PhD Course: Models and Methods for Materials and Environmental Sciences PhD student: Manuela Deiana
Tutors: Dr Francesco Ronchetti, Prof. Alessandro Corsini Title: Stable and ustable isotopes applied to deep rock-slide
Abstract: The PhD project aims to the application of stable (δ18O, δ2H, 87Sr/86Sr, 11B/10B ) and unstable (3H) isotopes to deep-seated rock-slide in order to provide a better understanding of groundwater processes. The conservative nature of stable isotopes renders them applicable tracers to define the groundwater origin, the landslide recharge area and the groundwater flow paths (δ18O, δ2
H); moreover underground structures, water-rock interaction and mixing phenomena can be identify by 87Sr/86Sr, 11B/10B as well time travel and groundwater age can be quantify using 3H. Hydrogeochemistry, often coupling with isotopic investigation, allows to characterize the geological reservoir in which groundwater processes take place.
During the second year of PhD hydrogeological monitoring, groundwater sampling, chemical and isotopic investigation have been carried out on three sites: the Berceto landslide, Montecagno landslide and Pietra di Bismantova (Fig.1). A first processing of collected data has been started and a period of 2 months was spent at the “Laboratoire de chrono-Environnement” (University of Franche-Comté- Besancon, France) in order to improve the geochemical modeling.
Fig. 1: Study sites; a) Berceto landslide; b) Montecagno landslide; c) Pietra di Bismantova.
Introduction: Since the second half of 20th century isotopes investigation have been used in hydrological studies at regional scale (i.e. global or regional rainfall isotopic composition (Craig , 1961; Longinelli et al., 2003), groundwater flow in regional aquifers or catchment areas (Vandenschric et al., 2002; Doveri et al., 2014).
In the last decade instead hydrogeochemistry and isotope geochemistry became relevant tools in local context studies as landslide phenomena (Bogaard et al., 2007; Peng et al., 2007; Cervi et al., 2012). Hydrogeochemistry (ions content, temperature and redox condition) allows to define the geological reservoir (landslide body) whereas isotope geochemistry (environmental isotopes) allows
a
to identify hydrological processes taking place inside the landslide body. Environmental isotopes such as oxygen, hydrogen and tritium are compositions of water whereas strontium and boron are acquired during water-rock interaction. In normal temperature condition environmental isotopes are hardly affected by fractioning processes after infiltration in underground hydrological circuit. The conservative behavior coupled with naturally occurrence in hydrological contests rends environmental isotopes a powerful tracer in hydrological studies also in local contexts.
Methods: hydrological monitoring was performed in 26 points of study sites in order to monitor, periodically and in continuous, groundwater level, spring and streams discharge and water physical parameters: Electrical Conductivity, Total Dissolved Solids, temperature and pH (Tab. 2). Sampling campaigns have been carried out to collect water from 26 points (Tab.1)
On sampled water 85 chemical analysis (Ca2+, Mg2+, K+, Na+, Cl-, SO4 2-, HCO3 -, NO3 -, Btot, Sr 2+ , Li+,NH4 + ), 120 analysis of δ18O – δ2H composition, 8 analysis of 87Sr/86Sr ratio and 12 analysis of 3H unstable isotope have been performed (Tab.1). The software PHREEQC was used to start the processing activity of data by means inverse geochemical modeling. This activity was developed
at the Laboratoire de
chrono-Environnement”(University of Franche-Comté- Besancon)
Tab.1: Chemical and isotopic investigations carried out on study sites
Tab.2: Hydrological and physical monitoring activity performed on study sites.
Results and discussion: All stable isotope δ18O – δ2H values obtained in study sites are used to analyze the relationship between the δ18O of content of sampled water and the respective elevation of catchment area.
Berceto landslide: the hydrological monitoring was performed in 8 points since November 2012 to date, periodically and in continuous way, in order to investigate groundwater levels. Sampling water
Study site Monitored points (spring, stream, piezometer) Activity C h emi ca l A n al y si s δ 18O – δ 2H 87 Sr/ 86 Sr 3H Berceto 8 55 45 8 6 Montecagno 14 20 50 n.d . 5 Pietra di Bismantova 4 10 25 n.d . 1
Study site Monitored points
Periodic Monitoring Continuous Monitoring
Level Q EC TDS T Ph Level Q EC TDS T Ph Berceto 8 x x x x x x Montecagno 14 x x x x x x x x x Pietra di Bismantova 4 x x x x x x x x x
was performed periodically in spring, piezometer and river. To date 55 chemical analysis, 45 δ18O – δ2
H analysis, 8 87Sr/86Sr ratios and 6 3H analysis have been carried out.
Results allow to distinguish inside the landslide body a calcium-bicarbonate groundwater type and a sodium-bicarbonate groundwater type. Stable isotopic values of δ18O – δ2H are aligned with the Nord Italy Metoric Line (Longinelli et al., 2003) revealing a meteoric origin for these water. The coupling of δ18O – δ2H results and 87Sr/86Sr values allow to deny an hydraulic connection between Berceto landslide and River Baganza located upstream. Moreover 3H results highlight the presence below the Berceto town of a semi-confined aquifer showing time travel equal or older than 60 years. According to the continuous piezometric monitoring in this aquifer prevails transfers pressure rather mass transfer, with transfer velocity of few meters to year.
The data processing by means inverse geochemical modeling in PHREEQC allowed to clarify water-rock interaction in this semi-confined aquifer.
