Jacques Mudry

Carbon 13 of TDIC to quantify the role of the unsaturated zone: the example of the Vaucluse karst systems (Southeastern France)

The total dissolved inorganic carbon (TDIC) and 13CTDIC have been used as chemical and isotopic tracers to evaluate the contribution of different water components discharging at the Fontaine de Vaucluse karst spring near Avignon. At the same time they have been used to separate its flood hydrograph. Waters flowing from unsaturated zone (UZ) and saturated zone (SZ) show similar concentration in TDIC. In UZ and SZ water rock interactions do not obey to the same kinetic. The mixing rate between water coming from the UZ characterised by a short residence time and water from the SZ with a longer residence time has been evaluated in the spring discharge. In a hydrodynamic system, which is rather complex as it is open to the soil CO2 in UZ and closed to the same CO2 in the SZ, 13CTDIC has excellent characteristics as an environmental tracer. In order to better describe the inwardness of mass movements within the aquifer, the apparent contrasting information obtained using two different isotopes (18O of water molecules and 13C of TDIC) must be combined. 18O informs whether the hydrodynamic system acts as piston flow (PF) or follows a well mixing model (WMM). Conversely, 13C gives more complete information on the UZ contributes to the total discharge.

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.

Utilisation du rapport Br/Cl pour déterminer l'origine de la salinité des eaux souterraines: exemple de la plaine du Souss (Maroc)

Abstract The use of bromide anion contents is an essential complement to chloride measurements in order to explain the salinity anomalies in groundwaters. Thanks to the relationship Br = ƒCl) and to the molar ratio Br/Cl, it allows marine influence areas (the Ocean, precipitation, aerosols, sedimentary paleosalinity) to be distinguished from evaporitic or anthropogenic influence areas, which are impoverished in Br as opposed to the oceanic domain. Applied to the Souss alluvial plain, and thanks to this methodology, we can interpret phenomena which are responsible for conductivity anomalies in groundwaters: present salt encroachment, sedimentary paleosalinity or wastewaters downstream; evaporites, recycling of irrigation waters in the mean Souss.

Post‐seismic permeability change in a shallow fractured aquifer following a ML 5.1 earthquake (Fourbanne karst aquifer, Jura outermost thrust unit, eastern France)

[1] Following a M L 5.1 earthquake in eastern France, a post-seismic 12-day long electrical conductivity increase was recorded in the water discharging from a karst aquifer located 3 km from the epicentre. We attribute this to a permeability enhancement which allowed long-residence time water from low-permeability fractures located in the saturated zone to be expelled. The permeability enhancement shows that shallow aquifers can be significantly deformed for several days by moderate magnitude earthquakes.

Isotopes and groundwater management strategies under semi-arid area: case of the Souss upstream basin (Morocco).

This study concerns the Souss upstream basin. The objective is to investigate the characteristics of surface water and groundwater, to assess the impact of artificial recharge as reinforcement of the natural replenishment and assess the renewal of groundwater under semi-arid area. Two major water types are observed: (i) surface waters and upstream springs (least mineralized) and (ii) all groundwater samples (prevailing calcium and magnesium bicarbonate water type). Water isotopes show a low evaporation of precipitations during infiltration. Impoverishment in heavy isotopes is the characteristic of mountain rainfalls, or of a climate colder and wetter than present. Carbon-14 activities (34-94 pmc) indicate a long residence time. The artificial recharge is low compared to the reservoir volume, due to which the renewal rate is also low.

Signal pluie et traçage par les isotopes stables en Méditerranée occidentale. Exemple de la région avignonnaise (Sud-Est de la France)

Abstract An isotopic survey of rainwater has been carried out in Avignon during the two periods 1978–1989 and 1997–1998. This monthly study allowed observation of the evolution of rainwater isotopic content during the year and its lack of relation with meteorological parameters, as well as definition of the local relationship between oxygen-18 and deuterium. Moreover, a comparison of the weighted mean isotopic value of rainwater (–6,21‰) with groundwater values provides a first approach to the estimation of the mean regional input function.

