Theoretical cross-section diagram of a borehole and management thresholds: reinforced monitoring (50% of the total possible exploitation range) and alert (30%)
© BRGM
The need
The Ille-et-Vilaine Joint Public-Private Syndicate for the Management of Drinking Water (SMG Eau 35 – Syndicat Mixte de Gestion de l'Eau Potable de l'Ille-et-Vilaine ) commissioned BRGM to develop an easy-to-implement method for linking the piezometric levels observed in the facilities operated to any potential pumping difficulties. The aim was to ensure the sustainable management of water resources by introducing reinforced-monitoring and alert thresholds.
The results
In order to define the management thresholds, it was first necessary to have a good understanding of the geometry and characteristics of the aquifer in question, and to have piezometric-monitoring data concerning the production facilities.
A number of criteria, which needed to be taken into account before calculating the management thresholds, were therefore identified in consultation with the hydrogeologists of the departmental drinking water syndicates of the Grand Ouest region (Ille-et-Vilaine, Côtes d'Armor, Morbihan, Manche, Orne, Loire-Atlantique and Vendée départements):
- availability and quality of piezometric-monitoring data;
- position of the equipment at the facility: pump, pump heads, weep holes, etc.;
- aquifer geometry: water inlet, aquifer roof, admissible groundwater lowering, pyrite levels, zero NGF (nivellement général de la France - equivalent to Ordnance Datum Newlyn, i.e. zero altitude above sea level) in coastal areas.
These criteria were then entered into a grid drawn up by BRGM to determine the minimum admissible piezometric level, based on the information available about the facility in question.
Furthermore, several methods were proposed for defining reinforced-monitoring and alert thresholds for the underground structures used to supply drinking water (filling rate between the minimum admissible level and the maximum level, consideration of drawdowns and summer decreases in productivity, analysis of volume data).
Piezometric measurement in a drinking water catchment point (Balazé, 2023)
© BRGM
Using the results
BRGM proposed to test one of the methodologies as part of this study, using the calculation grid. Reinforced-monitoring and alert thresholds were defined using the filling rate method for three facilities in Ille-et-Vilaine. However, data records for these facilities do not go back very far and are sometimes unreliable, so the relevance of these thresholds stills needs to be verified and linked to potential pumping difficulties. If need be, these thresholds could then be adjusted in a few years' time to adapt them to the operation of each individual facility. Consequently, certain adjustments to the proposed methods are to be expected, taking into account the results of the tests that will be carried out over longer, more reliable time-frames.
Thanks to the recommendations set out in the BRGM report, SMG Eau 35 can already start implementing a process to improve the reliability of the monitoring data and enter the latter in a database, and then test the management-threshold definitions for the various facilities in the département.
Thanks to its expertise in groundwater monitoring, BRGM understood our expectations in terms of managing production facilities. In our region, the use of drinking water resources is becoming a crucial issue and requires in-depth expertise in the field of groundwater and aquifers. Starting from scratch, in collaboration with the local community of hydrogeologists, we have developed an essential tool to ensure sustainable, optimum management of this precious resource.
The partner
- Syndicat Mixte de Gestion de l’eau potable de l’Ille-et-Vilaine (SMG Eau 35)
