NRP61/SWISSKARST PROJECT. TOWARD A PRAGMATIC MAPPING OF KARST HYDROLOGY

Hydrogeological maps in a karst environment often do not meet the requirements of users. Although formal and international guidelines have been proposed and accepted for hydrogeological maps, their application in karst show some limitations due to the specific hydraulic functioning of these aquifers whose the main one is the concomitancy of a slow-flow component and a fast-flow component organized in stream/conduits network. Additional specificities are:

- The unsaturated zone can be very thick in karst, from dozens to hundreds meters;

- Groundwater level may considerably rise during high water conditions. Isolated groundwater bodies at low-flow can merge together at high water stages and water may be diverted towards another or several other outlets.

- The delineation of groundwater catchment areas therefore includes regions from which flow may diverge.

In the frame of the Swiss National Research Program 61 (www.nrp61.ch) and thanks to the Federal Office for Environment and the Bernese Office for Water and Waste, SISKA develops a set of dedicated tools and approaches – basically in 3D – providing a pragmatic documentation of karst aquifers. In the meantime, improvements have been operated on the way of mapping as results can be described on maps (1/50’000; 1/25’000). Resulting maps display the extension and organization of the main flow systems, their catchment area (including diverging zones), the delineation of groundwater bodies (phreatic zone) as well as parts of unsaturated aquifers. It also shows confined part of the aquifer and outlines the most conducive location of the main underground drainage axes. Main discharge areas (springs or overflows) are also displayed. The main weakness of a 2D (map) representation is the visualisation of vertical exchanges between superimposed aquifers, as evidenced by the 3D-analysis. This is especially significant in folded/overlapped contexts. Questions also remain concerning the mean depth considered in the hydrogeological interpretation (e.g. interactions between deeper aquifers). Recent applications in Swiss cantons show the efficiency und usefulness of this mapping approach.