Elsevier

Applied Geochemistry

Volume 131, August 2021, 105030
Applied Geochemistry

Characterization of groundwater circulations in a headwater catchment from an analysis of chemical concentrations, Sr-Nd-U isotope ratios, and CFC, SF6 gas tracers (Strengbach CZO, France)

https://doi.org/10.1016/j.apgeochem.2021.105030Get rights and content

Highlights

  • Geochemical typology of deep water in the Strengbach critical zone observatory.

  • Specific chemical concentrations and Sr–U isotope ratios relative to surface water.

  • Evidence of longer residence times (water ages > 50 years) for the deep water.

  • Different circulation histories for surface and deep water in the Strengbach CZO.

Abstract

In order to characterize the chemical composition and the age of the water circulating in the critical zone of the Strengbach catchment (Vosges mountains, France), water samples from springs, 10–15 m deep piezometers and 50–120 m deep boreholes were collected and analyzed in elementary concentrations, in Sr, Nd, and U isotopic ratios, and in chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) concentrations. The results evidence a clear distinction between surface water (<≈10–15 m) and deeper water. The latter has much higher conductivity and cationic loads and is marked by lower Sr isotopic ratios and higher U activity ratios. Such a water typology suggests that the spring and piezometer water flows within the same shallow subsurface aquifer while deep borehole water belongs to a different circulation system, which flow path is controlled by the bedrock fracture network. The CFC data show that these two circulation systems are marked by contrasted residence times with a short residence time for the surface water and a longer residence time (water ages > 50 years) for the deep water. These results confirm different circulation histories for surface and deep water in the Strengbach catchment. They also suggest that the higher degree of chemical saturation of the deep water compared to the surface water is caused more by longer water residence times in the deep circulation systems than by differences in the primary minerals involved in each of the water-rock interaction systems. Our results also point that in the Strengbach granitic catchment, the SF6 concentrations cannot be used for water dating due to their lithogenic production in granitic bedrocks.

Introduction

The identification of the main water circulation levels in the Critical Zone (CZ) and the determination of the characteristic times of these circulations are among key parameters controlling the CZ functioning and its vertical extension (i.e., Riebe et al., 2017). These parameters also control its future evolution in response to environmental modifications, whether related to global climate change or to human activities of more local impact. This information, in particular, the partitioning between deep and subsurface circulations, is also important to achieve a sustainable management of the world's water resources and to model their future evolution in response to ongoing climate change (e.g., Gleeson et al., 2016; Ameli et al., 2017; Liu et al., 2017; O'Geen et al., 2018; Weill et al., 2019; Ackerer et al., 2020).

Surface and subsurface water circulations, especially in small headwater watersheds, are studied more than deep water circulations occurring in fractured bedrock (e.g., Viville et al., 2012; Brantley et al., 2017). The relatively recent setting up of several tens of meters-deep boreholes on such watersheds, especially those belonging to the so-called Critical Zone Observatories, is now allowing a much better exploration of water circulations and weathering processes in the deeper parts of the CZ (e.g., Brantley et al., 2013; Buss et al., 2013; Chabaux et al., 2017; Hahm et al., 2019; Holbrook et al., 2019; Ranchoux, 2020). These latter studies have systematically evidenced water circulation at great depths in the various explored sites. The hydrogeochemical characteristics of these waters, in particular, their specificity compared to surface waters, should be further clarified to better identify the water-rock interaction processes occurring in the deep CZ and the water circulation residence times in these deep levels.

The purpose of this study, conducted on the Strengbach CZO, France, is to determine the nature and residence time of the various waters circulating in the bedrock of the Strengbach catchment from the surface to ≈100m depth. The Strengbach basin has been the subject of relatively extensive hydrogeochemical studies in the past for surface waters (springs, soil solutions, streams), which today allows us to have a fairly good knowledge of their geochemical composition in major elements as well as Sr, Nd, and U isotopic ratios. These elements and ratios are relevant tracers when we seek to decipher the main water-rock interactions involved in biogeochemical and hydrogeochemical cycles (e.g., Aubert et al., 2001; Riotte and Chabaux, 1999; Tricca et al., 1999; Chabaux et al., 2011, 2013, 2019; Durand et al., 2005; Schaffhauser et al., 2014; Pierret et al., 2014, and references therein). All these data have led us to propose a first order relatively solid interpretative framework on the nature of water-rock interactions controlling the geochemical composition of surface waters in the Strengbach catchment.

