Groundwater surveys in some Italian areas with different geological and geodynamical features were performed in order to evaluate dissolved rare gases (222Rn, He and Ar) as potential tracers of fault systems and groundwater evolution (fluid origin, water-rock-gas interaction, fluid migration and mixing phenomena). The obtained results showed that the highest values of Rn and He were found along

important fault systems cutting the investigated areas, where the fault-related permeability increases deep-seated fluid circulation. However, dissolved-radon anomalies may be due to: a) a rapid ascent of Rn-bearing fluids, equilibrated with lithologies richer in U than outcropping formation; b) a local enrichment of U due to the action of low enthalpy groundwater or strong carrier gas (CO2) discharges usually occurring along the fault systems

Guerra, M.

Lombardi, S.

Pizzino, L.

Quattrocchi, F.

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IL NUOVO CIMENTO Maggio-Agosto 1999VOL. 22 C, N. 3-4Radon and helium as pathfinders of fault systemsand groundwater evolution in different Italian areas (*)F. QUATTROCCHI (1)(**), M. GUERRA (1), L. PIZZINO (1) and S. LOMBARDI (2)(1) Istituto Nazionale di Geofisica (ING) - Via di Vigna Murata 605, 00143, Roma, Italy(2) Università “La Sapienza” - Piazzale A. Moro 5, 00100, Roma, Italy(ricevuto il 9 Giugno 1998; revisionato il 18 Gennaio 1999; approvato il 19 Febbraio 1999)Summary. — Groundwater surveys in some Italian areas with different geologicaland geodynamical features were performed in order to evaluate dissolved rare gases(222Rn, He and Ar) as potential tracers of fault systems and groundwater evolution(fluid origin, water-rock-gas interaction, fluid migration and mixing phenomena).The obtained results showed that the highest values of Rn and He were found alongimportant fault systems cutting the investigated areas, where the fault-relatedpermeability increases deep-seated fluid circulation. However, dissolved-radonanomalies may be due to: a) a rapid ascent of Rn-bearing fluids, equilibrated withlithologies richer in U than outcropping formation; b) a local enrichment of U due tothe action of low enthalpy groundwater or strong carrier gas (CO2 ) dischargesusually occurring along the fault systems.PACS 91.30.Px – Phenomena related to earthquake prediction.PACS 92.40.Kf – Groundwater.PACS 91.25.Ey – Interactions between exterior sources and interior properties.PACS 01.30.Cc – Conference proceedings.1. – IntroductionNoble-gas geochemistry is currently recognised as a reliable tool to investigate theorigin and processes involving fluids during their ascent to the surface. A betterknowledge of the nature and evolution of deep fluids establishes a fundamental basisfor geological studies, including fault tracing. In fact, fault zones are known to act aspreferential pathways, that can readily transmit a variety of fluids, including gases, tothe surface (e.g., Banwell & Parizek, 1988; Dall’Aglio et al., 1995). Field data haveshown the reliability of dissolved-gas surveys for locating brittle deformations of the(*) Paper presented at the “Fourth International Conference on Rare Gas Geochemistry”,Rome, October 8-10, 1997.(**) E-mail: geochimicaHing750.ingrm.itG Società Italiana di Fisica 309F. QUATTROCCHI, M. GUERRA, L. PIZZINO and S. LOMBARDI310Earth’s surface, even in areas where fault-related surface expressions are difficult torecognise by traditional tools (e.g., Lombardi et al., 1997).Many parameters affect noble-gas content of sampled fluids. In particular the Ucontent in rocks, their porosity or degree of fracturing, water-rock re-equilibrationnear surface, degassing and mixing processes constrain the concentration in water ofsome radiogenic species as Rn, 4He, 40Ar. Moreover some different physico-chemicalFig. 1. – Cumulative frequency diagram of radon. Changes of slopes split every plot into threepopulations. The upper population (III) represents the anomalous values that are related tofaulted zones. The intermediate-value population is considered as the background for that area.The dashed lines mark the range of theoretical equilibrium of water with rocks (see text). Asidethe location of sampled areas is sketched. For each site the number of samples of Rn (upper) andof other gases (lower) are reported.RADON AND HELIUM AS PATHFINDERS OF FAULT SYSTEMS ETC. 311characteristics of noble gas such as solubility, stability and fugacity may originatemisleading interpretation if a comprehensive examination of a large number ofparameters is not properly considered.This communication summarises