Sophie Violette

Noble gases as natural tracers of water circulation in the Paris Basin: 2. Calibration of a groundwater flow model using noble gas isotope data

Using the rare gas concentrations in the aquifers of the Paris Basin (see part 1 of this series), a numerical model of a two-dimensional cross section of the entire Paris Basin was built to simulate groundwater flow and the transport of 3He, 4He, and 40Ar isotopes. The model included seven aquifers separated by seven aquitards in a steady state flow regime. Transport of the gases is by advection, diffusion, and dispersion in steady or transient states. The 4He transport was simulated first and made it possible to calibrate both the crustal flux of this isotope and the average permeability of each aquifer, which were then favorably compared with measured values. These values present a high variability from aquifer to aquifer, between 8.5 × 10−7 and 3.5 × 10−4 m s−1. The water velocities and average residence times were also estimated. Average turnover times for the different aquifers are highly variable, ranging from 8700 years for the shallowest one (Ypresian) to 30 Myr for the deepest one (Trias). The calibrated model was also able to correctly represent the distribution of 3He and 40Ar in the basin. Diffusion proved to be an important mechanism for vertical transfer through the aquitards of the helium isotopes, as opposed to 40Ar, which is transported mainly by advection. On the basis of the (4He/40Ar) radiogenic ratio a constant value of 10−l1 m s−1 was attributed to the permeability of all the aquitards. A sensitivity study showed that the permeability of the aquitards situated in the lower part of the basin (Lias and aquitards in the Triassic and Dogger) could not be higher than 10−11 m s−1 given the observed distribution of the radiogenic 4He/40Ar ratio, but a lower limit could not be defined. The crustal fluxes of 3He, 4He, and 40Ar in the basin were estimated at 4.33 × 10−13 mol m−2 yr−1, 4. × 10−6 mol m−2 yr−1 and 2.52 × 10−7 mol m−2 yr−1, respectively. The simulation of the 3He and 4He transport showed that theR/Ra ratio (value of the measured R = 3He/4He ratio normalized to the atmospheric ratio Ra) entering at the base of the Trias from the bedrock remained constant while crossing the basin except in the zones close to the recharge areas where it is influenced by the atmospheric component. This constancy is due to the low radiogenic/nucleogenic production rate of these isotopes inside the basin, as compared to the crustal flux.

Hydrologic and thermal modeling of an active volcano: the Piton de la Fournaise, Reunion

Abstract The hydrology of active volcanic islands and the interactions between cold groundwater flow and hot magmatic fluids are, as yet, poorly understood. In this study of the Piton de la Fournaise, Reunion, we try to estimate the different terms in the hydrologic balance and describe the nature and importance of groundwater flow in an active volcano. A challenging aspect of this study was that only limited data were available, a common feature of such systems. There were some volcanologic data, few hydrologic data and a deep geothermal borehole where a thermal profile had been established. First, we develop a global hydrologic model of the Piton de la Fournaise by making certain assumptions concerning groundwater pathways. This model is calibrated on the available hydrologic data. We then build a thermal model of the volcano based on hypotheses of its internal structure and on the groundwater flow pattern derived from the first model. This model is fitted on the thermal profile. We conclude that groundwater flow inside the volcano can be described as a succession of local perched aquifers, which eventually release water to a continuous basal aquifer close to the coast, with diffuse outlets to the ocean. The infiltrating water is kept away from the magma chamber by a thick impervious shell surrounding the chamber, probably created by silica precipitation in the fractured basalt. Heat is essentially conveyed to the ocean by groundwater flow. This model is likely to be valid for other active hot-spot volcanoes.

Experimental device for chemical osmosis measurement on natural clay-rock samples maintained at in situ conditions: implications for formation pressure interpretations.

In order to characterize the so-called coupled processes occurring in compacted clay rocks, the coupling coefficients must be identified. For this purpose, an original device which allows such measurement for undisturbed (natural) samples in their in situ conditions was developed. The present experimental device minimizes the fluid leaks improving the accuracy of the coupling parameter determination. Three chemical osmotic tests were performed on a cylindrical sample of Callovo-Oxfordian argilite. Room temperature variations during the chemical osmosis experiments required the implementation of temperature effects in the numerical model used for the interpretations. These variations offered the opportunity of an alternative method to estimate the compressibility of the fluid in the circuit connected to a measurement chamber located in the center of the sample. An osmotic efficiency of almost 0.2 for a concentration of 0.094 mol L(-1) is obtained for the Callovo-Oxfordian argilite. This value would explain only some part (approximately 0.10-0.15 MPa) of the overpressures (0.5-0.6 MPa) relative to the surrounding reservoirs measured in this formation. Others processes, such as thermo-osmosis, hydrodynamic boundary condition changes due to climate variations or creep behavior of the shale, could explain the remainder of the overpressures.

