Poitiers

POITIERS

POITIERS

Managers of the Poitiers site: Jacques Bodin, Pierre Fischer, Gilles Porel

Context and scientific objectives

The Poitiers Hydrogeological Experimental Site (HES) was developed by the IC2MP/Hydrasa team (UMR CNRS 7285) as part of the National Network of Hydrogeological Sites (SNO H+) and the ‘EAUX’ programs of the former Poitou-Charentes region (CPER 2002-2006 and 2007-2013).

Located 2 km to the east of the Science Campus of the University of Poitiers, the HES covers an area of 12 hectares on a land belonging to the University (University Botanical Garden – Deffend site).

The themes developed at the HES can be divided into two main axes:

Axis 1 : Characterization of preferential flow structures

  • Characterization and modelling of flows in highly channeled heterogeneous environments: The spatial density of boreholes at the HES is used to develop new data acquisition approaches that can be used to stress the network of preferential flow channels at different scales: the hydraulic properties of the preferential flow channels are characterized using (i) long (daily) or short (high frequency (2 Hz)) time hydrodynamic data, (ii) data from artificial tracings under pumping conditions, (iii) data from cross-borehole slug tests, and (iv) data from cross-borehole flowmeter tests.
  • 3D spatial distribution of heterogeneities: The individual geometry of discrete structures of high relative permeability is explored using geophysical imaging techniques (seismic, electrical, magnetic, etc.).
  • Boundary conditions: Parametrization of the boundary conditions (inlet/outlet) is tricky given the high channeling of flows, and it is difficult to specify the distance beyond which the influence of pumping can, at a given time, be considered ‘negligible’. The medium- and long-distance propagation of pressure disturbances outside the HES is investigated by setting up ‘remote’ sensors and carrying out pumping experiments with periodic signals.

Axis 2: Transfer of water and dissolved elements to the water table

  • Hydrological balance and recharge monitoring: Micrometeorological monitoring using a weather station and a flux tower at the HES allows quantification of evapotranspiration and water transfers to the underground environment (infiltration). The characterization of infiltration fluxes over time and space is studied using (i) a profile of time-lapse electrical resistivity tomographic measurements, (ii) pits equipped with water content probes and buried electrical quadrupoles. These hydraulic and electrical measurements are linked by a petrophysical relationship based on site observations.
  • Origin and dynamics of selenium in carbonate aquifers: The presence of selenium in groundwater at concentrations above the current standard for drinking water (10 µg/L) is a problem encountered in several regions of France, particularly in the Dogger aquifer in the Vienne region (HES aquifer). Lithochemical analyses carried out on core samples from boreholes at the HES indicate that significant levels of selenium are trapped in pockets of paleokarst fills scattered throughout the carbonate reservoir. Studies are being carried out to analyze the validity of this hypothesis, based on (i) detailed physico-chemical characterization (XRD, SEM, XPS) of the speciation of the selenium present in the filling pockets, (ii) characterization of the selenium release mechanism(s) from the solid matrix using laboratory experiments (static and dynamic) coupled with thermodynamic simulations, and (iii) numerical modelling of reactive flows at the reservoir scale.

Location

From a geological point of view, the HES is located on the northern slope of the ‘Seuil du Poitou’ geological structure, a vast Mesozoic carbonate plateau marking the transition between the Aquitaine and Parisian sedimentary basins.

Location of the HES
Geological log

The Jurassic limestones, which are laid on a crystalline Hercynian basement, contain two superimposed aquifers: (i) the Lower and Middle Lias aquifer (10 to 15 m thick), and (ii) the Dogger aquifer (100 m thick). These two aquifers are separated by the Toarcian marl aquitard (20 m thick). The studies carried out on the HES focus mainly on the Dogger aquifer.

The HES is located in the watershed and hydrogeological catchment area of the Clain, a river crossing the Vienne and Charente regions and flowing northwards to reach the Vienne River in Châtellerault. The Lias and Dogger aquifers supply the Clain River with groundwater throughout the year. The majority of the catchment area is used for farming (~70%, mainly cereals, oilseeds, protein crops and fodder) and forestry (~15%). The terrain is relatively flat, forming plateau with an average altitude of 115 m between the Clain and Vienne Rivers, whose run-off have dug the limestone.

Clain watershed.
Left: Outcropping geological formations ; Right: Land use

The climate around Poitiers is semi-oceanic. The average annual temperature in Poitiers was 11.8°C between 1990 and 2010, rising to 12.2°C between 2000 and 2020. The average annual rainfall of 690 mm in Poitiers has remained more or less constant since 1950.

