ITALIANO_2_corretto_Layout 6 815 Geochemical features and effects on deep-seated fluids during the May-June 2012 southern Po Valley seismic sequence Francesco Italiano1, Marcello Liotta1, Mauro Martelli1, Giovanni Martinelli2, Riccardo Petrini3, Anna Riggio4, Andrea Luca Rizzo1, Francesca Slejko3, Barbara Stenni3 1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy 2 Università di Trieste, Dipartimento di Matematica e Geoscienze (DMG), Trieste, Italy 3 Agenzia Regionale Prevenzione e Ambiente (ARPA) dell'Emilia Romagna, Sezione di Reggio Emilia, Reggio Emilia, Italy 4 Istituto Nazionale Oceanografia e Geofisica Sperimentale (OGS), Sgonico (Trieste), Italy ANNALS OF GEOPHYSICS, 55, 4, 2012; doi: 10.4401/ag-6151 1. Introduction A periodic sampling of the groundwaters and dissolved and free gases in selected deep wells located in the area af- fected by the May-June 2012 southern Po Valley seismic se- quence has provided insight into seismogenic-induced changes of the local aquifer systems. The results obtained show pro- gressive changes in the fluid geochemistry, allowing it to be established that deep-seated fluids were mobilized during the seismic sequence and reached surface layers along faults and fractures, which generated significant geochemical anomalies. The May-June 2012 seismic swarm (mainshock on May 29, 2012, M 5.8; 7 shocks M >5, about 200 events 3 > M > 5) induced several modifications in the circulating fluids. This study reports the preliminary results obtained for the geo- chemical features of the waters and gases collected over the epicentral area from boreholes drilled at different depths, thus intercepting water and gases with different origins and circulation. The aim of the investigations was to improve our knowledge of the fluids circulating over the seismic area (e.g. origin, provenance, interactions, mixing of different compo- nents, temporal changes). This was achieved by collecting samples from both shallow and deep-drilled boreholes, and then, after the selection of the relevant sites, we looked for temporal changes with mid-to-long-term monitoring activity following a constant sampling rate. This allowed us to gain better insight into the relationships between the fluid circu- lation and the faulting activity. The sampling sites are listed in Table 1, along with the analytical results of the gas phase. 2. Geo-hydrological background The Po sedimentary basin is filled by sediments of Alpine and Apennine origin. North-verging Apennine fronts that are buried under this cover overthrust the Padanian basement. Along with the north Apennine outcropping units, these buried structures represent a seismically active belt, where stresses connected in a complex way to the Neo- gene or earlier deformation history are still active [Boccaletti et al. 2011]. Plio-Quaternary stress field evaluations indicate the presence of a tectonic regime that is characterized by a N-NE compressive axis, and average deformation rates that range from 4 mm/year to 7 mm/year have been measured in the area [Cenni et al. 2012]. The local tectonics drove the sedimentation processes, allowing the filling of the sedi- mentary basin by alternate layers of sand and clay [Ori 1993, and references therein]. Gravel is only present near the Apen- nine chain in the alluvial fan areas, while the bottom of the Quaternary sediments is located at depths of 100 m to 700 m. These multilayered sequences constitute the Po aquifer sys- tem [Regione Emilia-Romagna and ENI-AGIP 1998]. In the part of the basin close to the Apennine chain (the belt of alluvial fans), aquifers are located in unsatured sedi- ments and are subjected to