Unsaturated zone can play an important role in karst hydrology. Investigating its structure and hydrodynamic functioning is a very challenging task, but it is also a key to better assess and manage groundwater resources.
Classically, hydrogeological studies combine hydrodynamic and hydrochemical measurements. Flows in unsaturated zone are accessed through boreholes or within natural or artificial tunnels. In this work, we highlight representativity issues of flow dynamics derived from boreholes monitoring in karst unsaturated zone. We show that combination of borehole observations with non-invasive hydrogeophysical monitoring is required to assess hydrodynamic and structural properties at site scale.
Our study is implemented in the LSBB (Low Noise Underground Laboratory, France), an artificial tunnel of 3.8km dugged within the karst unsaturated zone of the fontaine de Vaucluse aquifer. We combined hydrogeological, hydrochemical and isotopic measurements performed at an outflow within the tunnel with hydrogeophysical surface-based measurements, hydrodynamic monitoring and logging of three boreholes at the surface above the tunnel.
Results show that : a) boreholes located in similar geophysical context with small in-between distance (18m) may exhibit very different hydrodynamic behaviour, which questions the spatial representativity of the flows derived from boreholes monitoring; b) while artificial tunnel and boreholes provide access to karst unsaturated zone, their implementation may disturb natural flow pathways and thus hydrodynamics of the medium; c) combination of classical hydrogeological measurements with surface- and borehole-based geophysical measurements can provide relevant insights in flow dynamics, but implementation of hydrogeophysical measurements is time-consuming, and remains constrained by field conditions.
The authors thank SNO H+, PACA region and TOTAL for co-funding this R&D project. The authors thank TOTAL for giving permission to publish this paper.