High resolution climate and vegetation simulations of the Mid-Pliocene, a model-data comparison over western Europe and the Mediterranean region

Here we perform a detailed comparison between climate model results and climate reconstructions in western Europe and the Mediterranean area for the mid-Piacenzian warm interval ( ca 3 Myr ago) of the Late Pliocene epoch. This region is particularly well suited for such a comparison as several quantitative climate estimates from local pollen records are available. They show evidence for temperatures significantly warmer than today over the whole area, mean annual precipitation higher in northwestern Europe and equivalent to modern values in its southwestern part. To improve our comparison, we have performed high resolution simulations of the mid-Piacenzian climate using the LMDz atmospheric general circulation model (AGCM) with a stretched grid which allows a finer resolution over Europe. In a first step, we applied the PRISM2 (Pliocene Research, Interpretation, and Synoptic Mapping) boundary conditions except that we used modern terrestrial vegetation. Second, we simulated the vegetation for this period by forcing the ORCHIDEE (Organizing Carbon and Hydrology in Dynamic Ecosystems) dynamic global vegetation model (DGVM) with the climatic outputs from the AGCM. We then supplied this simulated terrestrial vegetation cover as an additional boundary condition in a second AGCM run. This gives us the opportunity to investigate the model’s sensitivity to the simulated vegetation changes in a global warming context. Correspondence to: A. Jost (anne.jost@upmc.fr) Model results and data show a great consistency for mean annual temperatures, indicating increases by up to 4 C in the study area, and some disparities, in particular in the northern Mediterranean sector, as regards winter and summer temperatures. Similar continental mean annual precipitation and moisture patterns are predicted by the model, which broadly underestimates the wetter conditions indicated by the data in northwestern Europe. The biogeophysical effects due to the changes in vegetation simulated by ORCHIDEE are weak, both in terms of the hydrological cycle and of the temperatures, at the regional scale of the European and Mediterranean mid-latitudes. In particular, they do not contribute to improve the model-data comparison. Their main influence concerns seasonal temperatures, with a decrease of the temperatures of the warmest month, and an overall reduction of the intensity of the continental hydrological cycle.

Hydraulic and chemical pulse tests in a shut‐in chamber imbedded in an argillaceous formation: Numerical and experimental approaches

Hydraulic and chemical pulse tests were performed in four shut-in chambers to obtain hydraulic and chemical parameters of the Callovo-Oxfordian shale. The osmotic tests, i.e., chemical pulses, were carried out by exchange of the formation water for a different solution either more or less concentrated. Two sets of two osmotic tests were done in March and October 2006. These osmotic tests are superimposed on a purely hydraulic response because of the drilling of the well causing a pressure drop. The pressure records (hydraulic and osmotic responses) were interpreted using a 2-D model to obtain the hydraulic and chemical parameters. The osmotic efficiency inferred for the Callovo-Oxfordian shale is on the order of 0.012 for a concentration of 0.43 mol L-1 and 0.12 for a concentration of 0.086 mol L-1. These results suggest that the Callovo-Oxfordian shale behaves as a weakly semipermeable membrane and only 0.1-0.15 MPa of the 0.2-0.5 MPa can be explained by these osmotic efficiency values.

Estimating thermo-osmotic coefficients in clay-rocks: II. In situ experimental approach

Water flow in compacted shales is expected to be modified by thermo-osmosis when a thermal gradient exists. However this coupled-flow process is poorly characterized since no experiments on non-remoulded clay-rocks are found in the literature. This paper presents a set of thermo-osmosis experiments carried out in an equipped borehole installed in the Liassic argillite at the Institut for Radiological protection and Nuclear Safety (IRSN) underground research laboratory (URL) of Tournemire (southeastern France). A numerical model - including coupled-flow equations, mass conservation laws, thermal expansion and changes of water properties with temperature - was developed for the interpretation of these experiments. A thermo-osmotic response was deduced from the pressure evolution in the test interval after temperature pulses (+2.5, +5.1, and +9 degrees C). The values of thermo-osmotic permeability determined during the experiments range between 6x10(-12) and 2x10(-10)m(2)K(-1)s(-1), depending on the pulse temperature and uncertainties on the model parameters. A sensitivity analysis on several model parameters was performed to constrain these uncertainties.

Fluid palaeo-circulation generated by the Pyrenean orogeny and its potential role in the formation of lead-zinc deposits in the Cévennes region: a modelling approach

