Christophe Emblanch

Dissolved organic carbon of infiltration within the autogenic Karst Hydrosystem

This work aims at using the Dissolved Organic Carbon (DOC) as a tracer of infiltration in karstic aquifers with only diffuse recharge. Regular measurements of DOC have been carried out in three karstic systems located in South-eastern France during an hydrogeological cycle. Considering that the DOC is mainly located in the soil, it might be used as a tracer of the infiltration. The behavior of the tracer considerably varies between the high and low flow periods. The low concentrations of DOC occur when the flow is mainly due to a long residence time of the water in the saturated zone, whereas the concentration of DOC increases with the contribution of recent waters during the flood period. So the DOC can be used as a tracer of the infiltration. A comparison between the magnesium and the DOC allows to demonstrate the ability of using the DOC as a tracer of the water residence time.

Le marquage de la zone non saturée du karst à l'aide du carbone 13

Abstract By using 13C as a tracer in karstic aquifers, one can distinguish between water from the unsaturated and the saturated zones, because the system behaves as one open to the biogenic CO2 in the unsaturated zone. This method shows that a significant reserve remains in the unsaturated zone at the end of the minimal flow. It is therefore likely that the unsaturated zone contributes to the outflow during this period.

Feasibility and Limits of Electrical Resistivity Tomography to Monitor Water Infiltration Through Karst Medium During a Rainy Event

The common hydrogeological concepts assume that water enters in karst media by preferential pathways. But it is difficult to identify these pathways, particularly if soil or scree covers the karst features. When and where does water enter in the hydrosystem? How fast? A unique large-scale Electrical Resistivity Tomography (ERT) surface-based time-lapse experiment was carried out during a typical Mediterranean autumn rainy episode (230 mm of rain over 17 days). A total of 120 ERT time-lapse sections were measured over the same profile during and after this event (30 days). The main goal was to evaluate efficiency and limits of the ERT to monitor water infiltration, under natural conditions. Apparent (directly measured) and inverted resistivity’s variation during the rainy event highlights some interesting zones. They could be interpreted as preferential pathways, where water dynamic seems quicker in term of moistening and drainage. Nevertheless, these results have to be interpreted reasonably because ERT does not provide enough precision to determine exact pathways geometry and functioning. In addition, forward modeling provided relevant data treatment limitations mainly for the deeper parts of the sections.

Water in karst hydrosystems unsaturated zone; MRS evidences within an integrated hydrogeophysical approach.

Karst unsaturated zone (UZ) is recognized to play a key role in karst hydrosystems recharge and contaminant attenuation processes. Its characteristics are identified as an important factor for karst groundwater vulnerability assessment and resource management. A large scale hydrogeophysical experiment was undertaken over the last 4 years combining several surface-based geophysical methods and measuring techniques in a unique experimental site within the Fontaine de Vaucluse (FDV) hydrosystem in southern France. The aim was to apprehend the UZ structure and functioning of typical urgonian karstified mediterranean limestone. Geological structure is being well recognized by electrical, electromagnetic and seismic measurements. Magnetic Resonance Sounding (MRS) results clearly identify water presence and evidence seasonal variations within the karst UZ. Due to these results a conceptual hydrogeological model is proposed. On May-June 2014 a drilling campaign is being programmed and a cross-validation of the MRS results is also planned.

Process-Based Vegetation Models Improve Karst Recharge Simulation Under Mediterranean Forest

Assessing underground hydrosystem recharge is crucial to characterizing their hydrogeological functioning. The common questions arising from a poor understanding of hydrogeological mechanism are about parts of the gross rain amount that evapotranspire and get temporarily stored within the soil. Evapotranspiration and soil water storage are largely influenced by the structure and the function of the aboveground vegetation, which is generally composed of heterogeneous forest layer in Mediterranean karstic systems. However, most models used to compute karst hydrosystem recharge rely on simplistic formulations of evapotranspiration (ET) that do not account for vegetation functioning. In this study, we used the vegetation process-based model CASTANEA to improve water transfer in the higher horizon of the karst system and recharge simulations. Effective infiltration was computed with CASTANEA or with a classical approach (based on precipitation minus ET) for a well-documented holm oak site in southern France. We then compared simulation results with outflow data measured at 33 m below ground. We found significant differences between the two calculation methods, up to 200 % of annual recharge in the case of a very dry year. The comparison of modelled effective infiltration with outflow data indicated that using CASTANEA improved assessment of the temporal dynamics of water recharge in this karst system compared to a more classical approach. Our approach constitutes a promising way to improve the simulation of karst hydrosystem recharge.