Articles | Volume 22, issue 2
Hydrol. Earth Syst. Sci., 22, 1563–1592, 2018
Hydrol. Earth Syst. Sci., 22, 1563–1592, 2018
Research article
 | Highlight paper
01 Mar 2018
Research article  | Highlight paper | 01 Mar 2018

Imaging groundwater infiltration dynamics in the karst vadose zone with long-term ERT monitoring

Arnaud Watlet1,2,*, Olivier Kaufmann1, Antoine Triantafyllou1,3, Amaël Poulain4, Jonathan E. Chambers5, Philip I. Meldrum5, Paul B. Wilkinson5, Vincent Hallet4, Yves Quinif1, Michel Van Ruymbeke2, and Michel Van Camp2 Arnaud Watlet et al.
  • 1Geology and Applied Geology Unit, Faculty of Engineering, University of Mons, Place du Parc 20, 7000 Mons, Belgium
  • 2Seismology-Gravimetry, Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Uccle, Belgium
  • 3Laboratoire de Planétologie et Géodynamique – Nantes (LPGN), UFR Sciences et Techniques, Université de Nantes, UMR-CNRS 6112, Rue de la Houssinière 2, BP92208, 44322 Nantes CEDEX 3, France
  • 4Department of Geology, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium
  • 5British Geological Survey, Environmental Science Centre, Keyworth, Nottingham NG12 5GG, UK
  • * Invited contribution by Arnaud Watlet, recipient of the EGU Hydrological Sciences Outstanding Student Poster and PICO Award 2016.

Abstract. Water infiltration and recharge processes in karst systems are complex and difficult to measure with conventional hydrological methods. In particular, temporarily saturated groundwater reservoirs hosted in the vadose zone can play a buffering role in water infiltration. This results from the pronounced porosity and permeability contrasts created by local karstification processes of carbonate rocks. Analyses of time-lapse 2-D geoelectrical imaging over a period of 3 years at the Rochefort Cave Laboratory (RCL) site in south Belgium highlight variable hydrodynamics in a karst vadose zone. This represents the first long-term and permanently installed electrical resistivity tomography (ERT) monitoring in a karst landscape. The collected data were compared to conventional hydrological measurements (drip discharge monitoring, soil moisture and water conductivity data sets) and a detailed structural analysis of the local geological structures providing a thorough understanding of the groundwater infiltration. Seasonal changes affect all the imaged areas leading to increases in resistivity in spring and summer attributed to enhanced evapotranspiration, whereas winter is characterised by a general decrease in resistivity associated with a groundwater recharge of the vadose zone. Three types of hydrological dynamics, corresponding to areas with distinct lithological and structural features, could be identified via changes in resistivity: (D1) upper conductive layers, associated with clay-rich soil and epikarst, showing the highest variability related to weather conditions; (D2) deeper and more resistive limestone areas, characterised by variable degrees of porosity and clay contents, hence showing more diffuse seasonal variations; and (D3) a conductive fractured zone associated with damped seasonal dynamics, while showing a great variability similar to that of the upper layers in response to rainfall events. This study provides detailed images of the sources of drip discharge spots traditionally monitored in caves and aims to support modelling approaches of karst hydrological processes.

Short summary
Understanding water infiltration in karst regions is crucial as the aquifers they host provide drinkable water for a quarter of the world's population. We present a non-invasive tool to image hydrological processes in karst systems. At our field site, the injection of electrical current in the ground, repeated daily over a 3-year period, allowed imaging changes in the groundwater content. We show that specific geological layers control seasonal to rainfall-triggered water infiltration dynamics.