Recent studies of the host rock palaeomagnetism of the lead-zinc deposits on the Cevennes margins pointed towards regional fluid circulation from the early to middle Eocene. The hypothesis has therefore been put forward that mineralising fluids might have migrated as a consequence of the Pyrenean uplift. Based on this assumption, a digital model was developed to describe this palaeo-circulation along two reconstituted cross-sections, in early Eocene times. One of them extends from the Gulf of Lion to the Les Malines deposits at the southern end of the Cevennes mountains; the other one connects the Montagne Noire to Les Malines in order to test the hypothesis of a more localised fluid circulation. The modelling of heat and fluid circulation along these cross-sections is constrained mainly by fluid-temperature data, derived from analyses of fluid inclusions. The maximum recorded temperatures are about 150°C. The METIS code (Ecole des Mines, Paris) was used to test the transport scenarios while prescribing hydrodynamic characteristics in the series that would allow fluid flow. Gravity-driven flow is initiated at a high point, either the Montagne Noire or the Pyrenees. Drainage occurs at depth. The permeable formations concerned are: Cambrian dolomite in the cross-section beginning in the Montagne Noire, and Triassic and Liassic carbonate or sandstone formations in the other one. The fluids converge at the deposit site through faults on the margins of the Cevennes horst. The highest temperatures reproduced by the digital simulations in a steady-state regime are in the order of 80°C at the deposit site for each pathway. A sensitivity test showed that higher temperatures, in the order of 150°C, could only be reached with a heat flux of 120 mW.m-2 and by optimising such parameters as permeability, aquifer geometry and thermal conductivity. However, such a parameter set does not seem geologically feasible. The modelling demonstrates that circulation must have occurred at greater depths in the case of gravity-driven fluid flow. The most probable explanation is that the fluid migrated in the deep crustal basement and that, during its ascent along the faults bordering the Cevennes heights, it mixed with basinal brines migrating through shallower aquifers.

Geochemical study of surface waters in mountain granitic area. The Iskar upper watershed: Massif of Rila, Bulgaria

The aim of this study is to determine the characteristics of the Iskar upper watershed surface waters as a first step of a seasonal evolution study. The geochemical and petrochemical characteristics of the Rila massif are well-suited to study the relative significance of different weathering mechanisms occurring along the year in a mountain aquatic system. In such an area the atmospheric input contribution to the surface water quality could be assessed. A first sampling survey in summer-time allowed us to get some data on major and minor elements in the soluble phase. The results show that the Iskar upper watershed is well-suited because of no mixing with long residence time groundwaters and of the low population density which results in insignificant anthropic input.

Atmospheric, weathering and biological contributions in the chemical signature of stream water: the upper Iskar Reka watershed, Bulgaria

Abstract The chemical signature of stream water is influenced by several factors: atmospheric input, weathering of bedrock and soils, biological uptake, soil storage, and decomposition of organic matter. The importance of weathering and biological activity, according to season, can be assessed by subtracting atmospheric input. In the upper Iskar Reka watershed, Bulgaria, results show that Na and Ca are mainly exported out of the basin (weathering is dominant), while nitrate and sulphate are consumed/stored. The behaviour of K suggests that biological activity is dominated by vegetation uptake in spring and by bacterial activity in autumn. In summer, the weathering load of Mg is compensated by reactions with clays. The low nutrient input from weathering due to low reserves in the soil may lead to a biomass production closely linked to a rapid internal cycle and also to a relative sensitivity of the ecosystem to any change in vegetation cover and atmospheric input. Editor Z.W. Kundzewicz Citation Gassama, N. and Violette, S., 2012. Atmospheric, weathering and biological contributions in the chemical signature of stream water: the Upper Iskar Reka watershed, Bulgaria. Hydrological Sciences Journal, 57 (3), 535–546.

Chemical characterization of aquifers with the same geological origin

Physical characteristics of the Bicaz dammed lake are described as follows: water volume of 1.150x 109 m 3, 31.1 km in length, 2 km in width. The sluice is at 45 m above the maximal altitude of the lake. The Bistrita River is the main tributary of the lake: mean water contribution of 1.721x109 m3/a, mean suspended matter flow of 291000 t/a. This river drains a catchment rich in formerly mined mineralizations of polymetallic sulfides. The aim of this study is to determine if the lake can be considered as a sink toward trace elements. We analyzed major and minor elements, TOC, organic N, and some trace elements (e.g. As, Ba, Cd, Cr, Co, Cu, Mo, Ni, Pb, Sb, Sn, V, U, Zn) in filtered and bulk water samples. We worked on three locations, an upper site, a middle site, and a down site. Here we present the data for July and October 2005. Some trace elements have smaller dissolved concentrations in lake water than in the Bistrita River. Common associations can be defined in this lake such as: Co is associated with Mn; As is released in solution as Fe oxi-hydroxide is dissolved; V and U are following the same behaviour, etc. However, their distribution is not as regular as in a natural lake. The stratification of the water column at all 3 sampled sites is not as regular as expected. At the upper site (water depth of 33 m), the water column is homogeneous until 20 m, then the redox condition allows having Mn as Mn (+ II ) and Fe as Fe (+III). For some elements a perturbation can be noticed at about 10 m. At the middle site (water depth of 59 m), there is a water section richer in dissolved and particulate Fe and Mn, with a dominant particulate form. This can assess this section, in between 30 to 50 m it is more oxidizing. At the down site, the reverse figure can be seen between 30 and 50 m depth. This redox anomaly is more noticeable for elements with a high solubility difference according to their oxidation rate. Other parameters do not follow this figure. This section could be linked to the sluice drawing off. In the whole lake, the stratification is more and more visible from the up to the down stream but it is not so pronounced than in natural lakes. Water column structuring seems to be disturbed by a lateral flow, induced by the sluice drawing off.