0.3 Ma. The present stress feld around Quaternary volcanoes of central Italy has a NE to ENE directon of extension, in agreement with the alignment of Quaternary volcanoes and earthquake fault plane solutons, with T axes preferentally oriented between NE and ENE. The geothermal reservoir is located in fractured Mesozoic carbonate rocks of the Tuscan se- ries, forming a structural high, whose top is located at 500-600 m depth. In the central area of the feld, in the upper part of the reservoir, a gas cap is found consistng essentally of CO 2 , which has long been exploited. The impermeable cover of the reservoir is provided by al- lochthonous Units of the Ligurian fysch. The surface of volcanic rocks host an important cold acquifer extensively exploited for civil uses. The geothermal fuid contained in the reservoir is a medium enthalpy (T = 140 ° C) hot water with TDS = 5000 ppm, containing about 2% by weight of dissolved CO 2 . The CIRM Commission of the Ministry of Economic Development approved the work program of two geothermal pilot plants, each of 5 MWe included under Legislatve Decree 22/2011, which released the use of ORC binary systems; total reinjecton of the fuid extracted in the same reservoir, and no emission of fuid to the atmosphere. The most advanced project is Castel Giorgio in Umbria Re- gion for which the Ministry of Environment approved the environmental compatbility. Gas geochemistry of Torre Alfna geothermal wells and Torre Alfna and Latera emissions In the Torre Alfna geothermal area, there is only one gas manifestaton, called “Solfanare”, located at North of Torre Alfna. The chemical and isotopic compo- siton of the gas emited is shown in table 1 where for comparison are also reported data from Enel geothermal wells and those of the gas manifestaton of the nearby area of Latera. The dominant gas is CO 2 whereas CH 4 content is low compared to the high enthalpy geothermal felds of Tuscany. The helium isotopic compositon of Torre Alfna is the same of that of Latera and despite Its relatvely low values (R / Ra = 0.34 to 0.44) suggests a likely source of mantle (or mag- ma) metasomatzed in subducton process. Monitoring of CO 2 soil degassing: frst results In summer 2011, a frst wide survey of soil CO 2 fux was carried out at Torre Alfna (1336 measurements over 12.6 km 2 by accumulaton chamber). The resultng CO 2 map (see fgure) shows that CO 2 emission is very low in most of the in- vestgate area and even in the proximity of wells (A1, A7, A13), i.e. above the gas cap, remaining mostly at values be- low the threshold established background to 48 g /m 2 day. CO 2 soil fux anomalies are found only in the proximity of “Solfanare” area where geothermal gases upraise through a NW-SE fault. Six target areas for regular monitoring of soil CO 2 fux from the soil has been identfed and reoccupied 7 tmes from April 2013 to June 2015 (with an almost seasonal frequency). All measured values are always within the background threshold, as shown in the log-probability plot in fgure. Three populatons may be recognized with slightly increasing values that characterize the diferent soil types of the investgated area, respectvely not vegetated, plowed and vege- tated. The lack of soil CO 2 fux, established with three years of study, show the efcacy of the impermeable layer upon the cap rock and represent an important background informaton. Based on an agreement with the operator, the Natonal Insttute of Geophysics and Volcanology (INGV) must ensure the following monitoring of the project area:  Geochemical Monitoring: periodic monitoring of soil CO 2 fux in the proximity of wells to be drilled in order to ensure that no abnormal gas emission will occur during drilling and exploitaton; monitoring of CO 2 and H 2 S air concentratons at the ORC plant and well heads; chemical monitoring of the water mass to check for possible contaminaton by geothermal fuid.  Geophysical Monitoring: installaton and management of a local seismic network for monitoring ante -operam seismicity, which the aim of recognize and study the possible future seismicity induced by fuid extrac- ton or re-injecton (the frst 6 statons with 3 components are operatng from May 2014); monitoring of possible subsidence through a network of GPS sensor and InSAR satellite monitoring (the system will be actva- ted in autumn 2015). The microseismic network The microseismic network of Castel Giorgio -Torre Alfna, was designed to study both the natural seismicity of the area and the possible seismicity associated with geothermal fuid extracton and reinjecton . Seismic monitoring began in May 2014, with the installaton of the three component seismic staton of “Torraccia”. Subsequentely the seismic network has been expanded to six three components statons. The network geometry is s uch to have a spacing between the statons reduced to 3 -4 km in the area corre- sponding to the geothermal reservoir, and wider moving away from the geothermal area. For the transducton of the seismic signal a SS-20 sensor has been adopted with three components (N-S, E-W and vertcal), short-period (2 Hz) with a usable band 0.1-250Hz. The SS-20 sensor, coupled to the digitzer SL06 / C3, an analog - digital 24-bit, is based on the Linux operatng system. During the period May - December 2014, in a preliminary study, almost 200 earthquakes have been located within the geothermal area. The maximum magni- tude observed in this period was Ml=2.8, with a locaton just a few km west of the geothermal area. Most earthquakes have Ml ≤1.0. In fact, these area has been subject to a seismicity which manifests itself mainly with the characters more typical of volcanic and geothermal areas: low hypocenters depth and tem- poral earthquakes distributon in "swarms". Based on the data obtained by INGV catalogues historical seismic actvity, more resentul in the area, are related to: Civita di Bagnoreggio (1695) with Mw 5.67 ±0.25; Acquapendente (1755, 1920, 1924) with Mw 4.50; Castel Giorgio (1957) with Mw 4.93 ±0.21 .