Hydrochemical dynamics of TOC and NO 3 − contents as natural tracers of infiltration in karst aquifers

Analysis of Total Organic Carbon (TOC) and NO3− contents in the water at six springs in the province of Malaga (Southern Spain), sampled under different hydrologic conditions, revealed two different hydrochemical behaviour patterns for these natural tracers of infiltration. TOC content increased during every recharge period, following the rapid arrival of water infiltrating through the soil. On the other hand, NO3− content only rose during the first flood episodes (normally in autumn), and fell during the winter and spring ones. This difference is consequence of the distinctive biogeochemical kinetics of nitrogen with respect to organic carbon, both in the soil and within the aquifer. Unlike the mineralisation undergone by TOC from the surface to the spring, the NO3− ion remains in the aquifer almost unaffected, due to the oxidizing conditions prevailing within the karst medium, which do not allow its denitrification and favour its preservation within the saturated zone. In non-polluted aquifers, TOC and NO3− have a common origin in the soil and can be used to determine infiltration processes and the hydrogeological functioning of karst aquifers. Their different hydrochemical evolution provides information about mineralization and degradation processes of organic matter within karst aquifers, which can be used to validate the vulnerability to contamination in this type of medium.

Bilan, fonctionnement et protection du système karstique de la Fontaine de Vaucluse (sud-est de la France)

ResumenL’aquifere karstique de la celebre Fontaine de Vaucluse a fait l’objet de nouvelles investigations hydrogeologiqurs dont les resultats portent sur la definition et le fonctionnement du systeme.Les donnees geologiques (lithologie, structure) nous ont permis la delimitation d’un impluvium de 1115 km2 dont l’altitude moyenne. 870 m, est confirmee par les traceurs naturels physico-chimiques et isotopiques.Le bilan hydrique, faute d’un reseau meteorologique representatif, a ete calcule a partir d’un modele climatique par tranches d’altitude, qui utilise les gradients pluviometriques et thermiques regionaux. On montre alors que les 3/4 des pluies efficaces proviennent des zones situees au-dessus de l’altitude moyenne de l’impluvium.Les etudes hydrodynamiques et physico-chimiques nous ont permis de demontrer la faible inertie du systeme malgre sa taille, donc sa bonne karstification, ce qui confirme a l’echelle de l’ensemble les observations speleologiques ponctuelles du plateau et de l’emergence.Malgre l...

Estimation of the water balance of alluvial aquifers in region of high isotopic contrast: an example from southeastern France

The much contrasted orographic and climatic characters of southeastern France serve to establish a hydrological balance of the porous aquifers in the region. Comparing a regional average gradient of 18O content versus elevation, which was calculated on low-water period karst waters content, with a −10.5‰ average d of Alpine rivers, an estimate of the percentages brought to these aquifers is proposed under the form of an abacus. It appears that the main groundwaters of the Var, the Durance and the Rhone are fed by an average of 20–30% of Provencal underground contribution, and 70–80% shallow contribution from the Alpine rivers. These figures reveal that local supplies range between 25 and 10% of the average yearly discharge flowing in the porous and shallow aquifers.

The karst system of the Fontaine de Vaucluse (Southeastern France)

The intake area of the Fontaine de Vauctuse system covers over 1,100 km2; its mean altitude is 870 m. The Lower Cretaceous limestones (1,500 m thick) give the system a very thick (800 m) unsaturated zone.Karstification is very well developed, both on the intake area (four sinkholes are more than 500 m deep) and on the lower part (sunken cave of 300 m depth under the spring). The bottoms of the sinkholes of the plateau do not reach the saturated zone of the karst, as their flows have chemical composition similar to seepage water. The maximum hydraulic gradient between the plateau and the spring is low, only 0.3%. Dye tracings allow assigning the Ventoux-Lure range (including its calcareous northern side with a southward dipping) and the Vaucluse Plateau to the intake area. The moisture balance, calculated for each altitude belt, shows that the effective rainfall strongly increases with altitude: 120 mm below 200 m, 1,300 mm over 1,800 m. The working of the system, studied by means of discharge, physical and chemical content, is one of a well-karstified milieu that reacts with a light inertia upon rainy periods. The system is made up of important reserves, peculiarly within the unsaturated zone, which maintain long decline and depletion episodes. Despite its large average discharge (21 m3·sec−1), the spring is not harnessed and thus no general protection exists on the intake area. Only local protection is provided by protection areas around some piped little springs of the plateau. A Biosphere Preserve will protect as a whole the higher part of the intake area—Mount Ventoux.