Rainfall distribution in the Vienne region (Source: MétéoFrance)

This rainfall is unevenly distributed across the Vienne region, with wetter areas in the south, towards the ‘Seuil du Poitou’, and drier areas in the north.

Instrumentation, monitoring and on-site experiments

The experimental framework, set up in 2002, comprises around forty boreholes, including three vertical and two oblique core holes. All the boreholes cross the entire Dogger aquifer (borehole depth = 125 m). Most of the boreholes are distributed in a regular grid within a 210 × 210 m square.

Location of HES boreholes around the University and in the Botanical Garden

The HES boreholes are either uncased or equipped with casings that are screened throughout the thickness of the Dogger aquifer. Therefore, the piezometric level in the boreholes corresponds to an average hydraulic head over the thickness of the aquifer. Under natural flow conditions, piezometric levels vary from 15 m to 25 m below the ground surface. During drilling, dry clayey limestone was systematically observed down to a depth of around 30 m, indicating that the Dogger aquifer is confined beneath this low permeable formation. Two additional boreholes were drilled down to the crystalline basement (boreholes C2 and IM1, approximately 160 m deep), in order to be able to record the hydraulic head in the Lower and Middle Lias aquifer during the hydraulic tests carried out in the other boreholes. No pressure disturbance was ever observed in the Lower and Middle Lias aquifer, demonstrating that the two aquifers are well isolated from each other by the Toarcian marls.

The Dogger aquifer is covered by a layer of soil of varying thickness at the HES, ranging from a few meters to around twenty meters. It is a Luvisoil over detrital clay-silt formations that are highly heterogeneous both laterally and vertically. This soil acts as a buffer at the soil-vegetation-atmosphere interface. The atmospheric fluxes of the water cycle (rain, evapotranspiration) is measured by a weather station installed in 2002 and a flux tower installed in 2023. Equipped pits, dug up to 2 m below the surface, are used to monitor water content every 30 cm vertically in the soil since 2024.

Since 2002, the investigations carried out at the HES have made it possible to collect a large amount of data concerning the Dogger aquifer. This dataset, now archived in the H+ database, concerns both continuous and one-off acquisitions.

Continuous acquisitions :

  • Piezometric monitoring at 10-minute intervals in HES boreholes
  • Micrometeorological monitoring at 30-minute intervals: rainfall, temperature, humidity, wind speed, solar radiation, real evapotranspiration, etc.
  • Monitoring of soil water content and temperature at 30-minute intervals, every 30cm to a depth of 2m (in 4 pits close to the micrometeorological station)

One-off acquisitions :

  • Groundwater dynamics under ‘natural’ and/or induced flow conditions: pumping tests and hydraulic shocks
  • Transport dynamics (non-reactive) in pumping condition: fluorescein tracing experiments
  • Hydrogeophysical imaging of the unsaturated zone: one-off or time-lapse electrical resistivity tomography profiles
  • Flow structure in boreholes: vertical flowmetry; temperature-conductivity logs
  • Geological structure of the reservoir: stratigraphy/lithology on core drillings, 3D seismic imaging of the reservoir, gamma ray logging, acoustic imaging, high-resolution camera
  • Petrophysical properties of carbonate rocks: laboratory measurements of porosity and permeability on core samples

These various investigations have made it possible to achieve an unequalled degree of characterization at the national scale for this type of environment.

Data availability

Main datasets

In this OSURIS catalog you can find links to key datasets of the site together with metadata:

Water cycle: https://www.osuris.fr/metadata/SNO-HPLUS-POITIERS-WATER-CYCLE

Geochemical cycle: https://www.osuris.fr/metadata/SNO-HPLUS-POITIERS-GEOCHEMICAL-CYCLE

Borehole hydrogeophysics: https://www.osuris.fr/metadata/SNO-HPLUS-POITIERS-BOREHOLE-HYDROGEOPHYSICS

All datasets

To help finding general datasets, predefined database requests have been created and are regularly executed. Results of the requests can be downloaded from the links available here:

Poitiers

Water cycle
Geochemical cycle
Borehole hydrogeophysics
Spatialized hydrogeophysics
Experiments
Experimental bench
Boreholes
Stations

* From August 2020, 9 wells m7, m8, m13, m15, m20, m22, mp4, mp6 and mp7 become m7b, m8b, m13b, m15b, m20b, m22b, mp4b, mp6b and mp7b respectively due to a change in their technical equipment.

KMZ viewer

The viewer below offers a comprehensive site visualization and information on available public data such as types, numbers and dates of measurements, locations of sites, wells and stations, as well as data providers information. This interface also provides an overview of available geophysical maps and cross-sections.

To visualize all data in the Google Earth software, you can download the following KMZ file: Poitiers.kmz

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