seasonal recharge processes. North of this area, the hydrogeological features are quite dif- ferent: fine sediments host confined and/or semiconfined aquifers that are characterized by low circulation velocities (10 m/year). Isotopic data of 18O/16O and 2D/1H indicate that the deep groundwaters in this area are of Alpine origin, while 3H, 14C and 36Cl data indicate ages of 15,000 to 30,000 years [Martinelli et al. 1998, and references therein]. These ancient groundwaters are found mainly in sediments of Alpine origin, from 100 m in depth to the Quaternary base- ment [Dondi et al. 1982, Regione Emilia Romagna and ENI- AGIP 1998]. Field data indicate that inside these sediments that are located at 100 m to 300 m in depth, the hydrostatic load increases linearly with depth, following a regular hy- draulic gradient. On the other hand, at depths shallower than Article history Received July 24, 2012; accepted August 22, 2012. Subject classification: Fluids, Geochemistry, Faults, Temporal changes. 2012 EMILIA EARTHQUAKES 50 m, load fluctuations are observed in connection with rain- fall and barometric pressure, although lenses of confined aquifers are known at shallow depth too. The above-men- tioned features suggest that the meteorological effects on the piezometry of deep aquifers can be considered to be negli- gible in the part of the basin that borders the Po River in the north and the belt of the Apennine alluvial fans in the south [Albarello and Martinelli 1994, and references therein]. The chemical composition of the aquifers of the Emilia- Romagna Plain is largely due to the stratigraphy of the Plio- Quaternary sedimentary successions, where the great prevalence of CaHCO3 groundwaters characterizes the plain at the foot of the Apennine range. The positive buried struc- ture of the Mesozoic limestones, known as the Ferrara anti- cline, largely affects the composition of the shallow aquifers above it, which become of the NaHCO3 type all along the structure, due to the precipitation of calcite or to water–rock interaction processes with Al-silicates. The shape of this structure is clearly reflected by the chemical composition of the aquifers of the Emilia-Romagna Plain, where NaHCO3 type waters follow the WNW-ESE structural trend, as al- ready established through seismic surveys [Pieri and Groppi 1981]. Saline, NaCl, type waters are limited to the north- eastern sector of the plain, and to the area north of Modena. They appear to be related to evaporitic material deposited in lagoonal areas north of the Ferrara anticline during the Pliocene [Conti et al. 2000] or to the uprising of marine brines trapped in Pliocenic layers. The O-H isotopic compo- sition of the NaCl waters has a clear meteoric signature, and no inflows of present day seawater appear necessary to jus- tify such a composition, although mixed phenomena be- tween meteoric originated groundwaters and brines of the Pliocenic or pre-Pliocenic layers cannot be excluded. To bet- ter understand the role of deep-seated fluids during the May- June southern Po Valley seismic swarm, a periodic sampling was carried out, including the groundwaters, dissolved gases, and free gases. Six wells, which include phreatic (ca. 10-20 m deep) and deep groundwaters (boreholes drilled to depths >100 m; Table 1) were considered during the first sampling campaigns. Three of the six wells sampled, which represent deep ground- waters, were considered for regular periodic sampling. These wells are located in the municipalities of Mirandola (depth 190 m, characterized by weak bubbling activity), Concordia [AGIP Mineraria 1977, Gorgoni and Tosatti 2004] (1,700 m deep AGIP