Montecagno landslide: the hydrological monitoring was conducted in order to obtain hydrogeological units of geological reservoir (permeability K, Storativity S) by means pumping or slug test. Moreover groundwater levels, spring discharge, water Electrical Conductivity, Temperature, Total Dissolved Solids and pH was monitored periodically and in continuous way. Moreover to try to identify groundwater flow paths a tracer test using salt solution has been realized.
Sampling activity was carried out in order to monitor chemical composition and stable isotope δ18O – δ2H content. A relation between δ18
O values and elevation of catchment area has been highlighted with the aim of identify recharge area of landslide body.
Pietra di Bismantova: in this study site activity concerned continuous spring discharge, Electrical Conductivity and temperature monitoring as well periodic monitoring of pH, TDS, T, EC. Moreover periodic sampling campaigns were achieved in order to determine chemical composition and stable isotopic δ18O – δ2H content.
Conclusions: The research activity during the second PhD year using different isotopic investigations (δ¹⁸O, δ²H, ⁸⁷Sr / ⁸⁶Sr, ³H) highlights relevant results. δ18
O – δ2H have allowed the reconstruction at large scale of the relationship between δ¹⁸O content and relative basin elevation in order to define the recharge area of landslide bodies. ⁸⁷Sr / ⁸⁶Sr coupled with δ¹⁸O-δ²H helped to deny hydraulic connection between River Baganza and Berceto landslide; moreover both δ¹⁸O-δ²H and ⁸⁷Sr / ⁸⁶Sr allowed to better understand groundwater flow paths inside the Berceto landslide. δ³H contributed to define the time travel within the Berceto landslide instead PHREEQ helped to identify geochemical water-rock reactions inside the Berceto landslide.
Next activities will be aimed to carry out additional stable isotopic analysis (δ¹⁸O, δ²H, ⁸⁷Sr / ⁸⁶Sr) in landslide sampling point and δ¹⁸O, δ²H in rainfall. Tracking test will be realized and the hydrological monitoring will continue. In the third year processing of data with PHREEQC will continue and in addiction a statistical analysis of data will be carry out.
Riferimenti bibliografici
Bogaard, T., Guglielmi, Y., Marc, V., Emblanch, C., Bertrand, C., and Mudry, J. (2007): Hydrogeochemistry in landslide research: a review. B. Soc. Geol. Fr., 178, 113-126.
Cervi, F., Ronchetti, F., Martinelli, G., Bogaard, T. A., Corsini A. (2012): Origin and assessment of deep groundwater inflow in the Ca’ Lita landslide using hydrochemistryy and in situ monitoring. Hydrol. Earth Syst. Sc., 16, 4205-4221.
Craig H., (1961): Isotopic Variations in Meteoric Waters. Science, Vol. 133 no. 3465 pp. 1702-1703.
Doveri M. and Mussi M. (2014): Water Isotopes as Environmental Tracers for Conceptual Understanding of Groundwater Flow: An Application for Fractured Aquifer Systems in the “Scansano-Magliano in Toscana” Area (Southern Tuscany, Italy). Water, 6(8), pp. 2255-2277.
Longinelli A. and Selmo E. (2003): Isotopic composition of precipitation in Italy: a first overall map. Journal of Hydrology 270, pp. 75–88.
Peng TR, Wang CH, Lai TC, Ho SK. (2007): Using hydrogen, oxygen, and tritium isotopes to identify hydrological factors contributing to landslides in a mountainous area, central Taiwan. Environmental Geology 52, pp. 1617–1629.
Vandenschrick G., van Wesemael, Frot E., Pulido-Bosch A., Molina L., Stie´venard M., Souchez R. (2002): Using stable isotope analysis (d D–d 18O) to characterise the regional hydrology of the Sierra de Gador, south east Spain. Journal of Hydrology 265, 43–55.
Activities of the second year
Data collected during these two years of PhD project have been presented at the following congresses and seminars:
Partecipation at congress
-EGU - European Geosciences Union General Assembly 2015 - Vienna (M. Deiana, F. Cervi, A. Corsini, M. Mussi, M. Pennisi, and F. Ronchetti. "Water isotopes in landslide research (WIsLaR)" Oral Contribution.
-5 Congresso Nazionale AIGA - Associazione Italiana di Geologia Applicata e Ambientale 2015 - Cagliari (M. Deiana, F. Bonacini, F. Cilona, M. Formicola, M. M. Nistor, A. Corsini & F. Ronchetti "Preliminary water flow monitoring of a vertical fractures system in a sandstone rock mass") Poster presentation.
-42 Internation Association of Hydrogeologist - AQUA Congress 2015 - Roma (Manuela Deiana, Alessandro Corsini, Mario Mussi, Giuseppe Ciccarese , Riccardo Giusti & Francesco Ronchetti "Monitoring and characterization of a spring in a fractured sandstone slab") Poster presentation.
Delivery of seminar
-Seminar: Isotopi stabili nel monitoraggio e caratterizzazione di frane profonde. Arpa Emilia-Romagna - Corso di formazione di nodo - Direzione Tecnica "Gli isotopi nel monitoraggio delle acque sotterranee: teoria, applicazioni e prospettive ", 25-06-2015, Bologna "
-Seminar: "Indagini isotopiche per lo studio della circolazione idrica sotterranea in movimenti franosi attivi"- Corso di aggiornamento professionale presso la fondazione dei Geologi della Toscana. 23-09-2015, Firenze.
