Nicolas Peyraube

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.

Trends of labile trace metals in tropical urban water under highly contrasted weather conditions

The spatio-temporal trend of trace metals (Cd, Co, Cr, Cu, Ni, Pb, and Zn) in a tropical urban estuary under the influence of monsoon was determined using diffusive gradient in thin films (DGT) in situ samplers. Three different climatic periods were observed: period 1, dry with dredging activity; period 2, intermediate meaning from dry to wet event; and period 3, wet having continuous rainfall. Conforming to monsoon regimes, these periods correspond to the following: transition from winter to summer, winter, and summer monsoons, respectively. The distinction of each period is defined by their specific hydrological and physico-chemical conditions. Substantial concentrations of the trace metals were detected. The distribution and trend of the trace metals under the challenge of a tropical climate were able to follow using DGT as a sensitive in situ sampler. In order to identify the differences among periods, statistical analyses were performed. This allowed discriminating period 2 (oxic water) as significantly different compared to other periods. The spatio-temporal analysis was then applied in order to distinguish the trend of the trace metals. Results showed that the trend of trace metals can be described according to their response to (i) seasonal variations (Cd and Cr), (ii) spatio-temporal conditions (Co, Cu, Ni, and Pb), and (iii) neither (i) nor (ii) meaning exhibiting no response or having constant change (Zn). The correlation of the trace metals and the physico-chemical parameters reveals that Cd, Co, Cu, and Cr are proportional to the dissolved oxygen (DO), Cd and Ni are correlated pH, and Zn lightly influenced by salinity.

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.