Stable isotope (2H, 18O and 87Sr/86Sr) and hydrochemistry monitoring for groundwater hydrodynamics analysis in a karst aquifer (Gran Sasso, Central Italy)

doi:10.1016/j.apgeochem.2005.07.008

Applied Geochemistry

Volume 20, Issue 11, November 2005, Pages 2063-2081

https://doi.org/10.1016/j.apgeochem.2005.07.008 Get rights and content

Abstract

This paper deals with chemical and isotope analyses of 21 springs, which were monitored 3 times in the course of 2001; the monitoring program was focused on the groundwater of the Gran Sasso carbonate karst aquifer (Central Italy), typical of the mountainous Mediterranean area.

Based on the hydrogeological setting of the study area, 6 groups of springs with different groundwater circulation patterns were distinguished. The hydrogeochemistry of their main components provided additional information about groundwater flowpaths, confirming the proposed classification. The spatial distribution of their ion concentrations validated the assumptions underlying the hydrogeological conceptual model, showing diverging groundwater flowpaths from the core to the boundaries of the aquifer. Geochemical modelling and saturation index computation elucidated water–carbonate rock interaction, contribution by alluvial aquifers at the karst aquifer boundaries, as well as impacts of human activities.

The analysis of ¹⁸O/¹⁶O and ²H/H values and their spatial distribution in the aquifer substantiated the hydrogeology-based classification of 6 groups of springs, making it possible to trace back groundwater recharge areas based on mean isotope elevations; the latter were calculated by using two rain monitoring stations. ⁸⁷Sr/⁸⁶Sr analyses showed seasonal changes in many springs: in winter–spring, the changes are due to inflow of new recharge water, infiltrating into younger rocks and thus increasing ⁸⁷Sr/⁸⁶Sr values; in summer–autumn, when there is no recharge and spring discharge declines, changes are due to base flow groundwater circulating in more ancient rocks, with a subsequent drop in ⁸⁷Sr/⁸⁶Sr values.

The results of this study stress the contribution that spatio-temporal isotope monitoring can give to the definition of groundwater flowpaths and hydrodynamics in fissured and karst aquifers, taking into account their hydrogeological and hydrogeochemical setting.

Introduction

Optimum protection and management of groundwater resources in karst areas is a priority objective (European Commission, 1995) in both industrialised and developing countries. Fissured carbonate aquifers host huge resources of high-quality groundwater. They also feed springs that are often used as sources of drinking water or that give rise to wetlands of high environmental value (Bradford and Watt, 1998). These springs or wetlands may be part of protected areas, as in this study.

Reconciling environmental protection with human needs requires an accurate and up-to-date assessment of available resources, based on hydrogeological studies. The availability of resources is influenced, in part, by anthropogenic factors (Custodio, 1997): water exploitation and construction work, such as the Gran Sasso motorway tunnel in Central Italy (Monjoie, 1975, Massoli Novelli and Petitta, 1997, Celico et al., 2004), may interfere with the natural hydrogeological setting.

Often, hydrogeological studies are not sufficient to shed light on the groundwater hydrodynamics of carbonate karst environments, since groundwater also flows through fractures and karst conduits. As described in this study, this knowledge gap can be bridged using hydrogeochemical investigation methods, some of which are based on stable isotopes, such as ¹⁸O, ²H and ⁸⁷Sr/⁸⁶Sr (Banner et al., 1994, Han and Liu, 2004, Johnson et al., 2000, Jorgensen and Banoeng-Yakubo, 2001, Kattan, 1993, Kattan, 2001, Katz and Bullen, 1996, Marfia et al., 2004, Walraevens and Cardenal, 1994, Thomas and Rose, 2003).

Isotope-based methodologies have become well established in the hydrological community for water resource assessment, development and management, and are now an integral part of many water quality and environmental studies (Clark and Fritz, 1997, Cook and Herczeg, 1999). These methodologies usually rely on “tracing” either isotope species naturally occurring in water (environmental isotopes) or on isotope tracers intentionally introduced into them (UNESCO, 2000).

Unlike the stable isotopes of ¹⁸O and ²H in water that have long been used in hydrology (Craig, 1961, Bison et al., 1989, Gonfiantini, 1978), Sr isotopes do not measurably fractionate in nature. Groundwater acquires dissolved Sr: (i) in its recharge area, through infiltration and percolation processes; (ii) along its flowpath, through dissolution or ion exchange with minerals. Hence, ⁸⁷Sr/⁸⁶Sr ratios give insight into water–rock interaction processes (Naftz et al., 1997). Various authors (Banner et al., 1989, Dogramaci and Herczeg, 2002, Frost and Toner, 2004, Katz and Bullen, 1996, Petelet-Giraud et al., 2003, Peterman and Stuckless, 1992, Barbieri and Sappa, 1997, Barbieri and Morotti, 2003) demonstrated the advantage of using Sr isotopes as tracers in hydrogeological applications.