In the current study, we propose to establish the hydrogeochemical typology of the deeper waters of the Strengbach CZO by a multi-method geochemical approach that combines the analysis of the above tracers (i.e., major and trace elements, Sr, Nd, and U isotope ratios) with the analysis of anthropogenic gases, namely, chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6). These latter elements are indeed suitable for determining the water ages and/or residence times in aquifers for relatively recent periods, 0–70 years or a little more, depending on the interpretative models used (e.g., Busenberg and Plummer, 1992; Corcho Alvarado et al., 2005; Plummer et al., 1998; Jankovec et al., 2017; De Montety et al., 2018; Cao et al., 2020). In particular, anthropogenic gases were successfully applied to estimate mean water transit time in fractured aquifers (Cook et al., 1996, 2005; Plummer et al., 2000; Bockgard et al., 2004; Ayraud et al., 2008; Jaunat et al., 2012; Roques et al., 2014a, b; Marçais et al., 2018). The results presented here on the Strengbach CZO highlight the interest of combining these different geochemical tracers to explore the hydrological functioning of the deep CZ.

Section snippets

Site presentation

The Strengbach catchment is a small 0.8 km2 granitic catchment in the Vosges mountains (northeastern France) at an altitude ranging between 880 and 1150 m a.s.l. with fairly steep slopes (mean slope of 15°) (Fig. 1). It is an equipped environmental observatory where meteorological, hydrological, and geochemical data have been recorded since 1986 (Observatoire Hydrologique et Géochimique de l’Environnement/Hydrological and Geochemical Environmental Observatory, OHGE; http://ohge.unistra.fr) and

Water sampling procedure

For the present study, 10 sampling campaigns, covering the diversity of hydrological contexts encountered on the catchment, were conducted over the period 05/2015–10/2018 (Fig. 2) for collecting water from springs CS1, CS2, CS3, CS4, RH3, and ARG, piezometers pz3, pz5, and pz7, and deep boreholes F5, F6, F7, and F8. Piezometer and borehole water were sampled with a pump descended to the chosen depth after generally 10 min of pumping, which is the time required to renew the water in the well.

Major element concentrations

Major element data for spring, piezometer, and borehole water analyzed for this study are presented in Supplementary Materials (SM Tables 2a and 2b; 3a, 3b; 4a, 4b).

Discussion

Geochemical data, whether elementary concentrations, 87Sr/86Sr and (234U/238U) ratios, or tracer gases (CFC and SF6), show a systematic distinction between the values measured in spring and piezometer water, on the one hand, and in deep borehole water, on the other hand. These data also show second-order variability between the different piezometers and between the different deep water sampled in the boreholes.

Conclusion

This study presents the water geochemical characteristic of the Strengbach catchment at different depths to bring better constraints of the deep alteration process, which occurs in a small granitic catchment. For this purpose, spring water (i.e., water circulating in the first meter of a saprolite aquifer), water from piezometers (i.e., from 10 to 15 m deep), and deep water (from 50 to 120 m-deep boreholes) were sampled and analyzed under different hydrological conditions (high, intermediate,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work benefited from fruitful discussions with V. Vergnaud, E. Chatton, G. Schaefer, A. D. Schmitt, S. Rihs, and colleagues from the CANTARE ANR Program, i.e., F. Gal, C. Dezayes, C. Lerouge, and P. Négrel. The study was financially supported by a Ph. D. scholarship awarded to C. Ranchoux by the University of Strasbourg, France. It was also supported by the French ANR Program under grant agreement ANR-15-CE06-0014 (Project CANTARE- Alsace). The drilling and equipment of the Strengbach

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  • Cited by (0)

    1

    Present address: HydroSciences Montpellier, Montpellier University, CNRS, IRD, 34000 Montpellier, France.

    2

    Present address: Institut des Géosciences de l'environnement, CNRS, Université Grenoble Alpes, Grenoble INP, IRD, 38000 Grenoble, France.

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