well characterized by intense bubbling activity), and San Felice sul Panaro (190 m deep well, characterized by a little bubbling activity). The results from one of these (the Vallalta site) are shown in Table 1 The waters were sampled using PVC bottles, while the dissolved gases were sampled in 250-mL glass vessels. When available, the bubbling gases were collected using an inverse funnel, driving the gas to a two-way sampling bottle. When free gas was not available, 240 mL water samples were col- lected in glass bottles sealed by a silicon rubber septa, using special pliers to extract the dissolved gas phase. All of the samples were collected, taking care to avoid even tiny bub- bles, to prevent atmospheric contamination (details in Ital- iano et al. [2009]). 3- Field work and laboratory results 3.1. Groundwaters The groundwaters were characterized by a circum-neu- tral pH, variable ionic strengths (electrical conductivity in the range 1-20 mS/cm) and a redox environment ranging from anoxic to slightly oxic. Based on their major constituent an- ions (Cl–, SO4 2–, HCO3 –) and cations (Mg2+, Ca2+, Na+, K+), most of the waters belong to the NaCl hydrofacies, with the exception of the sample from Camposanto Menozzi station, which belongs to the NaHCO3/Cl water-type (Figure 1a). This is consistent with the saline signature reported for wa- ters from wells in the northeastern sector of the Emilia-Ro- magna Plain and north of Modena [Martinelli et al. 1998]. Na and Cl show a linear correlation, with waters from the Mirandola site characterized by higher Na and Cl con- tents. On the basis of the molar concentrations, this trend deviates from a seawater dilution trend and from the 1:1 line as expected for halite dissolution. Waters appear to distrib- ute along a progressive dilution pattern for saline (65 g/l) waters found in fractured Mesozoic carbonates or in Pliocenic or pre-Pliocenic layers. This characterizes the structural high in the Modena-Ferrara low-plain area, which suggests possible admixing of a deep-seated component at different extents. In terms of the oxygen and hydrogen isotopic composi- tions (Table 2), the collected waters deviate from the line of meteoric precipitation reported for Apennine catchments and the easternmost part of the Region (Ravenna Plain), while they plot close to the meteoric water line for northern Italy [Longinelli and Selmo 2003]. This indicates a meteoric origin, as already suggested by Martinelli et al. [1998] and a long circuit for the local groundwaters. However, waters from the Mirandola site appear to deviate slightly towards a freshwater–seawater mixing line, which supports the possible contribution of formation waters of marine origin. Prelimi- nary Sr isotopic ratios have been measured on waters from the Mirandola, Concordia and San Felice sul Panaro sites. The Mirandola groundwaters sampled at different times overlap within the experimental error (87Sr/86Sr in the range of 0.708793 to 0.708848). Slightly lower 87Sr/86Sr values were obtained for San Felice sul Panaro (87Sr/86Sr, 0.708762), while the Concordia samples are characterized by a more radi- ogenic character (87Sr/86Sr, 0.709081). Assuming that the Sr- isotopic signature in the saline waters was not significantly altered by late-stage diagenetic processes, and that the high- ITALIANO ET AL. 816 817 FLUIDS GEOCHEMISTRY AND SEISMIC CRISIS Si te I. D . Si te B o re h o le D ep th M u n ic ip al it y D at e D is so lv ed g as es d 13 C O 2 d 13 C H 4 R / R a H e/ N e (m ) H e (c m 3 S T P /L ) H 2 (c m 3 S T P /L ) O 2 (c m 3 S T P /L ) N 2 (c m 3 S T P /L ) C O (c m 3 S T P /L ) C H 4 (c m 3 S T P /L ) C O 2 (c m 3 S T P /L ) 1 B as so li 70 Fi na le E m ili a 29 /0 5/ 20 12 1. 