# Groundwater reserves in a mountainous headwater catchment Insights from geophysics performed on the Strengbach Critical Zone Observatory

Nolwenn Lesparre, Camille Belloeil, Sylvain Pasquet, Jean-François Girard, Solenn Cotel, Clara Jodry, Cédric Champollion

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Nolwenn Lesparre, Camille Belloeil, Sylvain Pasquet, Jean-François Girard, Solenn Cotel, et al. Groundwater reserves in a mountainous headwater catchment Insights from geophysics performed on the Strengbach Critical Zone Observatory. eLTER Science Conference 2025, Jun 2025, Tampere, Finland. pp.e151704, 10.3897/aca.8.e151704. hal-05321989

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# Groundwater reserves in a mountainous headwater catchment Insights from geophysics performed on the Strengbach Critical Zone Observatory

Nolwenn Lesparre‡, Camille Belloeil‡, Sylvain Pasquet§,¶, Jean-François Girard‡, Solenn Cotel‡, Clara Jodry‡, Cédric Champollion¶†

‡ Université de Strasbourg, CNRS, EOST, ENGEES, Institut Terre et Environnement de Strasbourg, UMR 7063, Strasbourg, France

§ Sorbonne Université, CNRS, UAR 3455 OSU Ecce Terra, Paris, France

| Sorbonne Université, CNRS, EPHE, UMR 7619 METIS, Paris, France

¶ Géosciences Montpellier, UMR5243, CNRS, Université Montpellier, Montpellier, France

Corresponding author: Nolwenn Lesparre (lesparre@unistra.fr)

Received: 28 Feb 2025 | Published: 28 May 2025

Citation: Lesparre N, Belloeil C, Pasquet S, Girard J-F, Cotel S, Jodry C, Champollion C (2025) Groundwater reserves in a mountainous headwater catchment Insights from geophysics performed on the Strengbach Critical Zone Observatory. ARPHA Conference Abstracts 8: e151704. https://doi.org/10.3897/aca.8.e151704

## Abstract

### Introduction

Modelling the hydrological processes in mountainous areas is particularly challenging due to the strong heterogeneity of the underground medium in terms of hydrological properties, and the lack of groundwater observations. Here, we show how geophysical observations provide key information on the geometry of hydrofacies, the estimate of hydrological properties and the monitoring of the groundwater to study the critical zone in the Strengbach mountainous headwater catchment.

### Studied Site

The OHGE (Observatoire Hydrogéochimique de l'Environnement) is a headwater catchment of 0.8 km² that lies on a granitic bedrock (Pierret et al. 2018). This observatory
corresponds to the Strengbach catchment and is part of OZCAR, the French network of critical zone observatories. The OHGE is located in the Vosges mountains (northeastern France) with altitudes varying between 880 m and 1150 m (Fig. 1). The catchment topography shows steep slopes of 15° in average that reach up to 30° locally.

<figure><img src="image_3.png" /><figcaption>Figure 1. doi</figcaption></figure>

Localization of the acquired meteorological and hydrogeophysical data on the Strengbach catchment.

## Learning from scattered geophysical data

Meteorological and hydrological data are monitored since 1986 and six boreholes provide the distribution of geological facies at depths of 50 to 120 m (Chabaux et al. 2023). In addition, electrical resistivity and seismic refraction tomographies were acquired to estimate variations in soil and saprolite thickness. These data show soil thickness varying from 50 cm to 5 m, and saprolite thickness ranging from 1 to 16 m (Lesparre et al. 2024). The electrical resistivity tomographies also underline the spatial distribution of the geological facies, as one slope of the catchment shows significantly higher resistivity values than the other (Lajaunie et al. 2024). Despite the relatively thin saprolite, magnetic resonance soundings detected groundwater above the noise level, and revealed a region with higher water content (Lesparre et al. 2020). Gravity data acquired across the whole catchment shows that the method has the sensitivity to distinguish areas with distinct water storage dynamics (Chaffaut et al. 2022). In particular, a region upstream the Strengbach stream exhibits the highest values of water content and the largest variations in water storage.
We are using the geophysical data to develop a catchment-scale hydrogeophysical inversion. Hydrofacies geometries will be derived from the tomographies, and local hydrogeophysical experiments will be interpreted together with direct observations to estimate the range of hydrological properties. That information will serve as prior information of the inverse problem that will assimilate magnetic resonance and gravity data to complete piezometer and flow rate data.

**Keywords**

Geophysics, headwater catchment, groundwater, hydrofacies, imagery, monitoring

**Presenting author**

Nolwenn Lesparre

**Presented at**

**KEYNOTE**

**Conflicts of interest**

The authors have declared that no competing interests exist.

**References**

* Chabaux F, Négrel P, Gal F, Viville D, Ranchoux C, Van der Woerd J, Delay F, Ackerer J, Aquilina L, Diraison M, Masson F, Mériaux A, Labasque T, Lucas Y, Pezard PA, Reuschlé T, Schmitt A, Stille P, Wyns R, Lerouge C, Dezayes C (2023) Hard rock aquifer architecture and water circulation levels in the Strengbach critical zone observatory (France). Comptes Rendus. Géoscience 355: 185-206. https://doi.org/10.5802/crgeos.239

* Chaffaut Q, Lesparre N, Masson F, Hinderer J, Viville D, Bernard J, Ferhat G, Cotel S (2022) Hybrid Gravimetry to Map Water Storage Dynamics in a Mountain Catchment. Frontiers in Water 3 https://doi.org/10.3389/frwa.2021.715298

* Lajaunie M, Gance J, Sailhac P, Malet J, Warden S, Larnier H (2024) Hydrogeological structure of a granitic mountain catchment inferred from multi-method electrical resistivity datasets. Near Surface Geophysics 22 (1): 26-46. https://doi.org/10.1002/nsg.12268

* Lesparre N, Girard J, Jeannot B, Weill S, Dumont M, Boucher M, Viville D, Pierret M, Legchenko A, Delay F (2020) Magnetic resonance sounding measurements as posterior information to condition hydrological model parameters: Application to a hard-rock headwater catchment. Journal of Hydrology 587 https://doi.org/10.1016/j.jhydrol.2020.124941
* Lesparre N, Pasquet S, Ackerer P (2024) Impacts of hydrofacies geometry designed from seismic refraction tomography on estimated hydrogeophysical variables. Hydrology and Earth System Sciences 28 (4): 873-897. https://doi.org/10.5194/hess-28-873-2024

* Pierret M, Cotel S, Ackerer P, Beaulieu E, Benarioumlil S, Boucher M, Boutin R, Chabaux F, Delay F, Fourtet C, Friedmann P, Fritz B, Gangloff S, Girard J, Legtchenko A, Viville D, Weill S, Probst A (2018) The Strengbach Catchment: A Multidisciplinary Environmental Sentry for 30 Years. Vadose Zone Journal 17 (1): 1-17. https://doi.org/10.2136/vzj2018.04.0090