Investigation of matrix fluids in fractured aquifers through various gas analyse

International audienceWe investigated the matrix fluids in a fractured aquifer in French Brittany (Ploemeur, national observatory site). We used four wells close to each others, two of them being pumped for several hours. The four wells were monitored during the whole experiment for chemical composition (major and trace elements) and dissolved gases content: CFC, SF6, Rn, noble gases and He isotopes. After an equilibration phase, the chemical and gas concentrations evolve during the relaxation phase following pumping. The production of matrix fluids is evidenced through the following observations: * CFC concentrations decrease, indicating waters with longer residence time; * He, Rn and SF6 concentrations increase with time, indicating a potential production mechanism. SF6 and Rn production have indeed been measured in this aquifer. However, it is interesting to notice that these three elements do not have the same reactivity. they do not have similar production mechanisms and diffusion coefficients; * Some chemical elements such as silica increase indicating silicate alteration processes; * The redox conditions also vary with time during the relaxation phase, indicating more reduced conditions with U concentration increases; * Ar increases also indicate either an Ar production or a recharge temperature of 5°C. This experiment shows that fluids with various compositions in fractured aquifers are related to various degree of micro- and macro-porosity, and may be associated to various pumping conditions

Influence of diel biogeochemical cycles on carbonate equilibrium in a karst river

International audienceVariations in temperature, photosynthesis, and respiration force diel variations in pH and dissolved CO2 concentrations of surface streams, possibly controlling carbonate equilibrium between river water and carbonate stream beds. Diel cycles of water chemistry and ä13C of dissolved inorganic carbon (DIC)weremeasured to assess how biogeochemical processes affect dissolution and precipitation of calcite and thus channel development in Ichetucknee River, a large spring-fed river (discharge N6m3/s) flowing over carbonate karst terrain in north central Florida (USA). Sampleswere collected at a 4-h sampling interval during two one-week periods and at a 1-h interval during a single 24-h period. Simultaneously, temperature, pH, dissolved oxygen (DO) and NO3 . concentrations weremeasured using in situ sensors at 15-min or 1-h intervals. Ca2+, DICandNO3 .concentrations decreased during the day and increased at night causing diel changes of in-stream specific conductivity. These changeswere inversely related to diel changes in pH, PCO2 and DO concentrations. Thiswork shows that photosynthesis and respiration of subaquatic vegetation are the dominant processes influencing in-stream diel variation. During the day, a simultaneous increase of ä13CDIC and decrease in DIC indicates that photosynthesiswas the primary control on DIC concentrations. Calcite saturation indices ranged from0 to 0.5, with the highest value in daylight as a result of CO2 consumption causing carbonate precipitation. The water remained saturated with respect to calcite at night and ä13CDIC values decreased, indicating that CO2 production from ecosystem respiration was the dominant process affecting DIC concentrations but was insufficient to induce significant carbonate dissolution. At night outgassing maintained in-stream DIC concentrations lower than the supersaturated DIC springs source but a drop in ä13CDIC indicates that ecosystem respiration had a dominant influence over outgassing. Although CO2 outgassing occurs, it is shownto be aminor component of theDIC mass balancewhile carbonate precipitation represents 88% ofDIC loss. These results indicate that in-stream biological processes influence carbonate mineral diagenesis in large clearwater rivers

Water table variations in the hyperarid Atacama Desert : role of the increasing groundwater extraction in the pampa del tamarugal (Northern Chile)

In the hyperarid Atacama Desert (Northern Chile), the economic and social development is supported using fossil groundwater. The groundwater extraction (GWE) has significantly increased over the last 30 years, reaching similar to 4.2 m(3).s(-1) in 2018 (+1890%) at the Pampa del Tamarugal Aquifer (PTA). But opposite assumptions lead to uncertainties concerning the role of the increasing anthropogenic pressures and the ephemeral recharge events in the water table (WT) variations. This paper analyzes: (i) the long-term groundwater levels changes between the late 1950s (post Saltpeter Work) and the early 2010s, and (ii) the short-term response of groundwater levels, based on the analysis of the 1998-2018 WT time series at 10 observation boreholes. Results indicate that the WT variations in space and time are strongly related to the anthropogenic pressure changes. Since the late 1950s, the WT is declining in the major part of the PTA. Nevertheless, local reduction of GWE together with ephemeral recharge events in alluvial fans allowed local WT rises. But after a large GWE increase (+114%) between 2004 and 2006, all observation boreholes highlight a general WT decline (-9.8 +/- 5.8 cm.yr(-1)). Over the years, anthropogenic pressures became the dominant factor of the WT variations and led to overuse the aquifer

Recharge processes in karst aquifers inferred from noble gases: impact of the vadose zone – South of France. AGU 2019, 9-13 Dec. 2019, San Francisco, USA.

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Identifying origin of groundwater and flow processes in complex landslides affecting black marls in southern French Alps: insights from an hydrochemistry survey

International audienceThe Super‐Sauze mudslide is a persistently active slow‐moving landslide occurring in the black marl outcrops of the French South Alps. It has been intensively studied since the early 1990s. Geotechnical, geomorphological, geophysical and hydrological investigations have led to a better understanding of the processes governing the landslide motion. Water flows inside the system have been proven to have a major impact. To look closer at the processes involved and especially to gain a better idea of the origin and pathways of the waters, a hydrochemical study was carried out from May 2003 to May 2004. The groundwater was sampled during five field campaigns spread uniformly over the year. Groundwater from a network of boreholes was collected as well as spring waters from the fractured bedrock (in situ black marl) and from the moraine aquifer above the landslide. Results showed that the groundwater chemistry could not be fully explained by rainfall recharge or simple water–matrix equilibrium. A contribution of saline waters coming from the bottom of a thrust sheet overhanging the landslide was required to get the observed high mineralization. On a flow line, the hydrochemical evolution was related to both soil–matrix equilibrium and deep water sources coming up to the surface by means of major faults, the bedding planes and the schistosity. Hydrochemical anomalies made it possible to point out such contributions locally. It was shown that water chemistry and landslide activity were closely related. This hydrochemical investigation also enabled us to better define the hydrosystem limits.Copyright © 2006 John Wiley & Sons, Ltd

Impacts of irrigation on groundwater recharge, mixing and quality: sandyaquifer of the Limagne Bourbonnaise (central France).

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Outils de connaissance de la ressource en eau dans les milieux de socle - expériences de suivi d'aquifères bretons

7 p.International audienc

Approche pluridisciplinaire originale d'exploration de l'aquifère karstique du Lez : compartimentation hydrodynamique et résistance des communautés bactérienne aux antibiotiques

National audienc