9E -0 3 bd l 6, 5E -0 6 2, 1E -0 3 bd l 6, 6E -0 3 3, 8E -0 3 n. m . n. m . 0, 05 1, 06 3 B el le nt an i 19 0 Fi na le E m ili a 29 /0 5/ 20 12 5. 3E -0 4 6. 4E -0 3 2. 0E -0 6 1. 3E -0 3 0. 0E + 00 3. 7E -0 3 1. 0E -0 3 n. m . n. m . n. m . n. m . 20 /0 6/ 20 12 1. 4E -0 4 bd l 2. 7E -0 6 1. 1E -0 3 4. 6E -0 5 3. 2E -0 3 6. 6E -0 4 n. m . n. m . n. m . n. m . 4 M ir an do la 19 0 M ir an do la 22 /0 5/ 20 12 bd l bd l 1. 0E -0 6 2. 2E -0 4 9. 3E -0 5 4. 2E -0 3 1. 0E -0 3 n. m . n. m . 0. 55 0. 50 " 20 /0 6/ 20 12 bd l bd l 1. 4E -0 6 1. 7E -0 4 2. 7E -0 4 2. 5E -0 3 5. 2E -0 4 n. m . n. m . n. m . n. m . 5 M in oz zi 20 C am po sa nt o 26 /0 5/ 20 12 bd l bd l 8, 2E -0 6 8. 5E -0 4 bd l 6. 4E -0 4 9. 8E -0 3 n. m . n. m . 0. 74 0. 34 08 /0 6/ 20 12 2. 9E -0 4 bd l 2. 8E -0 6 9. 4E -0 4 0. 0E + 00 6. 3E -0 4 8. 2E -0 3 n. m . n. m . 0. 14 3. 32 6 Fe do ss a 20 C am po sa nt o 26 /0 5/ 20 12 3. 3E -0 4 bd l 3. 1E -0 6 1. 6E -0 3 0. 0E + 00 3. 1E -0 4 7. 1E -0 3 n. m . n. m . 0. 15 2. 88 7 Fr eg ni 20 C am po sa nt o 26 /0 5/ 20 12 bd l bd l 5. 9E -0 6 1. 2E -0 3 bd l 4. 7E -0 3 5. 0E -0 3 n. m . n. m . 0. 79 0. 32 M ia no 10 40 C or ni gl io 30 /0 5/ 20 12 5. 0E -0 4 bd l 3. 6E -0 6 2. 7E -0 4 5. 7E -0 4 2. 1E -0 3 2. 6E -0 4 n. m . n. m . 0. 11 4. 76 07 /1 0/ 20 04 bd l bd l 1. 6E -0 5 1. 5E -0 4 1. 9E -0 4 1. 0E -0 3 2. 1E -0 4 n. m . n. m . n. m . n. m . B . R om ag na 0 B . R om ag na 09 /0 6/ 20 04 2. 3E -0 4 1. 2E -0 3 4. 2E -0 6 4. 6E -0 4 2. 1E -0 4 2. 6E -0 3 1. 4E -0 4 n. m . n. m . n. m . n. m . B u b bl in g ga se s H e (p pm ) H 2 (p pm ) O 2 (v ol % ) N 2 (v ol % ) C O (v ol % ) C H 4 (v ol % ) C O 2 (v ol % ) 2 V al la lt a 17 25 C on co rd ia 27 /0 5/ 20 12 4 bd l 0. 01 11 .2 bd l 86 .8 3 0. 68 -9 .1 6 n. m . 0. 15 1. 94 29 /0 5/ 20 12 6 3 0. 07 11 .1 1 84 .0 5 0. 76 -7 .3 8 n. m . 0. 17 2. 53 01 /0 6/ 20 12 3 16 0. 07 11 .4 1 87 .4 0 0. 66 n. m . -7 4. 4 0. 22 2. 75 07 /0 6/ 20 12 3 bd l 0. 01 11 .1 0. 4 87 .2 6 0. 90 n. m . -7 4. 9 0. 24 2. 69 18 /0 6/ 20 12 4 12 0. 01 11 .2 bd l 84 .9 4 0. 79 n. m . -7 4. 7 n. m . n. m . 20 /0 6/ 20 12 4 27 0. 03 11 .2 1. 8 85 .3 4 0. 74 n. m . -7 4. 8 n. m . n. m . 26 /0 6/ 20 12 4 7 0. 02 10 .8 0. 6 84 .1 4 1. 23 n. m . n. m . n. m . n. m . 03 /0 7/ 20 12 5 15 0. 02 11 .1 0. 3 86 .3 6 0. 79 n. m . n. m . n. m . n. m . 13 /0 7/ 20 12 3 15 0. 02 10 .9 0. 7 85 .7 4 0. 91 n. m . n. m . n. m . n. m . T ab le 1 .C he m ic al a nd c ar bo n an d he liu m is ot op ic c om po si ti on o f di ss ol ve d an d bu bb lin g ga se s. C ar bo n is ot op es a re r ep or te d in d ‰ v er su s th e in te rn at io na l V ie nn a Pe e D ee B el em ni te (V -P D B ) s ta nd ar d. H e is o- to pe s ar e sh ow n as R a va lu es ( R a = a tm os ph er ic 3 H e/ 4 H e ra ti o = 1 .3 9 × 1 0– 6 ) . U ni ts o f m ea su re fo r bo th d is so lv ed a nd b u bb lin g ga se s ar e sh ow n. B dl , b el ow d et ec ti on li m it s; n .m ., no t m ea su re d. salinity component mostly contributed to the Sr budget in the waters studied, the isotopic composition of the Miran- dola waters would correspond to a paleo-seawater compo- nent of Mio-Pliocene age. 3.2. Gases The analytical results show that both bubbling and dis- solved gases have low atmospheric contamination (Table 1). The oxygen content is negligible, and CH4 is always the main gas species. He is enriched in all of the gas samples, and par- ticularly in the dissolved gases, where its concentration is greater than the equilibrium with air saturated water, by ca. 40-fold at the San Felice sul Panaro well, 50-fold at the Con- cordia well, and ca. 10-fold