Groundwater Monitoring in a Karst System using ground-based geodetic techniques

Jacob, Thomas 1; Bayer, Roger 2; Boudin, Frederick 2; Champollion, Cedric 2; Chery, Jean 2; Deville, Sabrina 2; Le Moigne, Nicolas 2; Vernant, Philippe 2
1: CIRES and Dept. of Physics, University of Colorado, Boulder, USA 2: Geosciences Montpellier, Université Montpellier II, Montpellier, France; thomas.jacob@colorado.edu

Water storage variations in a karst aquifer of ~ 100 km2 on the Larzac Plateau (South of France) has been monitored using a combination of ground-based gravity techniques and long baseline tiltmeters for over three years. Geodetic instruments are set up with the sole purpose of collecting novel information on the functioning of karst aquifers, whose structural heterogeneity makes them challenging to monitor with classical hydrological methods. Ground based gravity techniques comprise the following: - Monthly absolute gravity measurements at three sites, yielding water storage change (WSC) in the vicinity of the sites with no depth resolution. The recorded annual cycle is in the order of 10-20 µGal. - Twice-yearly surface to depth gravity measurements between the surface and 60 m depth, yielding complimentary information on the vertical location of WSC. - Twice-yearly microgravity surveying at 40 stations covering the karst recharge area, allowing for basin scale WSC to be monitored between 'wet' and 'dry' periods. Combining these gravity techniques allows for a better quantification of WSC from local scale (~50-100 m) to basin scale (~5-10 km). Recorded gravity signals are interpreted in the light of conceptual hydrological reservoir models. Long baseline tiltmeters set up in caves at two sites record pulse-like tilt signals after heavy precipitation events, with amplitudes of some ærads. These are attributed to the vertical transfer of water in nearby fractures, and allow for a quantification of infiltration dynamics.