the results obtained from an extensive surveyingproject performed by ING in some Italian areas with different geological (sedimentaryand crystalline environments) and geodynamical features (quiescent and high-seismicity areas). Selected areas were (fig. 1): Gargano and Calabria for sedimentaryenvironment, Central Alps and Sardinia for crystalline domains.2. – MethodologySampling was focused as much as possible along the main tectonic discontinuitiescutting each area. In addition samples were also collected throughout the areas toprovide a geochemical background.Radon concentration in groundwater was determined in the field by a-scintillationmethod (EDA Instruments, Toronto, Canada) after stripping by air bubbling.Dissolved-gas determination was performed in laboratory by coupling an extractionline to a quadrupole gas spectrometer (Lombardi et al., 1997).3. – Results and discussionsIn order to discriminate and evaluate the effect of brittle deformations (faults andfractures) upon dissolved-Rn distribution in different scenarios, a graphical methodproposed by Lepeltier (1969) was used. According to this author, slope changes andstraight line segments of the cumulative frequency diagram (fig. 1) are helpful for thedetermination of the background range, the recognition of multiple populations and theidentification of anomalous samples. Basically changes of slope split each plot intothree straight line segments representing three possible populations. As an example,for data set from Gargano (empty circle) two graphical thresholds can be set at about2-3 and 25 Bq/L. The population of the lowest Rn values (group I) is thought to resultfrom waters that likely have attained no secular equilibrium with rocks (for 222Rn about1 month) or that underwent a Rn depletion due to dilution or stripping phenomena. Thesecond group (group II) can be considered as the background population affected bythe features (i.e., power emanation, U content) of outcropping and reservoir rocksformations. Finally values greater than 25 Bq/L (group III) would be connected tosectors of enhanced Rn content, mainly linked to the presence of fault or fracture zonesand to changes of lithology. Data from Sardinia and Central Alps (empty square andtriangle, respectively), that were collected in crystalline domains (intrusive rocks),show a slight higher background ranges than those from sedimentary domains(carbonate in Gargano as well as carbonate and clays in Calabria). For crystallinedomains, the upper threshold could be set at approximately 40-50 Bq/L, as aconsequence of higher U content of rocks forming the crystalline basement (Carrara etal., 1974; Dall’Aglio et al., 1974).Graphical thresholds obtained from cumulative frequency diagrams may becompared with theoretical values computed according to the formula (modified afterF. QUATTROCCHI, M. GUERRA, L. PIZZINO and S. LOMBARDI312Wanty et al., 1992), that gives the generation of radon in rocks and transfer togroundwater:Rnw40.01253r3 g 12nnh3E3 [U ]R ,where Rnw is radon concentration in water (Bq/L); r is the bulk density (kg/m3 ); n andE the rock porosity and the emanation power, respectively (dimensionless); [U] is theuranium content, expressed as ppm by mass of rock. Since measurements of r, n, E arenot available, their values are assumed on the basis of literature data (Porstendorfer,1993; Schultz et al., 1992). For E40.26 0.05, r423006200, n40.256 0.05 and U(ppm)4 26 1, a range of Rn values (9–94 Bq/L) is obtained, consistent with the rockscharacterising the investigated areas. This computation states that the thresholdbetween background and anomalous population should occur in the range 9–94 Bq/L(although this is too wide due to lack of data), as verified for all our four surveys.If the anomalous samples are related to their own geological scenarios, thefollowing consideration can be drawn:– The anomalous samples from Gargano were collected along the CandelaroFault, an important NW-SE transtensive structure bounding the promontory whererelatively high values of the 3He/4He ratio (Hooker et al., 1985; B. Marty inGeochemical Seismic Zonation (GSZ) Project, unpublished data) point out a slightmantle signature, mainly at the Mt. Granata sites, at the crossing point with a NE-SWstructure.