Hydrogeochemical and isotope analyses can test and fine-tune a conceptual hydrogeological model. Furthermore, monitoring changes in stable isotopes over time and in space can give a better understanding of aquifer recharge and spring discharge that are essential for defining groundwater hydrodynamics. In addition, monitoring of stable isotopes proves to be a useful tool when urgent land planning decisions are to be made, such as those concerning conservation and management of water resources, as well as assessment of the quality of water for human use (Miroladov and Marjanovic, 1998).

Section snippets

Hydrogeological setting

The study area is the Gran Sasso massif (about 800 km² wide and with an elevation of 270–2912 m a.s.l.), a carbonate ridge consisting of Meso–Cenozoic units belonging to slope-to-basin lithofacies. The sequence has evidence of tectonic movements due to the Apennine orogenesis, such as overthrusts and successive extensional faults. The massif holds a regional aquifer, with high values of recharge (700 mm/a with respect to 1100 mm/a of precipitation) due to joints, faults and karst features. The

Methodology, sampling and analytical procedures

To obtain detailed information about groundwater flowpaths and spring recharge areas and to better understand the above-described hydrogeological setting (see Fig. 1), 3 spring water sampling surveys were conducted in 2001 (January, April and November), collecting data on in situ physico-chemical parameters and carrying out chemical analyses of main components and stable isotopes. Samples were collected directly from the spring outlets. During the January 2001 survey, all 21 identified springs

Major ions

As shown in Fig. 2, groundwater samples were classified by analysing their main groups of cations and anions and by determining their reaction values (relative percentages). All sampled waters were classified as of Ca⁺⁺ − Mg⁺⁺ $– {HCO}_{3}^{-}$ type (Chebotarev, 1955, Appelo and Postma, 1993), in line with the interpretation of a common origin from the Gran Sasso carbonate aquifer. Nonetheless, some of the waters had more pronounced Ca⁺⁺ + Mg⁺⁺ $– {HCO}_{3}^{-}$ characteristics (groups B, C, D, F and, partially, A),

Discussion

The results of hydrogeochemical and isotope analyses are consistent with the assumed conceptual model of groundwater flowpaths in the massif (Fig. 1). The integrated monitoring technique, adopted in this study, offers clues for a more in-depth understanding of the regional hydrodynamics, based on observed changes of the monitored isotopes over time and in space.

The Gran Sasso aquifer contains an active groundwater flow system that is supported by high recharge rates (about 700 mm/a) (Boni et

Conclusions

Qualitative groundwater circulation models based on water budgets can be enhanced through the use of chemical and isotopic tracers, particularly in karst aquifers. In the studied fractured karst aquifer, groundwater hydrogeochemistry of the main components was consistent with the proposed division of the springs into 6 groups, showing an increase in dissolved solids, mainly Ca⁺⁺ and ${HCO}_{3}^{-}$ , along groundwater flowpaths from the aquifer core to its boundaries. Groundwater chemistry also was

Acknowledgements

The Authors thank the Ministry of Universities and Scientific Research – MIUR (“Ambiente terrestre” project – “Cluster C11b” subproject – Law Decree no. 720/99) for its financial support, “Consorzio di Ricerca Gran Sasso” for its useful assistance, GEOKARST (Trieste, Italy) for δ²H and δ¹⁸O analyses, Dr. Giuseppina Benedetti for major element chemical analyses and E. Di Biasio for the chemical preparation of samples analysed by mass spectrometer (VG-54E).

The authors are indebted to Z.E.

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Stable isotope (2H, 18O and 87Sr/86Sr) and hydrochemistry monitoring for groundwater hydrodynamics analysis in a karst aquifer (Gran Sasso, Central Italy)

Abstract

Introduction

Section snippets

Hydrogeological setting

Methodology, sampling and analytical procedures

Major ions

Discussion

Conclusions

Acknowledgements

Geochim. Cosmochim. Acta

Appl. Geochem.

J. Hydrol.

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

Catena

Appl. Geochem.

Chem. Geol.

Geochemistry, Groundwater and Pollution

Tracing groundwater evolution in a limestone aquifer using Sr isotopes. Effects of multiple sources of dissolved ions and mineral–solution reactions

Geology

Contribution des isotopes des l’environnment à l’étude de la recharge naturelle des aquiféres fissurés du bouclier soudanien au Burkina-Faso

Hydrogeologie

Schema Idrogeologico dell’Italia Centrale

Memorie Società Geologica Italiana

Environmental isotope studies of limestone aquifers in central Italy

Metamorphism of natural water in the crust of weathering, Part I

Geochim. Cosmochim. Acta

Environmental Isotopes in Hydrogeology

Indagini idrogeologiche nella media e alta valle del Fiume Menotre, Umbria orientale

Atti Tic. di Scienze della Terra

Environmental Tracers in Subsurface Hydrology

Isotopic variation in meteoric water

Science

Stable isotopes in precipitation

Tellus

Stable isotope (²H, ¹⁸O and ⁸⁷Sr/⁸⁶Sr) and hydrochemistry monitoring for groundwater hydrodynamics analysis in a karst aquifer (Gran Sasso, Central Italy)