Geochemical evolution of groundwater in the unsaturated zone of a karstic massif, using the PCO2- Sic relationship

International audienceIn karstic environments, groundwater is strongly influenced by CO2 partial pressure variations of air present in the infiltration zone of these aquifers. In order to characterize the geochemical changes in groundwater as it moves through the infiltration zone, we monitored various rising springs in the perched karstic aquifer of Cussac (Dordogne, France), and measured the CO2 partial pressure in air of a nearby cavity (the Cussac Cave) for 24 months. Our method is based on the relationship between the saturation index with respect to calcite (SIc) and the CO2 partial pressure at atmospheric equilibrium with water. We distinguished a value for this last parameter when water is at equilibrium with respect to calcite (SIc = 0) called saturation CO2 partial pressure. The use of this parameter can provide information on flow conditions and relationships between water, air, and rock. Cussac aquifer is a suitable area to apply these methods because of its small size, numerous springs, and a cave that provides data for CO2 partial pressure condition inside the massif. Results show that most of the calcium-carbonate mineralization is acquired in the epikarst followed by a precipitation phase in the upper part of the infiltration zone. Groundwater reaches the saturated zone with some degree of saturation depending on CO2 partial pressure variations in air inside the massif

Estimation of epikarst air P CO 2 using measurements of water δ 13 C TDIC, cave air P CO 2 and δ 13 C CO 2

International audienceWhen present, an epikarst represents the starting point (the first karst compartment) of water flow through a karst system. The air characteristics in a karst, and especially in an epikarst, determine the initial water characteristics, e.g., water aggressiveness, which depends on the partial pressure of CO 2 ( P CO 2 ) in equilibrium with water. This paper proposes a method to estimate P CO 2 in epikarst air using spring water measures as HCO 3 - , temperature, pH and δ 13 C of Total Dissolved Inorganic Carbon (TDIC) and cave air measures as P CO 2 and δ 13 C of CO 2 . This method accounts for the TDIC variations of δ 13 C that are caused by CO 2 degassing and calcite precipitation from water. The calculations are based on the influence of pH variations and carbon loss on δ 13 C of TDIC. Measurements are taken at two sites: Lascaux cave and the Cussac cave sites located in Perigord, southwest of France. Four water springs are presented in this case study: two springs from an epikarst compartment, one spring from an unsaturated zone and one spring from a saturated zone. The P CO 2 in epikarst air is estimated to be from 4.4% (44,000 ppm) in winter to 10% in summer. These values are higher than the values of air P CO 2 measured in the soil (0.27–1.60%) or in the caves (0.30–3.1%, up to 7.50% in some parts of Lascaux). We show that in epikarst air, P CO 2 and δ 13 C CO 2 are not constant values but vary annually with high P CO 2 and depleted values (−22.31‰ VPDB) in the winter and higher P CO 2 and more depleted values in the summer (−24.20‰ VPDB)

Development of differently determined and differently targeted Cave Environment Protection Perimeters using hydrogeological basis

International audienceConserving a cave with important historical and archeological artifacts needs protection measures. Cave Environment Protection Perimeters (CEPP) operational framework is proposed as a conservation measure tool for the cave and its environment. In this study, Cussac Cave was examined. Water infiltration is the main identified threat as it can bring pollutants into the cave. Three nested CEPP zones were then identified covering three important resource safeguarding intentions. First is on determining the possible water path infiltration through fractures from surface to cave (CEPP 1). Second is on delineating watershed of water that can infiltrate above the cave after run-off (CEPP 2). And third is on determining the limits that can constrain groundwater circulation (CEPP 3). According to the objective, the CEPP were obtained using a combination of classical tools such as geomorphology, topography, hydrological parameters (water flow and chemistry), and artificial tracing. The immediate CEPP 1 is close and is small in size (0.5 km2) which can be prone to both chronic and acute pollution through direct infiltration above the cave. The hydrologic CEPP 2 is medium size (1.1 km2) which can entail risks concerning chronic and acute pollution that can be brought by surface run-off before infiltration. The hydrogeologic CEPP 3 is the largest of the three (3.9 km2). Recommendations in each zone on how to manage the cave environment are presented

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

Determination and quantification of major climatic parameters influencing the CO2 of Lascaux Cave

High and low CO2 partial pressure (Pco2) levels can induce consequences to the internal system of a karstic cave. This then can further affect the state of the cave. Subsequently, speleothem growths and the walls with prehistorical paintings and engravings will be affected with either calcite dissolution or precipitation. Thus, pinpointing the major components of Pco2 is found indispensable. In this paper, a method is proposed in order to (1) identify the major components of the Pco2 and (2) quantify each specific contribution to the Pco2. Pco2, air temperature, air pressure, and water flow were measured from 2008 to 2010 in Lascaux Cave (Southwest of France). Pco2 varied from 0.62% (6200 ppm) in summer to 0.04% (400 ppm) in winter. We identified three major components: (1) seasonal: component A; (2) low frequency: component B; and (3) high frequency: component C. The method is applied to estimate the contribution of each component. Component A was modelled with a sinusoid curve fitted to the Pco2 measures. This component was subtracted from the measured Pco2. Then, from the remaining value of Pco2, Fourier transform, frequency filtering, and inverse Fourier transform were applied to separate high- and low-frequency components. As a result, the seasonal component A contributes 77% of the measured Pco2, while components B and C supply 13 and 11%, respectively. The three identified components have links with environmental parameters such as temperature, water flow, and atmospheric pressure. Hypotheses were proposed to describe these links. Knowledge on these aspects can help in cave management and protection specifically in answering the question: which parameter should be given more attention

SIc–Abacus: An in–situ tool for estimating SIc and Pco2 in the context of carbonate karst

This article aims to provide an on the spot, handy field tool that can be used in estimating the Saturation Index with respect to calcite in karst aquifers in a limestone context. It relies on an abacus that gives the SIc by using measured pH and bicarbonate concentration as coordinates.The analytical expressions of the carbonate equilibrium equations are derived in order to calculate SIc from values that can be measured on field. Using these established relationships, we propose an Abacus built from the SIc=f(Pco2eq) reference frame, hence called SIc–Abacus.Accuracy of SIc calculation is also discussed. SIc is calculated with values measured considering uncertainty coming from pH (±0.05 pH units), temperature (±0.1 °C), bicarbonate and calcium concentrations (±2.5 mg/L and ±2 mg/L). This gives, for each sample, a range of possible SIc values of ±0.060 pH units (for a bicarbonate concentration close to 300 mg/L). The effect on SIc range of possible values given by bicarbonate and calcium uncertainties is about 10 times lower than the effect brought by pH uncertainty. Also, bicarbonate and calcium concentration can be estimated from electrical conductivity leading to a possible range of SIc value of ±0.087 pH units. In both cases, the range of possible values of SIc varies according to the bicarbonate concentration.Using Cussac and Lascaux sites, an evaluation was done. Results show the goodness of fit (R2=0.96) between the calculated SIc from measured pH, bicarbonate and calcium concentration and the estimated SIc values generated from the SIc–Abacus. Apart from calculating SIc, another method which is an alternative use of the generated SIc equation focused on obtaining pH using parameters such as electrical conductivity and dissolved CO2 measurements is presented.The study shows that SIc–Abacus can be a new in–situ tool for karst aquifer surveys. It can facilitate in making a swift decision that tackles carbonate karst issues (e.g. when selecting sites or in deciding if extensive monitoring should be performed or not based from a specific objective)