THE HYDROGEOLOGICAL OBSERVATORY OF PLOEMEUR (FRANCE) : LONG-TERM MONITORING AND EXPERIMENTATIONS

Characterizing and modeling biological, chemical, and physical processes in heterogeneous media is a very difficult challenge due to the scarcity of data and to the complexity of geological media. Hydrogeological observatories and research sites may be of great help in providing key data to characterize some of the processes involved and to acquire the amount of data necessary to validate relevant biochemical and physical models. The observatory of Ploemeur (French Brittany) belongs to the French network of hydrogeological site H+ (http://hplus.ore.fr/). This outstandingly productive crystalline rock aquifer is exploited at a rate of about 10⁶ m³ per year since 1991 as the main water supply for a town of 18,000 inhabitants. One of the main interests of the Ploemeur site is to be characterized by relatively fast flow velocities that allow a broad range of experimentations for groundwater studies and modeling. Here, we present few key actions which have provided great insights about groundwater flow and transport processes in heterogeneous media. Experiments were done for very different time and spatial scales.

Cross borehole flowmeter tests and pumping tests appear as an efficient method to identify and characterize preferential permeable flow paths all over the site. In parallel, the active structures of the aquifer as well as its hydromechanical properties were characterized through a microgravimetric and geodetic monitoring during a large scale pumping test. The evolution of groundwater chemistry, groundwater ages and the temperature monitoring allowed us to estimate the impact of groundwater abstraction at the watershed scale. In complement, some small-scale experimental sites were developed to test innovative imagery methods in fractured rock (Fiber-Optic DTS, hydrogeophysical methods...) and to characterize groundwater transport and reactivity (denitrification…) using conservative and reactive tracers. This comprehensive dataset and the facilities of the Ploemeur site are very useful for welcoming international collaborations to develop interdisciplinary studies across the critical zone.