A Numerical and Statistical Analysis of the Fractured Rock Aquifer System in Ploemeur, France: Using Time Series Data to Model Local and Regional Recharge
Résumé
Classification and modeling of fractured systems is highly challenging due to the heterogeneous nature of the material in conjunction with the often complex subsurface structure. Extensive groundwater data collection at differing time scales is helpful in constraining the spatial variability across such a system, and preferential flow paths are identified using cross borehole flowmeter tests and pumping tests.
We investigate the fractured crystalline rock aquifer system near Ploemeur, France, by characterizing infiltration and groundwater flow using a combination of surface deformation signals and time series hydraulic head and precipitation data collected at daily, monthly, yearly, and at decadal intervals. The recharge sites are of particular significance; annual withdrawal from the system nears one million cubic meters with minimal observed drawdown, raising questions as to the origins of the significant recharge necessary to sustain such a system. Extensive monitoring has shown a roughly two-month lag between seasonal water level and monthly precipitation, posing additional queries about groundwater movement through the unsaturated zone and fractured crystalline rocks. Surface deformation data may help to explain these factors, as we believe they may reflect various facets of the hydrologic system such as declining heads and loading of precipitation in the unsaturated zone.
The primary objective of this study is to evaluate the hydromechanical properties and the role of subsurface structures that have led to the development of the unique deformation and recharge signals at the Ploemeur site. We are using a combination of unsaturated and saturated zone modeling of the faulted system using MODFLOW and geostatistical tools to attempt to decipher the mechanisms constraining recharge and the lag between hydraulic head and precipitation, as well as the thickness of the unsaturated zone. The eventual addition of surface deformation measurements will be imperative to the accuracy of the model, and will be implemented in the final stages of work in order to distinguish the fracture geometry and provide insight to the structural and poromechanical controls on the system.