SR ISOTOPE ANALYSIS OF WATER SAMPLES AT THE RADIOGENIC ISOTOPE LABORATORY OF THE ISTITUTO NAZIONALE DI GEOFISICA E VULCANOLOGIA, SEZIONE DI NAPOLI - OSSERVATORIO VESUVIANO (INGV-OV)

Abstract

Since 2000, a Thermal Ionisation Mass Spectrometer (Thermo ScientificTM Triton TI® Mass Spectrometer) and a clean laboratory are operating at the Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Napoli, Osservatorio Vesuviano (OV) to measure strontium (Sr) and neodymium (Nd) isotope compositions of volcanic products for scientific purposes. In 2014 particular attention has been dedicated to set up the analytical procedure for extracting Sr and Nd and measuring their isotope compositions from groundwater and mineral water, due to its growing interest on environmental topics. Strontium is considered a trace element and Sr2+ preferentially substitutes for Ca2+ in most rock types since it geochemically behaves like calcium. Groundwater is enriched in Sr during water-rock interaction processes occurring within the saturated and/or unsaturated zones. Conversely, Sr is removed from water as a result of mineral precipitation and ion exchange reactions. The most common removal process is the coprecipitation of Sr2+ ions with calcium carbonate. However, this latter process does not fractionate Sr2+ [Faure and Powell, 1972]. Therefore, the Sr isotopic composition of groundwater records an integrated signal of water-rock interaction along flow path and dissolution/precipitation events, and can be used as a dynamic tracer to constrain subsurface flow in volcanic, non volcanic and geothermal areas. Water is a natural resource, which is renewed by different processes. The aforementioned geochemical processes and reactions with dissolving/precipitating minerals have a profound effect on water quality. Since the 1990s, Sr isotopes have been extensively used as a natural tracer of groundwater flow [Peterman and Stuckless, 1992; Bullen et al., 1996; Johnson and De Paolo, 1994; McNutt et al., 1990; McNutt, 2000; Frost et al., 2002; Gosselin et al., 2004; Klaus et al., 2007] because groundwater progressively acquires the 87Sr /86Sr isotopic ratio from the rocks with which it has interacted [Prasanna et al. 2009]. Furthermore, variable Sr isotope ratios and concentrations characterize different groundwater sources and are tracers of groundwater mixing [eg. Carucci et al., 2012]. In this work we present the first results obtained by analyzing the Sr isotope composition of two mineral waters, selected groundwater samples from Mt. Etna and a certified water sample from North America (National Research Council Canada - NRC, Certified Reference Materials - CRM TM-25.4). On the contrary, the Nd isotope ratios of such water samples have been not measured due to the low Nd content of the investigated samples. A statistically representative data set on certified international standards (NIST SRM 987, La Jolla and JNdi-1) was used to evaluate the quality of the analytical data produced at the INGVOV Radiogenic Isotope Laboratory since 2014. This methodological approach will allow us to isotopically characterize different water systems with the aim to trace back the water-rock interaction and mixing processes in different environments. This analytical procedure could be exported to other geological contests and applied to other types of waters (e.g. surface and thermal water).