Soil and weathered-rock characterization

People involved: Joaquin JIMENEZ-MARTINEZ (CNRS), Elise COULON (UR1), Tanguy LE BORGNE (UR1), Laurent LONGUEVERGNE (CNRS), Olivier Bour (UR1)), Nicolas LAVENANT (CNRS)

A complete characterization of the unsaturated zone, including soil and weathered rock, took place in the framework of the unsaturated zone monitoring activity (WP 1.2). It includes, for the different soil horizons defined, textural fractions (from sieve and laser method), bulk density (from undisturbed soil samples, steel rings), and infiltration test from Guelph and Double-ring permeameter to obtain hydraulic parameters, in this last case only for the first horizon.

Figure.Soil column and different defined horizons.

 

Table.Summary of the properties determined for each of defined soil horizons.

 

The infiltration test for the first soil horizon (A) gave a value of hydraulic conductivity ranges 10-5-10-7 m/s.

 

Geophysics

 

People involved:Laurent LONGUEVERGNE (CNRS), Pascal TARITS (UBO), S. HAUTOT (UBO),Rebecca HOCHREUTENER (CNRS), Clément ROQUES (UR1), Amandine Kehil (UR1)

Applied geophysics surveys have been carried out to better constrain the structure of Ploemeur and Saint Brice en Coglès sites. Several methods have been applied (seismic, electrical resistivity, electromagnetic mapping) to better constrain hydrologically active structures. In the same time, the observatories have become perfect sites for testing and validating emerging geophysical methods.

Ploemeur hydrogeological observatory

Both seismic and electrical resistivity methods have been applied. The figure shows inverted resistivity for 2 different ERT arrays, Wenner – Schlumberger and Gradient. While the general structure is identical a few differences are interesting and drive to a different interpretation of the geological structure. The Granite – Micaschist contact zone (at ~250 m) is showing stronger lateral gradients and deeper discontinuities. The Gradient array highlights a 50-m large conductive contact zone, instead of a horizontal resistive structure. The same resistivity profiles will be repeated over time with P. Tarits and S. Hautot within the framework of CLIMAWAT project to investigate soil moisture variations.

Figure. Inverted resistivity for 2 different ERT arrays, Wenner – Schlumberger and Gradient.

 

Saint Brice-en-Coglès test site

Saint Brice en Coglès can be considered as equivalent of Ploemeur hydrogeological observatory but at a pre-development stage. Indeed, RHAPSODI project highlighted here very high water yields in fractured context. Geologically speaking, permeable structures are located at the contact zone between granite and mica-schist. Hydrogeologically speaking, the site is a natural discharge area for a deep fractured aquifer system. Applied geophysics surveys were carried out to better understand subsurface structures associated with the discharging deep fracture. EM34 mapping has been applied and highlighted the position and orientation of granitic intrusions. Consequently, these granitic intrusions were ruled out and have not created permeable fractures. They were created by younger tectonic events.

Electrical profiles have also been repeated after the pumping experiment has been stopped to monitor the aquifer level rise. The figures below show electrical resistivity measured on Dec. 12, 2011, just before the pumped was stopped and the second one 7 days later. Groundwater levels measured in the wells are also indicated. It is evident that resistivity has decreased in the central part of the profile, at the same time, water level has increased. Inversion of the resistivity profiles will provide estimation of soil restoration.

Figure. Electrical resistivity measured on Dec. 12, 2011, just before the pumped was stopped (top) and the second one 7 days later (bottom).

Groundwater dating

People involved: Virginie VERGNAUD (UR1), Thierry LABASQUE (CNRS), Luc AQUILINA (UR1), Thomas HOUDAYER (UR1)

The planned analyses on similar experimental sites have carried out in order to characterize old and salted groundwater bodies, as well as their geographical extension. The sampling campaigns cover great part of the Armorica Massif. 

Note that all the selected sampling points’ present chloride excess indicating aquifer recharge temperatures colder than the current one. The interpreted temperature values are below 10°C, and even reaching values of 4°C. Finally, CFS-SF6 dating for the same water samples is taking place.

Figure.Ne-Ar relationship for several sampling sites (Ploemeur, Betton, St. Brice, and others) in Brittany. Sub-plot showing the relation between aquifer recharge temperature and air excess.

Borehole profiles monitoring

People involved: Rebecca HOCHREUTENER (CNRS), Audrey ANSELEMT (UR1),Tanguy LE BORGNE (UR1), Olivier BOUR (UR1), Vincent BOSCHERO (CNRS), Nicolas LAVENANT (CNRS), Christophe PETTON (CNRS)

Published articles:

 

Klepikova, M. V., Le Borgne, T., Bour, O., and Dreuzy, J.-R. d. (2013), Inverse modelling of flow tomography experiments in fractured media, Water Resources Research.

Klepikova, M. V., Le Borgne, T., Bour, O., Gallagher, K., Hochreutener, R., and Lavenant, N. (2013), Passive temperature tomography experiments in fractured media, Journal of Hydrology.

During CLIMAWAT project we have performed a series of temperature and electrical conductivity profile measurements in boreholes under different hydraulic conditions: ambient and pumping. On thePloemeur site more than 40 boreholes are monitored for their groundwater level. Also takes place about once a year a campaign to monitor the variation of physico-chemical parameters (temperature, conductivity, redox potentiel, pH etc.) in order to observe the impact of the pumping of water for the Ploemeur city. Figures bellow show the borehole logs realized during the last few years on the borehole F28 near to the pumping site. Results show that differences can be observed over the time.

Figure. Temperature profiles for different times at F28 observation well.

Figure. Electrical conductivity profiles for different times at F28 observation well.

To complete these data flow measurement were carried out on some boreholes using the heat pulse technique. Especially, the boreholes on the Stang er Brune site, an important experimental site which is located a few kilometers away from the pumping site, are monitored as these boreholes are also used for tracer tests. The figure below shows flow velocities of the borehole B2 situated in Stang er Brune (Ploemeur). The flow measurements are realized under ambient conditions but also with pumping either in the same borehole or in an adjacent well. Results show the depths where there is an important water arrival and give the flow of these fractures. These data is essential for the realization of tracer tests and to model the system as will be discussed further on.

Figure. Flow (velocity, V) profiles, under ambient and pumping conditions, for different times at B2 observation well.

Collection and monitoring climatic and groundwater data

People involved:Nicolas LAVENANT (CNRS), Laurent LONGUEVERGNE (CNRS), Olivier Bour (UR 1), Christophe PETTON (CNRS)

Published articles:

Jiménez-Martínez, J., Longuevergne, L., Le Borgne, T., Davy, P., Russian, A., and Bour, O. (2013), Temporal and spatial scaling of hydraulic response to recharge in fractured aquifers: Insights from a frequency domain analysis, Water Resources Research, 49(5), 3007-3023.

The water levels are recorded on more than 40 boreholes for the Ploemeur and Guidel experimental sites.

Figure. Water level evolution at F9 and F7 observations wells. m asl: meters above sea level.

 

In addition to groundwater levels, meteorological parameters are being collected: precipitation, temperature, evapotranspiration (Penman-Monteith).

 

Figure. Daily precipitation (top) and evapotranspiration (bottom) data collected at Ploemeur site.

The monitoring and data collection of hydrological and meteorological variables is carried out in order to build a complete data base for the experimental site as well as to be used as input for different numerical model developed in the frame of this project.