at Mirandola, which shows the deep provenance of the fluids feeding the selected sites. Both dissolved and bubbling gases are plotted in the tri- angular diagram of Figure 1b, which shows the relative gas contents. It is worth noting the CH4 content, which is always the dominant species, includes the gases dissolved in shallow waters, as a consequence of diffuse CH4 degassing at a re- gional scale that can remove the atmospheric components from the recharged waters. The carbon isotopic composition of CH4 (–79.7‰< d 13C < –74.4‰) collected at the deep boreholes (Table 1) highlights a biogenic origin for that methane from carbonate reduction. CO2 is always detectable in the bubbling gases, with a con- centration ranging between 0.66% and 0.9% at Concordia, ca. 1% at Massa Finalese, and 0.645% at San Felice sul Panaro. While the isotopic composition of CO2 from Massa Finalese is –17‰, the gases at Concordia displayed more positive val- ues, from –9‰ to –7‰ (Table 1). Considering together the information of the geochem- ical features of the gas phase, we argue that they originate from deep gas reservoirs, for which we exclude contributions from the mantle or mantle-derived sources, considering the isotopic ratio of He (in the range of 0.15-0.4 Ra). The sig- nificant amount of He, which is mainly composed of 4He, originates from radioactive decay of the parent radioactive elements distributed in the thick sedimentary layers of the Po Plain, and concentrated in the hydrocarbon reservoirs re- sponsible for the large release of CH4. High radon activity has never been detected, however, which includes the pre- liminary determinations carried out during the geochemical prospection. In contrast, temporal changes have been de- tected in other gas species, including He, CO2, N2 and CH4. 4. Earthquake-induced variations The crustal deformative processes associated with the occurrence of the seismic sequence induced a squeezing of deep-seated groundwaters associated with brines of the Miocenic-Pliocenic geological structures of the Ferrara buried anticlines. ITALIANO ET AL. 818 Figure 1a. Water geochemical features. Piper diagram for the water samples. Triangle, Mirandola; circle, Concordia; diamond, Bellentani; square, Menozzi; cross, Bassoli; star, Miano. 819 In the same sampling sites, and in particular during the month of May 2012, analysis of the dissolved gases and free gases detected a deep component that was characterized by high He concentrations, as above the atmospheric contribu- tion, and by 13C linked to deep geological layers. In coinci- dence with the May 29, 2012, M 5.8 event, the CH4 concentration in the bubbling gases from the deep well of the Vallalta site dropped sharply, with an increase in CO2 (Figure 2a). The CO2-rich component is also the main car- rier for He; thus, considering the radiogenic origin for the He component, we argue that the deep crustal component is represented by a CH4-dominated (>80% by volume), CO2-rich (1% to 5% by volume) gas phase. After the occurrence of the main earthquakes, the gas composition underwent modifica- tions, which were probably due to the long-term seismicity of the area. A behavior marked by a slow recovery of the former equilibrium conditions will help to define the background level for the area in terms of natural venting of deep and shal- low components and their mixing proportions. We expect that the general behavior might show similar effects as those al- ready observed in several sites around the epicentral area of the 1997-98 Umbria seismic sequence [Italiano et al. 2004] Preliminary speciation calculations at the Concordia site indicate that the waters