– The anomalous samples from Calabria were collected along the most importantfault systems of the northern and central Calabrian Arc: the Crati Graben, theSangineto, Cetraro-Rossano and Marchesato lines (this last located within the CrotoneBasin, see the Rn bubble chart in fig. 2). The Sila massif is characterised by a Rnanomaly too, likely as a consequence of prevailing crystalline lithologies (Calcara andQuattrocchi 1993; Calcara et al., 1996).Fig. 2. – Dissolved-radon distribution in the Crotone Basin. Rn values exceeding the thresholdvalue (20 Bq/L) obtained from the cumulative frequency distribution (in full) were found along thewell-known N-S trending Marchesato Fault.RADON AND HELIUM AS PATHFINDERS OF FAULT SYSTEMS ETC. 313– The anomalous samples from Central-Eastern Alps are related to theTonale-Pejo and Giudicarie lines; the greatest values (up to 2000 Bq/L) were found atthe intersection of these structures (Bagni di Mezzo spring site).– The anomalous data set from Sardinia is referred to the Capoterra Fault zone(fig. 3), located in the southernmost sector of the western Campidano Graben. Herebrittle deformations cut U-rich granite sandstone (Marcello et al., 1978), the couplingfurther enhances Rn values.Additional constraints about the evolution of sampled fluids and their relationshipwith fault zones are obtained if the relative content of dissolved He, N2 and Ar isreported in a ternary plot (fig. 4) as proposed by Giggenbach (1991). Data show for eacharea a common meteoric origin of sampled fluids as suggested by the clustering aroundthe ASW (Air Saturated Water) end-member and in accordance with stable isotopeanalyses (G. M. Zuppi, personal communication: GSZ Project, unpublished data). Inaddition most of points are aligned along the ASW-He line giving evidence of aprogressive water-rock interaction, that supplies He by the decay of radioactiveFig. 3. – Dissolved-radon distribution in the Campidano Graben. Also in this case the classes wereobtained from the cumulative frequency distribution. The highest values were recorded in thesouthernmost sector of the western border of the graben (Capoterra area). Faults locations (fullline) were obtained from Pala et al. (1982).F. QUATTROCCHI, M. GUERRA, L. PIZZINO and S. LOMBARDI314Fig. 4. – Ternary plot of dissolved He, N2 and Ar. Data show a common meteoric origin of sampledfluids as suggested by the clustering around the ASW (Air Saturated Water) end-member. Inaddition most of points are aligned along the ASW-He line, giving evidence of a progressivewater-rock interaction that supplies He by the decay of radioactive isotopes of the 235U-238U series.This is evidenced for samples collected in areas where crystalline outcrop prevails (Sardinia,Central Alps) or in areas affected by high seismicity that reactivates faults (Calabria).isotopes of the 235U-238U series. This is evidenced both for samples collected in areaswhere crystalline outcrop prevails (Sardinia, Central Alps) and in areas affected byactive tectonics, recent faulting and high seismicity (Calabria, Gargano), that improvesthe fault-related permeability (i.e., along the Candelaro Fault at the crossing pointwith a NW-SE line), even if, in Gargano, relative He weighted data do not exceed the65% of considered gases vs. the 90% of the other areas. Deviations from the ASW-Heline, i.e., changes of the N2/Ar ratio, can originate as a consequence of atmosphericcontamination due to the excess air (Heaton & Vogel, 1981), or during sampling andanalysis procedures as well as during a N2 production linked to the alteration oforganic matter (samples located above the line). For samples with a N2 /Ar ratio lesserthan the ASW reference, an overestimation of Ar values at the time of analysis isheavily suspected.RADON AND HELIUM AS PATHFINDERS OF FAULT SYSTEMS ETC. 3154. – ConclusionsThe results obtained in different geologic domains are evocative of the importantrole played mainly by structural patterns and lithology upon radon distribution ingroundwater. Dissolved-radon analysis, based on frequency distribution diagram, hasshown for each area the occurrence of three populations representative of theboundary conditions. The population of the lowest Rn values is interpreted as resultingfrom waters that have attained no secular equilibrium with rocks or that underwent aRn depletion due to dilution or stripping phenomena. A second threshold divides thebackground range from the anomalous population connected to areas of enhanced Rnproduction mainly linked to the presence of fault or fracture systems and to changes oflithology. However, dissolved-radon anomalies may be originated by a rapid ascent ofRn-bearing fluids (i.e., Candelaro Fault), equilibrated with lithologies richer in U thanoutcropping formation; and/or by a local enrichment of U, mostly if coupled withgranite sandstone (i.e., Capoterra area), enhanced locally by low enthalpy acidicgroundwater or by huge