Roland Lastennet

Characterization of karst systems using SIc-Pco2 method coupled with PCA and frequency distribution analysis. Application to karst systems in the Vaucluse county (Southeastern France)

Estimating vulnerability of a karst system to pollution is a major current issue. However, to assess vulnerability, an understanding of the karst system functioning is necessary. The aim of this paper was to characterize the degree of karst network development used by flows (i.e. karst system functioning) to have a better knowledge on the resource and its vulnerability to any possible contamination. This characterization can be done with an analysis of water chemistry emerging from a system. Carbon dioxide partial pressure (Pco2) and saturation index with respect to calcite (SIc) are two significant parameters that can be used to study water chemistry of karst systems through calcium-carbonate equilibriums. These enable the implementation of the SIc–Pco2 method. In this article, a methodology is proposed based on the SIc–Pco2 method coupled with frequency distribution analysis and principal components analysis (PCA). This study considers three karst systems with a developed saturated zone and one epikarst spring, located near the Fontaine de Vaucluse spring (Southeast France). Results show that different water types and flowing conditions in the karst system are identified. From this acquired information, karst network development can be estimated. Finally, results can aid in the vulnerability assessment of karst systems.

Hydrochemical and Hydrodynamic Behavior of the Epikarst at the Lascaux Cave (Montignac, France)

This study focuses on determining the hydrodynamic and hydrogeochemical behavior of the vadose zone of Lascaux karst (Dordogne, France). This work is of particular interest for the conservation of the famous prehistoric paintings of the Lascaux Cave. Indeed, unsustainable infiltration into the cave influences the microclimate parameters (temperature, humidity) and may play a role in aerologic phenomena (condensation, drying) that can affect the decorated walls. A continuous monitoring device for physical and chemical parameters (conductivity, pH, temperature, dissolved O2) and fluorescence of organic matter are installed in the epikarstic spring located inside the cavity. In addition, weekly samples are performed at the epikarstic and Haute Fageotte springs. Weekly rainfall (major ions, water isotopes) and soil water (major ions, organic carbon) samples are performed to determine the input chemical signal in the karst system. The results show that water with shorter or longer residence time can be identified during the floods, considering electrical conductivity and the dissolved organic carbon as natural tracers. Moreover, when the system is highly water saturated, water with long residence time is solicited highlighting an increase of the area participating to the flow at the epikarstic emergence. The study of the calco-carbonic equilibrium of the epikarstic spring also confirms these observations. The calco-carbonic equilibrium shows interannual and seasonal variations related to climatic conditions, carbon dioxide concentration in soils and epikarst, and the flow conditions in the vadose zone. The relations between SIc and PCO2eq allow us to recognize the climate influence on the recharge geochemical signal of the Lascaux karst. It would be useful to estimate the global warming impact on the geochemistry of karst water.

Examining the Functioning of a Multilayer Karst System: The Case of Toulon Springs (Dordogne, France)

Hydrodynamic and hydrochemical responses of Toulon springs are examined to identify the functioning of the karst system. First, analysis of the recession curve coupled to univariate and bivariate analyses is conducted to determine storage capacity and relationship between rainfall and discharge. Results show a complex system which could provide fast hydrodynamic responses to rainfall solicitation, but have an ineffective drainage network and then high storage capacity. In addition, temporal evolution of hydrochemical parameters leads to identify quick variation of hydrochemical responses during flood events. Furthermore, a principal component analysis (PCA) is conducted on 61 samples and confirms that springs are fed with waters from different origins.

Decomposition of CO2 Signal in Lascaux Cave

Air CO2 partial pressure (Pco2) in one of the monitored parts in Lascaux cave (Hall of the Bulls, near the entrance) is variable. For the year 2009, three major components of Pco2 were identified: a seasonal component, a low-frequency component and a high-frequency component. As a first step to understand the presence of Pco2 in caves, this study aims to separate these components and quantify their contributions to the Pco2 signal. Seasonal component is calculated with a sinusoid curve fitted to the measured Pco2. This component is subtracted from the measured Pco2. Then, from the remaining value of Pco2, low- and high-frequency components were separated using Fourier transform, threshold filtering of the frequency and Inverse Fourier transform. Seasonal component is the main contributor (82 % on the average); high-frequency component participates 12 % on the average. Low-frequency component is not present all the year; the average participation is 6 %. Finally, to link these three components with other measured parameters such as temperature, water flow and atmospheric pressure, hypotheses were proposed.

Characterization of the Functionality of Karstic Systems Based on the Study of the SIc–Pco2 Relation

This paper aims at characterizing the functionality of karstic systems. In order to do so, an extend of a method based on the saturated index with respect to calcite (SIc) and CO2 partial pressure (Pco2) is used. The initial method already applied by Peyraube et al. (2012) uses the Pco2 of water at equilibrium with atmosphere (Pco2_eq) and the Pco2 of water for a SIc equal to zero (Pco2_sat). The Pco2_sat variation gives information on flows conditions within the karstic system. It describes the degree of karstification of the system. Systems with developed saturated zone are characterized by under-saturated waters as well as saturated ones with variable Pco2_sat. At last, fissured system has Pco2_sat which vary only slightly. Moreover, variations of Pco2_sat show homogenization capacities of a karst.