Interests of long-term hydrogeological observatories for characterizing and modelling heterogeneous groundwater systems at multiple temporal and spatial scales: the example of Ploemeur, a crystalline rock aquifer (Brittany).
Résumé
Characterizing groundwater flows and surface interactions in heterogeneous groundwater systems such as
crystalline fractured rock is often extremely complex. In particular, hydraulic properties are highly variable while
groundwater chemical properties may vary both in space and time, especially due to the impact of groundwater
abstraction. Here, we show the interest of hydrological observatories and long-term monitoring for characterizing
hydrological processes occurring in a crystalline rock aquifer. We present results from the site of Ploemeur
(French Brittany) that belongs to the network of hydrogeological sites H+ and the research infrastructure OZCAR,
and where interdisciplinary and integrated research at multiple temporal and spatial scales has been developed
for almost twenty years. This outstandingly heterogeneous crystalline rock aquifer is also used for groundwater
supply since 1991.
In particular, we show how cross-borehole flowmeter tests, pumping tests and a frequency domain analysis
of groundwater levels allow quantifying the hydraulic properties of the aquifer at different scales. In addition,
groundwater temperature evolution was used as an excellent tracer for characterizing groundwater flow. At the
site scale, measurements of ground surface deformation through long-base tiltmeters provide robust estimates
of aquifer storage and allow identifying the active structures, including those acting during recharge process.
Finally, a numerical model of the watershed scale that combines hydraulic data and groundwater ages confirms
the geometry of this complex aquifer and the consistency of the different datasets. In parallel, this hydrological
observatory is also used for developing hydrogeophysical methods and to characterize groundwater transport and
biogeochemical reactivity in the sub-surface. The Ploemeur hydrogeological observatory is a good example of the
interest of focusing research activities on a site during long-term as it provides a thorough understanding of both
hydrological and biogeochemical processes that can be extended to many heterogeneous aquifers.