flux of carrier gas (CO2 ), usually occurring along the faultsystems (i.e., Tonale-Pejo line). The distribution of radon in groundwater is alsoaffected by the characteristics of the groundwater flow path (e.g., origin of fluids, flowvelocity, water-rock interaction, mixing phenomena), that can be recognised andpossibly evaluated mainly if considering a large set of variables (GSZ unpublisheddata).In this research Ar, N2 and He analysis supported and strengthened the reliabilityof noble-gas analysis methods for locating faults. This is demonstrated by high valuesof radiogenic helium (4He) recorded along the faulted areas as a consequence of thedeepening of flow path linked to the enhanced fault-related permeability.* * *We wish to acknowledge the EC Directorate General XII for Science, Research andDevelopment, Climate, Natural Hazards and Seismic Risk, for partly funding thisresearch (Geochemical Seismic Zonation EC Project, Contract n. ENV4-CT96-0291).We are indebted to Prof. R. CIDU and Dr. F. FRAU (University of Cagliari) for theirassistance during field data acquisition in the Campidano Graben. We would like tothank Prof. G. DAL PIAZ and Dr. S. MARTIN for the collaboration in central Alpssurvey.R E F E R E N C E SBANWELL G. M. and PARIZEK R. R., Helium 4 and radon 222 concentrations in groundwater andsoil gas as indicators of zones of fracture concentration in unexposed rock, J. Geophys. Res.,93 (1988) B1.CARRARA C., GHIARA E. and GRAGNANI R., Distribuzione dell’Uranio in alcuni bacini dellaSardegna, Rendic. Soc. Ital. Min. Petr., 30 (1974) 1175-1191.CALCARA M. and QUATTROCCHI F., Sulla scelta di siti idonei al monitoraggio geochimico ai finidella sorveglianza sismica della Calabria Centro-Settentrionale (Valle del Crati-Piana diSibari), Proceedings of the XII GNGTS Assembly 1993, Rome (CNR) 1993, pp. 463-480.CALCARA M., GUERRA I., MORETTI A., PIZZINO L. and QUATTROCCHI F., Il gas Radon nelle sorgentie nei suoli della Sila: implicazioni geotettoniche e di rischio ambientale, Proceedings of theIX GNGTS Assembly 1993, Rome (CNR) 1996, pp. 449-454.F. QUATTROCCHI, M. GUERRA, L. PIZZINO and S. LOMBARDI316DALL’AGLIO M., GRAGNANI R. and LOCARDI E., Geochemical factors controlling the formation ofthe secondary minerals of Uranium, in Formation of Uranium ore deposits,IAEA-SM-183/21 (1974) 33-48.DALL’AGLIO M., QUATTROCCHI F. and TERSIGNI S., Geochemical evolution of groundwaters fromthe Iblean Foreland (Sicily, Italy), after the December 12, 1990 earthquake (M45.4), Ann.Geofis., 38 (1995) 309-329.GIGGENBACH W. F., Chemical techniques in geothermal exploration, in Applications ofgeochemistry in geothermal reservoir development (CNR, Rome) 1991.HEATON T. H. E. and VOGEL J. C., “Excess air” in groundwater, J. Hydrol, 50 (1981) 201-216.HOOKER P. J., BERTRAMI R., LOMBARDI S., O’NIONS R. K. and OXBURGH E. R., Helium-3anomalies and crust-mantle interaction in Italy, Geochim. Cosmochim. Acta, 49 (1985)2505-2513.LEPELTIER C., A simplified statistical treatment of geochemical data by graphicalrepresentation, Econ. geol., 64 (1969) 538-550.LOMBARDI S. (PROJECT LEADER) et al., Geochemical Seismic Zonation. Seismic HazardZonation: a multidisciplinary approach using fluid-geochemistry methods. 1st AnnualReport (Contract No. ENV4-CT96-0291), DGXII, Bruxelles, Belgium (1997).MARCELLO A., PRETTI S. and SALVADORI I., Carta Metallogenica della Sardegna 1:200.000, ServizioGeologico d’Italia (1978).PALA A., PECORINI G., PORCU A. and SERRA S., Schema geologico strutturale della Sardegna, inRicerce geotermiche in Sardegna con particolare riferimento al Graben del Campidano, inCNR-PFE-RF 10 Report (CNR on Pisa) 1982.PORSTENDORFER J., Properties and behaviour of radon and thoron and their decay products inthe air, in Fifth International Symposium on Natural Radiation Environment, edited byLOWDER et al., (CEC Report EUR 14411 EN, 1993) 69-150.SCHULTS A., WIGGS C. R. and BROWER S. D., Geologic and environmental implications of highsoil-gas radon concentrations in the Great Valley, Jefferson and Berkeley Counties, westVirginia, Geological Society of America, Special Paper 271 (1992).WANTY R. B., LAWRENCE E. P and GUNDERSEN L. C. S., A theoretical model for the flux of radonfrom rock to groundwater, Geological Society of America, Special Paper 271 (1992).

Radon and helium as pathfinders of fault systems and groundwater evolution in different Italian areas

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Radon and helium as pathfinders of fault systems and groundwater evolution in different Italian areas

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