underwent an abrupt calcite undersat- uration pattern, followed by a trend towards new saturation FLUIDS GEOCHEMISTRY AND SEISMIC CRISIS Figure 1b. Gas geochemical features. Triangular diagram showing the relative concentrations of He, CO2 and CH4 for both bubbling (crosses) and dis- solved (empty triangles) gases. Samples from two sites located far away from the epicentral area (M, Miano di Corniglio; BR, Bagno di Romagna) are shown for comparison. Sample I.D. as in Table 1. Site name Date d18O dD Bassoli 29/05/2012 -10.03 -68.29 Bellentani 01/06/2012 -10.79 -74.93 07/06/2012 -10.81 -74.63 29/05/2012 -10.81 -74.09 Minozzi 08/06/2012 -8.42 -54.55 Mirandola 29/05/2012 -7.71 -52.99 01/06/2012 -8.12 -55.73 07/06/2012 -8.68 -60.29 Vallalta 29/05/2012 -10.68 -73.81 01/06/2012 -10.70 -74.96 07/06/2012 -10.69 -74.99 Table 2. Isotopic composition of stable isotopes (O2 and deuterium) for some waters collected during the seismic crisis. Data in d‰ versus Stan- dard Mean Ocean Water (SMOW) international standard. conditions (Figure 2a). This can be attributed to calcite disso- lution in contact with CO2-charged waters in a closed system, which indicates possible transients of CO2-rich gases. After June 15, the deep component lowered together with the water flow rate observed at the San Felice sul Panaro sampling site. The only way towards the contemporary generation of both groundwater chemical anomalies and gaseous anom- alies is the existence of faults that cross the Quaternary and Mio-Pliocenic deep layers. This hypothesis is supported by the preliminary data on gas and water geochemistry changes. Longer time series are required to confirm the proposed model of fluid/fault relationships. Further investigations will better highlight possible contributions of pre-Miocenic groundwaters hosted in carbonate reservoirs. ITALIANO ET AL. 820 Figure 2. Temporal changes that occurred in the liquid and gas phases. (a) Temporal changes in the calcite saturation state between May 29, 2012, and May 20, 2012. Symbols as in Figure 1a. (b) Temporal changes recorded in the gas phase (bubbling gases of the 1,725-m-deep Vallalta borehole). Variations in CO2 and CH4 concentration (vol%) with respect to the seismicity of the area (bar chart based on ISIDE-INGV data). Crosses, CO2; empty circles, CH4. The May 29, 2012, M 5.8 event occurred in coincidence with the main changes recorded in the chemical compositions of the vented gases. The inverse correlation shown by CO2 (solid line) and CH4 (dashed line) is of note. a b 821 5. Conclusions This geochemical prospection and the periodic sampling of both liquid and gaseous phases in selected groundwaters of the southern Po Valley have revealed the occurrence of tectonic discontinuities that can allow the fast uprising of deep-seated fluids. The fluids are of deep origin and their movement towards the surface is enhanced during seismic sequences. The lowering with time of the main deep-origi- nated components of the gaseous and water phases followed the lowering of the seismic rate and magnitudes. Further sampling will allow a better description of the ongoing tec- tonic process. Acknowledgements. The authors wish to thank Prof. G. Yuce and Prof. G. Bianchini for their useful comments and suggestions. 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Riserve idriche sotterranee della Regione Emilia-Romagna, G.M. Di Dio (ed.), S.EL.CA., Firenze. *Corresponding author: Francesco Italiano, Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy; email: f.italiano@pa.ingv.it. © 2012 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. 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