Abstract
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.
Highlights
Karst regions provide drinking water for a quarter of the world’s population (Ford and Williams, 2007; Mangin, 1975)
Watlet et al.: Imaging groundwater infiltration dynamics in the karst vadose zone aquifers is driven by water infiltration through the vadose zone (White, 2002)
The thickness of this vadose zone varies from one karst system to another but is commonly described as two entities: (i) its uppermost layer, the soil joined with the so-called epikarst which is characterised by high weathering and porosity of carbonate rocks, overlaying the (ii) infiltration zone
Summary
Karst regions provide drinking water for a quarter of the world’s population (Ford and Williams, 2007; Mangin, 1975). In parallel, Kaufmann and Deceuster (2014) have demonstrated the applicability of using ERT to image the porous matrix associated with karstification processes These studies demonstrate the applicability of such techniques with regard to hydrological purposes in karst, they spotted real challenges: the heterogeneity of the subsurface making the interpreta-. The monitoring site focuses on a small part of the karst area, at the entrance of the Lorette cave Such a local-scale approach supports the need to study karst hydrology on all scales (Hartmann, 2016) to build extensive data sets available for strengthening hydrological models
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