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

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Assessing the sensitivity of the Southern Ocean’s biological carbon pump to climate change (BELCANTO II): final report

BELCANTO (BELgian research on Carbon uptake in the ANTarctic Ocean) is a longterm project aiming at applying and developing process-level studies, geochemical proxy tools and numerical tools for assessing and understanding the present-day functioning of the CO2 biological pump in the iron-limited Southern Ocean (S.O.) and for predicting its evolution in response to scenarios of increasing atmospheric CO2. Over the last four years BELCANTO implemented and developed several multi-proxy approaches for assessing nutrient consumption and carbon fluxes, including 234Thdeficit and natural silicon isotopic composition. High quality results were obtained for whole water column δ29Si-silicate and δ29Si-opal, including first ever results for the low silicate Subantarctic region. Results show that the δ29Si signature of diatoms appears to be homogeneous in the mixed layer and between diatom species. The preferential uptake of light isotope by diatoms is well reflected in the vertical distribution of silicate δ29Si, as well as in the upper ocean silicate δ29Si values which increase northwards in parallel with the decrease of silicate concentration. However, the relationship between δ29Si and silicate concentration appears complex and depends on variations in vertical and horizontal supply of silicate. Results based on other proxy tools (Baxs 234Th-deficit, f-ratio & new production) indicate relatively high particulate carbon export and absence of strong mesopelagic mineralization in the Subantarctic Zone, but relatively low export and enhanced mesopelagic mineralization further south in the Polar Front Zone and the southern ACC. Furthermore, remineralization was clearly enhanced during summer as compared to spring. Our observations of 234Th-based carbon export and new production along cross frontal transects also appear to challenge the widely accepted idea of enhanced primary and export production in the Subantarctic Zone and the Polar Frontal Zone, compared to more southern areas. Laboratory-controlled experiments on two widespread phytoplankton species (diatoms-Thalassiosira gravida and Phaeocystis Antarctica) showed an effect of Fe addition on the morphological form, enhancing the presence of the colonial form compared to the free-living cells for Phaeocystis and increasing the appearance of long chains of diatoms vs. free living cells. Under Fe enriched conditions, the maximum photosynthesis and growth rates increased. In addition the quality of the organic matter was modified, enhancing bacterial remineralization. Consequently, Fe addition may impact on the fate of the phytoplankton in the planktonic food web and the resulting efficiency of the biological pump. Accordingly, during the EIFEX large scale iron enrichment experiment, iron addition fosters a diatom bloom which appeared to broke up rather fast and to sink rapidly accordingly to 234Th measurements. This in turn triggered mesopelagic mineralization as evidenced in Baxs measurements. N-uptake experiments with Fe-limited and Fe-replete natural algal communities indicate that the effects of ammonium and iron on f-ratio and new production are not simply cumulative and that the enhancement of the f-ratio due to Fe addition depends on the ambient ammonium concentration. However the relationship between this enhancement and ammonium concentration is at present not fully understood. Our results imply that there is no simple relationship between export production and iron availability. Ammonium appears to counter the effects of iron addition on export production, particularly for HNLC areas such as the Southern Ocean. The meso-scale iron enrichment experiment performed during the EIFEX cruise, clearly showed that iron addition induced a diatom bloom but that the latter did not persist for long and collapsed soon after, thereby inducing a massive export of carbon as witnessed from significant deficits in 234Th activity. This export in turn triggered mesopelagic organic carbon mineralization as evidenced by excess Baxs contents. Measurements of pCO2, scaled using remote sensing data (sea surface temperature, chlorophyll and wind stress) and related CO2 fluxes, suggest that previous budgets of atmospheric CO2 uptake in the S.O. may be overestimated. However, our observations indicate that sea ice cover can act as an additional CO2 sink not taken into account in previous CO2 budgets. This sea-ice CO2 sink function results from physical and biogeochemical processes prevailing within the sea ice itself. Results obtained with SWAMCO-4, an idealized 1-D model of the marine planktonic system calculating C, N, P, Si, Fe cycling within the upper ocean, the export production and air-sea CO2 fluxes, suggest that the sink for atmospheric CO2 will increase in response to the raising atmospheric CO2 concentration. SWAMCO-4 simulations show that the amplitude of the predicted CO2 sink displays large regional and inter-annual variations which are related to local hydrodynamics and the dominant phytoplankton species. This is particularly acute in the Phaeocystis-dominated marginal ice zone of the Ross Sea. There the predicted annual CO2 sink appears positively related to the length of the sea ice cover period. This results from the accumulation of iron within the ice and its sudden release in the water column at the time of ice melting, favoring algal growth. The past and future evolution of the sea ice cover was conducted using the ORCA2- LIM and ECBILT-CLIO 3D ice-ocean models. The influence of the Southern Annular Mode on zonal integrated sea surface temperature and ice concentration seems to be small due to counteracting effects. Over the last 250 years, the annual mean ice coverage decreased in response to both natural and anthropogenic forcing, with the impact of the latter forcing clearly being enhanced over the last 150 years. Nowadays, the decrease of ice cover is more acute in the northern hemisphere as compared to the southern hemisphere, and this difference is due to thermal inertia of the S.O. However, model outputs predict a more abrupt decrease of S.O. sea ice extent in the future, resulting in similar decreases of annual mean sea ice extents in both hemispheres by the end of the century. In parallel, the seasonal amplitude of sea ice extent will increase. Preliminary runs with the coupled ice-ocean-biogeochemical model ORCA-LIMSWAMCO- 4, predict that the efficiency of the S.O. biological pump will be very sensitive to changes in the seasonal amplitude and the mean extent of the ice cover

Integrated study of southern ocean biogeochemistry and climate interactions in the Anthropocene “BELCANTO III”. Final Report

The overall objective of the BELCANTO III project was to conduct targeted process studies and develop new proxies to construct and validate a realistic 3D ice-ocean biogeochemical model for the area south of latitude 30°S, based on improved understanding of the different factors regulating interactions between the atmosphere, ocean circulation and biogeochemical cycles and on synthesis/collection of existing/new data sets. The specific objectives were: - to improve our understanding about the iron cycling, with focus on the significance of iron-organic matter interactions for iron bioavailability and the efficiency of the biological pump (WP1) - to quantify nutrient consumption, CO2 uptake and C-export by assessing seasonality and interannuality of nutrient (Si, N) uptake; reconstruct carbon uptake and export from the surface layer via a multi-proxy approach; provide a relevant data set for 3D-NEMO-SWAMCO model validation (WP2) - to characterise the biogenic particles exported from the surface to the twilight zone and study their fate by quantifying remineralisation rates (WP3) - to model biogeochemical cycles in the modern Southern Ocean including seaice biogeochemistry (WP4) - to estimate impact of future climate change in the Southern Ocean through process-studies and modelling approaches (WP5)